Michael A. Filler, Georgia Institute of Technology
Kimberly Dick Thelander, Lund University
Anna Fontcuberta i Morral, École Polytechnique Fédérale de Lausanne
Jordi Arbiol, ICREA and Institut Català de Nanociència i Nanotecnologia
NM03.01: Nanowire Photonic Devices: Photodectors and Sensors
Monday PM, November 26, 2018
Sheraton, 2nd Floor, Back Bay D
8:30 AM - *NM03.01.01
GaN/AlN Nanowire Photodetectors—From the UV to the IR
Eva Monroy1,Akhil Ajay1,Maria Spies2,Jonas Lähnemann1,Martien den Hertog2
CEA-Grenoble1,Institut Néel2Show Abstract
Nanowire photodetectors attract broad interest due to their low dimensionality, small electrical cross-section, and ultrahigh photocurrent gain. In the ultraviolet region, ZnO and GaN nanowires have been intensively studied as spectrally-selective photodetectors. For this application, III-nitride nanowires present advantages in terms of heterostructuring possibilities and stability against chemical, mechanical or electrical stress. In a single-GaN-nanowire UV photodetector, the efficiency can be enhanced by the insertion of a GaN/AlN heterostructure [1,2], which leads to an increase of the responsivity by about two orders of magnitude, improved linearity, and the possibility to select the detected wavelength, while maintaining a UV/visible contrast larger than six orders of magnitude. Furthermore, devices with a linear photoresponse to the optical power can be implemented by using nanowires with a thickness below a certain threshold . On the other hand, the insertion of quantum dots in nanowires is also interesting for infrared photodetection using intraband transitions. Therefore, intraband transitions in GaN/AlN nanowire heterostructures have been investigated, varying the geometry and doping level of the GaN insertions . Based on this research, we present the first single-nanowire quantum well infrared photodetector (NW-QWIP), operating at the 1.55 µm telecom band . Finally, the study has been extended to cover the mid-infrared spectral range, up to around 6 µm, using intraband transitions in GaN/Al0.4Ga0.6N dots-in-a-wire.
 J. Lähnemann et al., Nano Lett. 16, 3260 (2016)
 M. Spies et al., Nano Lett. 17, 4231 (2017).
 M. Spies et al., Nanotechnology 29, 255204 (2018).
 A. Ajay et al., Nanotechnology 28, 405204 (2017).
 J. Lähnemann et. al., Nano Lett. 17, 6954 (2017).
9:00 AM - NM03.01.02
Fully CMOS-Compatible Synthesis and Photodetector-Integration of Ultrathin, Parallel-Aligned ZnO Nanowire Arrays by Infiltration Synthesis Derived from Atomic Layer Deposition
Brookhaven National Laboratory1Show Abstract
Semiconductor nanowires with reduced diameters enable high-performance chemical sensors and photodetectors owing to their large surface-to-volume ratios and enhanced surface band bending. Synthesis of nanowires and their device integration by CMOS (complementary metal-oxide-semiconductor)-compatible processes however remain a formidable challenge. Here we report fully CMOS-compatible synthesis and ultraviolet (UV)-photodetector-integration of ultrathin (~30 nm diameter), perfectly parallel-aligned, polycrystalline ZnO nanowire arrays by using infiltration synthesis, a new type of material hybridization technique derived from atomic layer deposition (ALD), where vapor-phase organometallic precursors are infiltrated into polymer templates, forming inorganic-infiltrated hybrid nanocomposites that can be directly converted into monolithic inorganic nanostructures inheriting the positional registry and morphological features of starting polymer templates by ashing the polymer matrix. Specifically, the ultrathin ZnO nanowire array is generated by infiltrating diethylzinc and water vapors into lithographically patterned polymer nanowire template made of a negative-tone photoresist SU-8. The integrated ZnO nanowire array photodetectors feature ultralow dark currents <20 fA invariant with the number of nanowires, over 6-decade photocurrent on-off ratios leading to >120 dB dynamic range, and unusual superlinear photoconductive responses, enabling increasing photodetector performance parameters for a higher incident light power. Considering the temperature-dependent field-effect transistor characteristics of the ZnO nanowire arrays, the observed superlinear photoconductivity can be explained by a new type of photoelectrochemical thermionic charge emission mechanism involving the reaction of chemisorbed oxygen and photo-generated charge carriers at grain boundaries. The demonstrated ultrathin nanowire synthesis and device fabrication methods have potentials for fully CMOS-compatible integration of nanowire sensor devices and circuitries. The identified photoelectrochemical grain boundary thermionic emission mechanism provides an improved understanding on the superlinear photoconductivity observed in nanostructured materials.
