Symposium Organizers
Tony van Buuren Lawrence Livermore National Laboratory
Leonid Tsybeskov New Jersey Institute of Technology
Susumu Fukatsu University of Tokyo
Luca Dal Negro Boston University
Fabrice Gourbilleau CIMAP, UMR CNRS
MM1: Light Emission and Photonic Devices
Session Chairs
Monday PM, December 01, 2008
Room 309 (Hynes)
9:30 AM - **MM1.1
Nanostructures for a Silicon Photonics.
Francesco Priolo 1
1 Dept. Physics & Astronomy, CNR-INFM MATIS & University of Catania, Catania Italy
Show AbstractRecent efforts towards a Si-based nanophotonics will be reviewed. It will be shown that Si nanostructures embedded in silica represent an extremely promising route. In particular a detailed correlation between structural and optical properties will be presented. We will demonstrate that, in contrast to what generally believed, properties of films grown by different methods are indeed very different as a result of the agglomeration properties. In particular, while Si-rich oxides deposited by sputtering present an almost full agglomeration of the Si excess atoms and strong luminescence at temperatures of about 1100 °C, films grown by plasma enhanced chemical vapor deposition (PECVD) present agglomeration of only a small fraction of the Si excess and require very high annealing temperatures to maximize luminescence. Amorphous Si nanoclusters may represent an interesting alternative for the monolithic integration of optical functions in Si technology. In particular, amorphous nanoclusters exhibit an intense room temperature electroluminescence (EL) with the advantage to be formed at a temperature remarkably lower than the temperature needed for the formation of Si nanocrystals. In addition, low operating voltages are required. The doping of these structures with rare earth ions will also be discussed. The introduction of Er ions on these structures produces a preferential transfer of the energy from an exciton in the nanocrystal to the rare earth. We will show that the different agglomeration of the excess Si in a SiOx matrix prepared by PECVD or sputtering can strongly affect the Er luminescence. Er-doped Si-nanocluster based light emitting devices operating at 1.54 microns will be demonstrated. We will show that the electrical and optical properties of the devices strongly depend on the technique used to grow the active layer. In particular, efficient devices based on sputtered films operate at much lower current densities with respect to PECVD ones. The waveguiding, confining and emission properties of active silicon-on-insulator (SOI) slot waveguides and photonic crystal (PhC) slabs containing Er and silicon nanocrystals are also studied in details. It is shown that the vertical emission properties of the slot waveguides are changed dramatically by patterning the waveguide core with a two-dimensional PhC lattice with an enhancement of the near-normal emission by more than two orders of magnitude with respect to the unpatterned slab. Finally, the advantages of using erbium silicate compounds as silicon compatible efficient emitters will be demonstrated. It is shown that carrier mediated excitation is achievable in these systems when a silicate/silicon multilayer is fabricated. Moreover electroluminescent devices based on Er silicate compounds will be demonstrated.These data are presented and the route towards electrically-driven amplifiers and lasers will be discussed.
10:00 AM - MM1.2
Parametric Study of the Optical Gain in Structures Containing Silicon Nanocrystals.
Dimitri Koshel 1 , David Barba 1 , Francois Martin 1 , Guy Ross 1
1 , INRS-EMT, Varennes, Quebec, Canada
Show AbstractSi-laser development is critically dependant on the optimization of optical gain in silicon structures. We present measurements of optical gain for samples containing silicon nanocrystals (Si-nc), produced by ion implantation, with different sizes and depth distributions providing a wide spread of optical emission and wave guiding characteristics. The effect of the ion fluence, thermal annealing temperature, and hydrogen passivation on the optical properties of the samples has been investigated. Measurement of optical gain in Si-nc structures is very challenging, requiring a very careful approach. Special attention must be given to pumping beam uniformity, diffraction effects, detector optical coupling and sample degradation at high pump laser power, factors that are critical to the accuracy of VSL measurements. Detailed measurements of the amplified spontaneous emission (ASE), for wavelengths in the 600 to 950nm range, have been performed using a high-resolution variable stripe length (VSL) approach with CW excitation. The dependence of optical gain on pumping power has been investigated. Gain measurements using the pump and probe (PnP) method were performed in support of the VSL measurements. The net modal gain has been evaluated for different spectral wavelengths and compared to the spectral dependence of the photoluminescent emission (PL). Generally, highest optical gain is at the peak wavelength PL emission.The photoluminescence spectra recorded during VSL measurements do not exhibit significant spectral distortion or energy shifts. This strongly suggests that the stimulated emission process is associated with an inhomogeneous optical amplifier medium. The parametric dependence of optical gain on ion implantation, thermal annealing and passivation will be discussed.
10:15 AM - MM1.3
Time-resolved Measurements of Optical Gain in Brightly Emitting Silicon Nanocrystals Embedded at High Densities in an SiO2 Based Matrix.
Katerina Dohnalova 1 , Lukas Ondic 1 , Katerina Kusova 1 , Ondrej Cibulka 1 , Ivan Pelant 1 , Karel Zidek 2 , Frantisek Trojanek 2 , Petr Maly 2 , Bernd Honerlage 3 , Olivier Cregut 3 , Pierre Gilliot 3 , Jean Oberle 4
1 Department of Thin Films, Institute of Physics, ASCR, Prague Czechia, 2 Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Prague Czechia, 3 GONLO, Institute de Physique et Chimie des Matériaux de Strasbourg, Strasbourg France, 4 CPMOH, Université Bordeaux 1, Talence France
Show AbstractVarious hybrid approaches to build-up laser diode for silicon photonics has been recently successfully realized, as e.g. various III-V materials integrated within the silicon chip or near-infrared emitting Er-coupled silicon nanocrystals (Si-ncs) in SiO2. Lasing in a purely silicon-based material, however, has not yet been reported and represents an important keystone for the basic semiconductor research and further possible application as a low cost silicon laser source. One of the promising materials for this purpose appears to be a material based on oxidized silicon nanocrystals (Si-ncs) embedded at high densities in SiO2 based matrix. The high volume fraction, small diameter and blue-shifted emission of Si-ncs are theoretically required for the observation of the stimulated emission. The high density requirement, however, does not match within (the present technology state-of-the art) the high optical quality standard necessary for the optical feedback application. In our previous publications we have made a step closer towards the lasing in silicon nanocrystals [1-4], however, the optical quality of the samples was rather poor. In this contribution we wish to report on our recent study of the time resolved optical gain in a new samples generation, prepared by electrochemical etching and intense H2O2 post-etching of Si wafers. Such samples contain small Si-ncs (2-3 nm in diameter) embedded at very high densities (~10^19 Si-ncs/cm3) in an SiO2 based sol-gel matrix. They exhibit higher optical quality and emit below 600 nm. We investigate behavior of the optical gain between 500 and 650 nm. We brought out standard VSL (variable stripe length) and SES (shifting excitation spot) measurement using two different excitation laser systems to confirm our measurements. Namely we used femtosecond amplified Ti:sapphire laser (100 fs pulse length, rep. rate 200 kHz) and Q-switched Nd:YAG laser with nanosecond pulses (7 ns pulse length, rep. rate 10 Hz). We observed in both cases optical gain in the spectral region 500-600 nm, blue-shifted compared to PL spectrum. Material with such wide gain spectra can be interesting for its potential using in optoelectronics as a tunable laser light source.[1] Appl. Phys. Lett. 84 (2004), 3280[2] J. Appl. Phys. 99 (2006), 116108[3] Appl. Phys. Lett. 88 (2006), 251105[4] New Journal of Physics 10 (2008), 063014
10:30 AM - **MM1.4
Si Nanostructures for Efficient, Tunable and Modulable Light Emission Devices.
Blas Garrido 1 , Olivier Jambois 1 , Josep Carreras 1 , Mariano Peralvarez 1 , Yonder Berencen 1
1 Electronics, University of Barcelona, Barcelona, Barcelona, Spain
Show AbstractWe will review in this contribution our recent developments on silicon based light emitting diodes (Si-LEDs). Si-LEDs of silicon nanoclusters (Si-nc) emitting in the red will be first introduced and different solutions, like pulsed polarization and/or nitride-oxide double stack will be discussed to increase efficiency (up to 0.1%) and reliability. A simulator for the electrical injection and conduction in these materials has been developed, and its results will be compared with experiment and theory. Doping Si-nc in SiO2 with other impurities is shown to shift the emission wavelength, and results will be presented on high power white tunable Si-LEDs (co-doping with C) and 1.5 μm emitting Si-LEDs (co-doping with Er). Finally, other device architectures will be shown to have unique properties, like Si-LED MOS transistor, which shows fast built in modulation capabilities of the current (and emission) of the gate, due to the wide frequency response of the transistor transconductance and the fast quenching of the emission of the Si-nc by Auger non-radiative recombination.
11:30 AM - **MM1.5
Plasmon Photodiode for On-Chip Optical Interconnect.
Keishi Ohashi 1 2 , Junichi Fujikata 1 2
1 , NEC Corporation, Tsukuba, Ibaraki, Japan, 2 , MIRAI-Selete, Tsukuba, Ibaraki, Japan
Show AbstractConventional optical components are too large for LSIs when compared with electronic components such as transistors. The fundamental difference in size comes from the fact that the wavelength of light (~1 μm) is longer than the de Broglie wavelength of electron (~10 nm). One of the approaches to fill the gap of this size is to use near-field optics which size is not constrained by the diffraction limit. Strong near-field can be created by the use of surface-plasmons, and they offer an approach to nano-scale photonic device. The size of the confined near-field region is comparable with the featured size of the materials. We have developed several types of surface-plasmon antennas for photodetectors. A small semiconductor structure is located near the antenna to absorb near-field light. The size of the structure can be made as small as that of the Schottky depletion layer, and hence the separation between electrodes can be reduced almost to the size of the near-field region.Silicon has not been a popular material for high-speed photodetectors in spite that it has been the best material for LSIs. One of the main reasons for this is that the absorption length of silicon is as large as 10 μm or more, resulting in a long carrier drift time and slow response. We have demonstrated a plasmon photodiode or a “Si nano-photodiode” that uses a small volume of near-field light to reduce the size of the silicon. The size of Schottky region in silicon can be reduced almost to the size of the near-field region (10-100 nm long) and the transit time of the carriers is fairly short accordingly. The full-width at half-maximum of the impulse response was as fast as 20 ps even when the bias voltage was less than 1 V. The surface plasmon antenna acts as a resonant cavity that temporarily reserves optical energy. The cavity function makes sense when the resonance is achieved in a shorter time than the response time of the photodiode.We have also developed a waveguide-integrated photodiode with a surface plasmon antenna for on-chip optical interconnects. A pair of interfacial periodic metal–semiconductor–metal Schottky electrodes was fabricated. It also works as a surface plasmon antenna. A high speed response time of 17 ps was obtained. We have demonstrated the operation of on-chip optical clock system by using this technology.
12:00 PM - MM1.6
Modulation of Light Emission Form Si Nanocrystals by Quantum Confined Stark Effect.
Mustafa Kulakci 1 , Ugur Serincan 2 , Ceyhun Bulutay 3 , Rasit Turan 1
1 Physics, Middle East Technical University, Ankara Turkey, 2 Physics, Anadolu University, Eskisehir Turkey, 3 Physics, Bilkent University, Ankara Turkey
Show AbstractQuantum confined Stark effect (QCSE) in Si nanocrystals can be utilized for the production of Si based electro-optical devices sucs as optical modulators and waveguides, since light absorption and emission can be controlled by using an external bias. Optical sources based on waveguide devices such as ring resonators can be fabricated by making use of QCSE that can provide tunable light for optically functional circuits. Successful fabrication of such devices may lead to brakethroughs in the fabrication of integrated electo-optical systems. In this work, Quantum confined Stark effect (QCSE) on excitons confined in Si nanocrystals embedded in SiO2 matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. PL peak shifts to lower energies with increasing electric field are recorded as expected from carrier polarization within the quantum dot. Reducing the measurement temperature further enhances the QCSE due to improved localization at the lowest energy states of the quantum dot. The variation of the PL intensity with applied voltage and temperature are also studied to understand the effect of other mecahisms such as carrier escape from the nanocrystals and Auger recombination. It is shown that the emission intensity remains constant for a wide range of applied voltage indicating that the observed energy shift is related to QCSE rather than carrier population of the nanocrystal. We have also found that the effect depends on the polarity of the applied voltage. This dependence is studied through C-V measurements in which the charge injection into the oxide from the substrate can be observable.We have shown that experimental results agree well with the simpified theoretical approaches which commonly suggests the quadratic dependence of the energy shift on the applied electric field. Moreover, we have performed rigorous theoretical calculations based on an atomistic pseudopotential Hamiltonian solved using the linear combination of Bloch bands technique. We identify that the nanocrystals should have a diameter of about 5.6 nm according to the peak emission wavelength when matched to the calculated effective optical gap. Our theoretical results reproduce the peak emission shift as a function of the applied electric field without any fitting parameters. Furthermore, the theoretical analysis reveals that most of the Stark shift is due to valence states. The nonmonotonic behavior of the PL peak intensity is also reproduced by the theoretical calculations at 30 K and 300 K. In summary, this is the first demonstration of QCSE in silicon nanocrystals which is also supported by rigorous theoretical analysis.
12:15 PM - MM1.7
Effects of an Applied Electric Field on Silicon Nanocrystals Photoluminescence.
Alexandre Lacombe 1 , David Barba 1 , Félix Beaudoin 1 , François Martin 1 , Guy Ross 1
1 , Institut National de la Recherche Scientifique (INRS) - Énergie, Matériaux et Télécommunications (ÉMT), Varennes, Quebec, Canada
Show AbstractSilicon nanocrystals (Si-nc) obtained by silicon implantation in silicon oxide (SiO2) are known to exhibit either photoluminescence (PL) or electroluminescence (EL) under proper excitation. However, EL intensity is typically 100 to 1000 times weaker than PL intensity. Resistivity of the surrounding SiO2 matrix and the effect of the high electric field applied are suspected to be responsible for the weak EL. These hypotheses are being investigated by applying selected bias voltages (i.e. electric fields) to MOS-like devices containing Si-nc during the PL process. In this experiment, an amorphous SiO2 layer with a thickness of ~60 nm has been thermally grown on an n-type silicon substrate, then two successive Si+ ion implantations have been made to ensure good uniformity of Si-nc within the oxide. Si+ ions were implanted with an energy of 25 keV to a fluence of 2.5 x 1016 cm-2, for the first implantation, and with an energy of 12 keV to a fluence of 1.0 x 1016 cm-2 for the second implantation. A 15 nm semitransparent gold electrode has been deposited on top of the oxide to allow for optical measurements while biasing the device. Photoexcitation is generated by a 405 nm laser diode directed at 45° on the top electrode of the samples, while the PL signal is collected normally to the electrode. Variable DC voltages have been applied between the top electrode and the back silver electrode, the latter forming an ohmic contact with the doped silicon substrate.The measurements show that in all cases, the PL intensity is significantly affected by the applied electric field, while no clear energy shift is observed. In forward bias, increasing the voltage enhances the PL intensity until a specific field strength is reached (~0.4 MV/cm), then gradually quenches the PL signal. In reverse bias, however, increasing the voltage always reduces the PL intensity. Also, the electric current, I, flowing through the device has been measured as a function of the applied voltage V, yielding I-V curves with the expected rectification behavior typical of a diode. However, in reverse bias, the laser photoexcitation induces a strong enhancement of the current along with the PL quenching described above, while no noticeable change occurs in forward bias. Finally, for all biases, the rate of change of both the PL intensity and the current, I, with respect to voltage, V, is high for electric field strengths under ~0.4 MV/cm and becomes lower at higher fields. These results show that an applied electric field has indeed important effects on the luminescence processes in Si-nc. These effects are discussed in terms of photogeneration, injection and recombination of charge carriers, and pave the way to a stronger EL signal in Si-nc embedded in a SiO2 matrix.
12:30 PM - **MM1.8
Dielectric vs. Plasmonic Cavities for Si Nanostructures.
Mark Brongersma 1
1 Department of Materials Science and Engineering, Stanford University, Palo Alto, California, United States
Show AbstractMM2: SiGe Nanostructures II
Session Chairs
Monday PM, December 01, 2008
Room 309 (Hynes)
2:30 PM - **MM2.1
Three-dimensional Silicon-Germanium Nanostructures for CMOS Compatible Light Emitters and Optical Interconnects.
David Lockwood 1 , Jean-Marc Baribeau 1 , Leonid Tsybeskov 2
1 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 2 Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States
Show Abstract3:00 PM - MM2.2
Composition and Strain Contrast of Si1-xGex (x = 0.20) and Si1-yCy (y ≤ 0.012) Epitaxial Films on (100) Si in Annular Dark Field Images.
Xiaohua Wu 1 , Jean-Marc Baribeau 1
1 Institute for Microstructural Sciences, National Research Council Canada, Ottawa, Ontario, Canada
Show AbstractHetero-epitaxy provides a means to combine lattice-mismatched semiconductor materials without introduction of defects. This approach makes it possible to engineer the electronic and opto-electronic properties of layered materials through the control of the composition and strain. Si1-xGex on Si is among the best studied hetero-epitaxial strained systems, and conventional transmission electron microscopy (TEM) played an important role in the observation of layer morphology and misfit dislocation in relation to the strain relaxation in this system. Contrary to conventional high resolution TEM lattice images where Si and Ge atoms can hardly be distinguished, annular dark field (ADF) imaging obtained in a scanning electron microscope (STEM) provides a means to probe atomic distribution due to the Z (atomic number) contrast nature of this technique. The ADF-STEM image contrast is known to be dependent on the average atomic number, Z, in a simple Zn power-law relationship. For most microscope geometries, n is in the range of 1.6 to 1.9. Contradictory to this compositional contrast prediction, ADF-STEM imaging of dilute GaNyAs1-y strained films showed that the lower average atomic number strained GaNyAs1-y layers were brighter than the higher average atomic number neighboring GaAs ,This phenomenon was explained by the local atomic displacement around substitutional N [Wu et al, J. Phys: Condens. Matter, 20, 075215 (2008)]. Here the ADF-STEM technique is applied to the study of compression strained Si0.8Ge0.2 layers and tensile strained dilute Si1-yCy (y ≤ 0.012) epitaxial layers grown on (100) Si substrates by molecular beam epitaxy (MBE). A series of ADF images were obtained with detector inner semi-angle ranging from 29 – 92 mad, and sample thickness ranging from 50 – 300 nm. The observed contrast of the ADF-STEM images is discussed in relation to the different strain status in Si1-xGexand Si1-yCy epitaxial layers. The intensity line profile of the ADF-STEM images is analyzed to obtain the composition profile in the film. The results obtained for different sample thicknesses and ADF detector angles agree very well with composition profiles obtained with analytical TEM techniques: energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), as well as secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES).
3:15 PM - MM2.3
Suppression of Nonradiative Auger Recombination in Si1-xGex/Si Superlattices Under High-Density Photoexcitation.
Takeshi Tayagaki 1 , Susumu Fukatsu 2 , Yoshihiko Kanemitsu 1 3
1 Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan, 2 Department of Pure and Applied Science, The University of Tokyo, Tokyo Japan, 3 Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto Japan
Show AbstractThere have been numerous attempts toward efficient light emission and lasing in the various types of Si nanostructures. Dynamics of high-density carriers in Si-based nanostructures play an essential role in light emission and optical gain processes. Under the high-density excitation, the nonradiative Auger recombination causes the saturation of the photoluminescence (PL) intensity in Si-based nanostructures. The understanding of the mechanism of the Auger recombination in the Si nanostructures is one of the most essential and crucial issues for the physics of highly dense carriers in nanostructures and the design of Si-based laser. In this paper, we report the PL dynamics in the Si1-xGex/Si superlattice (SL) and single quantum wells (SQW) under high-density excitation at low temperatures. The Si1-xGex/Si heterostructures are a material system compatible with the standard Si processing technology and provide us with a unique opportunity to investigate dynamics of high-density carriers in the artificially-controlled nanostructures. The samples were the 99 period strained Si1-xGex/Si SL with x=0.15 and the SQW. The time-integrated PL spectra and the PL dynamics under 3.0 eV excitation were measured at 10 K by a InGaAs array detector and a gated-photon counting system with a photomultiplier. In the SL, three PL peaks, assigned to the TO, TA, and no phonon-assisted transition, are observed. With increasing the excitation intensity, the PL lines show the broadening to the higher energy side, and the blueshift of the PL peak energy appears. These phenomena are explained by the saturation of the localized exciton and the redistribution of excitons to the delocalized states. Moreover, the saturation behavior of the PL intensity and the rapid PL decay are observed under high-density excitation. These behaviors are due to Auger recombination. To extract the characteristics in the SL, we compare the PL intensities in the SQW with the SL under the same excitation conditions. While the PL intensities in the SL are proportional to the excitation intensity up to 106 W/cm2, those in the SQW show the saturation behavior, indicating the appearance of the Auger loss, even under the weak excitation of 102 W/cm2. Although the carrier sheet density is about 102 times smaller than that in the SQW by considering the well periods in the SL (99 pairs), the significant difference indicates that the Auger recombination in the SL is suppressed compared to the SQW. This suppression is believed to be related to the delocalized states in the SL. Due to the electronic coupling between the wells, the delocalized state in the SL extends over the Si barrier layer. The Auger rate in the bulk Si was calculated to be smaller than that in the Si1-xGex well layer. Therefore, the high-density carriers in the SL show the small Auger loss, compared to the SQWs. The superlattice structures have many advantages over the bulk crystals and SQWs, because of their small Auger rate and strong PL.
3:30 PM - MM2.4
Raman Characterization of Active Impurities in Single Boron-doped Si1-xGex Alloy Nanowires.
Chiharu Nishimura 1 , Go Imamura 1 , Takahiro Kawashima 2 , Minoru Fujii 1 , Tohru Saitoh 2 , Shinji Hayashi 1
1 , Kobe University, Kobe Japan, 2 , Matsushita Electric Industrial Co.,Ltd., Moriguchi Japan
Show AbstractSi1-xGex alloy nanowires (SiGeNWs) synthesized by the vapor-liquid-solid(VLS) growth process have been attracting increasing interests because the electronic properties can be tailored in an extended range compared to those of SiNWs, and they can be used as transistors, chemical sensors, and light emitting devices. For these applications, precise control of the composition and the impurity profiles, and the development of the technique to characterize the profiles are indispensable. In this work, we employ micro Raman spectroscopy to characterize concentration and distribution of Ge and electrically-active impurities in in-situ boron (B)-doped SiGeNWs synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD).In Raman spectra of SiGeNWs, three major peaks are observed at 500-520, 400-410, and 280-290 cm-1 due to the vibrations of adjacent Si-Si, Si-Ge, and Ge-Ge pairs, respectively. The Ge composition can be estimated from the intensity ratio. Furthermore, the active B concentration can be estimated from the spectral shape. We show that the Si-Si mode of B-doped SiGeNWs has a long tail towards a high-wavenumber side. This spectral shape is attributed to Fano resonance between discrete phonon Raman scattering and continuous electric Raman scattering caused by the excitation of holes in the valence band. To evaluate the concentration of active B atoms from the asymmetric spectral shape, the spectra are fitted by Fano resonance formula and asymmetry parameters are extracted. From the comparison of the asymmetry parameter with those obtained for reference samples, i.e., B-implanted Si, the concentration of active B atoms is roughly estimated. We show that there are distributions of the Ge and the active B concentrations within individual B-doped SiGeNWs. The comparison of these distributions in SiGeNWs reveals that there is strong correlation between the concentrations, i.e., high Ge concentration region is always more heavily B doped. The correlation strongly suggests that supply of B2H6 during VLS growth enhances conformal deposition of high Ge and B concentration layers and the conformal deposition results in the distribution of the Ge and the B concentration within SiGeNWs.
