Browsing by Author "Zlatko Sitar, Member"

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A Path Towards GaN-based Vertical Superjunction Devices.
(2021-11-04) Khachariya, Dolar; Spyridon Pavlidis, Chair; Ramon Collazo, Member; B. Baliga, Member; Zlatko Sitar, Member
Barrier Properties of Plasma Deposited SiOx Transparent Coatings on Polymer Substrates
(1998) Erlat, Ahmet Gün; Richard Spontak, Chair; Benny Freeman, Member; Zlatko Sitar, Member
Charaterization of the Growth of Aluminum Nitride and Gallium Nitride Thin Films on Hydrogen Etched and/or cleaned 6H-SiC(0001) Surfaces.
(2000-10-16) Hartman, Jeffrey David; Robert F. Davis, Chair; Robert J. Nemanich, Co-Chair; Zlatko Sitar, Member; Salah M. Bedair, Member
The surface morphology and atomic structure of nitrogen doped, n-type 6H-SiC(0001)Si wafers before and after various surface preparation techniques were investigated. As-received wafers were exposed to in-situ cleaning with or without excess silicon to obtain either a (rt3 x rt3)R30° or a (3 x 3) reconstructed surface. The resulting surfaces were characterized using reflection high-energy electron diffraction, photo-electron emission microscopy, and atomic force microscopy. An atomically clean, reconstructed surface was obtained via thermal annealing at 950°C. Cleaning with excess silicon resulted in the formation of silicon islands on the surface. The surface morphology of hydrogen etched wafers depended upon their doping concentrations. Wafers with doping concentrations of greater than or equal 2.5 x 10E18 and less than 7 x 10E17 (ND-NA)/cm3 were investigated with the former exhibiting more surface features. The microstructure of all the samples showed regions with full and half unit cell high steps. An atomically clean, ordered, stepped surface was achieved via annealing at 1030 degrees Celcius. Chemical vapor cleaning resulted in the formation of silicon islands. The initial growth of AlN and GaN thin films on the cleaned, hydrogen etched 6H-SiC(0001) substrates were investigated using PEEM and AFM. The AlN films nucleated immediately and coalesced, except in the areas of the substrate surface which contained half unit cell height steps where pits were observed. The GaN films grown at 800°C for 2.5 minutes exhibited nucleation and three-dimensional growth along the steps. The GaN films deposited at 700° C for 2 minutes grew three-dimensionally with coalescence of the film dependent upon the step structure. Almost complete coalescence occurred in regions with unit cell high steps and incomplete coalesce occurred in regions with half unit cell height steps. Films of AlN grown for 30 minutes via GSMBE on hydrogen etched surfaces exhibited two-dimensional growth and had an RMS roughness value of 4 Å. Films grown at 1000 ° C exhibited an SK growth mode and had rocking curve FWHM of 150-200 arcsecs. MOCVD grown films on hydrogen etched wafers had an RMS roughness value of 4 Å and a XRD rocking curve FWHM of approximately 260 acrsecs.
Deposition and Electrical, Chemical and Microstructural Characterization of the Interface Formed between Pt, Au and Ag Rectifying Contacts and Cleaned n-type GaN (0001) Surfaces.
(2000-09-27) Tracy, Kieran M; Robert F. Davis, Chair; Robert J. Nenanich, Co-Chair; Robert M. Kolbas, Member; Zlatko Sitar, Member
The characteristics of clean n-type GaN surfaces and the interface between this surface and Pt, Au and Ag, have been investigated. Gallium-terminated (0001) surfaces of GaN, free of carbon and oxygen within the detection limits of XPS have been achieved by annealing in ammonia at 860°C for 15 minutes. Additional, in-situ surface analysis indicated a flat, stoichometric, and unreconstructed surface free of other contaminants. The electron affinity of this surface was 3.1 ± 0.2 eV. The valence band maximum was located 3.0 ± 0.1 eV below the Fermi level, indicating the presence of a surface state near the valence band maximum. Individual layers of Pt, Au or Ag were deposited in-situ on the cleaned surface and the interfaces characterized using XPS, UPS, LEED and TEM. All as-deposited metal/GaN interfaces were abrupt and unreacted; the Pt and Au were deposited epitaxially. The Schottky barrier heights obtained from photoemission measurements were 1.2, 0.9 and 0.5 ± 0.2 eV for Pt, Au and Ag, respectively. Values of the metal work function from UPS results were 5.7, 5.3 and 4.4 ± 0.2 eV for Pt, Au and Ag, respectively. Schottky barrier heights determined via ex-situ current-voltage measurements were 1.15, 0.88 and 0.56 ± 0.05 eV for Pt, Au and Ag, respectively. Capacitance-voltage measurements yielded barrier heights of 1.25 and 0.96 ± 0.05 eV, for Pt and Au, respectively. These results indicate that the Fermi level of the cleaned surface is not pinned. Upon annealing the aforementioned contacts from 400 to 800°C for 3 minutes each. The rectifying behavior of the Pt and Au contacts degraded as a function of temperature during annealing at 400, 600 and 800°C for 3 minutes each until they became ohmic. This was correlated with TEM of the annealed interfaces, which displayed increased chemical reaction and roughening as a function of temperature.
