Chairs
Hideki Hasegawa
Res Ctr for Interface Quantum Elect.
Hokkaido Univ
Sapporo, 060 JAPAN
81-11-757-1163
Minghwei Hong
Bell Labs, Lucent Technologies
Rm 1T-204
Murray Hill, NJ 07974-0363
908-582-7755
Zheng-hong Lu
Dept of Materials Science
Univ of Toronto
Energenius Ctr for Adv Nanotechnology
Toronto, ON M5S 3E4 CANADA
416-978-5638
Stephen Pearton
Materials Sci & Eng
Univ of Florida
132 Rhines Hall
Gainesville, FL 32611-6400
352-846-1086
Symposium Support
*American Xtal Technology, Inc.
*Johnsen Ultravac
*Plasma-Therm, Inc.
*Riber, Inc.
*University of Florida - Dept of MS&E
1999 Spring Exhibitor
Proceedings published as Volume 573
of the Materials Research Society
Symposium Proceedings Series.* Invited paper
SESSION Z1: FUNDAMENTALS OF SURFACES AND THEIR PASSIVATION
Chair: Zheng-hong Lu
Monday Afternoon, April 5, 1999
Golden Gate B1 (M)
1:30 PM *Z1.1
THEORY OF THE SULPHUR-PASSIVATED InP(100) SURFACE. Laurent J. Lewis , Département de physique et Groupe de recherche en physique et technologie des couches minces (GCM), Université de Montréal, Québec, CANADA; Chandré Dharma-wardana, Institute of Microstructural Studies, National Research Council of Canada, Ottawa, Ontario, CANADA.
We present a detailed and comprenhensive theoretical investigation of the sulphur-passivated (100) surface of InP. First, the ground-state structure is determined using density-functional methods, including full relaxation of the surface. The lowest-energy structure at 0 K is a striking reconstruction with the S atoms displaced from the bridge sites to form short and long dimers, belonging to two distinct sublayers. This surface structure is used to calculate the backscattering Raman spectrum; two peaks arising from surface-layer vibrations are observed experimentally. Next, our first-principles calculations are extended to the study of a number of other stable states of the surface that can arise upon annealing. For this purpose, we construct and relax several higher-energy states of the surface, and calculate the corresponding core-level photoemission spectra. A remarkable sequence of structures is found to unfold from the fully S-covered ground state as they become energetically accessible. The surface S atoms exchange with bulk P atoms, forming new -- and strong -- S-P bonds while dissociating pre-existing S-S dimers. The predicted core-level spectra are found to be entirely consistent with the experimental measurements; our calculations indicate that the annealed (at about 700 K) surface is a structure containing two S and two P atoms per unit cell. Finally, we have used the predicted stable surface structures to calculate the photo-emission and inverse photo-emission spectra. The calculated spectra agree well with experiment if the surface is assumed to consist of a mixture of the above ground-state and annealed structures.
2:00 PM Z1.2
HYDROGEN ADSORPTION ON INP (001) SURFACES. L. Li , Q. Fu, B.K. Han, C. Li and R.F. Hicks.
The adsorption of atomic hydrogen on (2x1) and (2x4) surfaces of InP (001) films has been studied by multiple internal reflection infrared spectroscopy, scanning tunneling microscopy and low energy electron diffraction. These films were prepared by metalorganic vapor-phase epitaxy and transferred in situ to a surface analysis system. Scanning tunneling micrographs of the (2x1) reveal that this reconstruction is terminated with a complete layer of dimers, while the (2x4) consists of rows of trimers separated by trenches. Hydrogen adsorption on the (2x1) surface produces vibrational bands at 2260 and 2340 cm-1, which are due to phosphorous monohydrides. Evidently, the surface is covered with a complete layer of phosphorous dimers. This structure has not been observed previously, and exhibits properties which in many ways are analogous to Si (001) (2x1). On the other hand, hydrogen adsorption on the (2x4) surface produces two broad infrared bands between 1135 and 1360 cm-1. These features are characteristic of bridged indium hydrides. These hydrides are generated by H atom attack of the In dimers in the second layer. Two phosphorous hydride bands are observed as well, consistent with a P-P or In-P dimer in the top layer. Our results provide much needed chemical information on the passivation of indium phosphide surfaces with hydrogen. Furthermore, in combination with STM images, they allow us to unequivocally identify the atomic structures of the (2x1) and (2x4) surfaces.