9:15 AM - NM03.01.03
Plasmonic Au/ZnO Nanowires for Room Temperature NO2 Detection
Bo Zhang1,Jiyu Sun1,Puxian Gao1
University of Connecticut1Show Abstract
Plasmonic Au-ZnO nanostructures with a size less than the incident light wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that may lead to strong absorption, scattering, and local field enhancement.1 These resonances, associated with noble metal nanostructures create sharp spectral absorption and scattering peaks as well as strong electromagnetic near-field enhancements.2 However, operation of ZnO gas sensors is limited to elevated temperature, which leads to enhanced energy consumption and large sensor size. Thus, reducing operating temperature or room temperature gas detection become significant in future sensor development. In this work, by utilizing the wavelength tunable photo-irradiation, selective gas detection has been demonstrated at room temperature based on Au/ZnO nanowire arrays. The Au/ZnO nanowires were synthesized by the microwave-assisted hydrothermal deposition of ZnO nanowires followed by Au nanoparticle (NP) dip-coating process. Compared to pristine ZnO, the Au-ZnO nanowire sensor performance was enhanced in both UV and visible regions, especially with highly enhanced sensitivity observed at 550 nm. The sensitivity towards 20 ppm NO2 could reach as high as 250%, and the detection limit is determined to be around 1 ppm at 25 °C. The sensitivity enhancement resulted from UV is due to the migration of photo-generated electrons from Au NPs to ZnO. On the hand, the sensing mechanism in the visible region is primarily due to the LSPR effect of Au. The oscillated electrons become more sensitive to the charge density and dielectric environment of Au. Besides, a large selectivity was found for NO2 gas over CO, NH3 and O2 at 330 nm UV irradiation. The ratio of cross sensitivity towards target gas and interfering gases is larger than 300. It is clear that with tunable light irradiation, room temperature NO2 gas detection could be achieved using plasmonic Au/ZnO nanowires with high sensitivity and selectivity.
1. Gogurla, Narendar, et al. "Multifunctional Au-ZnO plasmonic nanostructures for enhanced UV photodetector and room temperature NO sensing devices." Scientific reports 4 (2014): 6483.
2. Mayer, K. M. & Hafner, J. H. Localized surface plasmon resonance sensors. Chemical reviews 111 (2011): 3828-3857.
9:30 AM - NM03.01.04
UV Sensitivities of Catalyst-Free Grown ZnO 1-D Nanostructures on High Crystallinity Atomic Layer Deposition Seeds
Yun-Yi Chu1,Shang You Tsai1,Chun-Chi Chen2,Fu-Hsiang Ko1
National Chiao Tung University1,National Applied Research Laboratories2Show Abstract
Zinc Oxide is a nontoxic material, with a wide direct band gap (3.4 eV), high exciton binding energy (60 meV) and good thermal stability, making it suitable for UV LEDs, photo catalysts, UV sensors and gas sensors. At the surface of zinc oxide, a layer of positive space charge is usually formed to cancel out the charge of surface oxygen species. Removal and addition of said surface oxygen species may alter the electrical properties at the surface. Utilizing this mechanism, applications such as UV and gas sensors were studied by others. Here, we explored the various factors that may affect the UV absorbing and sensing properties of ZnO 1-D nanostructured devices, including crystallinity, contact junctions, and morphology. Regarding morphology, 1-D nanostructures have high surface area ratios and can exhibit surface properties in larger scales, as the structures can be viewed as wrapping surfaces around lines.