MM3: Si Nanostructures I
Session Chairs
Monday PM, December 01, 2008
Room 309 (Hynes)
4:15 PM - **MM3.1
Si Nanocrystals in SiO2 for Efficient Energy Management.
Dolf Timmerman 1 , Wieteke de Boer 1 , Tom Gregorkiewicz 1
1 , University of Amsterdam, Amsterdam Netherlands
Show AbstractSi nanocrystals (NCs) embedded in a SiO2 matrix are widely investigated in view of their potential photonic, optoelectronic and photovoltaic applications. In spite of that, many important unresolved issues remain. A particularly challenging one concerns relaxation and recombination processes of hot carriers in Si-NCs.In this contribution, we present optical investigations of electron/hole relaxation in Si-NCs. The study has been performed on solid dispersions of small Si-NCs (diameter 3.1 – 3.3 nm) embedded in a SiO2 matrix. Photoluminescence (PL) and absorption spectroscopies have been used.In time-resolved photoluminescence experiments we look at the exciton-related emission. In addition to the long microsecond radiative decay, we observe also a (sub)-nanosecond recombination whose relative amplitude shows pronounced dependence on pumping wavelength and power, indicating that it originates from a non-radiative process. In samples additionally co-doped with Er, the Er-related emission provides a convenient way to monitor excitation state of Si-NCs. Using that method, we have identified two energy transfer channels between Si-NCs and Er ions.In order to compare the (relative) quantum efficiency of different emission processes, the PL results have been scaled to absorbance measured for the same samples with linear absorption. With this approach, we demonstrate a threshold excitation energy, above which efficiency of emission is increased, becoming superlinear with absorbance (the “quantum cutting” effect [1]). The fast non-radiative processes are investigated in detail with time-resolved transient absorption spectroscopy, which is used to monitor relaxation of carriers inside the Si-NCs. This is accomplished by performing pump-probe experiments with femtosecond time resolution. In that experiment, the effect of the primary pulse on the absorption of the secondary pulse is followed. The results reveal ultrafast carrier relaxation processes with kinetics in subpico- to nanosecond scale. In the contribution, we combine the results obtained by emission and absorption spectroscopies in order to establish a link between emission enhancement and (ultrafast) carrier cooling. We consider different carrier recombination processes taking place in Si-NCs. In particular, we address the multiple exciton generation and the possibility of carrier trapping at defect states.[1] D. Timmerman et al., Nature Photonics 2, 105 (2008).
4:45 PM - MM3.2
Size Controlled Si Manocrystals: A Model Tool for Basic Understanding and Application Development.
Margit Zacharias 1 , Daniel Hiller 1 , Mihaela Jivanescu 2 , Andre Stesmans 2 3 , Stefanie Godefroo 3 , Victor Moshchalkov 3 , Manus Hayne 4
1 IMTEK, Faculty of Appl. Science, University of Freiburg, Freiburg Germany, 2 Department of Physics, University of Leuven, Leuven Belgium, 3 INPAC-Institute for Nanoscale Physics and Chemistry, K.U.Leuven, Leuven Belgium, 4 Department of Physics, Lancaster University, Lancaster United Kingdom
Show AbstractSi nanocrystals are prepared in many ways, but only a few fabrication methods offer independent size and position control. The preparation of SiO/SiO2 superlattices followed by thermal annealing results in layered arranged crystals. The influence of defects and quantum size based luminescence is still a matter of debate. The talk will summarize and evaluate the evidence for both contributions demonstrated so far [1]. An extensive electron spin resonance (ESR) analysis has been carried out on the size controlled Si nanocrystal structures with the intent to reveal and quantify occurring paramagnetic defects [2]. The results of a detailed study comprising hydrogen annealing and reactivation of defects by UV excitation and their signatures in photoluminescence and ESR will be discussed. Possible applications of nanocrystals will be discussed.[1] S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. Lebedev, G. Van Tendeloo and V. V. Moshchalkov, Nature Nanotechnology 3 (2008) 174.[2] A. Stesmans, M. Jivanescu, S. Godefroo, and M. Zacharias, Appl. Phys. Letters, in press.
5:00 PM - MM3.3
Phosphorus Donors in Silicon Nanocrystals: Doping Efficiency and Electronic Confinement.
Rui Pereira 1 2 , André Stegner 1 , Hartmut Wiggers 3 , Martin Brandt 1 , Martin Stutzmann 1
1 Walter Schottky Institut, Technische Universität München, Munich Germany, 2 Institute of Nanostructures, Nanomodeling and Nanofabrication, University of Aveiro, Aveiro Portugal, 3 Institut für Verbrennung und Gasdynamik, Universität Duisburg-Essen, Duisburg Germany
Show AbstractSi nanocrystals (Si-NCs) are emerging materials for a new generation of electronic devices that exploit nanoscale phenomena, from optoelectronics to solar cells. After the first step of preparing macroscopic amounts of high quality undoped freestanding Si-NCs has been accomplished [1, 2], we can now exploit the new physical phenomena and benefits resulting from applying these materials. A promising technology is to use Si-NCs to fabricate semiconductor inks for printable electronic circuitry [3]. Such printed NC devices are more economical both in terms of price and conservation of resources. The ubiquitous role of impurity doping in electronic devices raises the question as to whether the electrical and optical properties of Si-NCs can as well be tailored via doping [4, 5]. Very recently the first proof of electronic doping of freestanding Si-NCs via incorporation of impurity atoms has been reported [6, 7]. However, this research is still in a very early state and fundamental issues such as doping homogeneity and efficiency are not understood. In this work, we use low temperature electron paramagnetic resonance (EPR) in conjunction with secondary ion mass spectroscopy to investigate the efficiency of P doping in Si-NCs, with a mean diameter selected in the range of 3-40 nm, produced by microwave-induced decomposition of silane and phosphine in a low-pressure microwave plasma reactor. Our results show that although the efficiency of incorporation of P atoms into the NCs during growth does not depend on the NC size, the concentration of electronically active (paramagnetic) P states decreases when the size of the NCs is reduced. We find that this effect is due to segregation of P atoms at the NC surface and compensation of charges by surface dangling bond defects. In real applications, the efficiency of electronic doping at room temperature is also influenced by the ionization energy of dopants, which is a function of the localization of charge carriers in NCs. To investigate the donor localization effects, we measured the NC size dependence of the EPR hyperfine splitting associated with isolated P donors when going from the bulk to the nanoscale using electrically-detected magnetic resonance of thin films of P-doped Si-NCs. A sizable localization of donors is observed for NCs as large as 15 nm, which can be explained alone by a reduction of the dielectric screening in NCs.[1] J. Knipping, et al., J. Nanosci. Nanotech. 4, 1039 (2004).[2] L. Magolini, E. Thimsen, U. Kortshagen, Nano Lett. 5, 655 (2005).[3] L. Mangolini, U. Kortshagen, Adv. Mater. 19, 2513 (2007).[4] D. J. Norris, A. L. Efros, S. C. Erwin, Science 319, 1776 (2008).[5] S. C. Erwin, et al., Nature 436, 91 (2005).[6] A. R. Stegner, R. N. Pereira, K. Klein, R. Lechner, R. Dietmueller, M. S. Brandt, M. Stutzmann, H. Wiggers, Phys. Rev. Lett. 100, 026803 (2008).[7] R. Lechner, H. Wiggers, A. Ebbers, J. Steiger, M. S. Brandt, M. Stutzmann, phys. stat. sol. (RRL) 1, 262 (2007).
5:15 PM - MM3.4
Surface and Shape Modification of the Electronic Structure of Silicon Nanocrystals.
Robert Meulenberg 1 , April Montoya-Vaverka 1 2 , Subhash Risbud 2 , Louis Terminello 1 , Tony van Buuren 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , UC Davis, Davis, California, United States
Show AbstractAlthough the quantum confinement (QC) effect is the most widely studied phenomenon related to the size dependent evolution in the electronic structure of nanostructured materials, these changes may be complicated by many other factors, such as surface termination and particle shape. Using element specific x-ray probes (x-ray absorption and soft x-ray emission spectroscopy), we examine the effects of particle shape and surface termination on the overall electronic structure of silicon nanocrystals. Isolated unpassivated Si nanocrystals show weak QC effects while strong shifts in the band edges are observed when the Si nanocrystals are passivated with alcohol groups, oxygen, or hydrogen. Changing the particle shape (i.e. aspect ratio > 1) or introducing strain also induces changes in the electronic structure of these systems. The results are compared with recent theory and suggest the nanocrystal electronic structure is very complex and is comprised of many different factors. This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344
5:30 PM - MM3.5
Plasma Synthesis and Size-selected Deposition of Countable Silicon Nanoparticles.
Ingo Pluemel 1 , Klemens Hitzbleck 1 , Christof Schulz 2 , Hartmut Wiggers 2
1 Institute for Combustion and Gasdynamics, University Duisburg-Essen, Duisburg Germany, 2 Institute for Combustion and Gasdynamics and CeNIDE, University Duisburg-Essen, Duisburg Germany
Show AbstractAdvances in nanoparticle technology enable the production of new types of electronic devices, catalytic systems and complex functional surface coatings. For most of these applications, random deposition or self-assembled arrangements of particles on surfaces are sufficient. However, an increasing number of potential applications such as single electron transistors and quantum computers require exact placement of countable nanoparticles with a specific size. For this purpose, a low-pressure microwave plasma reactor was developed as a gas-phase nanoparticle source. After formation, the nanoparticles are deposited onto a pre-structured substrate using particle mass spectrometry (PMS). Nanoparticles in the size regime between 3 and 10 nm are synthesized between 30 and 200 mbar by gasphase-decomposition of Silane within a nearly thermal plasma. This reactor has the unique advantage of generating single crystalline particles with defined size distribution, structure, and morphology. Due to extremely steep temperature gradients and coulomb repulsion during particle formation and growth a very low degree of particle agglomeration and sintering can be achieved. Separated single particles are extracted by means of a particle laden molecular beam. A mass filter consisting of the PMS coupled to the reactor is used to select nanoparticles of a specific size according to their mass, charge and kinetic energy. After selection, the nanoparticles can be deposited onto almost any type of substrate. Using this principle, nanoparticle deposition becomes compatible with epitaxial growth methods and enhances their potentials with nanoparticle technology. As a result, sub-monolayer as well as multilayer consisting of single, semiconducting nanoparticles with almost identical size can be deposited on any substrate. Ex-situ methods like TEM, SEM, FTIR, and XRD are used to characterize morphology, crystallinity, and chemical composition of the particles. By depositing low densities onto a substrate, µ-photoluminescence measurements of single nanoparticles are performed. The STM-investigation of the electronic properties of single nanoparticles is in progress.
5:45 PM - MM3.6
Laser Ablation in Liquids: Applications in Synthesis of Group IV Semiconductor Nanostructures.
Guowei Yang 1
1 Physics, Zhongshan University, Guangzhou, Guangdong, China
Show AbstractMM4: Poster Session
Session Chairs
Tuesday AM, December 02, 2008
Exhibition Hall D (Hynes)
9:00 PM - MM4.10
Diffusion and concentration of E-centers in SiGe.
Alexander Chroneos 1 , Robin Grimes 1 , Hartmut Bracht 2 , Chao Jiang 3 , Blas Uberuaga 3
1 Department of Materials, Imperial College London, London United Kingdom, 2 Institute of Material Physics, University of Münster, Münster Germany, 3 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractElectronic structure calculations are used to investigate the stability, diffusion and relative concentration of E-centers in silicon germanium. These are pairs formed between lattice vacancies and donor atoms (phosphorus, arsenic or antimony) in silicon germanium. As silicon germanium is a random alloy the local environment surrounding the E-center is important. Here we have generated special quasirandom structures to describe the random silicon germanium alloys of three different compositions. By comparing the results to silicon and germanium it is predicted that the properties of E-centers do not vary linearly with the composition of the silicon germanium alloy. The impact of the germanium content of the alloys on the concentration and diffusion properties of the E-centers is in excellent agreement with the limited previous experimental results.
9:00 PM - MM4.11
Noble Photoluminescence from Ge/Si Nanocrystals Embedded in Silicon Oxide Matrix Annealed in Hydrogen Gas.
Naoki Kosaka 1 , Shin-ichiro Uekusa 1
1 School of Science and Technology, Meiji University, Kawasaki-shi Japan
Show Abstract Since the discovery of visible luminescence from porous silicon at room temperature in 1990’s, a semiconductor nanoparticle have been studied by many researchers. Because silicon (Si) which show visible luminescence and a lot of advantages of having abundant natural resources, no toxicity, and a high melting point is expected as substitute materials of the group ΙΙΙ-V compound semiconductor used for the current photonics. Futhermore, the germanium (Ge) which is a group ΙV as well as Si element was attracted by researcher’s attention. It was reported that violet (3.1-3.2eV) and blue (2.58-2.95eV) luminescence could be obtained from Ge nanocrystals embedded in silicon oxide matrix (SiO2) and red (1.3-1.5eV) luminescence can be obtained from Si nanocrystals embedded in SiO2. In this study, we prepared SiO2 thin films in which both Si and Ge were embedded and performed optical evaluation. We report a noble luminescence spectra of Ge/Si-SiO2 thin films.The Ge/Si-SiO2 samples were prepared on a p-type Si (100) substrate by RF-magnetron sputtering method with a composite target of 99.9999% purity SiO 2 plate, Si and Ge chips attached on the surface of the SiO2 plate. Deposition process was performed by the sputtering power 200W and times 30min. Subsequently, the as-deposited thin films were annealed in the temperature range from 700 to 900oC for 1 hour in argon (Ar) atmosphere. Furthermore, to examine the properties of Ge/Si-SiO2, we tried to change the annealing in hydrogen (H2) gas from Ar gas. We conducted a photoluminescence measurement (PL). The PL spectra of samples were measured by an excitation light of 325.0 nm of He-Cd laser and a liquid nitrogen cooled CCD detector.The PL spectra of the each annealed samples shows two main band peaks at around 500-540 nm (green band) and 400 nm (violet band). The green luminescence could be seen with naked eyes from all samples except the as-deposited thin film. Even, the PL intensity of green band of the samples annealed in H2 decreased in comparison with the samples annealed in Ar atmosphere. Now, we are considering that the green band was attributed to oxygen defect centers in the silicon oxide matrix, We systematically will discuss why such a phenomenon is caused.
9:00 PM - MM4.12
Raman Spectroscopy of Group IV Nanostructured Semiconductors: Influence of Size, Temperature and Stress.
Oscar Martinez 1 , Carmelo Prieto 1 , Alfredo Torres 1 , Juan Jimenez 1 , Andres Rodriguez 2 , Jesus Sangrador 2 , Tomas Rodriguez 2
1 Física Materia Condensada, Universidad de Valladolid, Valladolid Spain, 2 ETSI Telecomunicación, Universidad Politécnica de Madrid, Madrid Spain
Show Abstract9:00 PM - MM4.13
Colloidal Solution of Organically Capped Si Nanocrystals in Xylene: Efficient Photoluminescence in the Yellow Region.
Katerina Kusova 1 , Ondrej Cibulka 1 , Aliaksei Vetushka 1 , Katerina Dohnalova 1 , Ivan Pelant 1 , Anna Fucikova 2 , Karel Zidek 2 , Petr Maly 2 , Jan Valenta 2 , Pavel Matejka 3 , Snejana Bakardjieva 4
1 , Institute of Physics, AS CR, v.v.i., Prague 6 Czechia, 2 , Faculty of Mathematics and Physics, Charles Unversity, Prague Czechia, 3 , Institute of Chemical Technology, Prague Czechia, 4 , Institute of Inorganic Chemistry, AS CR, Rez Czechia
Show AbstractSilicon nanocrystals (Si-nc) represent a perspective light-emitting material for silicon photonics. From this point of view, brightly emitting ensembles with minimized light-scattering losses are desirable. Closely packed Si-nc in solid matrices usually exhibit quite strong light scattering due to larger Si-nc agglomerates, which represents losses affecting adversely e.g. optical gain measurements. Therefore, contrary to this 'standard' approach of the preparation of Si-nc assemblies, in this contribution we focus on the preparation and investigation of colloidal solutions of Si-nc. Such solutions provide a chance to modify the Si-nc surface in order to prevent agglomeration. Moreover, tailoring the surface chemistry of nanocrystals in solutions can affect favorably their electronic and photoluminescence (PL) properties. We have found that xylene can be a suitable solvent to achieve this goal.Si-nc in this study are prepared by electrochemical etching of porous silicon followed by mechanical pulverization, yielding Si-nc powder with natural oxygen passivation and orange PL (maximum at 600-650 nm, depending on etching parameters and post-etching treatment). Then, the Si-nc powder is mixed with the solvent and is kept under constant magnetic stirring. During stirring, the original orange PL peak disappears and a new yellow one appears instead. This change in PL reflects the modification of the surface chemistry of Si-nc as discovered by FTIR-ATR measurements: the original oxide shell is gradually replaced by the encapsulation with organic molecules from the solvent, which drastically influences optical properties of the colloidal solution. The resulting transparent, though yellowish colloidal solution exhibits bright yellow PL (maximum around 560 nm, quantum efficiency approximately 20%) and negligible light scattering. Apart from the change in the PL peak position, PL radiative lifetime is also dramatically reduced, decreasing by several orders of magnitude when compared with the original oxide-passivated powder down to approximately 20 ns, while the PL spectral position still remains inside the "S-band".
9:00 PM - MM4.16
Optical Gain Measurements of Silicon Nanocrystals.
David Barba 1 , Dimitri Koshel 1 , Chabha Dahmoune 1 , François Martin 1 , Guy Ross 1
1 EMT, INRS, Varennes, Quebec, Canada
Show AbstractLight amplification is a prerequisite for the fabrication of silicon laser source. Both net modal gain and intrinsic optical gain have been observed by variable stripe length (VSL) and pump and probe (PnP) measurements. On the other hand, optical losses have been evaluated by means of the shifting excitation spot (SES ) technique. The results are obtained in fused silica samples containing silicon nanocrystals (Si-nc) prepared by silicon implantation followed by a thermal annealing and hydrogen passivation. Our works provide information regarding the nature of the amplification medium including the optical loss and aims to optimize the optical gain of Si-nc systems as a function of the fabrication parameters. Our experimental investigations were carried out using adapted setups, ensuring a micrometer positioning of each optical component and the computing of large data acquisition. These systems take into account the numerous artefacts that can perturbate the measurements, namely : the sample inhomogeneity, the waveguiding effect and the laser pumping diffraction, responsible for an artificial overestimation of the gain values.The data recorded by VSL and PnP techniques can be related by taking into account both the filling factor (Γ) and the optical loss. Therefore, the former has been evaluated using results from chemical and structural characterization of the samples. A relative good consistency has been obtained with the amplified stimulated emission models of Si-nc. Finally, after a carefully data analysis, gain spectra are compared with Si-nc photoemission and photoluminescence measurements collected along the propagation direction of the Si-nc emission, which exhibit a strong waveguided and amplified signal.
9:00 PM - MM4.17
Intermixing During Heteroepitaxy of High Ge Content Ge(Si)/Si(100) Islands.
Sutharsan Ketharanathan 1 , Nick Jungwirth 1 , Peter Crozier 2 , Jeff Drucker 1 2
1 Dept. of Physics, Arizona State University, Tempe, Arizona, United States, 2 School of Materials, Arizona State University, Tempe, Arizona, United States
Show Abstract The Si distribution in high Ge content Ge(Si) islands formed by depositing pure Ge on Si(100) using solid source molecular beam epitaxy (MBE) is being investigated. We are using a combination of selective wet chemical etching, atomic force microscopy (AFM), quantitative electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), and Rutherford backscattering spectrometry (RBS) to determine the 3D composition profiles of Ge(Si)/Si(100) islands. Using AFM to monitor the surface evolution of individual islands during successive etches that are selective for different Ge contents has proven to be a useful method for mapping concentration distributions. When combined with cross-section EELS in STEM, which is a quantitative real-space method that projects the average concentration along the beam direction into 2D images, ~nm spatial resolution 3D concentration maps can be formed. We have extended the capability of the selective etching method to high (XGe > 0.7) Ge contents by exploiting the water solubility of Ge oxides. Successive 1 min. dips into boiling H2O followed by an air drying step etches SiGe at a ~1 nm/dip rate that decreases by a factor of 2 as the Ge content changes from 0.95 to 0.70. This etch rate is ~100 times slower than that of the more commonly employed NH4OH:H2O2 etch chemistry at Ge contents > 0.85 and should allow quantitative investigation of island composition evolution at low growth and annealing temperatures. We are investigating two sets of samples grown at 400 < T < 625°C at a deposition rate of 2.75 ML / min. One set is unannealed and the other has been annealed for 10 < t < 120 min at the growth temperature. The Ge coverages have been chosen so that intermixing each of the characteristic island morphologies: pyramids, domes and dislocated domes can be simultaneously investigated. Preliminary results indicate that the boiling H2O etch reveals both lateral and vertical composition variations within individual domes grown at temperatures as low as 400°C. However, the signature of Si enrichment at the island perimeter, apparent as a characteristic 'doughnut' shape after etching, does not appear until T ≥ 500°C. These results suggest that Si enrichment at the island perimeters may not be operative for growth temperatures less than 500°C.
9:00 PM - MM4.18
The Nature of Damage Cascades Produced During Heavy Ion Irradiation of Crystalline Silicon.
Philip Edmondson 1 , Duncan Riley 1 , Robert Birtcher 2 , Stephen Donnelly 1
1 Institute for Materials Research, University of Salford, Salford United Kingdom, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show Abstract9:00 PM - MM4.19
The Luminescence Origin for Plasma Power controlled Silicon Nitride Films Fabricated by Plasma-enhanced Chemical-vapor Deposition.
Seunghun Jang 1 , Changhun Ko 1 , Kiyoung Jeong 1 , Moonsup Han 1
1 Physics, University of Seoul, Seoul Korea (the Republic of)
Show AbstractVisible light emission of silicon with nano-sized structures has been reported at room temperature. For this enormous potential for silicon optoelectronic device application, the various nano-sized silicon composites have been studied by many researchers, also several models were proposed to explain the luminescence properties. It has been suggested that dominant mechanism for light emission from the Si QDs in silicon nitride (SiNx) films is the quantum confinement effect (QCE). However, as the observed luminescence from Si QDs has the red range, the QCE is not accepted for the unique mechanism to explain the visible photoluminescence (PL) [1]. The effect of surface or interface states due to the hydrogen passivation [2] could be a considerable factor affecting the luminescence. In this work, we investigate the origin of visible light emission for the nano-structured silicon nitride films. We fabricated the silicon nitride films by using plasma-enhanced chemical-vapor deposition (PECVD). We could have controlled the peak position of PL in the red-visible range by adjusting the plasma power of reactant gas. We report that the PL peak shifts toward the higher energy and the PL intensity becomes stronger with increasing the plasma power from 20 W to 60 W. In order to investigate the effect of chemical states on the luminescence properties, we measured core-level spectra of related elements using x-ray photoelectron spectroscopy (XPS). Investigating the chemical state change of silicon 2p core level, we found that bulk silicon content become more deficient as the plasma power increases. We also employed high resolution transmission electron microscopy (HRTEM), Raman scattering spectroscopy (RSS), Fourier transform infrared spectroscopy (FTIR) to interpret more deeply the luminescence mechanism for the plasma power controlled silicon nitride films.References[1] M. H. Wang, D. S. Li, Z. Z. Yuan, D. R. Yang, and D. L. Que, Appl. Phys. Lett. 90, 131903 (2007).[2] X. X. Wang, J. G. Zhang, L. Ding, B. W. Cheng, W. K. Ge, J. Z. Yu, and Q. M. Wang, Phys. Rev. B 72, 195313 (2005).