2:15 PM Z1.3
IN SITU SURFACE PASSIVATION OF GaAs BY THERMAL NITRIDATION USING METALORGANIC VAPOR PHASE EPITAY. Jingxi Sun , F.J. Himpsel1, A.B. Ellis2, T.F. Kuech, Department of Chemical Engineering, 1Department of Physics, 2Department of Chemistry, University of Wisconsin, Madison, WI.
In this study, the ammonia-based, in situ passivation of GaAs surfaces,within a metalorganic vapor phase epitaxy (MOVPE) system is present. The shift of the GaAs surface Fermi level, and hence the surface charge density, resulting from this in situ passivation, has been studied using photoreflectance (PR)spectroscopy. Samples consisting of an undoped GaAs layer on highly doped n-GaAs (UN+) and p-GaAs (UP+) structures allow for the determination of the surface Fermi level position using PR. These structures were grown by MOVPE and in situ passivation was performed after growth within the MOVPE system without exposure to air. After passivation, the surface Fermi level can be shifted by 0.23 eV towards the conduction band edge for UN+ structures and by 0.11 eV towards the valence band edge for UP+ structures from the normally mid-gap `pinned' positions. The shift of surface Fermi level toward band edges for both UN+ and UP+ structures is attributed to the reduction of surface state density. A nitride-based layer on the nitrided surface was identified from synchrotron radiation photoemission spectra of the surface, attributing to the passivation effect. In situ passivation of the GaAs surface after growth allows for a controlled chemical process on a surface free of oxide contamination. The in situ passivation also provides the possibility of depositing subsequent layers for device processing, such as silicon nitride or silicon dioxide, immediately after passivation of the GaAs surface.
2:30 PM *Z1.4
X-RAY STANDING WAVE ANALYSIS OF PASSIVATED GaAs(001) SURFACES. M. Sugiyama , Y. Watanabe, NTT Basic Res. Labs., Atsugi, JAPAN; S. Maeyama, Kochi Womens Univ., Kochi, JAPAN; M. Oshima, Univ. of Tokyo, Tokyo, JAPAN.
The soft x-ray standing wave (soft-XSW) technique is capable of locating the light-element atoms, such as S, Si, and P, adsorbed on semiconductor surfaces. This is because large cross-sections of synchrotron radiation soft x-rays for the light-element atoms ensure high emission intensity from monolayer-order quantities of these atoms. To study the adsorbate structure of compound semiconductor surfaces, we developed an ultra high vacuum soft-XSW analysis apparatus equipped with a molecular beam epitaxy (MBE) growth chamber. By using this apparatus, we have studied sulfide solution treated and in-situ sulfur adsorbed GaAs(001) surfaces at the synchrotron radiation beamline 1A at the Photon Factory. It is found that these surfaces are covered with stable Ga-S-Ga bridge bonds after annealing in vacuum. On the other hand, a submonolayer Si adsorbed GaAs(001) surface and P2 molecular beam exposed GaAs(001) surfaces prepared by MBE were also studied by the soft-XSW. We will also discuss the adsorption behavior of Si and P atoms on GaAs(001)-(2x4) clean surfaces.
3:15 PM *Z1.5
SURFACES OF GALLIUM NITRIDE AND GALLIUM ARSENIDE. John Northrup , Xerox PARC, Palo Alto, CA; J. Neugebauer, Fritz-Haber-Institute, Berlin, GERMANY.
Total energy calculations performed within the local density functional approximation and employing the first-principles pseudopotential method provide insight into the structure and energetics of GaN and GaAs surfaces. It is known that for GaAs it is possible to form stable surfaces with a high concentration of surface As. In contrast, the corresponding GaN surfaces with high N content are unstable. On the other hand Ga adlayers are stable on GaN but not on GaAs [1,2]. For the GaN(000-1) surface the scanning tunneling microscope images obtained by Smith et al demonstrate the existence of 1x1, 3x3, 6x6 and c(6x12) structures. Our current understanding of the structural sequence will be presented. A Ga adlayer model is proposed for the 1x1 surface and an adatom-on-adlayer structure is proposed for the 3x3 surface [1]. The stability of the 1x1 Ga adlayer is traced to the large bonding interaction within the adlayer. 1. A. R. Smith, R. M. Feenstra, D. W. Greve, J. Neugebauer, and J. E. Northrup, Phys. Rev. Lett. 79, 3934 (1997). 2. J. E. Northrup and J. Neugebauer, Phys. Rev. B 53, 10477 (1996).
