AVS1997 Session EM-TuA: Photonic and Electronic Materials and Processing

Tuesday, October 21, 1997 2:00 PM in Room C1/2

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	EM-TuA-1 High Performance Selectively Oxidized Vertical Cavity Laser Arrays K.D. Choquette, H.Q. Hou, K.M. Geib, M. Hagerott Crawford, B.E. Hammons (Sandia National Laboratories) Revolutionary advances in the performance of vertical-cavity surface emitting lasers (VCSELs) have recently been achieved, including ultralow threshold current (¾ 100 microamp) and high power conversion efficiency (> 50%). The improvements in VCSEL performance have been enabled by advances in the epitaxial growth and device fabrication. The fabrication technologies that have been employed to fabricate air-post, ion implanted, and oxide-confined VCSELs will be reviewed. In particular the use of selective wet oxidation of AlGaAs alloys to form buried oxide apertures within monolithic VCSELs will be emphasized. Important oxidation issues include the oxidation rate dependency on process parameters (e.g. layer composition and thickness, temperature, steam partial pressure, etc.), and the impact of selective oxidation on VCSEL reliability. Uniform and reproducible oxidation uniformity is critical for high density VCSEL arrays. The impact of the fabrication technologies on VCSEL performance will also be discussed. For example, optical/electrical loss and confinement can be engineered within selectively oxidized VCSELs to design specific characteristics, such as single mode or high power output.
2:40 PM		EM-TuA-3 Electromechanical Tuning of Lasing Wavelength of Vertical Cavity Lasers F. Sugihwo (Stanford University); M.C. Larson (Hitachi, Japan); J.S. Harris (Stanford University) Vertical-cavity surface-emitting lasers (VCSELs) combine low power with array integration capability, which is useful for applications such as wavelength division multiplexing (WDM). Conventional WDM laser arrays consisting of staggered-wavelength nontunable laser diodes suffer from thermal cross-talk and environmental changes in addition to issues pertinent to long term reliability of the array caused by the non-interchangeability of devices within the array. Redundancy can be added at a cost; however, propagated redundancy throughout the signal path dramatically increases system expense. Wavelength tunable VCSELs circumvent these shortcomings since every component in the array is interchangeable with other components in the array. Due to their tunable nature, these lasers are also robust to environmental changes. We use a deformable membrane as the top mirror of the micromachined tunable VCSEL. The membrane is held by four flexible legs anchored to four rigid contact pads. The membrane is released by selectively etching an Al_0.85Ga_0.15As sacrificial layer. We use dilute HCl for the release etch since HCl has nearly infinite selectivity to the GaAs layer cladding the AlGaAs sacrificial layer. Overhanging photoresist rails are used to prevent membrane from sticking during release. The membrane is made of 686.8Å GaAs/2694.4Å Si₃N₄/2.5 pairs of 1260Å SiO₂ and Si₃N₄/1500Å Au. The membrane legs are intentionally thinned to reduce their stiffness which reduces the required tuning bias. By applying bias between the membrane contact pads and the substrate, we can electrostatically modulate the air gap between the membrane and the laser cavity. With the deformable membrane approach, we have fabricated tunable lasers with 30 nm continuous wavelength tuning range. Membrane bias range required is 14.1 V. Mechanical response of the deformable membrane has been characterized using both electrical and optical methods and is on the order of 1-2 microseconds.