9:00 PM - MM4.2
Collective Transport in Silicon Nanocrystal Assembly Prepared by Laser Ablation Method.
Akira Sugimura 1 2 , Manabu Gibo 2 , Mitsuru Inada 3 , Tadashi Saitoh 3 , Ikuro Umezu 1 2
1 Department of Physics, Konan University, Kobe Japan, 2 Quantum Nanotechnology Laboratory, Konan University, Kobe Japan, 3 Department of Pure and Applied Physics, Kansai University, Osaka Japan
Show AbstractSilicon nanocrystal assembly is an interesting material which is applicable to quantum devices. Conduction mechanism through the nanocrystal assembly, however, has not been understood sufficiently, although its knowledge is inevitable in device designing. In this paper, we investigate collective transport properties in Si nanocrystal assembly which is prepared by laser ablation method. We deposited Si nanocrystals having a diameter of about 5nm on n-type Si substrate by using pulsed laser ablation method in hydrogen atmospheres [1]. The deposit shows columnar structure with a thickness of 100nm. The conduction properties of the Si nanocrystal assembly between the electrodes deposited on both sides of the sample were measured in the temperature range between 30K and 290K.When the temperature is higher than 180K, the conductance s depends on the temperature T as expressed by s~exp(-B/T), which is the formula for the hopping conduction. The B value corresponds to the activation energy caused by the quantized energy difference between the adjacent nanocrystals. When the temperature is decreased to the range between 120K and 180K, the temperature dependence changes to the relation described by s~exp{-(A/T)^(1/2)}, the formula for the variable range hopping. Taking into account the estimated value for the electron localization length, we can conclude that the transport mechanism in this temperature region is Efros-Shklovskii type hopping, in which electron confinement is stronger than that in Mott type variable range hopping. When the temperature is further decreased down to the range from 40K to 120K, measured result for the current-voltage (I-V) characteristics shows a clear voltage threshold, suggesting Coulomb blockade behavior. When we shift the observed I-V curves by Vt(T), all the I-V curves collapse on each other in the same manner as in Au nanocrystal assembly [2]. We analyzed the experimental date and found that the I-V curve can be expressed by Middleton and Wingreen’s scaling law as I~{V-Vt(T)}^g [3]. The exponent g is obtained to be about 3.15, which is consistent with the dimension of the present nanocrystal assembly. It is also shown that Vt(T) is a linear function with T. These results indicate that the Si nanocrystal assembly studied here behaves as a network of Coulomb-blockade-type tunnel junction.In conclusion, we elucidated the transport properties of Si nanocrystal assembly in the wide temperature region. It is found that the Si nanocrystal assembly behaves as a network of tunnel junctions between quantum dots which strongly confine electrons inside themselves. This result suggests that we can expect new devices based on many body effects in each QD in the present system. Reference[1] MRS Proc.832 F7.24(2005)[2] R. Parthasarathy et al., Phys. Rev. Lett. 92, 076801 (2004).[3] A. Middleton and N. Wingreen, Phys. Rev. Lett. 71, 3198 (1993).
9:00 PM - MM4.20
Controlled Blue Photoluminescence of Nanocrystalline Porous Silicon Treated by High-Pressure Water Vapor Annealing.
Bernard Gelloz 1 , Romain Mentek 1 , Kota Nishikawa 1 , Nobuyoshi Koshida 1
1 , Tokyo Univ. A&T, Tokyo Japan
Show AbstractNanocrystalline porous silicon (PS) is an attractive material for achieving various optoelectronic device applications. Luminescence of PS, however, has been suffering from insufficient efficiency and stability as well as a spectral emission mostly limited to the red part of the visible range. Recently [1], a reasonable solution has been found to obtain high efficiency and stability for the red-orange photoluminescence (PL) of PS. An external quantum efficiency of 23% has been demonstrated by using a post-anodization high-pressure water vapor annealing (HWA). The enhancement is caused by complete surface passivation with high-quality thin SiO2 film and significant reduction of both interfacial non-radiative defects and structural stress. The electroluminescence [2] has also been effectively stabilized by HWA as in the case of other photonic structures [3]. Under appropriate HWA conditions, a blue PL band can also be generated in PS. In this paper, the luminescence and photonic properties of blue-emitting HWA-treated PS are reported in details.The PS samples were formed using p-type Si wafers by anodization in HF solution. The influence of electrochemical oxidation (ECO) and thermal oxidation at high temperatures was studied, and then HWA was carried out in similar way reported previously [1].With ECO-treated PS, HWA induces a strong red PL band. This emission band is attributed to radiative transitions of localized excitons in nc-Si dots. HWA of as-anodized PS also leads to strong red PL emission. However, for appropriate conditions of HWA and PS formation conditions, a blue PL band can also be obtained, either coexisting or not with the red band. This blue band is clearly related to the oxide grown during HWA. It is not present in layers that were treated by ECO prior to HWA because the oxide grown during ECO prevents extensive oxide formation during the HWA step.The effect of rapid thermal oxidation was investigated in conjunction with HWA. The level of oxide in PS can be adjusted by controlling the oxidation temperature. The balance between the level of oxide and that of the nc-Si dots density can be varied. As a result, the respective contributions of the red and blue bands obtained after HWA can be adjusted. Almost fully thermally oxidized PS layers emit only very weak PL. However, after HWA, bright blue PL was obtained at room temperature, with no contribution of the red PL band.The blue emission band has been studied in terms of the polarization memory, lifetime, and excitation spectra, including their temperature dependence. The results show that this band is very specific. It exhibits features that are promising for the enabling of Si-based short-wavelength optoelectronics.[1] B. Gelloz, A. Kojima, and N. Koshida, Appl. Phys. Lett. 87, 031107 (2005)[2] B. Gelloz, T. Shibata, and N. Koshida, Appl. Phys. Lett. 89, 191103 (2006)[3] B. Gelloz, T. Shibata, R. Mentek, and N. Koshida, Mater. Res. Soc. Symp. Proc. 958, 0958-L08-02 (2007)
9:00 PM - MM4.21
Reduction of Threading Dislocations in Pure Ge on Si (100) Prepared by the Thermally-driven Relaxation of the Ge seed Layer.
Sang Hoon Kim 1 , Dong-Woo Suh 2 , Ji-Ho Joo 2 , Gyung-Ock Kim 2
1 MIT DevicesTeam, New Devices & Materials Research Department, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of), 2 Silicon Photonic Module Team, RF & Optical Devices Research Department, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show Abstract9:00 PM - MM4.22
Photoluminescence Property of Nitrogen-terminated Nanocrystalline Silicon Particles Dispersed in Pure Water.
Yuichi Murata 1 , Masaki Hiruoka 1 , Keisuke Sato 2 , Kenji Hirakuri 1
1 Faculty of Science & Engineering, Tokyo Denki University, Saitama Japan, 2 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Ibaraki Japan
Show AbstractStability of photoluminescence (PL) properties from the semiconductor nanoparticles depends on the environment conditions such as air storage or solution dispersion since it entirely reflects the modification elements on their surfaces. The visible luminescence from the hydrogen-terminated nc-Si particles rapidly degrades by aging after a short period of time despite the stable luminescence just after dispersion in pure water. This was closely related to the formation of the defects due to the hydrogen-desorption from the silicon-hydrogen bonds on the particle surface.In this paper, the photoluminescence (PL) property of the nitrogen-terminated nc-Si particles well-dispersed in pure water is discussed. The nitrogen-terminated nc-Si particles were prepared on the Si substrate by co-sputtering of Si3N4 chips/Si chips/silica mixture targets and post-annealing at temperature of 1100 °C. The nitrogen concentration on the particle surface was controlled by changing the number of Si3N4 chips in the mixture targets. The annealed samples were then treated in hydrofluoric (HF) acid steam to appear the nc-Si particles on the substrate. After that, the nc-Si particles were dispersed in pure water by ultrasonic vibration process. The surface composition and the luminescence property in pure water of the samples were estimated by using a fourier transform infrared (FT-IR) spectroscopy and PL measurements.The quantities of silicon-nitrogen bonds on the particle surfaces were increased with the number of Si3N4 chips from 4 to 18 chips. The increase of the adsorption quantity of nitrogen brought about the long-term stability of PL intensity after dispersion in pure water. These results indicated that the nitrogen-termination to the nc-Si particles is one of the effective techniques on the realization in the stable PL property from the dispersion solution.
9:00 PM - MM4.23
Toxicity Effect of Cancer Cell Labeled with Visible Luminescent Nanocrystalline Silicon Particles and Visualization Observation in Vivo.
Keisuke Sato 1 2 , Naoki Fukata 1 , Masaki Hiruoka 2 , Kenji Hirakuri 2 , Kohki Fujioka 3 , Kenji Yamamoto 3
1 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 Department of Electronic and Computer Engineering, Tokyo Denki University, Hatoyama, Saitama, Japan, 3 Department of Medical Ecology and Informatics, International Medical Center of Japan, Shinjuku, Tokyo, Japan
Show AbstractLuminescent semiconductor nanoparticles such as cadmium sulfide (CdS) and cadmium selenide (CdSe) have been widely studied for application to biomedical engineering fields. These nanoparticle materials, however, have some problems for the safety to living organism and the fluidity or excretion in living organism due to the increase of the particle size. Therefore, it is expected to develop the new nanoparticle materials with some features of non-toxic and minimal particle size. We have been fabricated the nanocrystalline silicon (nc-Si) particles, which continuously emits from red light to blue light by reducing the size to 2.5 nm or less. In this paper, toxicity test of cancer cell labeled with the nc-Si particles and the visualization observation in vivo will be discussed.The nc-Si particles having a diameter of 2.2 nm were formed in SiO2 films by radio frequency sputtering method and subsequent thermal annealing. The sample was treated with hydrofluoric acid solution to extract the particles from the SiO2 films. After HF treatment, the nc-Si particles were uniformly dispersed in the pure water using supersonic vibration technique. The nc-Si particles in the pure water then added in HeLa cells which were sensitive to toxicity test, and were co-cultured for 48 hours. The concentration of nc-Si particles varied from 1.12 to 112 µg/mL. The cytotoxicity tests was examined from the viability and cellular membrane damages of HeLa cells with the nc-Si particles as a function of the particle concentration using the mitochondrial activity and lactate dehydrogenate release assays, respectively. Moreover, it was also performed the cytotoxicity tests of nc-Si particles after ultraviolet (UV) light irradiation for 2 hours in order to investigate the toxicological effect of UV exposure. On the other hand, the visualization observation in vivo was examined by injecting the nc-Si particles of 4.5 mg into the lymphatic vessel of mouse.The viability and cellular membrane damages of cells did not quite depend on the particle concentration. These tendencies were almost same as those of cells without nc-Si particles. Furthermore, when the nc-Si particles after UV exposure added in cells, the cellular membrane damages were less in addition to the stable viability compared with the cells having the particles before UV exposure. This was due to the good biocompatibility between the harmless crystalline silicon and cellular tissue. Moreover, the cells with nc-Si particles exhibited green luminescence by irradiating UV light. On the other hand, the visualization of nc-Si particles in vivo could be clearly confirmed for the circulation from the lymphatic vessel to the lymph node.
9:00 PM - MM4.24
Controlled Assembly and Nanoscale Doping of Epitaxial Si(Ge) Quantum Dot Nanostructures.
Jeremy Graham 1 , Copeland Kell 1 , Jennifer Gray 2 , Stuart Wolf 1 , Jerry Floro 1 , Lothar Bischoff 3 , Robert Hull 1 4
1 Materials Science & Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Mechanical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 Institute of Ion Beam Physics and Materials Research, Research Center Dresden-Rossendorf Inc., Dresden Germany, 4 Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractSelf-assembled Si(Ge) quantum dots are a candidate for use in potential nanoelectronic device architectures such as quantum cellular automata and magnetic spin exchange switches. In order to apply QDs in such nanoelectronic devices they must be spatially ordered into patterns of varying complexity and posses the electronic or magnetic properties required for device operation. We have previously demonstrated the use of Ga focused ion beam (FIB) templating of Si surfaces prior to molecular beam epitaxy (MBE) growth in order to fabricate patterns of Si(Ge) QDs of any desired complexity. However, the use of Ga ions leads to undesirable doping of the resulting QDs and surrounding substrate. The current work employs a mass-selecting FIB to template QD structures grown via hyperthermal MBE, and to electrically or magnetically dope them at a dot-by-dot level. Ions can be mass-selected according to isotope mass and charge state by using a Wien filter. Suitable liquid metal alloy ion sources then provide the ability to template a Si substrate with electrically non-invasive ions (e.g. Si from AuSi) and implant dopant ions for electronic or magnetic activation (e.g. with B from AsPdB or Mn from GeMn), with resolution of < 50nm and doses down to a few ions per dot. We also demonstrate templated growth using B ions, which may effectively result in p-type doping of the as-grown QDs through hole capture.
9:00 PM - MM4.25
Electronic Structure of Functionalized Diamond Clusters (Diamondoids) in the Molecular Size Limit.
Lasse Landt 1 , Matthias Staiger 1 , David Wolter 1 , Trevor Willey 2 , Jeremy Dahl 3 , Robert Carlson 3 , Andrey Fokin 4 , Peter Schreiner 4 , Thomas Möller 1 , Christoph Bostedt 1
1 , Technische Universität Berlin, Berlin Germany, 2 , Lawrence Livermore National Lab, Livermore, California, United States, 3 , MolecularDiamond Technologies, Richmond, California, United States, 4 , Justus-Liebig-Universität, Giessen Germany
Show AbstractDiamondoids are the smallest members of the nanodiamond series. They are ideal clusters that can be perfectly size and shape selected [1]. They are fully sp3-hybridized due to their complete H-terminated surface and therefore perfectly superimpose to the diamond lattice. We investigated the electronic structure of pristine and surface functionalized diamondoids with a focus on thiol groups [2]. Thiols are of great interest because they form self-assembled monolayers (SAMs) on, e.g., gold surfaces. This opens the door to a variety of possible applications, e.g., in nanoelectronics [3]. The data are compared to band edge and optical data of pristine diamondoids. Thiolation has little effect on the lowest unoccupied states (LUMO) which are dominated by the hydrogen surface [4]. On the occupied side of the gap it adds an additional broad state that lies energetically above the highest occupied states (HOMO) of the corresponding pristine diamondoid. This leads to a reduction of the band gap in functionalized diamondoids that is observed in the optical spectra. The influence of diamondoid size and functionalization site is discussed.[1] J.E. Dahl, et al., Science 299, 96-99 (2003); [2] B.A. Tkachenko et al., Org. Lett. 8, 1767 (2006); [3] W.L. Yang et al., Science 316, 1460 (2007); [4] T.M. Willey, C. Bostedt, et al., Phys. Rev. Lett. 95, 113401 (2005).
9:00 PM - MM4.26
Si Nano Dots Formation for Solar Cell Application.
Branko Pivac 1 , Pavo Dubcek 1 , Robert Slunjski 1 , Ivana Capan 1 , Nikola Radic 1 , Hrvoje Zorc 1 , Sigrid Bernstorff 2 , Branislav Vlahovic 3
1 , R. Boskovic Institute, Zagreb Croatia, 2 , Sincrotrone Trieste, Trieste Italy, 3 Physics Department, North Carolina Central University, Durham, North Carolina, United States
Show Abstract9:00 PM - MM4.27
Ab initio Computationally Generated Nanoporous Carbon and its Comparison to Experiment.
Cristina Romero 1 , Ariel Valladares 1 , Alexander Valladares 2 , A. Calles 2 , R. Valladares 2
1 Materia Condensada, Instituto de Investigaciones en Materiales, UNAM, Mexico, D.F., Mexico, 2 Departamento de Fisica, Facultad de Ciencias, UNAM, Mexico, D.F., Mexico
Show AbstractNanoporous carbon is a widely studied material due to its potential applications in hydrogen storage or for filtering undesired molecular products. Most of the developments have been experimental although some simulational work has been carried out based on the use of graphene sheets and/or carbon chains and classical molecular dynamics. Here we present an application of our recently developed ab initio method [1] for generating group IV porous materials. The method consists in constructing a crystalline diamond like supercell with 216 atoms of carbon with a density of 3.546 g/cm3, then lengthening the supercell edge to obtain a density of 1.72 g/cm3, yielding a porosity of 51.5 %. We then subject the resulting supercell to an ab initio molecular dynamics process at 1073 K, in order to be able to compare with experimental results reported in the literature [2]. Our radial distribution function compares favourably with experiment validating to some extent our approach. [1] Computer modeling of nanoporous materials: An ab initio novel approach for silicon and carbon, Ariel A. Valladares, Alexander Valladares and R. M. Valladares, Mater. Res. Soc. Symp. Proc. (2007) Simposyum QQ. Accepted.[2] Local structure of nanoporous carbons, V. Petkov, R.G. DeFrancesco, S. J. L. Billinge, M. Acharya and H.C. Foley, Phil. Mag. B 79 (1999) 1519.
9:00 PM - MM4.29
Electrical Characterization of Microwave Synthesized Germanium Nanoparticle Ensembles.
Sonja Hartner 1 , Cedrik Meier 2 3 , Axel Lorke 2 3 , Hartmut Wiggers 1 3
1 Institute of Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg, NRW, Germany, 2 Experimental Physics, University of Duisburg-Essen, Duisburg, NRW, Germany, 3 CeNIDE, Center for Nanointegration Duisburg-Essen, Duisburg, NRW, Germany
Show AbstractGermanium is a promising alternative for the prevalent silicon, because of its better conductivity, higher charge carrier mobility and lower oxide barrier. The electronic transport and conductance through silicon nanoparticles is still insufficient for magnetotransport measurements and a lot of device applications. However, germanium nanopowder enables the measurement of nanoparticle ensembles in Hallgeometry to characterize electronic properties to receive information about majority charge carriers and the bandgap of the germanium nanoparticles calculated from the charge carrier concentration. The germanium nanoparticles were produced by gas phase synthesis in a microwave plasma reactor. The material was characterize with Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) and Fourier Transformed Infrared Spectroscopy (FTIR-Spectroscopy) and consist of highly crystalline particles with a diameter of about 22nm. For the transport properties the as-prepared powders were pressed into pellets with a green density of around 64%. The magnetotransport measurements are performed in a temperature range between 323K and 413K in the van der Pauw setup. The sample show ohmic behavior, p-type conductivity and a distinct Hall voltage. The mean free path of the charge carrier calculated from the thermal velocity is found to be around 37nm and increases slightly with rising temperature. For comparison, electrical transport was measured using impedance spectroscopy to support the result of the magnetotransport measurement. The measurements were carried out under synthetic air at temperatures ranging from 323K to 673K and show a negative temperature coefficient which is well known from semiconductors. The bandgap energy Eg=0.64eV for germanium nanoparticles calculated from the carrier concentration is similar to the bandgap energy Eg=0.66eV for bulk germanium and confirmed with measurements performed by impedance spectroscopy with Eg=0.71eV.
9:00 PM - MM4.3
Single Crystalline Ge1-xMnx Nanowires as Building Blocks for Spintronic Devices.
Machteld van der Meulen 1 , Nikolay Petkov 1 2 , Olga Kazakova 3 , Xinhai Han 4 , Kang Wang 4 , Ajey Jacob 4 5 , Justin Holmes 1 2
1 Department of Chemistry and the Tyndall National Institute, University College Cork, Cork Ireland, 2 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2 Ireland, 3 , National Physical Laboratory, Teddington United Kingdom, 4 Department of Electrical Engineering, University of California, Los Angeles, California, United States, 5 , Western Institute of Nanoelectronics and Intel Corporation, Los Angeles, California, United States
Show AbstractSingle crystalline Ge and Si nanowires1 have recently been synthesized, isolated and utilized in various charge-based nanowire devices2. However, operational nanowire spin-state devices have not yet been demonstrated. Theoretically, predictions show that such devices can be realized employing dilute magnetic semiconductors (DMS)3 as these materials combine complementary functionalities of both ferromagnetic and semiconductor materials systems4. Most research on manganese-doped germanium (Ge1-xMnx) DMS materials has focused on nanocrystals5 and thin-films6. However, nanowires are particularly attractive due to their ability in substantiating the semiconductor industries efforts in scaling. Here we report a supercritical fluid-liquid-solid (SFLS) method for producing single crystalline Ge1-xMnx nanowires. Elemental analysis, via Energy Dispersive X-Ray (EDX) Spectroscopy, reveals an average Mn concentration in the wires of 1.0 atomic %. Additionally, Mn maps obtained by Electron Energy Loss Spectroscopy (EELS) did not show clustering of the Mn atoms within the Ge lattice. High resolution transmission electron microscopy (HRTEM) images of the nanowires confirmed that they were single crystalline in nature. Secondary phases, nanoscale precipitates or large lattice distortions in the Ge lattice were not detected, suggesting that the Mn atoms are diluted within the Ge matrix. Extended X-ray Absorption Fine Structure (EXAFS) analysis at the Ge K-edge confirmed the single crystalline nature of the Ge lattice without any lattice distortion. EXAFS analysis at the Mn K-edge reveals Mn atoms occupy the substitutional site in the Ge lattice in agreement with theoretical calculations7.The magnetic properties of the nanowires were investigated by SQUID magnetometry. Soft room-temperature ferromagnetism was observed from the nanowires, originating from diluted Mn atoms within the Ge lattice. Recently high performance single nanowire field effect transistors (FET) were produced from the doped nanowires, with a hole mobility of around 340 cm2/Vs.(1)Holmes, J. D.;Johnston, K. P.; Doty, R. C.; Korgel, B. A.; Science, 2000, 287, 1471(2)Hang, Y.; Duan, X.; Wei, Q. ; Lieber, C. M.; Science, 2001, 291, 630(3)Ohno, H.; Chiba, D.; Matsukura, F.; Omiya, T.; Abe, E.; Dietl, T.; Ohno, Y.; Ohtani, K.; Nature, 2000, 408, 944(4)Dietl, T.; Nature Materials, 2003, 2, 646(5)Kang, J.-S.; Kim, G.; Wi, S. C.; Lee, S. S.; Choi, S.; Cho, S.; Han, S. W.; Kom, K. H.; Song, H. J.; Shin, H. J.; Sekiyama, A.; Kasai, S.; Suga, S.; Min, B. I.; Phys. Rev. Lett., 2005, 94, 147202(6)Passacantando, M.; Ottaviano, L.; D’Orazio, F.; Lucari, F.; De Biase, M.; Impellizzeri, G.; Prioli, F.; Phys. Rev. B: Condensed Matter and Material Physics, 2006, 73, 195207(7)Continenza, A.; Profeta, G.; Piccozi, S.; Phys. Rev. B: Condensed Matter and Material Physics, 2006, 73, 035212
9:00 PM - MM4.30
Pd Segregation at Orthorhombic-NiSi/Si (010) Epitaxial Interface Studied by Density Functional Theory.