3:45 PM Z1.6
CORE-LEVEL SPECTROSCOPIC STUDY ON THE CHEMICAL REACTIONS OCCURRING ON METAL AND COMPOUND SEMICONDUCTOR SURFACES. H.C. Chen, H.Y. Hwang, C.C. Chang , National Taiwan Univ, Dept of Chemistry, Taipei, W.H. Hung, Synchrotron Radiation Research Center, Hsinchu, TAIWAN.
The physisorption and chemical reaction of gas molecules on metal and compound semiconductor surfaces have been studied using core-level spectroscopy. The effect of the surface temperature and the gas exposure on the bonding state of the adsorbed molecules on the surface is measured as a function of the energy of the photoelectrons emitted from the surface under irradiation of synchrotron light. Details about the bonding sites of the adsorbates and the mechanisms of the subsequent chemical reactions can be obtained by examining the alteration of the intensity of the photoemission peak from the surface. For InP, the chemisorption of chlorine leads to the formation of various InClx (x = 1 - 3) and PCl species at 110 K. Etching of the surface is enhanced by irradiation of the sample with energetic ions. The reaction of H2S with InP leads to the formation of four different chemical states of S, which bonds to the In atom of the surface. Various hydrides are also formed through bonding with the surface P atom. Synchrotron radiation provides energy necessary to induce the replacement of P in InP by S, and to incorporate S into the InP lattice. A passivation layer, possibly in the form of InP1-xSx, is formed after cycles of H2S exposure and synchrotron radiation. For Pd, CO adsorption starts in both the high coordination and the two-fold bridge sites at low exposures at 100 K and then in the on-top site at high exposure. Substrate reconstruction occurs at increasing temperature, which causes a drastic decrease of the emission intensity from high coordination sites. The bridge sites in the groove of the surface becomes available for reaction at >350 K.
4:00 PM *Z1.7
STRUCTURE OF SINGLE-CRYSTAL Gd2O3 FILMS ON GaAs (100). A.R. Kortan , M. Hong, J. Kwo, J.P. Mannaerts and N. Kopylov, Bell Labs Lucent Technologies, Murray Hill, NJ.
We have used single crystal x-ray diffraction to determine the crystallographic structure of the epitaxially grown single-crystal Gd2O3 films on GaAs (100) substrate [1]. We identified a Mn2O3 isomorphous structure from a full momentum space analysis of the diffraction peaks of three different samples with 185, 35, and 15 film thicknesses. The Gd2O3 in this cubic phase grows epitaxially, by alligning its [001] and [10] axes with the [011] and [01] axes of GaAs within the plane, respectively. While 15 thick sample shows an elastically strained component in the film, thicker samples appear fully relaxed, possibly by misfit dislocations. [1] M. Hong, J. Kwo, A. R. Kortan, J. P. Mannaerts, A. M. Sergent, and M. C. Wu, Proceedings of MRS Fall Meeting, 1998, Symposium D, and paper submitted to Science.
4:30 PM *Z1.8
STRUCTURE OF CHEMICALLY PASSIVATED SEMICONDUCTOR SURFACES DETERMINED USING X-RAY ABSORPTION SPECTROSCOPY. A.P. Hitchcock , T. Tyliszczak, Brockhouse Institute for Materials Research, McMaster University, Hamilton, CANADA; Z.H. Lu, Dept. of Materials Science, University of Toronto, Toronto, CANADA; M.W.C. Dharma-Wardana, Institute for Microstructural Sciences, National Research Council, Ottawa, CANADA.
