Browsing by Author "Dr. Robert F. Davis, Committee Chair"

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Conventional and Pendeo-Epitaxial Growth of Non-Polar GaN(11-20) Thin Films on AlN/4H-SiC(11-20) Substrates and Their Characterization and Reduction in Defect Density
(2005-11-09) Wagner, Brian P.; Dr. Robert F. Davis, Committee Chair
Non-polar GaN(11-20) thin films were deposited on AlN/4H-SiC(11-20) substrates via metalorganic vapor phase epitaxy. Atomic force microscopy images revealed that the microstructure of the AlN buffer layer and the subsequently deposited GaN contained a highly oriented growth structure where parallel growth features were aligned in the [1-100] direction. Scanning electron microscopy showed that the interfaces between the substrate, buffer layer, and epi-layer were continuous. Cracking was observed in GaN films having a thickness greater than 800 nm. Planview transmission electron microscopy analysis revealed stacking faults and threading dislocations with densities of ~1.6x10ˆ6cmˆ-1 and ~ 3.3x10ˆ10cmˆ-1, respectively. X-ray diffraction confirmed that the GaN was deposited epitaxially in the same orientation as the substrate. The average on- and off-axis x-ray full-width half-maxima of the (11-20) and the (10-10) reflections were 948 arcsec and 5448 arcsec, respectively. Coalesced, non-polar GaN(11-20) ( films having volumes of material with reduced densities of dislocations and stacking faults has been achieved from etched stripes via the statistical and experimental determination of the effect of temperature and V/III ratio on the lateral and vertical growth rates of the GaN{0001} faces combined with pendeo-epitaxy. AFM of the uncoalesced GaN(0001) and GaN(000-1) faces revealed growth steps with some steps terminating at dislocations on the former and a pitted surface without growth steps, indicative of decomposition, on the latter. Coalescence was achieved via (a) a two-step route and the parameters of (1) T=1100°C and V/III=1323 for 1.5hrs and (2) 1020°C and V/III= 1323 for 1.5hrs and (b) a one-step route that employed T = 1020°C and a V/III ratio = 660 for six hours. The densities of dislocations in the GaN grown vertically over and laterally from the (11-20) stripes were ~4x10ˆ10 cmˆ-2 and ~2x10ˆ8 cmˆ-2, respectively; the densities of stacking fault in these volumes were ~1x10ˆ6 cmˆ-1 and ~2x10ˆ4cmˆ-1, respectively. Plan view AFM also revealed different microstructures and a reduction in the RMS roughness values from 1.2nm to 0.95nm in these respective regions.
Electrical, Chemical, and Structural Characterization of Au Schottky Contacts on Remote Plasma-Treated n-Type ZnO[0001]
(2003-10-15) Coppa, Brian Joseph; Dr. Robert F. Davis, Committee Chair
In situ cleaning procedures for ZnO( ) and ZnO(0001) surfaces have been developed and their efficacy evaluated in terms of the removal of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO( ) in pure oxygen at (600-650°C)±20°C and 0.100±0.001 Torr for 30 min. reduced but did not eliminate all detectable hydrocarbon contamination. However, annealing under similar conditions at 700°C±20°C for 15 min. caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits of X-ray photoelectron spectroscopy. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO( ) surface to a remote plasma containing an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, and 0.050 Torr resulted in smooth, highly-ordered, atomically-stepped and stoichiometric surfaces with a nearly flat electronic band structure. A 0.5 eV change in band bending was attributed to the significant reduction of the accumulation layer associated with surface contamination. The hydrocarbons were undetectable and only ~0.4±0.1 ML of hydroxide remained. The latter appeared to be more tightly bound to the ZnO(0001) surface due to unfilled dangling bonds associated with the polarity of this surface. This effect increased the optimal time and temperature of the plasma cleaning process to 60 min. and 550 °C, respectively, at 0.050 Torr. Approximately 0.4±0.1 ML of hydroxide also remained on this surface. Current-voltage results of Au contacts deposited on as-received, n-type ZnO(0001) surfaces showed an ~0.01 A/cm2 leakage current density to 4.6 V reverse bias and ideality factors (n) >2 before sharp, permanent breakdown. This behavior was due primarily to the presence of hydroxide on this surface, which typically increases the surface conductivity and forms an accumulation layer. Cooling in the plasma ambient caused the chemisorption of oxygen and the formation of a depletion layer of lower surface conductivity. These process steps produced smooth, highly-ordered, stoichiometric ZnO(0001) surfaces that possessed 0.3 eV of upward band bending. Sequentially deposited, unpatterned Au and the most rectifying gold contacts initially grew on these surfaces via the formation of islands that subsequently coalesced. The latter displayed a barrier height of 0.71±0.05 eV, a saturation current density of 4.04 µA/cm2, a value of n = 1.17±0.05, a significantly lower leakage current density of ~0.1 mA/cm2 to 8.5 V reverse bias, and a sharp, permanent breakdown at ~ –8.75 V. All measured barrier heights were lower than the predicted Schottky-Mott value of 1.0 eV, indicating that the interface structure and the associated interface states affect the Schottky barrier. However, the constancy in the FWHM of the core levels for Zn 2p (1.9±0.1 eV) and O 1s (1.5±0.1 eV), before and after sequential in situ Au depositions indicated an abrupt, unreacted Au/ZnO(0001) interface. Similar results were obtained for Schottky contacts deposited on remote plasma treated ZnO( ) surfaces, which possessed a step and terrace microstructure. Analysis of transmission electron microscopy results revealed the growth of epitaxial, single crystal Au forming an abrupt, unreacted interface.