Development of Device-quality InGaN Templates for use in Long Wavelength LEDs and Solar Cells.
(2024-03-28) Routh, Evyn Lee; Salah M. Bedair, Chair; Ramon Collazo, Member; Zlatko Sitar, Member; Claude Reynolds Jr, Member
Engineering Light Matter Interaction of Atomically Thin Transition Metal Dichalcogenide Materials.
(2016-08-18) Yu, Yiling; Linyou Cao, Chair; Kenan Gundogdu, Member; David Aspnes, Member; Zlatko Sitar, Member
Epitaxy of Polar Oxides and Semiconductors.
(2016-08-16) Shelton, Christopher Tyrel; Jon-Paul Maria, Chair; Daryoosh Vashaee, Member; Zlatko Sitar, Member; Jagdish Narayan, Member
GaN-Based Heterogeneous Heterostructure Devices.
(2023-05-25) Sengupta, Rohan; Spyridon Pavlidis, Chair; Frederick Kish, Member; Zlatko Sitar, Member; Ramon Collazo, Member
Gate Stack Design for Threshold Voltage Control of Gallium Nitride Power Transistors.
(2013-03-19) Kirkpatrick, Casey Joe; Veena Misra, Chair; Mehmet Ozturk, Member; Alex Huang, Member; Zlatko Sitar, Member
Investigation of ALD Dielectrics for Improved Threshold Voltage Stability and Current Collapse Suppression in AlGaN/GaN MOS-HFETs.
(2014-09-10) Ramanan, Narayanan; Veena Misra, Chair; Griff Bilbro, Member; John Muth, Member; Zlatko Sitar, Member
Modeling and Application of a Back-to-Back Diodes-Based Linear Variable Capacitor in Resonant Converters.
(2024-08-12) Pratik, Ujjwal; Zeljko Pantic, Chair; Srdjan Lukic, Member; Douglas Hopkins, Member; Jonathan Wierer, Member; Zlatko Sitar, Member
Modeling of Multistage Nernst-Ettingshausen Coolers.
(2018-04-02) Polash, Md. Mobarak Hossain; Daryoosh Vashaee, Chair; Zlatko Sitar, Member; Mehmet Ozturk, Member
N- and P-type Nano Bi(Sb)Te(Se) Alloy Materials with Enhanced Thermoelectric Figure of Merit and Device Performance.
(2012-09-21) Chan, Tsung-Ta; Carl Koch, Chair; Ronald Scattergood, Member; Zlatko Sitar, Member; Salah M. Bedair, Member
Point Defect Identification and Management for Sub-300 nm Light Emitting Diodes and Laser Diodes Grown on Bulk AlN Substrates
(2015-06-22) Bryan, Zachary A; Ramon Collazo, Chair; Zlatko Sitar, Member; Douglas Irving, Member; Mehmet Ozturk, Minor
Structural and Microstructural Characterization of III-Nitrides on 6H-SiC (0001) Substrates.
(2001-06-11) Preble, Edward Alfred; Robert F. Davis, Chair; Zlatko Sitar, Member; John Hren, Member; Mohamed Bourham, Member
Characterization of nitride films on 6H-SiC (0001) wafers via x-ray, TEM, and AFM was accomplished on standard GaN thin films with AlN or AlGaN buffer layers. TEM sample thinning capability was improved through the use of Nomarski in an optical microscope to gauge the thickness of the sample during preparation. TEM analysis was then completed of Au and Pt films deposited on chemical vapor cleaned GaN with annealed up to 800&$176;C. Chemical reactions were detected in x-ray measurements of the 800°C Pt samples and GaN/metal interface roughening were confirmed by TEM images in both metals. Interface roughening is attributed to the chemical reactions and interfacial stresses greater than the yield stress of the metal created during heat treatments by the difference in the thermal expansion coefficients of the GaN and the metals. The GaN rocking curves were found to track very closely to the values of the underlying substrate and changes in buffer layer growth temperatures were found to change the screw and edge dislocation populations of subsequent GaN layers. GaN grown on 1030°C AlN buffer layers showed the lowest edge dislocation populations when compared against buffers grown in the range of 1010-1220°C, even though the 1220°C AlN was much smoother. AlGaN buffer layers provided more edge dislocation reduction, with a 1090°C Al0.2Ga0.8N layer yielding the best GaN rocking curve values found in this work. GaN films with AlN buffer layers grown on hydrogen etched SiC substrates did not show rocking curve improvement when compared against samples with unetched substrates. The AlN layers showed extremely narrow, substrate limited, on-axis rocking curve values, but it is not clear as to whether additional defects are present that may broaden the off-axis rocking curves, causing the poorer results seen in the GaN films. Reciprocal space maps of uncoalesced, maskless pendeo epitaxy samples revealed that the wing regions are shielded from poor substrate material when compared against the seed material. The wing regions also have lower strain and rocking curve widths than the corresponding seed material.
Transition Metal Oxides for Infrared Optoelectronics
(2015-03-31) Sachet, Edward; Jon-Paul Maria, Chair; Stefan Franzen, Member; Zlatko Sitar, Member; Douglas Irving, Member