This talk will review recent progress in characterization of monolayer-passivated single crystal semiconductor surfaces by using advanced synchrotron radiation spectroscopies, in particular X-ray absorption fine structure spectroscopy (XAFS). The near edge and extended aspects of the XAFS signal, supported in some cases by first-principles calculations, have been used to investigate the structure of a number of systems relating to passivation of single crystal semiconductor surfaces by wet chemical methods. Systems investigated include Ge(111)-Cl; GaAs(111)-Cl; InAs(100)-S; Ge(111)-S; GaAs(111)A-S and GaAs(111)B-S. Use of a 9-element array of solid state Ge X-ray fluorescence detectors has led to a significant improvement in data quality and thus structural accuracy. The relationships between the derived surface structures and the utility of these surfaces for chemical passivation will be discussed.
SESSION Z2: NOVEL APPROACHES FOR SURFACE PASSIVATION AND DEVICE PROCESSING
Chair: Hideki Hasegawa
Tuesday Morning, April 6, 1999
Golden Gate B1 (M)
8:30 AM *Z2.1
A NOVEL SURFACE PASSIVATION STRUCTURE FOR III-V COMPOUND SEMICONDUCTORS UTILIZING A SILICON INTERFACE CONTROL LAYER AND ITS APPLICATION. Tamotsu Hashizume and Hideki Hasegawa, Research Center for Interface Quantum Electronics and Graduate School of Electronics and Information Engineering, Hokkaido University, Sapporo, JAPAN.
The present status of surface passivation research for III-V compound semiconductors utilizing a novel unique structure including a silicon interface control layer(Si ICL) is presented and discussed. The basic principle of passivation is to insert an ultrathin MBE-grown Si layer between the III-V compound semiconductor and a Si-based thick insulator so as to terminate the surface bonds of the III-V material with Si atoms and then transfer Si-bonds smoothly to those of the Si-based insulator. Such a principle works excellently for narrowgap materials such as InGaAs after suitable process optimization. For materials with wider enrgygaps, one has to take account of the fact that the coherently strained Si layer has a very narrow bandgap and band states of Si ICL behave as interface states within the energygap of the III-V material.This problem can be solved by further reducing the Si ICL thickness systematically such that the resultant ultranarrow Si surface quantum well has no quantum states due to the quantum confinement effect. Passivation of GaAs, InP, InGaAs, AlGaAs and InAlAs surfaces and their in-situ characterization have been carried out in a UHV-based multi-chamber system where solid source and gas source MBE chambers, photo- and ECR plasma CVD chambers, and XPS, UHV-STM, UHV contactless C-V and UHV PL chambers are connected by a UHV transfer chamber together with other chambers. The topics discussed in the paper include theoretical optimization of the passivation structure including a Si surface QW, UHV-STM study on atomic arrangements of Si ICL and the effect of the initial surface reconstruction, process characterization and optimization by combined use of contactless C-V, XPS and UHV-PL methods and application to MISFETs, insulated gated HEMTs and passivation of quantum wells and wires.
9:00 AM *Z2.2
THE (Ga2O3)1-x(Gd2O3)x OXIDES WITH 0 < x 1 FOR GaAs PASSIVATION. J. Raynien Kwo , M. Hong, A.R. Kortan, D.W. Murphy, J.P. Mannaerts, Y.C. Wang and A.M. Sergent, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Our previous discovery of novel Ga2O3(Gd2O3) oxide mixture for GaAs passivation has led to the first demonstration of the enhancement-mode GaAs MOSFET's with inversion in both n- and p-channel configurations. Depletion-mode GaAs MOSFET's with negligible drain current drift and hysteresis were also fabricated using this oxide as the gate dielectric. The basic mechanism responsible for such a low interfacial state density near the Ga2O3(Gd2O3)/GaAs interface has been a primary subject of recent investigations. Chemical analysis indicated substantial amount of Gd in the film. In order to elucidate the role of Gd2O3 in this composite oxide, we have examined the dependence of dielectric and structural properties of (Ga2O3)1-x(Gd2O3)x on the Gd (x) content. Contrary to the previous thinking that Gd is a problematic impurity to the oxide, we found Gd2O3 is a necessary component. Pure gallium oxide does not passivate GaAs, and x greater than 0.2 is needed to stabilize the gallium oxide in the 3+ fully oxidized state due to the electropositive nature of Gd+3. Furthermore, we also found that pure Gd2O3 film is an excellent oxide to passivate GaAs surface. The Gd2O3 film of a dielectric constant of 10 is grown epitaxially as a (110) Mn2O3 crystal structure on (100) GaAs. The atomically smooth interface and single domain structure of Gd2O3 ensure excellent dielectric properties with low leakage conduction and high breakdown strength even for films of only 1.5nm thick. The robustness of the epitaxial dielectric against post high temperature processing adds another attractive feature for device applications. We expect that epitaxial growth of the Mn2O3structure can be extended to other rare earth oxides, and to other semiconductor substrates like Si. Our findings thus suggest new opportunities of producing high gate dielectrics for GaAs as well as Si based MOSFETs
9:30 AM *Z2.3
DEVELOPMENT OF LOW TEMPERATURE SILICON NITRIDE AND SILICON DIOXIDE FILMS BY INDUCTIVELY-COUPLED PLASMA CHEMICAL VAPOR DEPOSITION. J.W. Lee , K.D. Mackenzie, D. Johnson, Plasma-Therm Inc., St. Petersburg, FL.