In this study, 1-D nanostructured ZnO UV sensors were developed on silicon dioxide using an atomic layer deposition (ALD) seed and chemical vapor deposition (CVD) nanostructure growth to investigate nanostructure properties. An ALD process was selected to deposit the seed layer due to the capability of uniform thickness growth and excellent crystallinity. The CVD process with vapor-solid (VS) growth provided a clean method to grow zinc oxide nanostructures without introducing unnecessary additives, by using a source of zinc metal powder and high purity oxygen. Compared to wet chemical methods and vapor-liquid-solid (VLS) growth of ZnO nanostructures, the VS processes would not have salts and metal particles, which may affect devices in undesired ways, such as salt interactions with humidity, Ohmic or Schottky contacts by metal particles with ZnO and stability issues of metal catalysts.
Different 1-D nanostructures such as crossing nanowires and free-standing nanorods were deposited through the control of seed layer quality, zinc source temperature and substrate undercooling. Characterization of ZnO structures was done by SEM, X-ray reflection (XRR) and XRD techniques for morphology, thickness and crystallinity; UV-VIS, Photoluminescence spectroscopy and a Keithley 2400 instrument were used for the absorption, emission spectrums and UV irradiated conductivity change. The ALD seed ZnO layer showed highly preferred (002) plane crystal growth by a strong XRD peak with d-spacing approximately 2.6 angstroms, while 1-D nanostructure-grown samples exhibited similar or slightly lower (002) preference, indicated by minor ZnO (100) and (101) peaks. Structures with conductivity responses after 5 minutes of 3V UVA LED (345nm to 425nm, < 0.5 mWatts) irradiation from 3% to 226% were observed, with diagonally crossing nanowires showing lower responses, and vertical free-standing nanorods of higher responses. Comparison of such materials showed the dependence of 1-D nanostructured device UV response properties on ZnO morphologies.
9:45 AM - NM03.01.05
Light Emitting Silicon Nanowires—From Photonics to Sensing Applications
Antonio Leonardi1,2,3,Maria José Lo Faro2,Dario Morganti1,2,Cristiano D'Andrea2,Barbara Fazio2,Paolo Musumeci1,Pietro Artoni1,Cirino Vasi2,Gerardo Palazzo4,Luisa Torsi4,Francesco Priolo1,2,5,Alessia Irrera2
Università degli Studi di Catania1,Consiglio Nazionale delle Ricerche2,Istituto Nazionale di Fisica Nucleare3,Università degli Studi di Bari Aldo Moro4,Scuola Superiore di Catania5Show Abstract
The scientific community has devoted an increasing interest to quantum confinement materials. In particular, silicon nanowires (Si NWs) are considered one of the most appealing resource to be employed in nanoscaled devices. Si NWs with an efficient room temperature (RT) light emission would represent a great industrial advancement, opening the route to a wide range of unexpected photonic applications. Nevertheless, to achieve a good control on quantum confined Si NWs fabrication is complex and challenging with the current technology. The most diffuses approaches such as lithography or Vapor-Liquid-Solid techniques suffers of different limits restraining the realization of quantum confined Si NWs. We demonstrated the realization of an ultradense array (1012 NWs/cm2) of light emitting Si NWs by using a modified metal assisted chemical etching without any type of mask or lithography. This method is fast, cheap and compatible with the standard Si technology. NWs achieved by this technique exhibited a very bright RT PL and EL tunable with NWs size in agreement with the occurrence of quantum confinement effect. With this method we demonstrated the realization of a 2D random fractal array of aligned Si NWs without any lithographic process or mask and by using a fractal gold layer realized by a Si technology compatible approach. We were able to control and tune the optical properties of the system by changing the fractal morphology of the Si NWs array . In-plane multiple scattering and very strong light trapping with diffuse reflectance below 0.1% related to the fractal structure were observed overall the visible range [1-2]. An innovative generation of Si NW-based optical biosensor is realized, which exploits the PL properties for the ultrasensitive and selective detection of proteins  in a wide range of concentrations. The occurrence of non radiative phenomena introduced by the target analyte on the NWs surface determines the quenching of the PL signal. In particular, we realized a sensor for C-reactive protein (CRP), which is crucial for heart-failure pathology. Cardiovascular problems are some of the major cause of death for both men and women. The availability of high sensitivity, low cost and reliable CRP sensors is a priority demand in clinical diagnosis for cardiovascular diseases. Si NWs sensors are fast, highly selective and offer a broad concentration dynamic range. Moreover, these sensors reach a fM sensitivity permitting non-invasive analysis in saliva . Si NWs open the route towards new optical label-free cheap sensors and a full compatible with the standard Si technology for primary health care diagnosis of biomarkers. Moreover, by changing the functionalization the use of Si NW sensors opens the route towards a new class of promising label free optical sensors for different application fields.