Polytype Stability, Microstructural Evolution, and Impurities at the Interface of Homoepitaxial 4H-SiC(1120) Thin Films Grown via Hot-Wall Chemical Vapor Deposition.
(2007-06-04) Bishop, Seann; Dr. Robert F. Davis, Committee Chair; Dr. Lew Reynolds, Committee Member; Dr. Raoul Schlesser, Committee Member; Dr. John Muth, Committee Member; Dr. Nadia El-Masry, Committee Member
The electronic properties of 4H-SiC make it a leading semiconductor material for high-power applications. Despite advances in SiC crystal growth, devices fabricated in 4H-SiC(0001) continue to be limited by defects like micropipes, dislocations and stacking faults. Investigations performed here on non-basal 4H-SiC have demonstrated micropipe-free, a-plane 4H-SiC films comparable with conventional 4H-SiC(0001). Improved device performance has been achieved for p-i-n rectifiers fabricated in a-plane 4H-SiC. Hot-wall chemical vapor deposition (CVD) is the most widely used growth process employed to meet the material demands for 4H-SiC-based high-power electronics; however, the suitability of this technique for the growth of thin films (<10 μm) is not well established. A detailed analysis of the epitaxial growth of a-plane 4H-SiC thin films by vertical, hot-wall CVD has been performed in this work. Two growth regimes were identified and termed sublimation growth and precursor growth. In the former, the SiC coating decomposes and results in the in-situ (sublimation) growth of epitaxial SiC films. It is proposed that in-situ layer aids in subsequent thin film growth from reactant gases. Transmission electron microscopy revealed areas without observable defects and an indistinguishable interface between the substrate and sublimation grown layer. Aluminum impurity concentrations to 3E18 per cc. were identified near the interface with the substrate. The influence of these impurities on the cathodoluminescence spectrum of 4H-SiC was studied. A model based on boundary layer theory was developed to explain the origin and the profile of the aluminum impurities. Secondary ion mass spectrometry revealed the SiC coating to be the major source of aluminum impurities. An argon diluent reduced the concentration of aluminum at the interface to 2E17 per cc. Films deposited via precursor growth exhibited specular surfaces. Optical and structural characterization showed the films replicate the polytype of the underlying substrate exactly. The microstructural evolution of a-plane 4H-SiC and III-nitride films was also investigated. 4H-SiC films showed evidence of step-flow growth in the sublimation-driven regime, while both step-flow and terrace nucleation and growth were observed in the precursor-driven regime. AlN initially grew via the Stranski-Krastanov mode on 4H-SiC , while the Volmer-Weber mode was observed for GaN on AlN.
Transmission Electron Microscopy Preparation and Characterization of Bulk ZnO and Au(111) / ZnO(000-1) Heterostructures
(2003-11-26) Kiesel, Sharon Mary; Dr. Robert F. Davis, Committee Chair; Dr. N. A. El-Masry, Committee Member; Dr. Salah Bedair, Committee Member
Several preparation techniques were investigated, including (a) dimpling and ion milling, (b) hand-held tripod polishing and (c) automatic tripod polishing, to prepare electron transparent samples of ZnO suitable for transmission electron microscopy (TEM). Thinned ZnO became prone to cracking below a thickness of ~20 microns during the use of each of these techniques. The extension of cracks, supposedly generated at micropores, was believed to be the cause of the brittle behavior of the material. A repeat dimple process was introduced to ameliorate this problem. In this procedure, the sample was dimpled while it is >200 microns, polished from the backside and then redimpled with a felt-polishing wheel. Ion milling with a low beam current (2-4), a low angle (8°-13°) and the use of a liquid nitrogen cold stage to reduce ion mill damage was subsequently employed. The investigation of an additional technique namely, focused ion beam thinning, generated high densities of defects in the ZnO that were extrinsic to the material and which prevented TEM studies of these samples. Micrographs of ZnO produced by the repeat dimple procedure confirmed the presence of dislocations and stacking faults in the material. TEM studies of gold contacts on the ZnO wafers confirmed epitaxial growth of the former and the delamination of the contacts during cooling from annealing in air at 175 degrees for 15 minutes.