High-density plasma technology is becoming increasingly attractive for the deposition of dielectric films such as silicon nitride and silicon dioxide. In particular, inductively-coupled plasma chemical vapor deposition (ICPCVD) offers a great advantage for low temperature processing over plasma-enhanced chemical vapor deposition (PECVD) for a range of devices including compound semiconductors. In this paper, the development of low temperature (< 200C) silicon nitride and silicon dioxide films utilizing ICP technology will be discussed. The material properties of these films have been investigated as a function of ICP source power, rf chuck power, chamber pressure, gas chemistry, and temperature. The ICPCVD films will be compared to PECVD films in terms of wet etch rate, hydrogen content, stress, and other film characteristics. Two different gas chemistries, SiH4 /NH3/He and SiH4/N2/Ar were explored for the deposition of ICPCVD silicon nitride. The ICPCVD silicon dioxide films were prepared from SiH4/O2/Ar. The wet etch rates of both silicon nitride and silicon dioxide films are significantly lower than films prepared by conventional PECVD. This implies that ICPCVD films prepared at these low temperatures are of higher quality. The advanced ICPCVD technology can be also used for efficient void-free filling of high aspect ratio (3:1) sub-micron trenches. Detailed results on this work will also be presented.
10:15 AM *Z2.4
SELECTIVELY OXIDIZED DIELECTRIC APERTURES AND MIRRORS FOR LOW THRESHOLD VCSELS AND SPONTANEOUS LIGHT EMITTERS. D.G. Deppe , D.L. Huffaker, L.A. Graham, S. Csutak, and Q. Deng, Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, TX.
Selective oxidation of AlAs (or AlGaAs) can be used to form buried, low refractive index apertures within high Q Fabry-Perot microcavities. These apertures provide electrical and optical confinement, and for vertical-cavity surface-emitting lasers (VCSELs), have resulted in ultra-low threshold room temperature lasing with threshold currents under 40 A. When used with quantum dot light emitters, the oxide-apertured microcavity can also be used to control spontaneous emission lifetime. In this talk we will review the microcavity fabrication based on selective oxidation to form both oxide-apertures and high contrast oxide/GaAs distributed Bragg reflectors. This type of high contrast microcavity can provide the route to new performance regimes of low power VCSELs and spontaneous light emitters. When used with quantum dot light active regions, it may also be possible to fabricate high speed, GaAs-based light emitters that operate at 1.3 m wavelength. Initial results of lasing from quantum dots at 1.3 m and oxide-confined VCSELs and microcavities operating beyond 1.1 m will be presented. The 1.3 m wavelength quantum dot active regions exhibit low threshold current density of 90 A/cm2at room temperature for edge-emitting lasers, and might be combined with the selective oxidation to fabricate high speed, long wavelength VCSELs and microcavity light emitting diodes. Our initial experiments on apertured microcavity controlled spontaneous lifetime show up to a factor of 2 increase in the spontaneous emission rate, and will also be described.
10:45 AM *Z2.5
DESIGN OF BURIED OXIDE PROFILES WITHIN SELECTIVELY OXIDIZED VCSELS. Kent D. Choquette , A.A. Allerman, K.M. Geib, Center for Compound Semiconductor Science and Technology, Sandia National Laboratories, Albuquerque, NM; D. Mathes, R. Hull, Dept of Materials Science, University of Virginia, Charlottesville, VA.