1. Light: Science & Applications 5 (4), e16062, 2016
2. Nature Photonics 11,170-176, 2017
3. ACS Photonics 5 (2), 471–479, 2018
NM03.02: Metal Halide Perovskite Nanowires
Monday PM, November 26, 2018
Sheraton, 2nd Floor, Back Bay D
10:30 AM - *NM03.02.01
Probing Fundamental Charge Carrier Dynamics in Metal-Halide Perovskite Mircowires
Aboma Merdasa2,Eva Unger1,2
Lund University1,Helmholtz-Zentrum Berlin für Materialien und Energie2Show Abstract
Metal halide perovskites exhibit favorable properties for optoelectronic devices. This talk will summarize results on metal halide perovskite wires illuminating fundamental properties of metal-halide perovskites.
Intermittency effects in the photoluminescence suggest the existence of photo-induced dynamic state in metal-halide wires that lead to non-radiative recombination. For wires longer than 10 µm, photoluminescence quenching at defined positions along the wire suggest localized states that periodically become activated, leading to efficient charge carrier quenching. From the gradient in photoluminescence, the charge carrier diffusion length can be estimated from this one-dimensional model system.
Structural defects also play a role in the observed phase coexistence and hysteresis during the tetragonal to orthorhombic phase transition in methylammonium lead iodide wires. The phase transition temperature appears to be dependent on the local defect concentration with more defective domains more readily transforming from the tetragonal to orthorhombic domain. This leads to charge carriers being funneled to lower energy tetragonal sites during the phase transition shown by photoluminescence microscopy and super-resolution imaging.
 Merdasa et al., ACS Nano, 11, 5391 (2017)
 Dobrovolsky et al., Nat. Comm. 8, 34 (2017)
11:00 AM - NM03.02.02
Orientation-Dependent Hybrid Perovskite Conversion of VLS-Grown Lead Halide Nanowires
Hyewon Shim1,Naechul Shin1
Inha University1Show Abstract
Organic-inorganic hybrid perovskites, such as methylammonium lead iodide (CH3NH3PbI3) have shown outstanding optoelectronic properties, promising extensive application in solar harvesting. Although most hybrid perovskite materials have been synthesized under solution process, recent demonstrations of the vapor phase synthesis of the hybrid perovskite nanostructures suggest there is an emerging interest in controlling their properties under highly-confined structures. In particluar, single-crystalline 1D lead halide nanowires can be prepared via vapor-liquid-solid (VLS) growth mechanism through the self-catalyzed growth mechanism, which then converted to hybrid perovskite. Since the conversion occurs on the preformed nanowire, electronic properties 1D perovskite depend on the original nanowire structure. In this study, we report the VLS growth of lead iodide (PbI2) nanowires on a c-sapphire (0001) substrate followed by conversion to CH3NH3PbI3 using methylammonium iodide, and confirm that the degree of perovskite conversion depends on the growth orientations of PbI2 nanowires. We observe two different growth directions; vertically-oriented  nanowires and kinked nanowires. Photoluminescence (PL) measurements on each growth direction suggest that the oriented nanowires exhibit a higher degree of conversion compared to the  oriented nanowires. In addition,  oriented nanowires exhibit the position-dependent degree of conversion, depending on the presence of the catalyst tip on top of the nanowire. In particular, the conversion is observed both on the catatlyst tip and the base of nanowire, suggesting methylammonium iodide incorporates into the nanowire either by vapor trasport and surface diffusion. Our observation indicates that the vapor phase conversion of PbI2 to CH3NH3PbI3 is a diffusion-limited process. This finding is an important step towards an structure engineering of perovskite nanowires.