Dae-Hee Kim 1 , Hwa-Il Seo 2 , Yeong-Cheol Kim 1
1 Materials Engineering, Korea University of Technology and Education, Chonan Korea (the Republic of), 2 Information Technology, Korea University of Technology and Education, Chonan Korea (the Republic of)
Show AbstractIt is expected that NiSi will be employed down to 22 nm node as a contact material, since its first introduction to 65 nm node last year. However, NiSi is less stable than the previously-used CoSi2 at high temperature. Some noble metals, such as Pd and Pt, have been added to NiSi to improve its thermal stability. Recently, Y. C. Kim et al. (Appl. Phys. Lett., 91, 113106, 2007) investigated Pd distribution in NiSi/Si structure in sub-nanoscale resolution using LEAP and found that Pd segregation at the interface. We employed a first principle calculation to understand the Pd segregation at the interface. An orthorhombic structure of NiSi was used to construct an orthorhombic-NiSi/Si (010). Lattice parameters along x- and z-directions in orthorhombic-NiSi were matched with Si. The strain energy of orthorhombic-NiSi was relieved along y-direction by 3.1 %. The optimized 1x4x1 orthorhombic-NiSi supercell and 1x2x1 Si (010) supercell were put together to construct the orthorhombic-NiSi/Si (010) with 10 Å vacuum along y-direction, and the structure was relieved in calculation to minimize its total energy. The optimized gap between orthorhombic-NiSi and Si (010) is 1.55 Å. Pd substituted in Ni and Si sites located near interface in orthorhombic-NiSi was energetically favorable, while Pd substituted in Si sites located near interface in Si substrate was not. Ni sites located near interface was more favorable for Pd substitution than Si sites (EPd→Ni = -0.75 eV, EPd→Si = -0.43 eV). Diffusion pathways for Pd in Ni sites to migrate from surface to interface were considered, and energy barriers of the diffusion pathways were calculated using nudged elastic band (NEB) tool.
9:00 PM - MM4.32
Surface Segregation of Shallow Donors in Nanocrystalline Silicon: Density-functional Studies.
Jose Coutinho 1 , Rui Pereira 1 , André Stegner 2 , Vitor J. Torres 1 , Sven Oberg 3 , Patrick Briddon 4 , Martin Brandt 2
1 Institute of Nanostructures, Nanomodeling and Nanofabrication, University of Aveiro, Aveiro Portugal, 2 Walter Schottky Institut, Technische Universität München, Munich Germany, 3 Department of Mathematics, Luleå University of Technology, Luleå Sweden, 4 School of Natural Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne United Kingdom
Show Abstract9:00 PM - MM4.33
Highly Strained Dislocation-Free Si(001) Nanomembranes.
Boy Tanto 2 , Max Lagally 1 2 , Don Savage 2 , Chanan Euaruksakul 3 , Deborah Cottrill 1
2 Physics, Univ of Wisconsin Madison, Madison, Wisconsin, United States, 1 Material Science and Engineering, Univ of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Electrical Engineering, Univ of Wisconsin Madison, Madison, Wisconsin, United States
Show AbstractHighly strained (≥~1%) Si has conventionally been made by Si deposition on relaxed thick compositionally graded SiGe layers. The SiGe relaxation proceeds through dis-location formation. While this process does create strained Si, the resultant surface is rough (from cross-hatch) and has non-uniform strain. Recently, we showed that Si/SiGe/Si membranes grown on SOI elastically strain share when released from the handle wafer, yielding dislocation-free tri-layer membranes with ≥~0.6% strained Si that can be readily transferred and bonded to a new host[1]. We extend the release/ strain sharing method with new approaches that allow generating much higher strains. The key step is the use of higher-Ge-concentration middle layers, something thought to be impossible. We present calculations that show the relationship of Si layer thick-nesses and Ge concentration in the SiGe layer that maximize the dislocation-free strain in the Si layers in a tri-layer structure. To obtain high strains, we use the opti-mized thicknesses and concentrations for a trilayer; release, transfer, and bond; and then remove the outer two membrane layers. We then regrow a new tri-layer using this strained-Si film as the starting substrate, but now with an even higher-Ge-concentration, but again optimized, middle layer. The whole process can be repeated until the desired strain is achieved. We readily achieve 1%-tensile-strain, dislocation-free Si. We discuss new processing issues that arise in the release, transfer, bonding, and etching of these high-strain, high-Ge-concentration membranes. We present Raman, AFM, LEEM, and x-ray measurements that provide experimental verification of high strain, absence of dislocations, and low surface roughness in these ultrathin Si membranes.[1] M.M. Roberts, L.J. Klein, D.E. Savage, K.A. Slinker, M. Friesen, G. Celler, M.A. Eriksson, and M.G..Lagally, Nature Materials 5, 388 (2006).Supported by NSF and DOE.
9:00 PM - MM4.34
Clustering Effects on the Luminescence from Dense Networks of Amorphous Silicon Nanoparticles.
Aaron Hryciw 1 2 , Al Meldrum 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Physics, University of Alberta, Edmonton, Alberta, Canada
Show AbstractAmorphous silicon (a-Si) nanocomposites fabricated via phase-separation of a silicon-rich oxide or nitride are an attractive class of materials for light-emitters in integrated optoelectronics due to their bright emission in the visible and near infrared, and the low, CMOS-compatible thermal budget required for their preparation. Despite recent demonstrations of photo- and electroluminescent devices based on this material system, however, the effect of a realistic a-Si nanostructure topology on the emission has not been investigated in great detail. In particular, when modeling the luminescence from ensembles of a-Si nanoclusters, an assumption of monodispersion is typical, and interparticle transport is often ignored. By developing a Monte Carlo implementation of a luminescence model for generalized spatially-confined amorphous semiconductor nanostructures, we investigate the emission from ensembles of a-Si particles with different packing fractions, simulating specimens ranging from collections of isolated particles to macroscopic percolation clusters. We compare results of the model to measured a-Si nanocluster photoluminescence spectra from silicon-rich oxide thin films annealed between 400 and 700 °C, incorporating experimentally-determined size distributions and packing fractions. It is shown that the internal quantum efficiency and emission spectra of specimens dominated by networks of a-Si nanoclusters differ dramatically from those consisting largely of isolated particles.
9:00 PM - MM4.35
Dissociation of H2O Molecule Adsorbed on Si (001) 2x1 Surface: A Theoretical Study.
Hyun-Chul Oh 1 , Hwa-Il Seo 2 , Yeong-Cheol Kim 1
1 Materials Engineering, Korea University of Technology and Education, Chonan Korea (the Republic of), 2 Information Technology, Korea University of Technology and Education, Chonan Korea (the Republic of)
Show AbstractDensity functional theory is used to investigate possible chemical reaction pathways for H2O molecule dissociation on a Si (001) 2x1 surface. The Si (001) 2x1 surface is modeled by a slab consisting of four layers of four Si atoms and a 2x1 surface reconstruction of buckled and zigzagged two Si dimers. An H2O molecule is adsorbed on a Si atom of a Si dimer of the Si (001) 2x1 surface. Adsorption of the H2O on the Si atom at lower height between the two Si atoms of the Si dimer is more favorable than that on the Si atoms at upper height. When initial rotation angle of the H2O molecule is 135° with respect to the [-110] dimer direction, the energy of the adsorbed H2O molecule is the most favorable. Energy advantage by adsorption of the H2O molecule on the Si (001) 2x1 surface is 0.66eV. After the H2O molecule is adsorbed on the Si atom at lower height of the dimer, one of its H atoms is dissociated from the H2O molecule and bonded to the other Si atom of the dimer. A dissociation energy barrier of an isolated H2O molecule is 5eV in theory, but the calculated dissociation energy barrier of the H2O molecule adsorbed on the Si (001) 2x1 surface is 0.2eV, indicating that the H-O bond of the H2O molecule is significantly weakened when the molecule is adsorbed on the Si surface.When two H2O molecules are adsorbed on the 2x2 Si supercell surface that contains two Si dimers, there are three possible arrangements: 1) two H2O molecules on two Si atoms of a Si dimer. 2) two H2O molecules on Si atoms at lower height of the two Si dimers (diagonal arrangement). 3) two H2O molecules on one-sided Si atoms of the two Si dimers (parallel arrangement). The relative energies of the three possible arrangements are 0, -0.78, -0.48 eV, respectively, indicating that the diagonal arrangement is energetically the most favorable. In addition, when the two H2O molecules adsorbed by the diagonal arrangement are dissociated to form two pairs of H:Si and Si:OH bonds, the H and OH ions are adsorbed on the diagonal positions, respectively.
9:00 PM - MM4.36
Using New Porous Nanocomposites for Photocatalytic Water Decontamination.
Maryam Zarei Chaleshtori 1 , Geoffrey Saupe 2
1 Environmental Science and Engineering, UTEP, El Paso, Texas, United States, 2 Environmental Science and Engineering, UTEP, El Paso, Texas, United States
Show AbstractCost-effective reductions of contaminated waters including organic and inorganic industrial chemicals are a well-known need in the world. Many industrial processes demand large amounts of water, which is released into the environment after use. If the wastewater is polluted with hazardous compounds, decontamination of the water becomes important and necessary, and clean-up expenses become a significant factor for industrial processes. Ordinary methods of decontaminating chemicals in water add considerable costs and delay to a production system.The search for economical ways to remove organic and inorganic contaminants has incited the study and exploration of heterogeneous photocatalyst-assisted oxidation methods, which have been shown to effect full mineralization of organic contaminants. Photocatalysis involving the use of UV light with wide band gap semiconductor metal oxides (such as TiO2) appears to be a promising clean-up alternative for the removal of contaminants.Heterogeneous catalysts that accelerate the photolytic destruction of organic contaminants in water are a potentially inexpensive and highly effective way to remove both trace-level and saturated harmful compounds from industrial waste streams and drinking water. Porous photocatalytic materials can have the combined qualities of high surface area and relatively large particle sizes, as compared with nanoparticulate catalyst powders. The larger particle sizes of the porous materials facilitate catalyst removal from a solution, after purification has taken place. We have synthesized new kinds of photocatalytic porous oxide materials that can be used to purify contaminated water by accelerating the photodegradation of any kind of organic pollutant. The new materials have very large open pore structures that facilitate the diffusion, the surface contact of contaminants, and solvent flow through the catalyst. These qualities enhance surface reactions important to the process. The new catalysts have shown robust physical and chemical properties that make them candidates for real applications in polluted water decontamination.
9:00 PM - MM4.38
Formation and Characterization of Diamondoid Monolayers on Silicon Surfaces.
Jason Fabbri 1 , Peter Schreiner 2 , Jeremy Dahl 3 , Robert M. K. Carlson 3 , Nicholas Melosh 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Justus-Liebig University Giessen, Giessen Germany, 3 , Chevron MolecularDiamond Technologies, Richmond, California, United States
Show Abstract9:00 PM - MM4.39
Obstacles and Progress in Nanofabrication of High Resolution and Nanofocusing X-ray Optics Based on Silicon and Diamond.
Abdel Isakovic 1 , Kenneth Evans-Lutterodt 1 , John Warren 2 , Aaron Stein 2 , Alec Sandy 3 , Suresh Narayanan 3
1 NSLS, Brookhaven National Laboratory, Upton, New York, United States, 2 CFN, Brookhaven National Laboratory, Upton, New York, United States, 3 , Argone National Laboratory, Chicago, Illinois, United States
Show Abstract9:00 PM - MM4.40
ZnS, CdS and ZnCdS Thin Films from Zn(II) and Cd(II) Complexes of 1, 1, 5, 5-Tetramethyl-2-4-dithiobiuret as Single Molecular Precursors.
Karthik Ramasamy 1 , Mohammad Malik 1 , Paul O'Brien 1
1 Chemistry, University of Manchester, Manchester United Kingdom
Show AbstractSynthesis of zinc and cadmium complexes of 1, 1, 5, 5-tetramethyl-2-4-dithiobiuret [M (tmtb)2] (M = Zn, Cd) is described together with X-ray single crystal of cadmium complex. Thermo gravimetric analysis of complexes showed clean single step decomposition into corresponding metal sulphides in the temperature range between 253 – 388 °C. Both zinc and cadmium complexes are air, moisture and light stable. These complexes have been used as a single molecular precursor for the deposition of ZnS, CdS and ZnCdS thin films by aerosol assisted chemical vapour deposition (AACVD) method. Adherent specular films of ZnS were obtained in the temperature range 300 - 450 °C, CdS films obtained in the range of 350 – 500 °C and ZnCdS ternary films at 400 °C by varying the precursor concentration. XRD of ZnS films showed cubic to hexagonal phase1 change above 350 °C. XRD pattern showed the deposition of hexagonal phases for CdS and ZnCdS films.2 SEM showed that the morphology of the films varied depending on the deposition temperature. The films were also characterized by electronic spectra (UV/Vis), and atomic force microscopy (AFM). To the best of our knowledge these complexes are the first in their class to be used as a single molecular precursor to deposit ZnS, CdS, ZnCdS thin films by AACVD method.Reference:1. R. D. Pike, A. Wold, T. N. Blanton, H. J. Gysling, Thin Solid Films,1993, 224, 221- 226. 2. R. S. Feigelson, A. N’Diage, S. Y. Yin and R. H. Bube, J. Appl. Phys., 1977, 48, 3162
9:00 PM - MM4.5
Composition Dependence of Thermoelectric Properties for Amorphous Si-Ge-Au Thin Films.
Hiroaki Takiguchi 1 , Makoto Abe 1 , Yoichi Okamoto 1 , Hisashi Miyazaki 1 , Jun Morimoto 1
1 Materials Science Engineering, National Defense Academy, Yokosuka, Kanagawa, Japan
Show AbstractWe have already reported that amorphous Si-Ge-Au thin films show extremely large thermoelectric power (~10-2 V/K) as well as low electrical resistivity of conventional semiconductor (10-3 ~ 10-6 Ωm). There are still many unsolved questions of appropriate composition of Si, Ge and Au. In this work, we investigate all three composition dependence for thermoelectric properties of amorphous Si-Ge-Au thin film. The samples were prepared by alternate deposition from three electron guns of Si, Ge and Au in the ultrahigh vacuum system. The samples have 4 layers structure of Si/Au/Ge/Au as 1 period. The artificial intervals, total number of periods and the total thickness were kept at 10 nm, 30 and 300 nm, respectively. Thermoelectric power and electrical resistivity were measured with various composition of Si, Ge and Au. The maximum value of thermoelectric power was ~10-3 V/K at 300 K (Si, Ge and Au were about 53, 45 and 2 at. %, respectively). Thermoelectric power and electrical resistivity decreased with increasing Au concentration. Electrical conduction type changed with varying composition and temperature. Conduction type of the samples are n-type when Au concentration is less than 7 at. % and the ratio of Si and Ge is almost equal. The experimental results suggest that both donor and acceptor levels exist in the thin films.
9:00 PM - MM4.6
Effects of Transparent SiCN Doping Layer on Performance of Si Nanocrystal Light-Emitting Diodes.
Chul Huh 1 , Kyung-Hyun Kim 1 , Jongcheol Hong 1 , Hyunsung Ko 1 , Wanjoong Kim 1 , Gun Yong Sung 1
1 , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractRecently, Si nanocrystals (nc-Si) have attracted a great interest due to their potential for applications in silicon-based optoelectronic devices. In our previous result [1], well-organized nc-Si embedded in silicon nitride films were grown by a plasma enhanced chemical vapor deposition. We also fabricated the nc-Si light-emitting diodes (LEDs) by applying an amorphous silicon carbide (SiC) doping layer and an indium tin oxide current spreading layer [2]. In order to further increase the performance of nc-Si LEDs, a detailed investigation on the transparent doping layer must be done. In the present work, we have investigated the influence of a transparent silicon carbon nitride (SiCN) layer on performance of the nc-Si LEDs. The optical band gap (Eg) of SiCN film shifted to a shorter wavelength with increasing N composition into the SiCN film, which was estimated from 2.2 to 2.6 eV by applying the Tauc model. The Eg values determined were 2.2 for SiC, 2.4 for SiCN (15 % N), and 2.6 eV for SiCN (18 % N) films, respectively. The electrical property of the nc-Si LED by applying a transparent SiCN film was enhanced compared to that of the nc-Si LED with a SiC film. This can be attributed to a reduction in the tunnel barrier of electrons into the nc-Si from the SiCN doping layer due to higher band gap of the SiCN doping layer than the SiC doping layer. The intensity of electroluminescence (EL) spectra of the nc-Si LED measured at room temperature was increased with increasing the input current. We found that both photoluminescence (PL) and EL showed a similar peak centered at 680 nm, suggesting that the EL and PL processes can be related to the same origin. The light output power of nc-Si LED with a SiCN doping layer was also improved compared to that of nc-Si LED with a SiC doping layer. The results suggest that a transparent SiCN doping layer is a very effective way to improve the performance of nc-Si LEDs. Moreover, we demonstrated the 8×8 μ-LED array with stable and uniform light emission.Acknowledgement.This work was partly supported by the IT R&D program of MIC/IITA [Ubiquitous Health Monitoring Module and System Development] and Top Brand R&D program of MOST [Basic Research for the Ubiquitous Lifecare Module Development] in Korea. References. [1] T. Y. Kim et al., Appl. Phys. Lett. 85, 5355 (2004). [2] C. Huh et al., Appl. Phys. Lett. 88, 131913 (2006).
9:00 PM - MM4.7
Mechanical Fabrication of Si-Ge-B Bulk Samples with Superior Thermoelectric Power.
Makoto Abe 1 , Hiroaki Takiguchi 1 , Youichi Okamoto 1 , Hisasi Miyazaki 1 , Jun Morimoto 1
1 , National Defense Academy, Yokosuka, Kanagawa, Japan
Show AbstractWe have already reported that Au or B doped SiGe amorphous thin films have superior thermoelectric properties which are attribute to amorphous phase. For the practical use, the bulk materials are required. In this work, we have tried and succeeded to fabricate Si-Ge-B amorphous bulk samples. First, fine particles were prepared by three kinds of mechanical process, such as roller milling method, mechanical alloying method and planetary milling method. It was intended to introduce a large amount of defects and strain into samples and/or “amorphouslization”. Then, the prepared fine particles were pressed and formed into 2 x 5 x 15 mm3 samples. Thermoelectric power and electrical resistivity of samples were measured by steady state measurement and four terminal method with temperature range of room temperature to 873 K in N2 flow at atmosphere pressure, respectively. Thermal conductivity was measured by photo-pyroelectric method in room temperature. Some samples have maximum value of thermoelectric power over 10-3 V/K. therefore≤ we were succeeded to prepare bulk materials which have higher thermoelectric power than that of conventional crystal materials. X-ray diffraction measurement and scanning electron microscope observation were performed on fabricated samples. From XRD measurement and SEM observation, the samples were in mixed condition of disordered microcrystals and amorphous state.
9:00 PM - MM4.8
Structural Relaxation of Voids in Si Substrates by High Temperature Hydrogen Annealing.
Reiko Hiruta 1 , Hitoshi Kuribayashi 1 , Ryosuke Shimizu 2 , Koichi Sudoh 3 , Hiroshi Iwasaki 3
1 , Fuji Electric Device Technology Co., Ltd., Nagano Japan, 2 , Fuji Electric Advanced Technology Co., Ltd., Tokyo Japan, 3 , The Institute of Scientific and Industrial Research, Osaka University, Osaka Japan
Show AbstractRecently, shape transformation of microstructures fabricated on Si substrates by surface diffusion during high temperature hydrogen annealing has been proven to be useful for fabrication processes of three-dimensional structures [1,2]. One of the significant applications of the shape transformation is formation of silicon-on-nothing (SON) structures, in which a plate-like void is formed under a thin Si layer, through structural relaxation of an array of deep holes in high temperature hydrogen ambient [3]. The SON structure, as the ideal silicon-on-insulator (SOI) structure, because the vacant space shows the lowest dielectric constant. In this paper, we have studied the structural relaxation of a buried void, which is observed in the process of the SON structure formation.A periodic square array of holes with the diameter 0.7µm, spacing 0.7µm, and depth 3µm, was fabricated on n-type CZ-Si (100) substrates (2 W×cm) by anisotropic reactive ion etching (RIE) with a SiO2 mask. The substrates were annealed in 60~760 Torr hydrogen gas ambient at 1100~1150 degree. We evaluated the profile of voids by scanning electron microscopy (SEM), cleaving the sample normal to the substrate surface along the (110) plane.During annealing of the hole array, first vertically long voids are formed in the substrate through closing of the hole apertures, and then they relax to an oval shape. The surfaces of the voids are composed of {111} {100} {113} {110} facets. We found that the {113} facets grow up widely, while the {100} and {110} facets shrink through the process of the structural relaxation. In the presentation, we will discuss the detailed mechanism of the observed structural relaxation. [1] H. Kuribayashi, R. Shimizu, K. Sudoh, and H. Iwasaki, J. Vac. Sci. & Technol. A22, 1406 (2004). [2] R. Shimizu, H. Kuribayashi, R. Hiruta, K. Sudoh and H. Iwasaki, Proc. 2006 Int. Symp. Power Semicon. Dev. & ICs, p.113 (2006). [3] T. Sato, I. Mizushima, S. Taniguchi, K. Takenaka, S. Shimonishi, H. Hayashi, M. Hatano, K. Sugihara and Y. Tsunashima, Jpn. J. Appl. Phys. 43, 12 (2004).
9:00 PM - MM4.9
Surface Preparation and Thin Film Epitaxial Growth on β-FeSi2 Substrate.
Haruhiko Udono 1 , Kensuke Akiyama 2 , Masaru Itakura 3
1 , Ibaraki University, Hitachi, Ibaraki, Japan, 2 , Kanagawa Ind. Tech. Cent., Ebina, Kanagawa, Japan, 3 , Kyushyu University, Kasuga, Fukuoka, Japan
Show AbstractSemiconducting iron disilicide, β-FeSi2, has received much attention as a suitable material for Si-based optoelectronic devices [1]. Up to date, a number of researchers have grown β-FeSi2 on Si substrate by reactive deposition epitaxy (RDE), molecular beam epitaxty (MBE), ion beam synthesis (IBS), ion beam sputter deposition (IBSD) and chemical vapor deposition (CVD) methods and succeeded to grow highly oriented β-FeSi2 films on Si(100) and (111) substrates [2]. However, rotational domains usually exist in the films at very high-density level. Recently, we have grown β-FeSi2 thin film on single crystalline β-FeSi2 substrate that is prepared from solution grown bulk crystals. In this paper, we report surface preparation of β-FeSi2 substrate and homoepitaxial growth of β-FeSi2 thin films by MBE.The substrates of β-FeSi2 (100) and (101) were prepared from the solution grown β-FeSi2 single crystals [3]. Typical size of the substrate was approximately 1.5 x 1.5 mm2 in area and 0.9 mm in thickness. Prior to the thin film growth, the substrate was etched by HF(50%):HNO3(60%):H2O solution. After the etching, the substrate was loaded into MBE chamber and then annealed in an ultra high vacuum (UHV). Condition of the substrate surface was observed by RHEED. The β-FeSi2 film of 50-nm-thickness was deposited on β-FeSi2 substrates at the growth temperatures between 620 and 900°C. The deposition rate of Fe was fixed at 0.6 nm/min, and that of Si was varied from 2.4 to 3.5 nm/min. β-FeSi2 films were evaluated by the RHEED, non-contact AFM, scanning electron microscope (SEM) and X-ray diffraction.From the RHEED observation before and after thermal annealing, we found that the surface of as-etched substrate was covered with thin native oxide layer, which would be introduced during the loading of substrate, and it was removed by annealing above 850°C. We also succeeded to grow β-FeSi2 epitaxial films on β-FeSi2 (100) and (101) substrates. Results of XRD measurement, SEM, TEM and AFM observations of the films will be presented.