3:00 PM		EM-TuA-4 Surface Reconstructions on Wurtzite GaN Studied Using STM A.R. Smith, R.M. Feenstra, D.W. Greve (Carnegie Mellon University); J. Neugebauer (Fritz-Haber-Institut der MPG, Germany); J.E. Northrup (Xerox Palo Alto Research Center) We report the first observations by scanning tunneling microscopy of surface reconstructions on MBE-grown wurtzite GaN surfaces. Using an RF plasma source to activate the nitrogen, a two-dimensional growth mode has been achieved on sapphire, resulting in a surface having large, atomically flat terraces separated by atomic height steps. We have identified four unique surface reconstructions on GaN(000-1): 1x1, 3x3, 6x6, and c(6x12). The 1x1 has the GaN c-plane lattice constant of 3.189 Å and acts as the starting surface for preparing the higher order reconstructions. The 1x1 is formed by annealing the as-grown film surface to a temperature above the Ga condensation temperature, which removes excess Ga atoms, most likely those not directly bound to a nitrogen atom in the last GaN layer. As Ga is deposited incrementally onto the 1x1 surface, the other reconstructions appear in the order previously indicated. First-principles calculations have been performed to identify the most theoretically plausible structural models, starting with the 1x1 reconstruction. The calculations have been performed including the Ga 3d electrons in the valence band. Out of several possibilities, the calculations most strongly support the model for the 1x1 as being a Ga adlayer sitting above the N-terminated face. Other possibilities, such as the Ga-terminated face or a Ga adlayer sitting above the Ga-terminated face, were found to be energetically unstable for all possible Ga chemical potentials. Work is also in progress to experimentally determine the polarity of the grown film.
3:20 PM		EM-TuA-5 Enhanced Schottky Barrier on InGaAs for High Performance Photodetector Application L. He, S. Liu (Northern Illinois University) Metal-Semiconductor-Metal (MSM) photodetectors have shown great potential in the application of optoelectronic integrated circuits (OEIC's) due to its planar nature. An important consideration in the design of high performance MSM photodetectors in the wavelength of 1.3 and 1.55µm is the metal Schottky barrier height. The material for the wavelength range is InGaAs. However, the Schottky barrier height to InGaAs is unfortunately low (about 0.2-0.3eV) which results in a large dark current. The conventional method to increase the metal/InGaAs Schottky barrier height is employing a very thin cap layer above InGaAs active layer, such as InP, InAlAs, GaAs, and AlGaAs, as a barrier-enhancement layer. It is believed that the additional layer, however, could slow the MSM photodetector response. From processing point of view, the additional layer also increase the technology complexity. In this paper, a newly developed technique, low temperature (LT = 77K) processing, was used for Schottky metal deposition on InGaAs. The metal/InGaAs Schottky barrier height was increased to be 0.7eV at an Ag/InGaAs contact comparing the same sample processed at room temperature with barrier height of 0.3eV. Computer simulations have been conducted based on the enhanced Schottky barrier height. The dark current density was reduced by the order of 6 which greatly increased the signal-to-noise ratio of the MSM device while maintaining high speed performance. Since the LT processing is of simple and convenient, it provide an excellent method to obtain high performance MSM photodetectors.
3:40 PM		EM-TuA-6 Study of SiO₂/GaAs Interface formed by UV Oxidation of Si on GaAs Z. Wang, D.M. Diatezua, H. Morkoc, A. Rockett (University of Illinois, Urbana-Champaign) The application of GaAs-based microelectronic devices has always been severely limited by the difficulties in growing a suitable insulator on a GaAs substrate with a minimum trap density at the interface. A breakthrough to improve the interface was the introduction of an interface controlled layer (ICL) of Si. However, the function of the ICL Si layer was questioned recently due to unsuccessful attempts to fabricate the devices using this technique. A logical solution to this problem was to eliminate the Si ICL after its growth and yet to preserve the interface with GaAs. In this study, a thin layer of Si was pseudomorphically grown on a p-GaAs (100) substrate. The Si layer was then oxidized under UV ozone for various lengths of time to consume the Si layer without damaging the substrate. The kinetics of the SiO₂ layer growth and interface information were obtained using X-ray photoelectron spectroscopy (XPS). The resulting SiO₂/GaAs structure was annealed in UHV and monitored in situ by XPS. The electrical properties of the interface were studied by capacitance-voltage and inductance-frequency techniques. Correlation is made between dielectric chemistry determined by XPS and trap densities found by C-V.
