Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N18: Focus Session: Wide Band Gap Semiconductors III |
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Sponsoring Units: DMP Chair: William Schaff, Cornell University Room: LACC 406A |
Wednesday, March 23, 2005 8:00AM - 8:36AM |
N18.00001: Semiconductor-Dielectric Interfaces: Structure, Defects and Mobility Invited Speaker: The semiconductor-dielectric interface is the key to a successful MOSFET technology and has played the essential role in the silicon revolution. Wide-band gap materials have presented a challenge to achieve the same degree of interface perfection as silicon, although considerable progress is underway. The SiC/SiO$_{2}$ interface is of particular scientific interest in this development because of its close relationship to silicon, both in processing and structure. Silicon carbide itself provides an intriguing scientific platform for understanding such materials structures due to the availability of many poly-types with different band-gaps, access to different crystal faces-polar and non-polar-, and with a fabrication process that is similar to the well-studied silicon structure. This talk will review recent work in understanding in the oxidation process in SiC. We describe the nature of the (heavily) defected intrinsic interface, the use of chemical modification to reduce the interface defect density and the understanding of these processes that emerges employing varying band-gap and crystal-face, within the same bulk material. Systematic use of chemical modification and processing, combined with a careful analysis of interfacial structure, results in significant progress in reducing defects and increasing charge and inversion layer mobility. Collaborators: S. Dhar, J. Williams, S. Pantelides, L. Porter, J. Cooper Supported by DARPA Contract No. N00014-02-1-0628 and ONR DEPSCoR Grant N00014-01-1-0616. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N18.00002: III-Nitrides on Ferroelectric Lithium Niobate: Impact of the Electrostatic Boundary Condition Kyoung-Keun Lee, Gon Namkoong, Walter Henderson, W. Alan Doolittle Lithium niobate (LN) is a promising substrate for GaN high electron mobility transistors (HEMTs) for ``smart'' integrated electronics with optical modulators in fiber optical systems. Recent efforts\footnote{W. A. Doolittle, G. Namkoong, A. Carver, W. Henderson, and A. Brown, Proc. of Mat. Res. Soc. Fall, Boston MA, Dec. 2-6, Vol 743, L1.4, (2002)}\footnote{S. M. Madison, W. Henderson, K. M. Patel, G. Namkoong, K.-K. Lee, S. E. Ralph, and W. A. Doolittle, 2004 Dig. IEEE Lasers and Electro-Optic Soc. (LEOS) Sum. Top. Meetings, November 2004} have demonstrated GaN high electron mobility transistors using LN substrates and that the strong polarization discontinuity between the LN/GaN layers can be used to control the polarity of the GaN, including the demonstration of periodically poled GaN on periodically poled LN. Herein, the origin within the substrate, the fundamental nature and strength of the polarization discontinuity for determining the polarity of the GaN epitaxial layers are described. Kelvin force microscopy, x-ray diffraction and chemical etching studies show a strong correlation between LN inversion domain density resulting from non-uniform lose of Li$_{2}$0 from LN when heated in vacuum. This affects the polarization of III-Nitride films grown on these multi-domain LN substrates, strongly influencing the channel mobility due to polar scattering. Methods for reducing the inversion domain density in GaN/LN will be described and correlated with the HEMT channel mobility. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N18.00003: Integration of Functional Perovskites with (0001) GaN Venu Vaithyanathan, Aaron Fisher, Darrell G. Schlom, Paul J. Shlichta Hybrid structures in which the functional properties of oxides can be exploited in combination with semiconductors offer exciting opportunities for devices. Perovskite oxides exhibit a wide range of functional properties motivating their integration with (0001) GaN. We used two basic criteria to select functional perovskite oxides for integration with GaN: thermodynamic stability and lattice match. Using the NIST-ICDD Crystal Data database of about 150,000 inorganic compounds, a comprehensive lattice match search was performed between all orientations of all known oxide perovskites and the (0001) face of GaN. The best lattice match was for a \textit {$\sigma $}$_{3}$ boundary between the (111) pseudocubic perovskite plane and the (0001) plane of GaN. We also performed extensive thermodynamic stability calculations between all binary oxides and GaN. Our analysis led us to believe that (111) ~SrTiO$_{3}$ would be a good buffer layer to grow on (0001)~GaN, from which the transition to one of many functional perovskite oxides could be made. Extensive attempts to integrate epitaxial (111)~SrTiO$_{3}$ on (0001) GaN were, however, unsuccessful. As the surface of GaN often contains a thin Ga wetting layer, we calculated the thermodynamic stability of Ga with all binary oxides. In contrast to the thermodynamic stability of SrO in contact with GaN, SrO is found to be thermodynamically \textit{unstable} in contact with Ga while TiO$_{2}$ is stable in contact with Ga. This is consistent with our experimental observations during the deposition of SrTiO$_{3}$ on GaN. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N18.00004: Polarization Effects on Heterojunction Band Offset Measurements of Oxide-GaN Interfaces R.J. Nemanich, C.C. Fulton, B.R. Rodriguez, C. Liu, S. Cho, H. Morkoc XPS has been used to determine the band offsets of both polar oxide-polar nitride and non-polar oxide-polar nitride heterojunctions for both N-face and Ga-face GaN. In the case of polar oxide-polar nitride heterojunctions (specifically, ZnO-GaN), it is expected that the spontaneous polarizations will align across the interface. ZnO epitaxial layers were prepared by MBE on unintentially doped GaN films grown on sapphire substrates. The valence band heterojunction band offset was deduced from the difference in the core level positions where the valence band of each material was determined from prior XPS and UPS measurements of clean GaN layers and bulk ZnO. We found the ZnO valence band to be positioned below the GaN valence band at 1.85 and 1.05 for the Ga-face and N-face heterojunctions, respectively. Assuming Eg = 3.4 for GaN and Eg = 3.3 eV for ZnO, the Ga- and N-face conduction band offset are 1.95 and 1.15 eV, respectively. The interface dipole for each configuration is deduced based on the deviation from the electron affinity model, and the band offsets are analyzed in terms of the interface bonding and polarity. It is found that polarization effects play an significant role in the band alignment and interface electronic structure. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N18.00005: Effect of various surface treatments on optical and morphological characteristics of homoepitaxially overgrown GaN layers and device structures Fatemeh Shahedipour-Sandvik, Vibhu Jindal, James Grandusky, Muhammad Jamil Effect of ex-situ treatment on morphological and optical quality of homoepitaxially grown GaN layers as function of layer thickness is reported. Treatments include use of organic solvents, Hydrochloric and Hydrofluoric acids. The properties of the grown GaN on template layers were observed to vary significantly due to different surface treatments. For un-treated (surface blown dry with UHP N$_{2})$ samples morphology of the surface becomes smooth with RMS roughness of 0.6 nm shortly after nucleation and becomes as smooth as the initial surface (RMS=0.3nm) after 1 $\mu $m of overgrowth. The surface treated with HCl, on the other hand, shows increase in roughness initially and then recovers after the growth of 1 $\mu $m while the HF treated surface does not recover up to 1 $\mu $m of growth. In addition to the morphological changes, photoluminescence measurements are also presented which characterize the difference in optical properties of the layers as a function of layer thickness for different surface treatments. We will also present the effect of various treatments on performance of violet light emitting diodes. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N18.00006: Reconstructions and adsorbates on polar and nonpolar GaN surfaces David Segev, Chris G. Van de Walle Recently, exciting progress has been made in the ability to grow GaN in nonpolar orientations. We report on a systematic study of the reconstructed GaN $a$ (1120), $m$ (1010) and c (0001) planes. Using first- principles calculations, based on density-functional theory within the local density approximation (LDA), we examine the structural and electronic dissimilarities between the polar and the nonpolar surfaces. Adatom energetics and doping issues have been also investigated for the different surfaces considered. We propose a simple way to overcome the band-gap underestimation problem inherent in density-functional theory. New features will be presented, in light of the available experimental data. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N18.00007: Self-heating study of an AlGaN/GaN-based high electron mobility transistor using visible and ultraviolet micro-Raman scattering I. Ahmad, V. Kasisomayajula, J.M. Berg, M. Holtz, S.R. Kurtz, C.P. Tigges, A.A. Allerman, A.G. Baca We report micro-Raman studies of self-heating in an AlGaN/GaN heterostructure field effect transistor using both visible (488.0 nm) and ultraviolet (363.8 nm) excitations. The $<$ 100 nm optical penetration depth of the UV light allows us to measure temperature rise ($\Delta T$) in the two-dimensional electron gas (2DEG) region of the device between source and drain, while visible light gives us the average $\Delta T$ in the GaN layer and that of the SiC substrate at the same lateral position. Combined, we depth profile the self-heating in the device. Measured $\Delta T$ in the 2DEG is consistently over twice the average GaN-layer value. Simulations are performed to describe the electrical behavior of the device. The results of electrical simulations are used for thermal simulations to describe the thermal behavior of the device. The presence of a hotspot, located at the edge of the gate in the 2DEG on drain side was observed. The measured temperature rise is related to the growth of the hotspot. Excellent agreement between experimental results and simulations is produced over the wide range of operating conditions. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N18.00008: Air-Stable Field-Enhanced III-Nitride Photocathodes Robert Strittmatter, Jordana Blacksberg, Shouleh Nikzad, Amir Dabiran, Andrew Wowchak, Peter Chow We report on recent investigations of Si delta-doping by molecular beam epitaxy (MBE) near the surface of p-type GaN films to attain high efficiency photocathodes for use in intensified ultraviolet imagers. These delta-layers are prepared to achieve effective negative electron affinity (NEA) without the use of low work function metal coatings, such as cesium, which are suitable only in ultra-high vacuum environments. Hall measurements, secondary ion mass spectrometry (SIMS), and capacitance-voltage (C-V) depth profiling reveal highly confined delta-layers with activated carrier densities in excess of 10$^{14 }$cm$^{-2}$ as close as 2 nm from the semiconductor surface. When the delta-layer is biased relative to the bulk, a large field-enhancement of the photoelectron yield is observed. In addition to UV spectroscopic quantum efficiency data, we will present total electron yield measurements for these photocathodes under electron-beam bombardment at various incident energies. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N18.00009: Isostructural phase transitions in GaN/ScN and InN/ScN superlattices V. Ranjan, S. Bin-Omran, L. Bellaiche, Ahmad Alsaad We predict the existence of pressure-induced isostructural phase transitions (IPTs) in GaN/ScN and InN/ScN superlattices from first principles. The IPTs in these superlattices are anomalous in the sense that they are associated with trivial order parameters and generate a dramatic change in many physical quantities. Furthermore, the {\it order} of the phase transition is found to be dependent on the superlattice period and on the non-transition-metal cation. We also reveal the reason behind, and consequences of, these unusual dependencies and IPTs. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N18.00010: Atomic and Electronic Structure of Polar Nitride/Oxide Interfaces: h-GaN(0001) and c-GaN(111) on MgO(111) Marija Gajdardziska-Josifovska, Vlado Lazarov, Justin Zimmerman, Yi Rong, Sau Ha Cheung, Michael Weinert, Lian Li Polarity can play an important role in atomic and electronic structures of surfaces and interfaces. In this work we show that MgO(111) surface polarity can be used as a parameter for controlled growth of both the hexagonal and the energetically less favorable cubic phase of GaN by electron- cyclotron resonance (ECR) plasma-assisted molecular beam epitaxy (MBE). The growth of cubic (111) (or hexagonal (0001)) GaN is achieved when N (or Ga) is first deposited on the polar MgO(111)-(1$\times $1) surface. High resolution transmission electron microscopy (HRTEM) and density functional theory (DFT) studies indicate that the cubic GaN(111)/MgO(111) interface structure is determined by Mg-O-N-Ga stacking, with each N atom bonded to O at top site. This specific atomic arrangement at the interface allows cubic stacking to more effectively screen the substrate and film electric dipole moment then the hexagonal stacking. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N18.00011: Defect engineering in Si substrate for strain reduction at GaN/Si interface Muhammad Jamil, James Grandusky, Fatemeh Shahedipour-Sandvik We report on a novel method that could potentially reduce the high dislocation density and cracks caused by lattice and thermal mismatch strain at the GaN/Si interface. In this method nitrogen ion implantation at an energy of 75 keV and various doses is employed to cause defects in the substrate allowing higher freedom for realignment of AlN nuclei at the interface. Although a great number of misfit dislocations are formed at the interface due to lattice mismatch, screw dislocations are mostly formed at the grain boundaries. We will report on optical and morphological measurements on over 50 GaN/Si samples, performed to elucidate the potential for this technique. Our preliminary results indicate high crack reduction of the GaN layer to crack distances of 120 $\mu $m in 2$\mu $m thick GaN layers for samples undergoing ion implantation and defect annealing scheme. Photoluminescence measurements at room temperature show an increase in bandedge to yellow luminescence ratio indicative of higher quality of the layers. [Preview Abstract] |
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