Symposium Organizers
Tony van Buuren Lawrence Livermore National Laboratory
Leonid Tsybeskov New Jersey Institute of Technology
Susumu Fukatsu University of Tokyo
Luca Dal Negro Boston University
Fabrice Gourbilleau CIMAP, UMR CNRS
MM5: Si and Ge Nanocrystals
Session Chairs
Tuesday AM, December 02, 2008
Room 309 (Hynes)
9:30 AM - **MM5.1
Silicon and Germanium Tunneling Nanotransistors as Ultimate Switches.
Alex Zaslavsky 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show Abstract10:00 AM - MM5.2
Effect of Indirect-to-Direct Gap Transformation on Auger Recombination in Germanium Nanocrystals.
Istvan Robel 1 , Ryan Gresback 2 , Doh Lee 1 , Uwe Kortshagen 2 , Jeffrey Pietryga 1 , Richard Schaller 1 , Victor Klimov 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractA large bulk exciton Bohr radius (~24 nm) and a small energy separation between the lowest indirect- and direct-gap band minima (~0.14 eV at 300 K) make germanium (Ge) well-suited for studies of the effects of quantum confinement on band structure, especially in the context of confinement-induced indirect-to-direct gap transformation. In addition to being of interest from a fundamental perspective, Ge nanostructures are promising candidates for applications in photovoltaic and light-emitting technologies that can benefit from low toxicity as well as tunability of absorption and emission across near-infrared and visible wavelengths.The effect of three-dimensional quantum confinement on radiative recombination in indirect-gap materials has been extensively analyzed in the literature. Specifically, relaxation of translational momentum conservation has been invoked to explain significant enhancement in emission efficiencies of nanoscale forms of silicon. The influence of strong quantum confinement on multiparticle processes such as Auger recombination in indirect-gap semiconductors has not been studied yet either theoretically or experimentally. The purpose of this work is to address this unexplored aspect of Auger recombination by performing a side-by-side comparison of Auger-decay rates in nanocrystals of Ge with those in nanocrystals of direct-gap materials such as PbSe and InAs.In our work we study colloidal Ge nanocrystals with sizes ranging from 3.5 nm to 10 nm. We observe that multiexciton recombination time constants scale linearly with nanocrystal volume. Surprisingly, the Auger constants measured for nanocrystals (up to ~3×10-28 cm6s-1) are orders of magnitude larger than those in bulk germanium (~10-31 cm6s-1). We explain this dramatic increase in Auger rates by relaxation of momentum conservation induced by strong three-dimensional spatial confinement. This effect leads to a change in the mechanism for Auger recombination, which instead of being a four-particle phonon-assisted process in the bulk becomes a "direct" three-particle process in nanocrystals. By comparing decay rates measured for Ge nanocrystals with those in nanocrystalline direct gap materials such as PbSe or InAs, we observe a remarkable similarity of Auger constants for particles of same sizes, despite a dramatic difference (by 4 to 5 orders of magnitude) in their respective bulk phases. The general conclusion of our work is that multiexciton dynamics in nanocrystals are primarily controlled by particle sizes but not the details of their band structure.
10:15 AM - MM5.3
Infrared Emitting Colloidal Germanium Nanocrystals.
Doh Lee 1 , Istvan Robel 1 , Jeffrey Pietryga 1 , Richard Schaller 1 , Victor Klimov 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWith a narrower band gap and higher absorption cross-sections than silicon, germanium (Ge) is a material of increasing interest for optoelectronic application, especially in solar cells. However, as Ge is an indirect band gap semiconductor, relatively low absorption and photoluminescence efficiencies of the bulk material limit its potential. Ge nanocrystals (NCs), on the other hand, have been reported to exhibit significant photoluminescence over a wide range of energies with varying degrees of correlation to NC size. While such emission is generally attributed to the effects of quantum confinement, the significance of confinement effects in each case remains unclear. As sample variability (in, e.g., size, shape, crystallinity and composition) complicates analysis of the role of confinement, a key to better understanding of the optical properties of Ge NCs is controlled synthesis. Colloidal growth has been successful applied to efficiently produce high-quality NCs of many materials with excellent size and shape control. Current colloidal methods for Ge NCs, while efficient, produce non-emissive NCs with moderate control over size and shape dispersity. In this study, we synthesized Ge NCs while monitoring the effects of variables such as solvents, reducing agents, reaction temperature, and capping ligands. The resulting NCs showed robust chemical stability and good surface passivation, as evidenced by the luminescent properties, including up to 10% quantum yield in infrared photoluminescence. Size analysis of the samples by transmission electron microscopy revealed that the size dependence of infrared photoluminescence is in close proximity with the data previously reported by other groups. Finally, time-resolved spectroscopy of photoluminescence at both visible and infrared region was employed to understand the dynamics of excitons in Ge NCs.
10:30 AM - MM5.4
Size distribution of self assembled Ge nanocrystals determined by photoluminescence
Nelson Rowell 1 , David Lockwood 1 , Isabelle Berbezier 2 , Antoine Ronda 2 , Pierre Szkutnik 2 , Alim Karmous 2
1 , NRC Canada, Ottawa, Ontario, Canada, 2 , Institut Matériaux Microélectronique Nanosciences de Provence, Marseille France
Show AbstractGermanium nanocrystals (NCs) were obtained from the dewetting process during in-situ thermal annealing of an amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 layer on Si(001). The Ge NCs were then capped with a thin layer of amorphous Si to prevent oxidation. The mean NC diameter – 2.5 to 60 nm – depends on the initial Ge layer thickness. Low temperature photoluminescence (PL) measurements were performed to investigate quantum confinement effects on the Ge nanocrystal energy gap and defect states. For the present range of particle sizes, the nanoparticle PL emission appeared primarily as a wide near-infrared band peaked near 800 meV. The peak energy of the PL band reflects the average NC size and its shape depends on the NC size distribution. The size dependence of the Ge NC band gap has been investigated theoretically by Delerue, Allan and Lannoo, who find a sharply increasing gap energy for NC diameters less than ~6 nm from both the k.p and tight binding models. Using these theoretical models we have analyzed the PL of each sample in terms of the NC size distribution required to reproduce the observed asymmetric band shape, which includes, for the smaller diameter NCs, band gap enlargement due to quantum confinement. Good agreement with the observed size distribution determined from transmission electron microscopy analysis is obtained when allowance is made for a nonlinear increase in the PL quantum efficiency with decreasing NC diameter. Given a good theoretical description of the system, this implies that it is possible to evaluate the size distribution of semiconductor NCs from their PL energy dependence.[1] C. Delerue, G. Allan, M. Lannoo, Phys. Rev. B 48 (1993) 11024.
10:45 AM - MM5.5
Germanium Nanocrystal Thin Films Exhibiting Quantum Confinement.
Zachary Holman 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractSemiconductor nanocrystals embedded in matrices and dissolved in solution have garnered much attention for their size-dependent optical properties; however, in order to be used in traditional thin film devices, nanocrystal films with tunable properties must be designed. The Group II-VI and IV-VI nanocrystal communities have had some success in achieving this goal by drying and then chemically treating colloidal particles, but this approach is difficult with Group IV nanocrystals which are not nearly as solution-friendly. We report on a novel plasma synthesis and gas-phase deposition technique for forming dense thin films of germanium nanocrystals (Ge-NCs) which exhibit quantum confinement.Germanium tetrachloride is dissociated in the presence of hydrogen in a nonthermal plasma to nucleate Ge-NCs. Transmission electron microscopy and X-ray diffraction indicate that the particles are nearly monodisperse (standard deviations of 10-15% the mean particle diameter) and the mean diameter can be tuned from 4-15 nm by changing the residence time of the Ge-NCs in the plasma. The particles can be made either crystalline or amorphous without otherwise altering their characteristics by controlling the plasma power.Downstream of the synthesis plasma, the Ge-NCs are accelerated by sonic gas flow through an orifice plate and impacted on a substrate rastered under the orifice. This results in the deposition of a highly uniform film of Ge-NCs on the substrate. By altering the total gas flow and substrate-orifice standoff distance, we have made films that are greater than 50% the density of bulk Ge, as determined by cross-sectional scanning electron microscopy and Rutherford backscattering.Absorption spectroscopy measurements show that the Ge-NC films are quantum confined, and the absorption coefficient blue-shifts as the constituent particles in the film are made smaller. The band gap of the films has been determined from the absorption spectra, and increases from the bulk value of 0.7 eV to over 1.1 eV for films of 4 nm GeNCs. Current-voltage measurements on as-deposited films yield conductivities on the order of 10^-7 S/cm in the dark and an order of magnitude higher under Air Mass 1.5 Global light. Investigations into solar cell devices utilizing these films are currently underway.This work was supported by NSF under NIRT grant CBET-0506672, IGERT grant DGE-0114372, and MRSEC grant DMR-0212302. Support was also provided by the UMN Center for Nanostructure Applications.
11:30 AM - **MM5.6
Opto-electronic Properties of Ge and Si Related Nanostructures on Ultrathin Si Oxide Covered Si Surfaces.
Masakazu Ichikawa 1
1 Department of Applied Physics, University of Tokyo, Tokyo Japan
Show Abstract Nanostructures have attracted much interest due to their quantum-confinement effects, which can cause the changes in material opto-electronic properties. We found that ultra-small Si and Ge nanodots with the size of ~5 nm and ultra-high density of ~1012 cm-2 grew on Si surfaces covered with ~0.3 nm thick ultrathin SiO2 films [1,2]. Deposited Si and Ge atoms on the surfaces reacted with the ultrathin SiO2 films by the chemical reactions of SiO2+Si→2SiO↑ or SiO2+Ge→SiO↑+GeO↑ to form Si and Ge nucleation sites. The subsequently deposited Si and Ge atoms were mainly captured by the nucleation sites due to the larger adsorption energies on the nucleation sites than those on the ultrathin SiO2 films. This resulted in Si and Ge nanodot growths with ultra-small and uniform size and with ultra-high density. Si and Ge nanodots had spherical structures which were different from that of Si homoepitaxial layers and that of Ge islands formed by the Stranski-Krastanov growth. This growth technique can apply to grow different material nanodots on Si substrates such as GeSn [3], β-FeSi2 [4] and GaSb nanodots. We have recently succeeded in forming Ge nandots with periodic arrangements on Si substrates by using block copolymer nano-masks on ultrathin SiO2 films. We investigated electronic properties of individual Ge [5], GeSn [6] and β-FeSi2 [7] nanodots by scanning tunneling spectroscopy. With decreasing the dot sizes, the energy band gaps increased due to the carrier quantum-confinement effects in the dots. We also observed the Coulomb-blockade effect featured by discrete tunneling current fluctuation on Ge nanodots at room temperature [8]. The discrete fluctuation was explained by a single electron trap in a nanodot and a single electron escape into a Si substrate. We further investigated optical properties of the structures of Ge nanodots embedded in Si films [9]. Intense PL and electroluminescence (EL) were observed at photon energies around 0.8 eV used for the optical fiber communication from the structures after high-temperature annealing. We have recently observed intense EL from the stacked structures of Ge nanodots embedded in Si films at room temperature [10]. [1] A. Shklyaev et al., Phys. Rev. B 62, 1540 (2000). [2] A. Shklyaev et al., Phys. Rev. B 65, 045307 (2002). [3] Y. Nakamura et al., J. Appl. Phys. 102, 124302 (2007). [4] Y. Nakamura et al., J. Appl. Phys. 100, 044313 (2006). [5] Y. Nakamura et al., Appl. Phys. Lett. 87, 133119 (2005). [6] Y. Nakamura et al., Appl. Phys. Lett. 91, 013109 (2007). [7] Y. Nakamura et al., Appl. Phys. Lett. 89, 123104 (2006). [8] Y. Nakamura et al., Appl. Phys. Lett. 90, 153104 (2007). [9] A. Shklayev et al., Appl. Phys. Lett. 88, 121919 (2006). [10] A. Shklayev et al.,unpublished.
12:00 PM - MM5.7
Ge Quantum Dots on Si Substrate Assembled in Large Area Complex Patterns by Focused Ion Beam Nano-templating.
Maria Gherasimova 1 , Robert Hull 1 2 , Mark Reuter 3 , Frances Ross 3
1 Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Materials Science and Engineering, Renssselaer Polytechnic Institute, Troy, New York, United States, 3 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractPrecise nanoscale control of quantum dot (QD) positioning on a substrate is necessary to enable many potential applications in microelectronics and photonics, such as the realization of quantum cellular automata and targeted incorporation of internal light sources in the photonic crystal cavities. We have achieved controlled nucleation of Ge QDs on Si substrates by encoding the target nucleation sites with low dose focused ion beam (FIB) pulses, and have investigated both the reliability statistics of large area QD arrays (typically tens of microns in lateral size, containing tens of thousands QDs), and the possibility of fabricating single QDs at particular locations on substrates. Ge islands are synthesized in an ultra-high vacuum environment inside a transmission electron microscope equipped with a video-rate data capture capability, immediately after the FIB implantation in an adjacent chamber. Our findings indicate that for large QD arrays, the array assembly fidelity becomes limited by the competition between the target nucleation sites for the available Ge adatoms in the sub-100 nm site separation regime, while nearly 100 % registration rates are obtained at larger distances. In this work, we investigate the pathways towards improving the QD assembly reliability on the scale of tens of nanometers by manipulating the adatom kinetics during synthesis, and by influencing the trapping ability of the encoded sites to promote rapid island nucleation on the closely spaced target locations. We will discuss the application of the nano-templating technique to form quantum cellular automata circuits and to enhance photonic crystal structures.
12:15 PM - MM5.8
Self-Assembly of Ge Nanocrystals on Prepatterned Substrates and Application to Ncs Memory.
Isabelle Berbezier 1
1 Materials, IM2NP, Marseilles France
Show Abstract12:30 PM - MM5.9
Preparation of Freestanding Si and Ge Nanocrystals by Ultrasonic Aerosol Pyrolysis for Next Generation Solar Conversion and Energy Storage Applications.
Michael Haag 1 , Fusheng Xu 1 , Brian Larsen 1 , Se-Hee Lee 1 , Conrad Stoldt 1
1 Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
Show AbstractA synthetic route adaptable for the continuous, large-scale production of silicon and germanium nanocrystals for emerging electronic and optoelectronic applications such as next generation solar conversion technologies is presented. Using an ultrasonic aerosol pyrolysis approach, relatively monodisperse Si and Ge nanocrystals with average sizes below 20 nm are synthesized, and the mean crystal diameter is precisely tuned by varying the Si or Ge precursor concentration. The nanocrystals result from the thermal decomposition of an ultrasonically generated aerosol containing the Ge or Si precursor, followed by their gradual cooling and subsequent capture as a colloid in organic solvent. Nanocrystal surface encapsulation and functionalization will be described, as well as fundamental material property measurements and initial device integration schemes. Results from two applications for these material systems will be described: (1) third generation solar conversion and (2) solid-state battery technologies based upon integrated Si and/or Ge nanocrystals.
12:45 PM - MM5.10
Photoluminescence and Optical Bleaching of Direct Band Gap Transition in Ge-on-Si.
Xiaochen Sun 1 , Jifeng Liu 1 , Lionel Kimerling 1 , Jurgen Michel 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractGermanium is a promising candidate for an on-chip Si-based light emitter which is crucial for the realization and monolithic integration of Si microphotonics on microelectronics circuitry. Ge is a Si-CMOS compatible material that can be grown epitaxially on Si and processed using VLSI technique. The wavelength range of the light emission from the direct band gap (0.8eV) transition of Ge falls into the desired telecommunication band (1550nm) that is also used in Si microphotonics. The occurrence of direct band gap light emission in intrinsic Ge is limited by the small amount of injected excess electrons in the direct conduction valley (Γ valley) due to the nature of its indirect band structure. However, both our theoretical study and experimental data have shown that biaxial tensile stress in thin film Ge epitaxially grown on Si can change the band structure by decreasing the difference between direct and indirect gaps. A 2% tensile strain is sufficient to equalize the direct band gap and the indirect band gap at 0.5 eV. However, such a high strain will shift the direct band gap light emission to a much longer wavelength of ~2500 nm. To obtain efficient light emission around 1550 nm from the direct transition of Ge, a moderate tensile strain can be introduced to Ge-on-Si films while the rest of the energy difference between the direct band gap and the indirect band gap can effectively be reduced by the occupation of the indirect L valleys with extrinsic electrons from n-type donors. A theoretical analysis has been done to show such band-engineered Ge is able to achieve optical net gain from direct band gap transition at carrier injection rate comparable to some III-V lasers. Ge-on-Si films at 0.2% tensile strain with various n-type doping levels were prepared and tested. The photoluminescence spectra show that the direct band gap luminescence peaks at around 1590nm and the intensities of the luminescence increase with the doping levels, consistent with our theoretical prediction. We will present non-degenerate (distinct pump source and probe source) pump-probe spectroscopy measurements that were performed on heavily doped tensile-stressed samples. The wavelength-dependent transmittance of the modulated tunable probe laser under continuous pump light was measured by an optical power meter. Optical bleaching in the wavelength range above the direct band gap energy was observed, indicating that the absorption of the material is reduced under optical injection. This bleaching effect is a precursor to optical net gain. These experiments support our theoretical prediction and point to Ge as a gain medium for on-chip Si-based light emitters.
MM6: Diamondoids
Session Chairs
Tuesday PM, December 02, 2008
Room 309 (Hynes)
2:30 PM - **MM6.1
Nanostructure Design for Silicon Performance.
Lionel Kimerling 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract3:00 PM - MM6.2
Thiol-modified Diamondoid Monolayers on Silver and Gold Studied with Near-Edge X-ray Absorption Fine Structure Spectroscopy.
Trevor Willey 1 , Jonathan R. Lee 1 , J. Fabbri 2 , D. Wang 3 , P. Schreiner 4 , A. Fokin 4 , N. Fokina 4 , B. Tkachenko 4 , Jeremy E. Dahl 5 , Robert M. Carlson 5 , N. Melosh 2 , T. van Buuren 5
1 Materials Science and Technology Division, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States, 3 , Virginia Tech, Blacksburg, Virginia, United States, 4 Institute of Organic Chemistry, Justus-Liebig University, Giessen Germany, 5 MolecularDiamond Technologies, Chevron, Richmond, California, United States
Show AbstractHigher diamondoids, hydrocarbon cages with a diamond-like structure, have largely evaded laboratory synthesis but can be purified from petroleum sources. This new class of nanometer-sized rigid hydrocarbon molecules shows promise in various areas of nanotechnology. Particularly, computation indicates individual diamondoids possess negative electron affinity, and diamondoid monolayers exhibit incredibly intense, monochromatic photoemission. Such surface-attached diamondoids have technological possibilities as high-efficiency field emitters in molecular electronics, as well as other nanotechnological applications, and fundamental studies of the properties of these monolayers are a necessary precursor.Methods to site-selectively functionalize diamondoids enable formation of controllable diamondoid self-assembled monolayers on surfaces. We have investigated a number of thiol-modified diamantanes, triamantanes, tetramantanes, and pentamantanes adsorbed on silver and gold, using near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and x-ray photoelectron spectroscopy (XPS). The results illustrate how thiol position, diamondoid size, and diamondoid shape affect the diamondoid self-assembled monolayer structure.
3:15 PM - MM6.3
Optical Properties of Pristine and Functionalized Nanodiamonds - Diamondoids.
Lasse Landt 1 , David Wolter 1 , Matthias Staiger 1 , Jeremy Dahl 2 , Robert Carlson 2 , Andrey Fokin 3 , Peter Schreiner 3 , Thomas Möller 1 , Christoph Bostedt 1
1 , Technische Universität Berlin, Berlin Germany, 2 , MolecularDiamond Technologies, Richmond, California, United States, 3 , Justus-Liebig-Universität, Giessen Germany
Show AbstractDiamondoids constitute a class of novel, structurally and chemically pure nanodiamonds adding fully sp3-hybridized members to the nano-carbon family [1]. We have measured the optical properties of pristine and C–H-bond functionalized diamondoids and observed an unexpectedly strong shape dependence of the optical properties for pristine diamondoids, which in some cases even outweighs the quantum confinement effects for the optical gap. For functionalized diamondoids we found the optical properties to depend on the functionalization site. Our data show that attaching a thiol group at the outermost edge of a diamondoid fixes its optical gap at ~6 eV for all investigated diamondoid sizes. Attaching the same functional group centrally preserves the size dependence of the optical gap. This could serve as a chemical switch for the quantum confinement effects of the gap in future applications. Further experiments show that diamondoids exhibit intrinsic luminescence. The emission is energetically broad and lies in the deep UV. These findings display the diamondoids’ potential for new photonic applications in the deep UV spectral range. The presented experimental data will be compared to earlier electronic structure measurements [2,3] and theoretical work [4].[1] J.E. Dahl, et al., Science 299, 96-99 (2003) ; [2] T.M. Willey, C. Bostedt, et al., Phys. Rev. Lett. 95, 113401 (2005); [3] K. Lenzke, L. Landt, et al., J. Chem. Phys. 127, 084320 (2007) ; [4] N. Drummond, A.J. Williamson, et al., Phys. Rev. Lett. 95, 096801 (2005).
3:30 PM - **MM6.4
Higher Diamondoids: From Petroleum to Carbon Nanomaterial.
Jeremy Dahl 1 , Robert Carlson 1
1 , MolecularDiamond Technologies, Chevron, Richmond, California, United States
Show Abstract4:30 PM - MM6.5
Tribology of Self Assembled Diamondoid Monolayers.
Bin Wu 1 , Jason Fabbri 1 , Nicholas Melosh 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show Abstract4:45 PM - MM6.6
Field Emission Studies on Au Coated Ge Nanowires Functionalized with Diamondoids.
Chenhao Ge 1 , Jason Fabbri 1 , Nick Melosh 1
1 , Stanford University, Stanford, California, United States
Show Abstract5:00 PM - MM6.7
Electronic Structure of Graphene-Oxide: Theory and Experiments.
Jahan Dawlaty 1 , Farhan Rana 1 , Dan Li 2 , Gordon Wallace 2
1 ECE, Cornell University, Ithaca, New York, United States, 2 Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2522, New South Wales, Australia
Show AbstractThe unusual electronic and optical properties of graphene have been at the forefront of the current research in condensed matter systems [1]. Recently, graphene-oxide has attracted significant attention [3,6]. Graphene-oxide is an oxygenated derivative of graphene containing hydroxyl and epoxide functional groups attached to the carbon atoms [6]. Studying graphene-oxide is pertinent to graphene research in many ways. For example, oxidation of graphene significantly changes its electronic structure and properties, such as the bandgap. Despite its importance, the electronic structure of graphene-oxide has not been studied extensively [5]. The accepted structural model of graphene-oxide consists of oxygen atoms attached to the carbon atoms in the plane of graphene in the form of hydroxyl and epoxide groups [6]. It has been shown that as a result of covalent bonding to oxygen, the sp2 geometry of the carbon atom changes [5,7]. Consequently, the hopping energy between two neighboring carbon is modified. Also, as a result of the covalent bond with the oxygen atom, the on-site energies of the newly hybridized orbitals are changed. Using these basic ideas, we have developed a tight-binding approach to study the electronic structure of graphene-oxide. We start with a Hamiltonian for a graphene structure of finite size (~5000 atoms). The effect of the incorporation of oxygen at random sites is modeled by changing the on-site and hopping energies at the site of the oxygen atom. These parameters, along with the percentage of oxygen coverage, are varied until reasonable agreement between the calculated density of states and experimental optical spectra is achieved. Our model has two distinct advantages over previous approaches (1) It is simple and allows for computation of larger structures and (2) It allows one to account for the randomness in the attachment of the oxygens. Optical spectra of the samples [3] were measured in the UV-visible region. We find that for ~40% oxygen coverage, change in on-site energy of -3.5 eV, and a reduction in the hopping energy of about 10%, a bandgap of about ~1.8-2.0 eV appears in the density of states. This agrees well with the measured optical transmission spectra of graphene-oxide films. We also observe that the band edges are not sharp and mid-gap states are present, as is expected for a disordered material. The bandgap is found to be tunable. We observe that a band-gap emerges with an oxygen coverage as low as 10% and the magnitude of the gap increases as the coverage is increased.[1] A. K. Geim et. al., Nature Materials, (6) 183 (2007)[2] O. Hod et. al., Nano Letters, (7) 2295 (2007)[3] D. Li et. al, Nature Nanotechnology, (3) 101 (2008)[4] S. Stankovich et.al. Journal of Materials Chemistry, (16) 155 (2006)[5] D. W. Boukhvalov et. al., arXiv.org:0804.0784 (2008)[6] H. He et. al. Chemical Physics Letters, (287) 53 (1998)[7] J. Nakamura et. al., Journal of Physics: Conference Series, (100) 052019 (2008)
5:15 PM - MM6.8
Protein Functionalization of Nanodiamond Particles and Surfaces For ``Smart" And Label-Free Biosensing.