11:15 AM - NM03.02.03
First-Principles Insights of Electronic Structure in Quasi-One-Dimensional Van der Waals Materials
Xiaofeng Qian1,Baiyu Zhang1
Texas A&M University1Show Abstract
Low dimensional materials have been one of the focus areas in materials science for more than three decades. In particular, the low dimensionality and reduced dielectric screening lead to pronouced optoelectronic properties such as excitons and polaritons. Here we demonstrate that quasi-one dimensional (1D) materials exhibit unique electronic structure and optical properties promising for low-cost photovoltaics and novel optoelectronics. First, we will present our first-principles study of a few quasi-1D van der Waals crystals with non-toxic and earth-abundant elements, and will elaborate the role of 1D structure and defects on their highly anisotropic optical and electronic properties. Our results shed light on new thin-film photovoltaic systems with excellent defect tolerance. Second, we will show that quasi-1D van der Waals crystals exhibit exciting nonlinear optical effects, and a microscopic picture is provided based on first-principles theory. We believe the theoretical findings presented here will open up many exciting opportunities in quasi-1D materials and nanostructures.
11:30 AM - *NM03.02.04
Growth and Optoelectronic Applications of Single-Crystal Nanowires of Metal Halide Perovskites
Dongxu Pan1,Song Jin1
University of Wisconsin1Show Abstract
The remarkable performance of lead halide perovskites in solar cells can be attributed to the excellent photophysical properties that are also ideal for lasers and light-emitting devices (LEDs). The chemical and structural characteristics of halide perovskites make their crystal growth behaviors very different from conventional inorganic semiconductor materials. Here we first report new insights on the crystal growth of halide perovskites and developed the solution growth of single crystal nanowires, nanorods, and nanoplates of methylammonium (MA), formamidinium (FA) and all-inorganic cesium (Cs) lead halides perovskites (APbX3) via a dissolution-recrystallization pathway. We also developed the vapor phase epitaxial growth of aligned CsPbX3 perovskite nanowires and single-crystal thin films. Moreover, nanostructures of metastable perovskite phases, such as FAPbI3 and CsPbI3, can be stabilized via new chemical strategies by using surface ligands. These single-crystal nanowires are excellent model systems to study the intrinsic properties of perovskite materials, such as carrier transport and ionic interdiffusion. We also demonstrated high performance room temperature lasing with broad tunability of emission color from 420 nm to 824 nm from single-crystal lead halide perovskite nanowires with estimated lasing quantum yields approaching 100%. LEDs can also be fabricated with nanoscale structures of 3D or 2D perovskites. The excellent properties of these single-crystal perovskite nanowires of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance semiconductor lasers, LEDs, and other optoelectronic applications.
NM03.03: Nanowire Quantum Devices
Anna Fontcuberta i Morral
Monday PM, November 26, 2018
Sheraton, 2nd Floor, Back Bay D
1:30 PM - *NM03.03.01
Epitaxy of Hybrid Bi- and Tri-Crystal Quantum Materials by Molecular Beam Epitaxy
Niels Bohr Institute1,Microsoft Quantum Materials Lab2Show Abstract
Interfaces are key elements in nanostructured device architectures. The quality of interfaces is particularly important for quantum devices where the device performance depends on the order, uniformity and purity of the interfaces that takes part in the quantum device structure. Hybrid nanowire materials with semiconducting, superconducting and magnetic components properties constitute some of the most promising candidates in the search for materials suitable for topological quantum computing [i]. I will discuss the mechanisms of hybrid epitaxy by Molecular Beam Epitaxy [ii] that lead to well defined interfaces between crystals of different structural and electronic properties.
I will present on new characterization schemes of how to extract information from the hybrid epitaxial materials and discuss the challenges and material requirements needed for realizing and eventually manipulating topological protected quantum states.