The use of selectively oxidized current apertures within vertical cavity surface emitting lasers (VCSELs) has produced dramatic performance advances due to improved electrical and optical confinement. With better understanding of the wet oxidation process, improved control in the fabrication of buried oxide apertures within VCSELs can be pursued to address specific laser properties. We will discuss the implementation of buried oxide apertures within monolithic VCSELs and will show that buried oxide apertures with specific profiles can influence the laser performance. VCSELs nominally emitting at 850 nm have been fabricated with various oxide aperture designs. Thin oxide layers positioned at nulls in the longitudinal optical standing wave exhibit reduced optical confinement and loss. The thinner oxide produces lower loss due to the lower threshold current density for VCSELs with small aperture sizes. This enables high efficiency operation for lasers with oxide aperture sizes as small as 0.5x0.5 m, which previously was only possible with oxide apertures offset several mirror periods from the active region. Moreover, the thinner oxide VCSELs also exhibit higher efficiency. For example, a 1x1 m thin oxide VCSEL exhibits 1.5 mW maximum power and >20% power conversion efficiency, whereas the same size device with a conventional quarterwave thick oxide aperture does not operate. Conversely, thick tapered oxides provide more optical confinement and loss. However, due to increased modal discrimination, this oxide structure can enhance the single mode output. We find that single mode operation can be maintained in larger diameter thick oxide VCSELs as compared to thin oxide lasers. The fabrication of the thin and thick oxide apertures is strongly influenced by the layers adjacent to those intended for oxidation. Sandia is a multiprogram laboratory operated by Sandia Corporation for the United States Department of Energy under contract No. DE-AC04-94AL85000.
11:15 AM *Z2.6
ALUMINUM AND GALLIUM-BEARING NATIVE OXIDE FORMED BY WET OXIDATION OF AMORPHOUS ALUMINUM OR GALLIUM ARSENIDE ON III-V COMPOUND SEMICONDUCTORS. Kuang-Chien Hsieh , Univ of Illinois, Dept of Electrical and Computer Engineering, Urbana, IL.
Amorphous -(Ga,As) and -(Al,As) compounds with 50 excess As have been deposited on GaAs, InP and GaP substrates by molecular beam epitaxy at about 100C. Upon oxidation at as low as 300C with water vapor, the -(Al,As) is converted to a truly amorphous native aluminum oxide layer as characterized by transmission electron microscopy (TEM). Auger electron spectroscopy (AES) profiles indicate a complete depletion of As in the oxide layer. Although -(Ga,As) also oxidizes to form an amorphous gallium oxide matrix at low temperatures, a competing solid state recrystallization mechanism has led the formation of GaAs microcrystallites on the order of 30 Å Complete removal of As in the gallium oxide layer requires further oxidation of the GaAs microcrystallites. Extensive wet oxidation of the amorphous (Ga,As)/(Al,As) heterostructures on GaAs, however, results in an As loss near the surface of the underlying GaAs substrate as characterized by TEM and AES. Similar degradation is also observed near the surface of InP substrates. In contrast, GaP substrates exhibit greater chemical stability against water vapor under similar oxidation conditions. Incorporation of a thin GaP barrier layer of about two monolayers on the GaAs substrate prior to the deposition of -(Ga,As)/(Al,As) compounds has proven effective to stabilize the GaAs interface. Oxidation at 400C for 30 minutes yields a uniform aluminum oxide without noticeable damage to the underlying GaAs surface. High resolution TEM shows an improved interfacial roughness on the order of less than 15 Å In addition, an enhancement of photoluminescence of more than three orders of magnitude as compared to the as-grown counterpart indicates a great reduction in interface electronic traps. With a reduced interface trap density and improved interface roughness, this amorphous native oxide has a potential use in GaAs-based metal-oxide-semiconductor field-effect transistors.
11:45 AM Z2.7 NANOSCALE STRUCTURAL AND CHEMICAL CHARACTERIZATION OF AlGaAs AND AlInP THERMAL OXIDES. D.T. Mathes , R. Hull, Dept. of Materials Science and Engineering, University of Virginia, Charlottesville, VA; R.D. Dupuis, R.D. Heller, Microelectronics Research Center, The University of Texas at Austin, TX; K.D. Choquette, A.A. Allerman, Sandia National Labs, Albuquerque, NM.
Abstract not available.