Sanju Gupta 1 , J. Mimisevich 2 , S. Grant 2
1 ECE, UMC, Columbia, Missouri, United States, 2 Biological Engineering, UMC, Columbia, Missouri, United States
Show Abstract5:30 PM - MM6.9
Heteroepitaxial Single-crystal-like Diamond Films on Flexible, Large-area, Low Cost, Single-Crystal-Like Substrates for Wide-ranging Electronic Applications.
Amit Goyal 1 , Lee Heatherly 1 , Leslie Wilson 1
1 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge , Tennessee, United States
Show Abstract5:45 PM - MM6.10
Design and Fabrication of SiN Nanocavities with Diamond Nanocrystals for Quantum Information Processing Applications.
Mughees Khan 1 , Murray McCutcheon 1 , Parag Deotare 1 , Marko Loncar 1
1 School of Engineering and Applied Sciences, Harvard, Cambridge, Massachusetts, United States
Show Abstract
Symposium Organizers
Tony van Buuren Lawrence Livermore National Laboratory
Leonid Tsybeskov New Jersey Institute of Technology
Susumu Fukatsu University of Tokyo
Luca Dal Negro Boston University
Fabrice Gourbilleau CIMAP, UMR CNRS
MM7: Ultrathin Si Layers
Session Chairs
Wednesday AM, December 03, 2008
Room 309 (Hynes)
9:30 AM - **MM7.1
Nanometer-thin Silicon Layers: Preparation, Properties, and Applications.
Philippe Fauchet 1
1 ECE, University of Rochester, Rochester, New York, United States
Show AbstractWe report on the preparation and properties of nanometer-thin silicon layers, as well as their potential applications in areas as diverse as photonics and biofiltration. These ultra-thin films are manufactured by RF sputtering, although some films are obtained after repeated, controlled thinning of thin SOI wafers. Typically, the films are sandwiched between nm-thin silicon dioxide films, and can be amorphous or nanocrystalline, and porous or not, depending on the growth and processing conditions. We will review recently published results [1,2,3,4] and discuss new, recent results.This work was supported by grants from NSF, NIH, SRC, and AFOSR.[1] C.C. Striemer et al., Nature 445, 749 (2007)[2] H.G. Yoo et al., Opt. Express 16, 8623 (2008)[3] H.G. Yoo and P.M. Fauchet, Phys. Rev. B 77, 115355 (2008)[4] E. Kim et al., J. Am. Chem. Soc. 130, 4230 (2008)
10:00 AM - MM7.2
Enhancement of Electrical Conductivity in Ultrathin Silicon-on-insulator via Modification of Interface States.
Sangkeun Ha 1 , Ming-Huang Huang 1 , Shelley Scott 1 , Joseph Lyding 2 , Max Lagally 1
1 Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractUltrathin silicon-on-insulator (UTSOI) consists of a very thin Si (001) layer separated from a bulk wafer by SiO2. The thinness of the top Si layer (5nm to 200nm) implies a very low volume and a large surface-to-volume ratio. The low volume makes the total number of bulk dopants very low, and the large surface area makes the conductivity of UTSOI highly sensitive to surface and interface states. The depletion of free mobile carriers in the thin Si layer by trap states at the Si/SiO2 interface causes the sheet resistance to be very high and dependent on the thickness of the Si layer. [1] The sheet resistance of the top Si film at appropriate thicknesses is thus a sensitive function of the density of interface states at the Si/SiO2 interfaces. If one can reduce the interface density, one should observe a reduced sheet resistance. We performed H2 thermal anneals to reduce the interface trap density at the oxide interfaces in UTSOI with a bulk dopant density of 1015/cm3, with several thicknesses of the top Si layer, ranging from ~10nm to ~500nm, and measured sheet resistances using the van der Pauw technique. For a ~50nm Si layer, we observed a reduction in sheet resistance by an order of magnitude after an anneal in pure H2, and a five-fold reduction with a forming-gas (5% H2 in N2) anneal. In contrast, for a 500nm membrane, for which the normal level of interface states cannot deplete the free carriers, the effect of reducing the interface state density is quite small. The results provide proof of our model of sheet resistance in thin UTSOI. [1] The thickness dependence of the sheet resistance allows us to estimate the reduction of interface trap density with various types of anneals. In all cases, simple diffusion relations establish that the anneal gas reaches the bottom interface. Comparison of anneals with different gases shows that the reduction of sheet resistance is H2-concentration dependent. Comparison of sheet resistances for different Si thicknesses at a given doping level for different annealing gases and annealing times allows us to estimate the efficiency of reducing interface states with these anneals.[1] P. Zhang et al. Nature 439, 703 (2006)Supported by DOE, NSF, and New Zealand FRST
10:15 AM - MM7.3
Van der Pauw and Hall Measurement on Ultrathin Silicon-on-Insulator.
Sangkeun Ha 1 , Weina Peng 1 , Hongquan Jiang 2 , Madhu Thalakulam 1 , Donald Savage 2 , Mark Eriksson 1 , Max Lagally 2
1 Physics, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractUltrathin silicon-on-insulator (UTSOI) at typical doping levels (1015 cm-3) is easily depleted by Si/SiO2 interface trap states. Because the Si (001) layer is very thin (<10nm to ~500nm), surface states, surface-induced band bending, and interface states will modify the conductivity and dominate the conductivity due to bulk charge carriers. Transport measurements provide a sensitive probe of the carriers. Our earlier measurements on UTSOI showed that Si/SiO2 interface traps deplete thin Si layers of mobile carriers. Sheet resistances can reach as high as 1011 ohm/sq for a 10 ~ 20nm thick Si layer [1]. Thus, modifications in surface states that induce even modest changes in carrier densities can be detected in transport measurements.We performed van der Pauw and Hall measurements on UTSOI with a variety of surface modifications, including H termination and epichlorohydrin surface modification.. The initial sheet resistance of UTSOI is 3 ~ 4 orders of magnitude lower (relative to oxide termination) after surface modification and slowly climbs as oxide forms and replaces the surface termination. van der Pauw and Hall measurements reveal the density and the sign of the carriers. We propose possible mechanisms for this enhanced conductivity. Modifying the Si/SiO2 interface state density allows us to manipulate the free carriers in the UTSOI and thus the degree of band bending that occurs with surface termination. The competition between effects at the back interface (oxide) and front surface states (H or other termination) can thus be investigated.[1] P. Zhang et al. Nature 439, p703, 2006Supported by DOE and NSF
10:30 AM - **MM7.4
Lateral Carrier Injection to Quantum Confined Ultra-Thin Silicon.
Shin-ichi Saito 1 2
1 Central Research Laboratory, Hitachi Ltd., Tokyo Japan, 2 SORST, JST, Tokyo Japan
Show AbstractThe indirect band-gap character of bulk Si can be relaxed by reducing the dimensionality (D) from 3D (bulk) to 2D (well), 1D (wire), or 0D (dot) structures [1,2]. By choosing the appropriate crystallographic orientation, low dimensional Si is predicted to become the direct band gap semiconductor, although the dipole matrix element for optical transition is orders of magnitude smaller than that of III-V compounds. Nevertheless, the enhanced photoluminescence (PL) spectral intensities due to the quantum confinements were reported in these systems [1-4]. The carrier injection to these low dimensional nano-structures is one of the major challenges towards a practical all-Si based light source. The surface of Si is chemically active to form the highly insulating layer of SiO2, and the injections are limited to the tunneling between adjacent Si nanostructures. Therefore, a trade-off between optical intensity and carrier injection efficiency is expected; reducing the dimensionality enhances the PL intensity, while it reduces the carrier injection efficiency.In order to avoid the trade-off, we proposed a device structure, where the active ultra-thin Si layer (~ 5 nm) is directly connected to the highly doped thick Si electrodes [3, 4]. The device was fabricated by the local-oxidation-of-silion (LOCOS) process using silicon-on-insulator (SOI) substrates. In this device, carriers of both electrons and holes are laterally injected to the ultra-thin Si, and the efficient electroluminescence (EL) is expected. The high current density (typically >100 kA/cm2) would partially compensate the intrinsic low optical dipole moment. According to the EL intensity profiles, the light emission was exclusively recognized in the ultra-thin Si region along the line of the pn-junction (line emission). The optical intensity can also be controlled by an application of the back gate voltages to the ultra-thin Si. The back gate voltage controls the carrier density by the ambipolar carrier injection to the active channel. As a result, the device operates as a Si Light-Emitting Transistor (LET).By using this LET, we demonstrated a very primitive on-chip optical interconnection. We applied an input voltage pulse between the pn-junction, and observed the photocurrents in the detector of none-doped Si pads. Although the response time is extremely low (~ a few seconds) due to our experimental set-up, we hope to improve the performance of the Si LET for future all-Si based on-chip optical interconnections.[1] L. T. Canham: Appl. Phys. Lett. 57, 1046 (1990). [2] Silicon Photonics, edited by L. Pavesi and D. J. Lockwood Springer, Berlin, (2004).[3] S. Saito, et. al., Jpn. J. Appl. Phys. Part 2 45, L679 (2006). [4] S. Saito, et. al., Appl. Phys. Lett. 89, 163504 (2006).
MM8: Si Nanostructures II
Session Chairs
Wednesday PM, December 03, 2008
Room 309 (Hynes)
11:30 AM - **MM8.1
Silicon/Silicon-Germanium Quantum Dots and Nanomembranes.
Mark Eriksson 1
1 Physics, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractSilicon, the material at the heart of modern classical electronics, also has properties that are well suited to quantum electronics. These include low spin-orbit coupling, the existence of zero-spin nuclear isotopes, and a low noise oxide. It is now possible to realize gated, tunable silicon quantum dots containing individual electrons, to monitor such dots through integrated charge sensing, and to measure pairs of coupled silicon quantum dots. In the first part of this talk I will discuss the results of recent measurements of spin-dependent electrical transport in silicon double quantum dots. We have observed spin blockade – the suppression of current through a double dot due to long lived spin states – when the transport current flows in one direction. Reversing the flow of current through the dots leads to the observation of a new effect, lifetime-enhanced transport, in which long spin lifetimes enhance, rather than suppress, the flow of current through the dots. In the second part of this talk I will focus on quantum transport in silicon membranes. Coherent strain sharing allows the generation of tensile strained silicon and compressively strained silicon-germanium without the incorporation of dislocations. I will present the results of transport measurements in such systems and discuss the implications for future work in silicon quantum devices.This work was performed in collaboration with C. B. Simmons, N. Shaji, M. Thalakulam, E. K. Sackmann, B. J. Van Bael, L.J. Klein, H. Qin, H. Luo, D. E. Savage, M. G. Lagally, R. Joynt, M. Friesen, R. H. Blick, and S. N. Coppersmith.Financial support was provided by the NSF through DMR-0520527 and DMR-0325634, by ARO and NSA through contract W911NF-04-1-0389, and by DOE through contract DE-FG02-03ER46028.[1] “Single-electron quantum dot in Si/SiGe with integrated charge sensing,” C.B. Simmons, Madhu Thalakulam, Nakul Shaji, Levente J. Klein, Hua Qin, R.H. Blick, D.E. Savage, M.G. Lagally, S.N. Coppersmith, and M.A. Eriksson, Appl. Phys. Lett. 91, 213103 (2007). [2] “Spin blockade and lifetime-enhanced transport in a few-electron Si/SiGe double quantum dot,” Nakul Shaji, C. B. Simmons, Madhu Thalakulam, Levente J. Klein, Hua Qin, H. Luo, D. E. Savage, M. G. Lagally, A. J. Rimberg, R. Joynt, M. Friesen, R. H. Blick, S. N. Coppersmith, M. A. Eriksson, arXiv:0708.0794, appearing in Nature Physics, doi:10.1038/nphys98.[3] “Elastically Relaxed Free-standing Strained-Si Nanomembranes,” Michelle M. Roberts, Levente J. Klein, Don E. Savage, Keith A. Slinker, Mark Friesen, George Celler, Mark A. Eriksson, Max G. Lagally, Nature Materials 5, 388 (2006).
12:00 PM - MM8.2
Spontaneous Emission Enhancement of Silicon Nanocrystals in Metal-oxide-silicon-oxide Slab Waveguides.
Aaron Hryciw 1 , Young Chul Jun 1 , Mark Brongersma 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractFor a dipole oscillator embedded in the thin dielectric layer of a metal-dielectric-semiconductor (MDS) planar structure, the spontaneous emission decay rate is calculated to exhibit a large (~10) enhancement with respect to a dipole in the bulk dielectric if this layer is sufficiently thin. In addition to a relatively sharp enhancement peak due to the surface plasmon resonance of the metal film, the model predicts a broad off-resonance enhancement for dipoles oriented perpendicular to the film interfaces which is linear with wavelength, similar to the effect recently calculated for metal-dielectric-metal (MDM) structures; in contrast to the MDM case, however, MDS structures with semi-infinite metal and semiconductor layers are characterized by leaky, rather than bound, modes. For a semiconductor layer of finite thickness on a dielectric substrate, the dipole emission can also be coupled into the slab waveguide modes of the structure, an attribute which suggests applications in monolithically-integrated plasmon-enhanced light sources. We investigate these phenomena experimentally via time-resolved emission studies of silicon nanocrystals embedded in the silica layer of planar silver-silica-silicon and silver-silica-silicon-silica structures.
12:15 PM - MM8.3
Optical Gain of Dislocation-related D12 Emission Line in Si.
Jun Igarashi 1 , Norishige Tana-ami 1 , Kiyoshi Kawamoto 1 , Susumu Fukatsu 1
1 Graduate School of Arts and Sciences, University of Tokyo, Tokyo Japan
Show AbstractSearch of efficient Si-based light-emitting materials and structures has been motivated by the surging demand in the field of silicon-photonics that a light emitting device compatible with Si must ride on an SOI-chip very soon. As opposed to simple extraction of incoherent light, which is based more or less on spontaneous emission, optical gain requires an inverson among the relevant electronic states. So it is natural that one resorts to band-edge states in view of considerable success of semiconductor lasers. However, as long as we stay on indirect-gap Si, this is hardly achievable. Interestingly, it has been pointed out that defect-related states can be free of the problems plaguing the band-edge states and hold promise for gain and lasing in Si. Nevertheless, this is not a commonly shared view, and so little attention has been paid to defects simply because they are unwanted except for a specific type with possible gain. In this work, an attempt is made to unveil the potentially useful properties of the defect-related emissions. Among the many well-known emission features observed in Si and have some bearing on Si-native structural defects, the dislocation-related emissions are representative. To date, the D1 feature located near to the telecom C-band has been most extensively studied: an internal quantum efficiency of emission amounting to a few % has been reported under current injection at room-temperature. However, there has been no report and/or attempt to seek gain for the D-line features. Here we report on the observation of optical gain in the near-infrared due to one of the dislocated-related luminescence bands: the D12 emission line (1.47 um) which is located in energy in between the well-known D1 and D2 bands has been found to be endowed with optical gain at cryogenic temperature. This is a generic study to probe into the very nature of defect-related states and so cryogenic operation does not compromise the significance of the observation of gain due to defects.The sample used was an SOI substrate carrying a 1.5-um thick device Si layer. A 0.18-um thick Si was grown on top the device layer by molecular beam epitaxy and was post-growth annealed around 570 K for one week. During such an extended annealing produced was the D12 line along with the D4 line (1.26 um) while the D1 through D3 features were totally missing. Single-pass on-off gain was evaluated in the pump-probe geometry using a pair of chips and a cooled Ge detector. A 3dB gain was obtained at 8K while the gain was quenched above 27 K. By contrast, no gain was observed for the D4 line, indicating that the D12 feature that has been ascribed to dislocations arises from a different class of origin as compared to the D4 line which is attributed to dislocations. Optical gain using the engineered defects under control is expected to provide us an alternative route to the first near-band-edge lasing in Si and a viable photonic device.
12:30 PM - MM8.4
Group-IV Semiconductors Incorporating Sn.
Jose Menendez 1 , John Kouvetakis 2 , Vijay D'Costa 1 , John Tolle 2 , Yanyan Fang 2 , Andrew Chizmeshya 2
1 Physics, Arizona State University, Tempe, Arizona, United States, 2 Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States
Show AbstractMM9/D6: Joint Session: Er Doped Si Nanostructures
Session Chairs
Wednesday PM, December 03, 2008
Room 309 (Hynes)
2:30 PM - **MM9.1/D6.1
Rare Earth Ion Beam Processing for Silicon Photonics.
Wolfgang Skorupa 1 , Lars Rebohle 1 , Slawomir Prucnal 1 , Charaf Cherkouk 1 , Manfred Helm 1
1 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden Germany
Show AbstractCombining silicon-based electronic circuits with optoelectronic functionality is one of the key challenges for the future semiconductor technology. Such work must not only be devoted to the “telecommunication” wavelength of 1.54 µm because there are much more applications requiring light sources from the UV to IR wavelength range. In our work we employed ion beam processing to embed different rare earth (RE) luminescent centers (Gd, Ce, Tm, Tb, Eu, Er) into the silicon dioxide layer of purpose-designed Metal-Oxide-Silicon-based Light Emitting Devices (MOSLEDs) with advanced electrical performance. Efficient electroluminescence was obtained from UV to infrared with a transparent top electrode made of indium-tin oxide. Several developments for improving the device stability will be proposed related to charge compensation and the elimination of defects in SiO2. The electrical and electroluminescence properties of these devices are discussed and evaluated in respect of possible applications for biosensing applications. As an example our recent effort to detect estrogens in drinking water will be discussed.
3:00 PM - MM9.2/D6.2
The Role of Hydrogen in the Luminescence-center-mediated Er3+ Excitation in Si-rich SiO2 with and without Si Nanocrystals.
Pieter Kik 1 , Oleksandr Savchyn 1 , Ravi Todi 2 , Kevin Coffey 2
1 CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida, United States, 2 AMPAC, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, Florida, United States
Show AbstractRecently we have shown that erbium excitation in Si-doped SiO2 films occurs predominantly through isolated silicon related luminescence centers (LC) in the oxide. This conclusion seemed to contradict studies in which hydrogen passivation of Si nanocrystals leads to a significant improvement of the Er3+ photoluminescence (PL) intensity. In order to study the role of hydrogen passivation in the luminescence-center-mediated excitation of Er3+, Er-doped Si-rich SiO2 films with significantly different microstructures were prepared. These samples were subsequently passivated in forming gas (5% H2, 95% N2) at different temperatures. Photoluminescence measurements on samples containing no detectable silicon nanocrystals (annealed at 600°C) revealed little change in the indirectly excited Er3+ PL intensity. In contrast, photoluminescence measurements on samples containing silicon nanocrystals (annealed at 1100°C) revealed a strong (factor ~10) increase in the Er3+ PL intensity as the passivation temperature is increased from room temperature to 500°C. This increase is accompanied by a similar increase in the Si nanocrystal emission intensity, suggesting that nanocrystal mediated excitation does play an important role in the Er3+ excitation mechanism. These observations are attributed to two interrelated effects, namely, (a) an increase in the density of fully passivated optically active nanocrystals due to the removal of silicon dangling bonds upon passivation, and (b) a concurrent reduction in nonradiative Er3+ relaxation from levels above the 4I13/2 level due to a direct interaction of excited Er3+ ions with silicon dangling bonds. It is argued that such an interaction is reasonable based on the known energies of the Er3+ excited states and the location of the Si dangling bond state in the silicon bandgap. These findings suggest that silicon sensitized Er3+ gain media must either be processed at low temperature to avoid the formation of a crystalline Si phase in the SiO2 host, or at high temperature followed by hydrogen passivation.
3:15 PM - MM9.3/D6.3
Er-doped Si Nanolayers for Generation of IR and THz Range Radiation.
Salvatore Minissale 1 , Tom Gregorkiewicz 1
1 Van der Waals Zeeman Institute, University of Amsterdam, Amsterdam Netherlands
Show AbstractEr-doped silicon structures have undergone extensive investigations for development of optical materials and devices. In that research, important drawbacks have been pointed out, which need to be overcome in order to achieve intense, and eventually also stimulated, room temperature emission. Mostly, low optical activity and multiplicity of optically active centers formed by an Er3+ in Si are identified as the major obstacles. More recently, Si/Si:Er multinanolayer structures have emerged as a novel material with superior optical properties, which could allow for realization of optical gain. Such a hope comes from the fact that the Er-related 1.5 μm emission from Si/Si:Er multinanolayers features a spectrum comprising only a few lines with a very narrow and homogeneous linewidth of ΔE < 8 μeV. This can be traced back to the presence of only a single type of Er-related optical center (labeled Er-1). Moreover, optical activity of Er dopants in these structures is higher than in thick Si:Er layers commonly used.In this presentation, we will review the unique properties of Si/Si:Er multinanolayers: the single center formation and its microscopic structure, the level of optical activity, and relation between optical and electrical properties. We will show that combining photoluminescence and magneto-optical spectroscopy it is possible to draw a detailed level scheme for the 4I15/2 ground state of the Er3+ ion involved in the Er-1 center. Transitions within the ground state, between individual sublevels split due to the Stark effect induced by the local crystal field on Er3+ ion, fall into the THz range, with the relevant wavelengths being expected at 43, 91, 144, and 169 μm. In full analogy to the 1.5 μm emission in the IR domain, a very small linewidth can be expected also for these THz transitions. In the second part of the presentation we will report on our recent investigations concerning the THz range transitions. The study has been conducted by the time-resolved pump-probe absorption technique, since the initial linear absorption measurements proved inconclusive. This was due to B dopants present in the substrate, whose relatively strong internal transitions between shallow acceptor states appear in the same range, and therefore preclude observation of the weaker (“forbidden” transition) Er-related signals.We will present results obtained by pump-probe experiments, conducted in the standard and also in the “transient grating” configuration, which conclusively identify an optical transition at 43 μm and determine the related time constant as τ≈50 ps. We argue that this value reflects a nonradiative multiphonon recombination within the 4I15/2 ground state of an Er3+ ion. To the best of our knowledge, this is the first observation of an optical transition within a multiplet of a rare earth ion. Finally, we will address the practical potential of Si/Si:Er nanolayers for development of a compact and electrically-driven all-Si THz source.