[i] Nayak et al. Rev. Mod. Phys. 80, 1083 (2008)
[ii] Krogstrup et al. Nature Mater. 14, 400-406 (2015)
2:00 PM - NM03.03.02
In Situ Patterned Superconductor/Semiconductor Nanowires
Martin Bjergfelt1,Damon Carrad1,Martin Aagesen1,Thomas Kanne1,Filip Krizek1,Peter Krogstrup1,Thomas Jespersen1,Jesper Nygård1
University of Copenhagen1Show Abstract
We present electrical and structural characterisation of superconductor/semiconductor hybrid nanowires consisting of previously unexplored materials combinations. Each superconductor was deposited in-situ after semiconductor growth, without breaking vacuum. This ensures a clean and transparent interface between the semiconductor and superconductor; a crucial requirement for inducing a ‘hard’ BCS superconducting gap in the nanowires. Additionally, we developed a shadow mask method for patterning the superconductor layer during deposition. This allowed us to fabricate functional devices without need to develop specific etches and/or lithography techniques for each material. The method thus streamlines growth and fabrication while minimizing the risk of materials degradation during post-processing. We have demonstrated several key architectures including tunnel probes, Josephson Junctions and Majorana islands. These devices facilitated characterization of induced superconductivity properties in InAs nanowires, and provided insights into the requirements for obtaining a hard gap. Also of interest were the nanoscale superconducting properties of each material since the critical temperatures and critical magnetic fields differ strongly from bulk values. In particular, the materials we studied show dramatically increased out-of-plane critical field compared to aluminium, which is highly desirable for many proposed applications in topological superconductivity. The ability to fabricate devices based on a wide variety of interfaces broadens the scope for future applications of superconductor/semiconductor hybrid devices.
2:15 PM - NM03.03.03
Spin-Orbital Kondo Effect in InAs Nanowire Crystal Phase Double Quantum Dots
Heidi Potts1,Malin Nilsson1,Sebastian Lehmann1,Adam Burke1,Kimberly A. Dick1,Claes Thelander1
Lund University1Show Abstract
Semiconducting nanowires offer a unique platform to study fundamental physical effects. Recent advances in crystal phase engineering enable switching between zinc-blende and wurtzite crystal phases in InAs nanowire growth. Using this knowledge, a quantum dot (QD) can be created consisting of a thin zinc-blende section between two wurtzite tunnel barriers . Furthermore, the QD can be split into two parallel coupled QDs using two local sidegates and a global backgate . This system can be considered an artificial molecule, for which the electron population on the two dots can be changed separately, and the tunnel coupling between the two dots can be tuned. This offers a great flexibility to study for example the interaction between electrons and spins located on the two dots .
In this work, we use the crystal phase InAs QD system to investigate Kondo transport. The Kondo effect is an extensively studied many-body phenomenon which can be observed in QDs with two degenerate levels containing only one particle. Experimentally, the spin-degeneracy is often used to study the Kondo-effect. Recently the degeneracy of two orbitals in parallel coupled QDs has also been used to explore the so-called orbital-Kondo effect [4-6]. Special interest has arisen in the condition where both the spin- and the orbital-Kondo effects are present, resulting in a SU(4) symmetry. The challenge in experimental studies is to distinguish between the spin- and orbital-Kondo effects.
The InAs parallel QD system allows us to study the crossing of two spin-degenerate orbitals, one from each QD. The large single QD orbital spacing results in even-odd level spacing, making it evident whether the electron population on the dot is even or odd. At zero magnetic field, we observe enhanced conductance at zero bias due to spin-Kondo transport when one (or both) of the QDs contains an odd number of electrons. We further observe a Kondo peak when two orbitals from the QDs are aligned and contain only one electron. The zero-bias peak due to the spin degeneracy splits in magnetic field (Zeeman effect). Similarly, we demonstrate that the zero-bias peak due to the orbital degeneracy splits when detuning the orbital energies. The large g-factor of InAs allows us to study the orbital-Kondo effect independently of the spin-Kondo effect, and to demonstrate that the zero-bias peak at the orbital degeneracy persists, even though the spin-degeneracy is lifted by applying a magnetic field.
In conclusion we demonstrate both spin- and orbital-Kondo effects in InAs nanowire QDs formed by crystal phase engineering, and clearly show how the system can be tuned to have either one or both effects present at zero bias.
 M. Nilsson et al. PRB 93, 195423 (2016)
 M. Nilsson et al. Nano Letters 17, 7847 (2017)
 M. Nilsson et al. arXiv:1803.00326 (2018)
 T. Delattre et al. Nature Physics 5, 208 (2009)
 Y. Okazaki et al. PRB 84, 161305(R) (2011)
 A. J. Keller et al. Nature Physics 10, 145 (2013)
2:30 PM - *NM03.03.04
Selective Area Epitaxy of a Materials Platform for Scalable Quantum Computing
Microsoft Quantum1Show Abstract
Nanoscale hybrid III-V semiconductor/superconductor heterostructures have demonstrated potential to advanced quantum transport physics, in particular to host the elusive Majorana quasiparticles. Virtually all progress on the materials side in the last 6 years has relied on high quality vapor liquid solid growth of III-V free-standing nanowires functionalized at a later stage by ex-situ deposited Al. Despite these early successes a more robust, scalable materials platform is still missing to enable the future topological quantum computer.