4:00 PM		EM-TuA-7 Causes of Surface States on Sulfide Passivated III-V Surfaces R.W.M. Kwok, W.H. Choy (The Chinese University of Hong Kong) Sulfide plays roles to assist the ordering of InP (100), GaAs (100) and GaAs (110) surfaces, to prevent the loss of group V elements during annealing, and to lessen of surface damage during dielectric or metal deposition. However, surface state densities of III-V semiconductors are still high when compared with the theoretical calculations of a perfectly sulfide terminated surface. In this study, the structural and chemical changes of the III-V surfaces induced by sequential processes of UV/O₃ exposure, HF etching, and solution phase or gas-phase polysulfide treatment were investigated by scanning tunneling microscopy (STM) in both air and ultrahigh vacuum. Studies were also performed using x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and thermal desorption spectrometry. The causes of the surface states after sulfide passivation were attributed to the surface roughness and the defective bonding on the III-V surfaces. The results give highlights to the routes for further reduction of surface states.
4:20 PM		EM-TuA-8 Deposition of Passivating GaS films on GaAs(100)-(1x1), (2x4), and (4x6) Surfaces using a Directional Exposure of ((t-Bu)GaS)₄* S.I. Yi, C.-H. Chung, W.H. Weinberg (University of California, Santa Barbara) Multilayer deposition of GaS on GaAs(100) surfaces has been studied using a directional exposure of a single source gallium cluster ((t-Bu)GaS)₄ in an ultra high vacuum chamber. Clean As-rich GaAs(100)-(1x1) and GaAs(100)-(2x4) surfaces are prepared by thermally desorbing arsenic capping overlayers. Thermal evolution of the hydrocarbon from the ((t-Bu)GaS)₄ deposited at a surface temperature of 100 K has been studied, using temperature programmed desorption and high resolution electron energy loss spectroscopy. The desorption species and the surface reactions are identified. The growth kinetics has been studied at varying surface temperatures via monitoring the sulfur coverage. The surface reconstruction is also examined as a function of both the exposure and the surface temperature, using low energy electron diffraction. For comparison, the deposition of ((t-Bu)GaS)₄ on a GaAs(100) surface with (2x1) sulfur overlayer formed by adsorption of H₂S has also been examined. The roles of the adsorption sites and the hydrocarbon removal in the multilayer growth mechanism are identified. The passivation performance of the resulting film has been examined against wet oxidation. *Supported by NSF Grant DMR-9504400.
4:40 PM		EM-TuA-9 Interaction of S with the GaAs(100) Surface and the Formation of Sulfide Thin Films X.-Y. Zhu (University of Minnesota); J. Yang, R.A. Hinton, Q.-S. Xin (Southern Illinois University) In an attempt to deposit a long term passivation layer on GaAs, we have investigated the extentive interaction of sulfur with the GaAs(100) surface. S atoms, generated from photodissociation or hot filament assisted dissociation of H₂S, react readily with the GaAs substrate in forming a sulfide thin film with film thickness ranging from nanometers to micrometers. We found that gallium sulfide is more stable than arsenic sulfide; the latter desorbs at temperatures below 800 K. As a result, the surface and near surface region consists exclusively of gallium sulfide. For thin sulfide films of a few nanometer thickness, the gallium sulfide layer is epitaxial to the GaAs(100) substrate. Both the kinetics of diffusive-controlled growth and the chemical nature of the sulfide thin film are determined.
5:00 PM		EM-TuA-10 Determination of 2-D Pair Correlations and Pair Interactions of In Atoms in Molecular Beam Epitaxially Grown InGaAs Alloys K.-J. Chao, C.K. Shih, D.W. Gotthold, B.G. Streetman (University of Texas, Austin) Intra- and inter- layer atom-atom correlations in molecular beam expitaxially grown dilute InGaAs alloys were studied using cross-sectional scanning tunneling microscopy. By imaging individual chemical constituents we construct a large ensemble of "atom maps" from which two-dimensional In-In pair correlation functions were deduced. We found a total absence of inter-layer pair correlation along [001] and a strong negative correlation for the nearest neighbor pair along [110]. In addition, a wea k long-range oscillation along [110] is also observed. From the pair correlations we further determine a repulsive interaction energy of 0.1 eV for the nearest neighbor In-In pairs along [110].