4:00 PM - **MM9.4/D6.4
Towards an Er Doped Silica Amplifier Sensitized by Silicon Nanoclusters.
Nicola Daldosso 1 , Daniel Navarro-Urrios 1 , Alessandro Pitanti 1 , Romain Guider 1 , Lorenzo Pavesi 1 , Larysa Khomenkova 2 , Fabrice Gourbilleau 2 , Richard Rizk 2
1 Physics, University of Trento, Povo - Trento Italy, 2 , CIMAP, UMR CEA/CNRS 6176, Caen France
Show AbstractThe use of erbium doped amplifiers for widespread integration presents some difficulties: Er ions pair interactions, small excitation cross-section, high power laser diodes as pump source,excited state absorption, …. The use of sensitizers for erbium ions can relax the stringent conditions for the pump source. A good sensitizer should present a high absorption cross section and has to transfer efficiently energy to Er. The capacity of silicon nanoclusters (Si-nc) to act as sensitizers has opened the route towards an all-optical (and even electrical) Si-based amplifiers operating in the third telecommunication window. In fact, Si-nc (either amorphous or crystalline) have broad absorption spectrum and very large absorption cross sections with respect to Er in stechiometric silica. Moreover, a clear breakthrough would be the possibility of having an electrical excitation of rare-earth ions through the Si–nc. Unfortunaltly, the main obstacles to achieve net optical amplification in Si-nc based Er doped waveguides are currently Carrier Absorption (CA) losses and the low number of Er ions coupled to Si-nc (few %). The reason for this low number is still under discussion. We present here our last results on the optical properties of reactive magnetron co-sputtered waveguides containing Si-nc and Er in a SiO2 matrix. The material has been optimised in terms of the increasing of PL intensity and lifetime (up to 5 ms) as well as the decreasing down to few dB/cm of the propagation losses in rib-loaded waveguides. We have reduced CA induced losses to less than 0.2dB/cm (at pump fluxes as high as 10(20)ph/cm2 s) both through an engineering of Si-nc size (i.e. by tuning the Si content and the annealing conditions) and coupling high percent of Er ions to Si-nc. Around 25% (the highest fraction up to date) of the optically active erbium population has been inverted through indirect excitation (pumping with a 476nm laser line), leading to internal gain coefficients of abouot 1dB/cm.By time resolved measurements we demonstrate that the energy transfer mainly occurs from Si-nc to the metastable level of Er ion and that the transfer time is extremely fast (tens of ns). Furthermore, we show that there are no traces of Auger back-transfer or Excited State Absorption mechanisms and that pair induced quenching is not an issue in our samples, being the limited interaction distance between Si-nc and Er ions the main reason.We believe that these results re-validate the feasibility of an optical amplifier based on this material since the sensitization of Er by Si-nc is not limited by fundamental physical processes. The major, if not the only, constraining factor in achieving full Er inversion is the limited interaction distance between Si-nc and Er ions, that can be solved by a careful material optimization. We acknowledge financial support by EC through the LANCER project (FP6-IST-2005-033574).
4:30 PM - **MM9.5/D6.5
Optimum Coupling between Er Ions and Si Nanoclusters Sensitizers for High Performance Integrated Photonics.
Richard Rizk 1 , Julien Cardin 1 , Khalil Hijazi 1 , Larysa Khomenkova 1 , Fabrice Gourbilleau 1
1 CIMAP, CNRS, Caen France
Show AbstractThe sensitizing role played by the broad-band high-absorbing Si nanoclusters (Si-nc) towards Er ions in silica is still the object of tremendous research activities. Such a coupling induces some 103-104 enhancement of the effective excitation of Er ions, and allows also the use of high power LEDs or electrical excitation instead of the expensive pump lasers. However, the energy transfer from Si-nc to Er ions is mainly governed by their separating distance which should be lower than a value as low as ~0.5 nm, hence limiting the fraction of coupled Er to 0.5-3%, as reported so far. The present contribution deals with the effort made to maximize this fraction of couple Er, through a careful engineering of the composition of the active material, together with the analyses of various features: sensitizers’ nature, energy transfer process(es), optical losses/gain. The Er-doped silicon-rich SiO2 layers were grown by magnetron co-sputtering of confocal targets, under a reactive (H2+Ar) or Ar plasma, before being submitted to thermal treatments. While the reactive approach deposition has allowed the increase of the fraction of coupled Er to about 11% of the total Er content, the film grown at 500-600°C by the co-sputtering of three confocal cathodes under pure Ar, has led the a further enhancement of this fraction and the majority of the optically active Er ions are now coupled to Si-nc. This paves the way to the achievement of a net optical gain and then light amplification. Such an unprecedented result would be due to the growth of very small and highly dense Si-based sensitizer entities, of few tens of atoms, acting as point-like defects inducing deep isolated electronic states instead of the electronic band structure of Si-nc containing thousand(s) of atoms. These ‘atomic’ scaled sensitizers are much more numerous than the usual Si-nc and their formation would be due to some specific growth kinetics at relatively high substrate temperature and subsequent thermal treatment, leading to three inherent advantages: very high density favoring a maximum coupling with the neighboring Er ions, very small or negligible confined carrier absorption inducing losses, and very fast energy transfer (tens of ns) induced by some trap-mediated excitation of Er that appears reminiscent of that in crystalline Si. Finally, such an optimum distribution of the Si excess has showed good transport of carriers injected by electrical excitation, as demonstrated by the observation of a high electroluminescence. We acknowledge financial support by EC through the LANCER project (FP6-IST-2005-033574).
5:00 PM - MM9.6/D6.6
Sensitized Erbium Emission in Silicon Nanocrystals-based Superlattice Structures.
Rui Li 1 2 , Joe Warga 1 2 , Selcuk Yerci 1 2 , Luca Dal Negro 1 2
1 Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States, 2 Photonics Center, Boston University, Boston, Massachusetts, United States
Show AbstractAt present, there is a strong need to engineer Si-compatible nanostructures for efficient light emission under optical and electrical excitation. The widely investigated material system, Si-nanocrystals (Si-nc) in SiO2 glass matrices, shows very efficient (60%) room- temperature light emission under optical pumping. However, the presence of an insulating SiO2 matrix significantly hinders the fabrication of stable and efficient electrically driven light sources. Therefore, it is important to explore novel CMOS-compatible solutions based on the excitation of Si-ncs dispersed in alternative, electrically injectable materials structures. We report on time-resolved Si-ncs emission, Er sensitization and electroluminescence in a novel Si-based, CMOS compatible quantum system based on Si/Si-rich nitride (SRN) nanocrystals arranged in a superlattice structure. The structure was fabricated by RF magnetron sputtering deposition followed by thermal annealing from 500°C to 1000°C. High-resolution TEM pictures show 2nm diameter Si clusters dispersed in SRN layers with controlled thickness between 4nm and 50nm. Erbium incorporation and activation is achieved by direct co-sputtering of the Si-rich layers. We will first discuss Si-N bond surface state related emission mechanism of SRN by low temperature photoluminescence measurements. Then we will discuss the important role of annealing temperature for SRN emission and Er activation in Si-nc-based superlattice structures (Si-nc-SLs) and show that energy transfer occurs on the nanosecond time scale. In addition, we will report on the ultra-fast time-resolved emission dynamics of superlattices, and estimate the maximum transfer rate and transfer efficiency in these novel systems. Our results from low temperature emission dynamics (4K) directly address the nature of energy transfer and as a phonon mediated non-radiative process. We believe that light emission from Si-ncs-based SLs can provide alternative routes towards the engineering of Si-based light sources for 1.54µm applications.
5:15 PM - MM9.7/D6.7
Significant Improvement of Photoluminescence Intensity and Lifetime from Er3+ Ions Coupled to Si Clusters in Si-rich-SiO2 Layers.
Larysa Khomenkova, 1 , Fabrice Gourbilleau 1 , Christian Dufour 1 , Julien Cardin 1 , Richard Rizk 1
1 CIMAP, ENSICAEN, Caen Cedex 04 France
Show AbstractThe role of silicon clusters whether crystallized or amorphous as efficient sensitizers of Er3+ ions is well established. Most of the works reported so far have stressed on the very small proportion of Er3+ ions benefiting from the ‘effective’ and efficient excitation through silicon clusters. This prevents the development of compact and CMOS-compatible photonic devices. The present work aims at exploring the ways to improve the coupling between silicon clusters and optically active Er3+ ions. The reactive magnetron sputtering from two confocal SiO2 and Er2O3 cathodes in argon-hydrogen plasma was used to deposit Er3+-doped Si-rich-SiO2 layers with controlled composition. The structural, compositional and photoluminescence properties of the layers were examined. The effect of the deposition parameters as well as subsequent annealing procedure was investigated in order to reach an optimum Er3+-related photoluminescence intensity and an improved lifetime. Careful analyses were made through the dependence of these parameters on the pumped photon flux and the excitation wavelength.It was found that the adopted approach allows significant enhancement of both PL intensity and lifetime. For example, the comparison with the best samples obtained up to now shows i) the increase of Er3+ emission lifetime from 5 ms to about 10 ms and ii) the increase of the PL intensity, at resonant (488nm) or non-resonant (476nm) excitations, by a factor of three. The overall results were analyzed in terms of coupling rate between the Er3+ ions and Si-nc, together with the impact of up-conversion processes. The project “LANCER” (IST-2005-033574) is acknowledged.
5:30 PM - MM9.8/D6.8
Investigation of the Er3+ Location in Si-rich Silicon Oxide by 3D Atom Probe Tomography.
Etienne Talbot 1 , Rodrigue Larde 1 , Fabrice Gourbilleau 2 , Richard Rizk 2 , Philippe Pareige 1
1 , GPM - Université de Rouen - UMR CNRS 6634, Saint Etienne du Rouvray France, 2 , CIMAP, UMR CNRS 6252, ENSICAEN, Caen France
Show Abstract5:45 PM - MM9.9/D6.9
Enhanced Erbium Near Infrared Emission in Doped Group IV Oxide Nanowires Using a GeOx Sensitizer.
Jeffery Coffer 1 , Ji Wu 1
1 Chemistry, Texas Christian University, Fort Worth, Texas, United States
Show AbstractGroup IV oxide nanowires provide efficient platforms for the confined modulation of light. As an active emitter in the near IR, however, such materials are limited by direct excitation of optically-active centers such as erbium (III) ions incorporated into the matrix with a corresponding low absorption cross section. To overcome these limitations, one logical approach is to add a sensitizer such as Si nanocrystals or Ag+ ions that have much larger absorption cross sections than Er+3 and can be employed to excite the rare earth ions more efficiently via a carrier-mediated process. We have developed facile routes to erbium-doped Group IV oxide nanowires and nanofibers using a combination of sol gel condensation chemistry and electrospinning. In the work described here, we focus on nanofibers of SnO2 as a host matrix, a highly optical transparent n-type semiconductor with a wide band gap of 3.6 eV at 300 K. Such nanofibers doped with erbium can ideally serve as building blocks to assemble more optoelectronic units on a single chip via a bottom-up approach. Since Er+3 ions can only be directly excited in as-prepared Er-doped SnO2 nanofibers, an effective sensitizer is required. Previous work published by our group has identified GeOx as an efficient sensitizer for erbium ions; therefore, it is logical to introduce this sub-oxide into Er-doped SnO2 nanofibers for the deliberate purpose of producing a carrier-mediated excitation enhancement pathway into a conductive host. In this study, we employ a vapor transport method and a corresponding in situ oxidation-reduction reaction to introduce various amounts of GeOx into the Er-doped SnO2 nanofibers. The composition and structure of these nanofibers were characterized using a combination of TEM, SEM, X-ray energy dispersive spectroscopy (EDX), and micro-Raman spectroscopy. It is demonstrated that the photoluminescence intensity of Er-doped SnO2 nanofibers at 1540 nm can be enhanced via a carrier-mediated mechanism by almost two orders of magnitude after the introduction of this germanium suboxide.
Symposium Organizers
Tony van Buuren Lawrence Livermore National Laboratory
Leonid Tsybeskov New Jersey Institute of Technology
Susumu Fukatsu University of Tokyo
Luca Dal Negro Boston University
Fabrice Gourbilleau CIMAP, UMR CNRS
MM10: Si Devices
Session Chairs
Thursday AM, December 04, 2008
Room 309 (Hynes)
9:30 AM - **MM10.1
Si Single-Electron SOI-MOSFETs: Interplay with Individual Dopants and Photons.
Michiharu Tabe 1 , Daniel Moraru 1 , Zainal Burhanudin 2 , Ratno Nuryadi 1 , Maciej Ligowski 1 3 , Ryszard Jablonski 3 , Miftahul Anwar 1
1 Research Institute of Electronics, Shizuoka University, Hamamatsu Japan, 2 , University Technology Petronas, Perak Malaysia, 3 , Warsaw University of Technology, Warsaw Poland
Show Abstract Recently, there have been increasing demands for controlling individual electrons, photons and dopants in developing nm-scale Si devices. Our most recent results on Si single-electron nano-devices will be presented. Si single-electron-tunneling (SET) or single- hole-tunneling (SHT) devices have a great potential in realizing a variety of new functions. So far, we have fabricated multi-junction Si SOI-MOSFETs by a couple of unique but simple methods, in which the channel region has a structure of mutually-connected Si dots or dopant-induced potential. By using these devices, most recently, we have obtained several important results of single-electron transfer, single-photon detection and single-ion (dopant) detection. Furthermore, we have succeeded in detailed observation of potential landscape in channel region by LT-KFM (Low Temperature-Kelvin Probe Force Microscopy), which will provide direct information of potential changes due to single-electron, single-dopant and photon absorption.References1)M. Tabe, R. Nuryadi, D. Moraru, Z. A. Burhanudin, K. Yokoi and H. Ikeda, “Si Multidot FETs for Single-Electron Transfer and Single- Photon Detection”, Acta Physica Polonica A, Vol.113, No.3, pp.819-822 (2008).2)Z. A. Burhanudin, R. Nuryadi and M. Tabe, “Detection of field-induced single-acceptor ionization in Si by single-holetunneling transistor”, Appl. Phys. Lett. Vol.91, No.4, pp.042103-1-3 (2007).3)D. Moraru, Y. Ono, H. Inokawa and M. Tabe, “Quantized electron transfer through random multiple tunnel junctions in phosphorous-doped silicon nanowires”, Phys. Rev. B, Vol.76, no.7, pp. 075332-1-5 (2007).4)R. Nuryadi, Y. Ishikawa and M. Tabe, “Single-photon-induced random telegraph signal in a two-dimensional multiple-tunnel-junction array”, Phys. Rev. B 73, pp.045310-1-7 (2006).5)Hiroya Ikeda and Michiharu Tabe, “Numerical study of turnstile operation in random-multidot-channel field-effect transistor”, J. Appl. Phys. 99, pp.073705-1-6 (2006).
10:00 AM - MM10.2
Nonvolatile Charge Storage Characteristics of a MOS Diode with Buried Silicon Nanocrystals and Interfacial Si Nano-pyramids.
Yu-Chung Lien 1 , Cheng-Tao Lin 1 , Chi-Kuan Lin 2 , Gong-Ru Lin 1
1 Graduate Institute of Photonics and Optoelectronics, National Taiwan University , Taipei Taiwan, 2 , National Chiao Tung University, Hsinchu Taiwan
Show Abstract10:15 AM - MM10.3
Use of Silicon Ink to form Polycrystalline Silicon Films for Solar Cells.
Xiaodong Pi 1 , David Rowe 1 , Mike Renn 2 , Stephen Campbell 3 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 2 , Optomec Advanced Applications Laboratory, Minneapolis, Minnesota, United States, 3 Electrical Engineering and Computer Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractSolar cells based on polycrystalline silicon films are intensely studied owing to the combined advantages of low cost and high energy conversion efficiency. In contrast to the conventional chemical vapor deposition (CVD) enabled preparation of polycrystalline silicon films, we produce polycrystalline silicon films by taking advantage of silicon nanoparticles (Si-NPs). Si-NPs are synthesized in nonthermal plasma by decomposing a silicon precursor (e. g., SiH4) [1, 2]. The conversion efficiency of silicon precursors is > 50 %, which is significantly higher than those (< 30 %) in CVD processes. After the synthesis Si-NP are dispersed in chloroform/chlorobenzene by means of sonication. The resultant dispersion forms a silicon ink [3]. Aerosol-jet printing and ink-jet printing are employed to deliver silicon ink onto various kinds of substrates. The printing process enables large-area formation of silicon films, which has the potential of reducing the cost of solar cells. The printed films are then annealed with laser irradiation. Significant grain growth has been observed after laser annealing. Si-NPs are doped with B and P in order to form p-type and n-type polycrystalline silicon films [4]. It is found that the doping of B and P into Si-NPs greatly increases the electrical conductivity of polycrystalline silicon films. Si-NPs are also doped with Ge to enhance the absorption of polycrystalline films. The effect of Si-NP crystallinity (amorphous/crystalline) on the formation of polycrystalline films is also investigated. Finally, we discuss the relationship between the initial Si-NP size and the final grain size in the polycrystalline films.This work is supported by the MRSEC Program of the NSF under Award No. DMR-0212302, NSF NIRT Grant No. CBET-0506672 and the Center for Nanostructure Applications at the University of Minnesota.
10:30 AM - MM10.4
Influence of Etching and Surface Functionalization on the Visible Photoluminescence of Silicon Nanocrystals.
Anoop Gupta 1 2 , Mark Swihart 3 , Hartmut Wiggers 1 2
1 Institute for Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg Germany, 2 Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Duisburg Germany, 3 Department of Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York, United States
Show AbstractIn this study, we have investigated the photoluminescence (PL) properties of silicon nanocrystals (Si-NCs) after etching and functionalization processes. The synthesis of Si-NCs was carried out in a low pressure microwave plasma reactor via pyrolysis of silane (SiH4). The surface of Si-NCs was stain etched in a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). The etched Si-NCs show a continuous blue shift in the emission spectrum with increasing etching time. Hence, we were able to obtain the full visible spectrum of Si-NCs. We observed that the air oxidation of etched Si-NCs profoundly affects their optical property, which causes a blue shift in the emission spectrum and a diminished intensity. In order to stabilize their PL, surface functionalization of etched Si-NCs was endeavored with various alkenes using photoinitiated hydrosilylation technique. We observed that the emission spectrum of functionalized Si-NCs was shifted compared to that of etched Si-NCs. The nature of the shift (red/blue) is dependent on the size of the etched Si-NCs. Furthermore, we found that the amount of shift depends on the UV exposure time and the type of ligands on the surface of the Si-NCs.
10:45 AM - MM10.5
Analysis of the Electronic Structure and Luminescent Pathways in Rare-earth Doped Silicon Dielectrics Through XANES and XEOL.
Tyler Roschuk 1 2 , Patrick Wilson 1 2 , Jing Li 1 2 , Jacek Wojcik 1 2 , Peter Mascher 1 2
1 Engineering Physics, McMaster University, Hamilton, Ontario, Canada, 2 Centre for Emerging Device Technologies, McMaster Unviersity, Hamilton, Ontario, Canada
Show AbstractMM11: Si Nanostructures III
Session Chairs
Thursday PM, December 04, 2008
Room 309 (Hynes)
11:30 AM - **MM11.1
First Principle Calculations of Silicon Nanostructures in Realistic Environments.
Giulia Galli 1
1 , University of California, Davis, Davis, California, United States
Show AbstractFirst principles calculations of the optical and thermal properties of Si and Si/Ge nanostructures will be presented. In particular, the role of interfaces between nanoparticles and embedding media (e.g. solid matrices) will be addressed and discussed.
12:00 PM - MM11.2
Strain Distribution in Oxide Embedded Silicon Nanocrystals: A Reactive Force Field Approach.
Dundar Yilmaz 1 , Ceyhun Bulutay 1 , Tahir Cagin 2
1 Physics, Bilkent University, Ankara Turkey, 2 Chemical Engineering, Texas A&M University, College Station, Texas, United States
Show Abstract The strain distribution in silicon nanocrystals (Si-nc) embedded in an oxide matrix is a subject of active debate. There is no consensus on the qualitative nature of the strain such as the ambiguity in its tensile or the compressive character which is usually caused by a blindfolded use of the phonon confinement model. Thus, the lack of a reliable strain characterization prevents a coherent interpretation of the experimental data. Strain has direct implications on the luminescence, infrared and Raman activity of the Si-nc.With the use of a realistic force field, the results of a molecular dynamics simulation may prove to be more valuable than the best available transmission electron microscopy image. On the other hand, there have been quite a few theoretical attempts for a microscopic understanding of the strain in S-nc in which the chemistry of the environment was poorly characterized, such as the use of Keating-like simplistic models. These potentials have the drawback of keeping a static bond topology without allowing for a charge transfer during the simulation. To address these shortcomings, we apply the recently-developed reactive force field by van Duin et al to the strain distribution in Si-nc. This enables the formation and the breaking of the bonds in the course of the simulation as well as the charge transfer through the electron equilibration method. We first characterize the radial strain in a 3 nm diameter Si-nc through the bond length distribution. This reveals that deep inside the nanocrystal, Si-Si bonds have no strain at all, but just in the vicinity of the surface, Si-Si bonds show tensile strain up to %3. Next we investigate the role of strain on the formation of the facets as the nanocrystal size increases. The diameters in the range from 1-5 nm are considered. Hence we have calculated strain on specific Miller Planes by projecting these distortions onto on to (100), (110), (111) and (121) surfaces. This forms the first reactive force field-based characterization of strain in realistic-sized Si-nc, which will be instrumental in gauging the widely-used phonon confinement models.
12:15 PM - MM11.3
Composition Maps in Coherent and Dislocated Self-Assembled Alloy Quantum Dots.
Nikhil Medhekar 1 , Vishwanath Hegadekatte 1 , Vivek Shenoy 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractKnowledge of composition profiles within self-assembled SiGe and InGaAs quantum dots is critical for applications in optoelectronic and memory devices as variations in composition at the nanoscale can substantially influence their electronic properties. Obtaining the quantitative description of composition profiles in a quantum dot is a challenging task due to the coupling between composition variations, shape and size of the quantum dots and long-range elastic interactions. In this talk, we present an efficient scheme that combines the finite element analysis with an optimization scheme [1] based on a quadratic programming method to determine equilibrium profiles in strained coherent and dislocated quantum dots. Composition profiles are found to strongly depend on the shape of the dots, as strain relaxation in dots with steeper sidewalls allows for segregation of the larger alloy component in the regions near the apex. Based on these observations, we have developed a phase diagram that shows the degree of segregation of the alloy components in the phase space spanned by the temperature (which governs chemical mixing) and the shape of the dot. Further, we find that the segregation of the alloy components can substantially reduce the critical dot size for the transition between the shapes with different facets. Moreover, as the quantum dot grows in size, dislocations are nucleated in the dot in order to reduce the strain energy. Using our scheme, we show that the coupling between the composition-dependent mismatch strain and the elastic fields due to interface dislocations can lead to a dramatic transformation in composition profiles due to presence of dislocations. [1] N. V. Medhekar, V. Hegadekatte and V. B. Shenoy, Phys. Rev. Lett. 100, 106104 (2008).
12:30 PM - MM11.4
On the Mechanism of Dislocation Ordering at the Ge/Si(113) Interface.