Here I will report on our progress in using molecular beam epitaxy (MBE) to grow both III-V semiconductor nanowire networks and a high quality epitaxial s-wave metal superconductor, using selective area epitaxy. I will first introduce the current state of the art, advantages and challenges of the selective area approach and MBE with respect to other growth techniques and geometries. Then a new method to successfully map the selective area epitaxy window in the ultra-high purity MBE environment will be detailed. Fundamental knowledge gained by growth studies will be applied to obtain reproducible, high yield advanced high spin-orbit III-V nanowire/superconductor networks. The materials properties and quality are assessed by a combination of morphological, compositional and structural analyses.
NM03.04: Narrow Bandgap Nanowires
Michael A. Filler
Monday PM, November 26, 2018
Sheraton, 2nd Floor, Back Bay D
3:30 PM - *NM03.04.01
High-Quality Narrow-Band Gap III-V Semiconductor Nanostructures Grown by Molecular-Beam Epitaxy
Institute of Semiconductors, Chinese Academy of Sciences1Show Abstract
Among the III-V group semiconductors, InAs and InSb nanostructures have attracted much attention since they exhibit narrow-band gaps, high electron mobilities, strong spin-orbit couplings and giant g factors. These unique properties make them ideal materials for applications in high-speed and low-power electronics, infrared optoelectronics and especially topological quantum computing. All these applications need not only a high crystal quality but also a high degree control of the material morphology. In my talk, I will firstly give a brief introduction about our work on the successful growth of free-standing single-crystalline InSb nanosheets/nanowires on one-dimensional InAs nanowires stems on Si (111) substrates by molecular-beam epitaxy (MBE) [1,2]. Then I will present the growth of the wafer-scale free-standing high-quality two-dimensional InAs nanosheets also by MBE. These InAs nanosheets show the outstanding electrical and optical properties. Finally, I will forward to our recent progress on the heterostructures composed with the narrow-band gap semiconductor nanostructure and a superconducting aluminum fabricated by an MBE system without exposing to the air during all the growth process .
 D. Pan, M. Q. Fu, X. Z. Yu, X. L. Wang, L. J. Zhu, S. H. Nie, S. L. Wang, Q. Chen, P. Xiong, S. von Molnár, and J. H. Zhao, Nano Lett., 14 (2014) 1214.
 D. Pan, D. X. Fan, N. Kang, J. H. Zhi, X. Z. Yu, H. Q. Xu, and J. H. Zhao, Nano Lett., 16(2016) 834.
 D. Pan, J. H. Zhao et al., to be published.
4:00 PM - NM03.04.02
Light Emission from Direct Bandgap Hexagonal Germanium
Erik Bakkers1,3,Elham Fadaly1,Alain Dijkstra1,Floris Knopper1,Luca Gagliano1,Marcel Verheijen1,2,Jos Haverkort1
Eindhoven University of Technology1,Philips Innovation Laboratories2,Delft University of Technology3Show Abstract
Group IV-Semiconducting materials, especially Ge- rich alloys, with a hexagonal crystal structure have been theoretically predicted to exhibit a direct band gap nature1,2. Density functional theory (DFT) calculations predict a 0.3 eV bandgap for hexagonal Ge and a range from 0.3 to 0.8 eV for hexagonal Ge-rich SiGe alloys. This opens new frontiers towards uniting the electronic and optoelectronic functionalities on a single chip. However, experimentally, hexagonal Ge is an unexplored territory structurally, since Ge and its alloys crystallize naturally in the cubic structure which is optically inactive due to its indirect bandgap nature.
Remarkably, the Nanowire (NW) geometry offers a unique platform for realizing new crystal structures which are inaccessible except under extreme conditions3. Recently, we have developed generic technique in which a core/shell NW template is utilized for transferring the crystal structure in achieving new crystal phases