Roland Kroeger 1 , Thomas Schmidt 2 , Torben Clausen 2 , Jens Falta 2
1 Department of Physics, University of York, York United Kingdom, 2 Institute of Solid State Physics, University of Bremen, Bremen Germany
Show AbstractSmooth and low-defect Ge layers were grown on Si(113) substrates using surfactant mediated epitaxy. A detailed study of the arrangement of interfacial misfit dislocations using transmission electron microscopy was performed for such films, which were deposited at different substrate temperatures. For the maximum growth temperature of 650°C a bimodal dislocation distribution with maxima at 7 and 13 nm, respectively, was identified in contrast to the monomodal distribution found for Ge films on Si(111) substrates [1]. Based on a model suggested by Atkinson and Jain et al. [2] the relative interaction forces of dislocations in an array was determined and used to calculate the equilibrium spatial distribution in a sufficiently large array. It was found that for Ge/Si(113) the screw dislocation component of interfacial dislocations with Burgers vectors b = ½<110> lead to interaction forces, which are either attractive or repulsive depending on the relative orientation of the screw component and resulting in two stable dislocation distances, which are in excellent agreement with the experimentally found values. The calculations also show that best agreement with the experimental values is found if only next neighbor interactions are taken into account. These findings demonstrate the importance of the screw component of the Burger vector for the interaction between interfacial dislocations and for the ordering and hence stress relief in these films.[1] Th. Schmidt, R. Kröger, J. I. Flege, T. Clausen, J. Falta, A. Janzen, P. Zahl, P. Kury, M. Kammler, and M. Horn-von Hoegen, Phys. Rev. Lett. 96, 066101 (2006).[2] A. Atkinson and S.C. Jain, J. Appl. Phys. 72, 2242 (1992).
MM12: Nanowires and Thermolelectrics
Session Chairs
Thursday PM, December 04, 2008
Room 309 (Hynes)
2:30 PM - MM12.1
Thermal Conductivity of Three-dimensional Si/SiGe Nanostructures.
Han-yun Chang 1 , Leonid Tsybeskov 1 , Andrei Sirenko 2 , David Lockwood 3 , Jean-Marc Baribeau 3
1 Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States, 2 Department of Physics, New Jersey Institute of Technology, Newark, New Jersey, United States, 3 Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada
Show Abstract2:45 PM - MM12.2
Atomistic Modeling of Thermal Transport in Realistic Si/Si1-xGex Superlattices.
Eric Landry 1 , Alan McGaughey 1
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractSuperlattices are composite, periodic structures containing alternating material layers with thickness on the order of 1-10 nanometers. Group IV semiconductor superlattices (e.g., Si/Si1-xGex) have the potential to increase the efficiency of thermoelectric energy conversion devices. This is because their cross-plane thermal conductivity can be reduced (through phonon scattering at interfaces and changes in the phonon dispersion characteristics) while maintaining good electron transport properties, resulting in high values of the thermoelectric figure of merit. A limited number of superlattice designs (all containing two layers in the unit cell) have been considered in previous studies. Therefore, it is likely that the full potential of superlattices has yet to be fully realized. We hypothesize that by optimizing the design of Si/Si1-xGex superlattices and considering more complex designs (i.e., with more than two layers in the unit cell) the superlattice thermal conductivity can be further reduced, leading to additional increases in the thermoelectric figure of merit. This hypothesis is supported by our recent theoretical study on model Lennard-Jones superlattices.In this work, molecular dynamics (MD) simulations are used to model thermal transport in realistic Si/Si1-xGex superlattices. The atomic interactions are modeled using the Stillinger-Weber interatomic potential. Because MD simulations are only strictly valid in the classical limit, the thermal conductivity predictions are made at a temperature of 500 K, where quantum effects are negligible. We incorporate the realistic effects of (i) lattice mismatch-induced strain, (ii) natural isotope concentration, (iii) deviations in the thickness of each superlattice layer, and (iv) species mixing at the superlattice interfaces into our model. In order to validate our prediction methodology, the thermal conductivity predictions of crystalline and amorphous Si, crystalline and amorphous Ge, Si1-xGex alloys, Si/Si0.7Ge0.3 and Si/Ge superlattices are compared to the experimentally measured values. Aside from the Si/Ge superlattices, the predicted trends are in agreement with experiment. For the Si/Ge superlattices, the thermal conductivity is predicted to be greater than an alloy of similar composition while the opposite relationship is observed experimentally. We attribute this discrepancy to additional phonon scattering in the experimental samples that results from defects not present in the MD model (e.g., misfit dislocations). We also perform a parametric study of the thermal conductivity dependence on period length and alloy composition for Si/Si1-xGex superlattices with x < 1 (where agreement between the simulation predictions and experimental measurements is expected). From these results, we suggest a superlattice design for minimizing the thermal conductivity, desirable for maximizing the efficiency of thermoelectric energy conversion.
3:00 PM - MM12.3
Thermoelectric Transport in Silicon Nanowires.
Hyuk Ju Ryu 1 , Clark Ritz 1 , Max Lagally 1 , Mark Eriksson 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractThe performance of silicon nanoelectronics is significantly affected by thermal issues such as hot spots and large temperature inhomogeneities. These effects become especially significant when the device dimensions are smaller than or comparable to the mean free path of phonons in the semiconductor host. In this context, silicon thermoelectrics offers attractive possibilities for the cooling of hotspots and for the conversion of wasted thermal energy into useful electrical energy. We present results on both the experimental measurement and the numerical modeling of thermoelectric properties of nanoscale silicon devices. For the experiments, sensitive detection of local temperatures and thermoelectric voltages is essential. We first present performance characteristics of nanoscale platinum heaters and sensors that have been fabricated on ultrathin SOI patterned into nanowires, and we demonstrate linear response of the sensors at temperatures between 30K and 310K. We also present measurements of the thermoelectric voltage in silicon nanowires induced by the temperature gradient. Highly boron-doped nanowires with cross-sectional dimensions of 17nm x 200nm reveal thermoelectric powers of 220µV/K and 100µV/K at temperatures of 300K and 200K, respectively. The thermoelectric properties of silicon nanowires can be modified by the incorporation of superlattices and gating. We compare the effects of silicon/silicide superlattices with silicon/silicon-germanium superlattices. Finally, we present the use of gated field-effect devices to modulate the thermoelectric power. Implications for use of these techniques in integrated systems will be discussed. This work is supported by DOE under Grant No. DE-FG02-03ER46028.
3:15 PM - MM12.4
Comparison of the Growth Mechanisms of Silicon Nanowires Synthesized with Au, In, Ga and Al as a Catalyst.
Anna Fontcuberta i Morral 1 2 , Ilaria Zardo 2 , Linwei Yu 3 , Jean-Pierre Alet 3 , Martin Frimmer 2 , Pere Roca i Cabarrocas 3
1 Institute of Materials, Ecole Polytechnique Federale de Lausanne, Lausanne Switzerland, 2 Walter Schottky Institut, Technical University of Munich, Garching Germany, 3 LPICM, Ecole Polytechnique, Palaiseau France
Show AbstractSilicon nanowires have attracted significant attention in the last few years in both fundamental and applied studies. One of the reasons is that silicon nanowires hold the promise for the realization of high device density chips and interconnects compatible with CMOS technology. One of most common method for synthesis of nanowires is the Vapor-Liquid-Solid method, in which a metal seed catalyst is required. In many studies gold (Au) is used as a catalyst, though gold is known to be a deep-level impurity in silicon. The optical and electronic properties of the nanowires could be much further improved and novel high-level applications realized, if alternative metal catalysts were used instead. Additionally, due to cross-contamination issues, the utilization of gold is incompatible with CMOS-technology. The search of new catalysts is therefore crucial for letting the nanowires enter the field of low dimensional semiconductor structures, future CMOS technology and nano-optoelectronic devices. It is therefore very important to understand the synthesis of silicon nanowires in systems alternative to the Au-Si system. In an effort to find alternatives to Au, we have studied the growth of nanowires using In, Ga and Al as catalyzers as the synthesis of silicon nanowires with one of these metals as a catalyst could open the way for the simultaneous fabrication of interconnects and nanowire transistors.Silicon nanowires have been synthesized by Chemical Vapor Deposition (CVD) and Plasma Enhanced CVD by using Au, Ga, Al, and In as a catalyst. As observed by Scanning Electron Microscopy and Raman Spectroscopy, the morphology and structure of the wires strongly depends on the synthesis temperature, pre-treatment of the surface before growth and metal used. At low temperatures the majority of the wires tend to be strongly kinked and bent, while at higher temperatures only straight nanowires are present. We have been able to obtain nanowires with all these metals. However, the temperature window for obtaining high quality wires is reduced if one compares to the standard gold.
3:30 PM - MM12.5
Ab-initio Calculation of Thermal Conductivity of Silicon-germanium Alloys.
Jivtesh Garg 1 , Nicola Bonini 2 , Nicola Marzari 2
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThermoelectric materials will become commercially viable for converting heat into electricity and for refrigeration once their figure of merit (ZT) is improved. One key approach to increase performance is to reduce the thermal conductivity - in alloys it is lower than the binary endpoints due to increased scattering induced by strain and disorder. Understanding the thermal conductivity of complex materials materials is also important for other applications - from reducing hot spot temperatures on electronic chips to better thermal-insulation materials. Here, we have calculated the thermal conductivity of silicon-germanium alloys using ab-initio density functional perturbation theory. The electronic structure of the alloy is studied with the virtual crystal approximation and the single mode relaxation time approximation; perturbation theory up to the third order provides phonon lifetimes, and disorder effects are taken into account by ensemble averages over configurations with random mass disorder.
MM13: Porous Silicon and Beyond
Session Chairs
Thursday PM, December 04, 2008
Room 309 (Hynes)
4:15 PM - MM13.1
Characteristics of Thermo-Acoustic Nanocrystalline Porous Silicon Ultrasound Generator as a Wide-Band Tweeter.
Bernard Gelloz 1 , Akira Asami 1 , Nobuyoshi Koshida 1
1 , Tokyo Univ. A&T, Tokyo Japan
Show AbstractUtilizing a big contrast of the thermal conductivity between single-crystalline silicon (c-Si) and anodized nanocrystalline porous silicon (nc-PS), a significant sound wave can be generated by thermo-acoustic transfer on the PS surface without any mechanical vibrations [1]. Its emission characteristics exhibit the completely flat frequency response in a wide range. Based on this advantage over the conventional piezo-electric transducer, various studies have been done for applications in air: impulse probe for 3-dimentiona object sensing [2], non-contact actuator [3], and reproduction of mouse pup vocalization calls [4,5]. To amplify the advantageous property of this silicon-based PS emitter further, a digital drive scheme has been applied to two-dimensionally arrayed PS emitters. The fundamental characteristics are reported here in the frequency range from audio- to ultrasound-band.The experimental 2×2 emitter arrays were fabricated on a p-type (100) c-Si wafer by a masking technique. The nc-PS device unit is composed of a heater electrode (5×5 mm2), an nc-PS layer, and a c-Si substrate. The nc-PS layer was prepared by anodization of the c-Si substrate in an ethanoic HF solution. The nc-PS layer thickness was adjusted to a value (25 μm in this case) that is larger than the thermal diffusion length in every frequency under study. After anodization, a thin metal film heater (50 nm thick) onto the nc-PS layer by RF-sputtering. The ultrasound pressure and waveform were measured by a microphone placed at a distance of 30-50 cm from the device surface. Due to the complete thermal-insulating property of nc-PS, surface temperature fluctuations generated by an electrical input supply into the heater electrode is very quickly transferred into air in close proximity to the surface. The induced air expansion and compression result in the generation of the acoustic wave. The emissive property of the broad acoustic wave throughout the frequency from audio to ultrasound band has been confirmed by the experiment in which the white noise is used as the input signal. It has been also demonstrated that the analog acoustic signal can be reproduced well by digital drive with a. practical constant level in the frequency range from 1 to 100 kHz. The present result verifies the usefulness of this device as a super tweeter including broadband digital speaker. 1. H. Shinoda, T. Nakajima, M. Yoshiyama and N. Koshida, Nature 400, 853 (1999).2. K. Tsubaki et al, Jpn. J. Appl. Phys. 44, 4436 (2005).3. J. Hirota, H. Shinoda and N. Koshida, Jpn. J. Appl. Phys. 43, 2080 (2004).4. T. Kihara et al, Appl. Phys. Lett. 88, 043902 (2006).5. A. Uematsu et al., Brain Research 1163, 91 (2007).
4:30 PM - MM13.2
Dichroic Rugate Filters Based on Birefringent Porous Silicon.
Nobuyuki Ishikura 1 , Minoru Fujii 1 , Kohei Nishida 1 , Shinji Hayashi 1 , Joachim Diener 2
1 Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe Japan, 2 Physik-Department E16, Technische Universität München, Garching Germany
Show AbstractThe possibility of fabricating planar optical circuits entirely from silicon (Si) is attractive, given the extensive use of Si in integrated electronic devices. One problem with crystalline Si is that its intrinsic birefringence and hence its ability to influence polarized light is small. The significant birefringence can be achieved by anisotropic nanostructuring of bulk Si e.g. two- or three-dimensional photonic crystals. Another approach is electrochemical etching of a (110) oriented Si wafer resulting in porous Si layers which exhibit strong in-plane birefringence [1]. Stack of the birefringent porous Si layers with alternating refractive indices and thicknesses result in a dichroic Bragg reflector with a transmission/reflection band depending on the polarization direction of the incident light [2]. However, the performance of the dichroic Bragg reflector is limited because of the appearance of unnecessary higher-order harmonics and interference oscillation (sidelobes). The purpose of this work is to significantly improve the porous-Si-based dichroic filters by introducing gradual refractive index profiles, i.e., the formation of porous-Si-based dichroic rugate filters. The sinusoidal refractive index profile combined with index-matching layers and apodization result in the reflection band having two distinct transmission bands depending on the polarization of the incident linearly polarized light without higher-order harmonics and sidelobes. As one of the applications of porous-Si-based dichroic rugate filters, we demonstrate a planar linear polarizer having the extinction ratio better than 20 dB (up to 25 dB) over a 100 nm band width with high transmittance. Furthermore, electrochemical etching process allows us to combine these elements easily and to produce much complicated devices without increasing the process cost. Therefore, with the present approach, a variety of Si-based polarizing elements can be realized at very low process cost. [1] N. Kunzner, D. Kovalev, J. Diener, E. Gross, V. Yu. Timoshenko, G. Polisski, F. Koch and M. Fujii, Opt. Lett., 26 (2001) 1265 [2] J. Diener, N. Kunzner, D. Kovalev, E. Gross, V. Yu. Timoshenko, G. Polisski and F. Koch, Appl. Phys. Lett., 78 (2001) 3887
4:45 PM - MM13.3
Three Dimensional Nanoscale Pattern Formationin Light Emitting Porous Silicon.
Ik Chun 1 2 , Edmond Chow 2 , Xiuling Li 1 2
1 Electrical and Computer Engineering, University of Illinois, Urbana, Illinois, United States, 2 Micro and Nanotechnology Laboratory, University of Illinois, Urbana, Illinois, United States
Show AbstractPorous silicon remains to be an area of focused research due to its ability to tune the refractive index through porosity, flexibility in surface functionalization, and its light emitting properties etc. Patterned porous silicon structures are potentially useful for integrated optical and sensing applications. Promising processing techniques for micron scale pattern formation have been developed, utilizing lithography with electrochemical resistant mask, or maskless localized treatment by electron beam, near-field photon flux, ion beam irradiation or implantation. The spatial resolution of these techniques is limited by lithography tools or the well-known fragility and reactivity during porous silicon formation or etching. As a result, a nanoscale (< 100 nm) large area fabrication method for patterned porous silicon has not been reported previously. Here we present a simple and efficient method to form nanoscale (10 – 100 nm) 3D patterns in light emitting porous silicon. In this method [1], nanoscale metal pattern is used to assist differential chemical etching under and in-between metals without any external bias. The metal in this case does not serve as a mask, rather as a catalyst for hole (h+) generation for semiconductor dissolution. Material removal occurs directly underneath the metal due to the fast catalytic etching reaction. This results in the imprint of the 2D lithography metal pattern into 3D silicon body. Due to the nature of the pattern development process, porous silicon formation is achieved in the same step, and a 3D nanoscale patterned porous silicon structure can be generated. Since the reaction front continues to move downward with the metal as the materials are removed underneath it, 3D nano-features fabricated using this method show vertical sidewalls with no undercut, in contrast to making patterned porous silicon using photoresist or ebeam resist as mask. In general, the 2D to 3D pattern transfer fidelity and resolution limit are dependent on how well the patterned metal features form a coherent interface with the semiconductor during pattern development. This nanoscale fabrication method, with modification, is expected to be applicable to other semiconductor materials.Specifically, various nanoscale metal patterns were written to PMMA on p-type Si substrates (~7 Ωcm), using electron beam lithography. Platinum (Pt) and Titanium (Ti) metals were evaporated and followed with standard lift-off. Metal patterned substrates were then etched in the H2O2 metal-HF (HOME-HF) etchant for 15 ~ 60 seconds. Morphology resulted from etching are examined with scanning electron microscope and atomic force microscope, and optical properties of the etched samples are characterized using photoluminescence spectroscopy and cathodoluminescence imaging. Detailed results will be presented and discussed.[1] I.S. Cun, E.K. Chow, and X. Li, Appl. Phys. Lett. 92, 19113 (2008).
5:00 PM - MM13.4
Nanoscale Multilayers in Porous Silicon for THz Phonon Engineering.
Gazi Aliev 1 , Paul Snow 1
1 Physics, University of Bath, Bath, Somerset, United Kingdom
Show AbstractThere has been much work in recent years investigating and exploiting multilayer optical mirror and microcavity structures in porous silicon. These are produced by electrochemical etching of silicon wafers and their manufacture relies on the variation of porosity that can be obtained during porosification by etching current control. The layers are normally an A/B repeating cell with a difference in porosity to create a refractive index step at the interface between layers. Typically, these mirrors demonstrate optical stop bands in the visible to near-infrared region and use porosities of 40 - 80% with layer thicknesses of ~100nm. Their uses range from optical filters to biological sensors. However, to produce the highest quality microcavity devices the surface roughness of the porosity interfaces have been carefully minimised by low-temperature etching and control of the viscosity of the etchant. This has produced values of the surface roughness of a few angstroms. It thus seems probable that the porosity of a multilayer system can be modulated with a period of a few nanometers. Even if the interfaces are relatively broad compared to the layer thickness, this will tend to produce a rugate-style sinusoidal porosity profile that has also been optically studied at visible wavelengths.We have explored the theoretical acoustical properties of a multilayer system with a conservative repeat distance of 12nm so that each layer is significantly thicker than the anticipated surface roughness. Such samples could not been characterised using standard optical reflectivity but the electron-density fluctuation of the porosity layers would be measurable by x-ray scattering to allow characterization of such a mirror. For acoustic properties, the Rytov model of an infinite multilayer structure, which has previously been widely and successfully used to model zone-folding of the phonon dispersion in GaAs/AlGaAs superlattices was employed. The acoustic impedance of each pSi depends only on density, which is linked to porosity, and the velocity of acoustic waves in the layers for which the dependence on porosity has been measured. The calculated impedance mismatches possible are much greater than those obtained in GaAs/AlGaAs or Si/Ge multilayer systems. It is found that the whole dispersion relation of the multilayers depends on only a few parameters and can be easily calculated. The results demonstrate that the zone-folding is accompanied by large bandgaps in the acoustic transmission spectra and this can be easily tuned by control of the layer thicknesses and relative porosities. We find that the fundamental band acoustic band gap can easily be moved to values of ~450GHz for a 8nm/4nm system with porosities of 40/80% and thus higher bands are at THz frequencies. It is proposed that inelastic light scattering measurements could be used to measure the zone-folding of the acoustic phonon dispersion to experimentally evaluate porous acoustic multilayers.
5:15 PM - MM13.5
Activity of Nanocrystalline Silicon Planar Ballistic Electron Emitter in Solutions.
Toshiyuki Ohta 1 , Shuichiro Ogawa 1 , Bernard Gelloz 1 , Nobuyoshi Koshida 1
1 , Tokyo Univ. A&T, Tokyo Japan
Show AbstractAmong the versatile functions of anodized nanocrystalline porous silicon (PS), the ballistic electron emission is based on generation of hot electrons in interconnected nanosilicon dot chains [1]. The most characteristic feature of this phenomenon is that energetic electrons are uniformly ejected with 5-7 eV in average at an applied voltage of 15-20 V. This enables it possible to use the PS ballistic emitter as an active electrode in atmospheric gas ambience [2] and even in solutions [3,4] as reported previously. In pure water, for instance, the ballistic emission causes hydrogen generation due to direct reduction of H+ ions at the device surface. To clarify the mechanism of the interfacial effects, the present paper reports on the characteristics in various solutions.The ballistic emitter is composed of a thin Au surface film, a PS layer, polycrystalline Si, a single-crystalline silicon wafer, and a back contact. A poly-Si film was formed by a LPCVD method on n+-Si substrate. The PS layer was formed by photo-anodization technique in an ethanoic HF solution. Then it was thermally oxidized to enhance the electric field effect. To improve the operation stability in solutions, annealing treatments and surface modification technique were applied to the PS layer. Finally, a thin Au film was deposited as a surface electrode. In solutions, the part excepting the active area was covered with water-proof material. The emission characteristics in solutions were evaluated by the cyclic voltammogram under the three electrode configuration. As experimental media, pure water, H2SO4 solutions, KOH solutions, and physiological salt solutions were used. Besides gas chromatography for assignment of gas species evolved during the emitter operation, analyses were made in terms of the pH value and the dissolved hydrogen content.In every solution, both the current increase and the gas evolution were seen under the operation of the electron emission, and then only hydrogen is generated with no bi-product like oxygen without using any counter electrode. The pH value tends to increase with increasing the operation time of the emitter. The hydrogen generation rate depends on both the H+ concentration in solution and the relationship between the mean energy of emitted electrons and the redox potential of solutions.The nanosilicon ballistic emitter operates as an active electrode supplying highly reductive electrons into solutions to produce substances or something. This suggests the technological potential of the ballistic emitter as a key device for productive electronics.[1] N. Koshida, X. Sheng, and T. Komoda, Appl. Surf. Sci. 146, 371 (1999). [2] T. Ohta, A. Kojima, and N. Koshida, J. Vac. Sci. Technol. B 25, 524 (2007).[3] N. Koshida, T. Ohta, and B. Gelloz, Appl. Phys. Lett. 90, 163505 (2007). [4] T. Ohta, B. Gelloz, and N. Koshida, J. Vac. Sci. Technol. B 26, 716 (2008).
5:30 PM - MM13.6
Structural and Optical Emission Properties of SiNx Films Formed by Reactive and Direct Sputtering.
Selcuk Yerci 1
1 Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States
Show AbstractThe structural and optical emission properties of non-stoichiometric silicon nitride films SiNx fabricated by N2 reactive and direct magnetron co-sputtering have been investigated by Raman, Fourier Transform Infrared (FTIR) and photoluminescence spectroscopy. Based on a systematic FTIR and Raman analysis of SiNx films, we will compare the chemical and structural properties of samples produced with both techniques. Moreover, we will discuss the role of Si nanocrystals nucleation on the stoichiometry of SiNx matrices fabricated with varying Si content and annealed in a broad temperature range from 300 to 1000 oC. In addition, the effect of Erbium incorporation on the structural properties of SiNx will be addressed. Finally, we will present our results on the correlation between the structural properties of SiNx films and their light emission properties as studied by steady-state and time-resolved photoluminescence techniques. These results can potentially lead to an understanding of the key materials parameters governing the light emission properties of SiNx films and Si nanocrystals in SiNx host matrices.