Browsing by Author "Zlatko Sitar, Committee Member"
Now showing 1 - 8 of 8
- Results Per Page
- Sort Options
- The Chemistry and Surface Microstructure of Si-Based Substrates and their Effect on the Evolution of the Microstructures of III-Nitride Films Grown via Metalorganic Vapor Phase Epitaxy(2005-03-31) Reitmeier, Zachary J; Robert F. Davis, Committee Chair; Zlatko Sitar, Committee Member; Mark Johnson, Committee Member; John Muth, Committee MemberThe present research was undertaken with the goals of understanding the evolution of defects and strain in heteroepitaxial AlN and GaN films deposited via metalorganic vapor phase epitaxy and minimizing those defects through manipulation of the substrate. As observed with atomic force microscopy (AFM), AlN initially grew in the form of flat-topped islands on as-received SiC substrates. Threading dislocations (TDs) observed in transmission electron microscopy (TEM) images initiated at the AlN/SiC interface as the result of defects at the surface of the mechanically polished substrate and/or condensation of point defects. GaN initially grew in the Stranski-Krastanov mode on AlN/SiC before transitioning to the dislocation-mediated step flow mode. The TDs in GaN resulted from the propagation of the TDs present in the AlN layer. The biaxial strain in the GaN layers varied with buffer layer material and layer thickness yet all samples investigated remained in residual compression due to incomplete relaxation of the coherent strain. The presence of strain during the initial growth of Al[subscript x]Ga[subscript 1-x]N layers directly on as-received SiC also resulted in phase-separated regions of Al-rich and Al-poor film. A high temperature hydrogen etch was then used to remove mechanical polishing scratches from the SiC substrates. Subsequently deposited AlN layers featured reduced pit density and the elimination of scratch-induced undulations. GaN layers deposited with AlN buffer layers on these substrates resulted in slightly reduced TD densities as observed by AFM, TEM, and high resolution X-ray diffraction (HRXRD). Regions of dramatically reduced dislocation densities were observed by HRXRD, TEM, and cathodoluminescence for GaN layers on stripe-patterned Si substrates. However, long growth times resulted in outdiffusion of Si from the substrate and subsequent film roughening. Finally, it was demonstrated that the presence of ammonia during heating of GaN templates to the growth temperature for homoepitaxy resulted in removal of carbon- and oxygen-based contaminants from the template surface.
- Direct Bonding of Gallium Nitride to Silicon Carbide: Physical, and Electrical Characterization(2003-08-08) Lee, Jaeseob; Zlatko Sitar, Committee Member; Salah M. Bedair, Committee Member; Robert. J. Nemanich, Committee Chair; Robert F. Davis, Committee Co-ChairThe direct bonding method is applied to the GaN/SiC system, and the processing conditions for successful direct bonding are clarified. Direct bonding of GaN/SiC is achieved at 900°C. The direct bonding of GaN to Si-face SiC is very dependent on the choice of chemical treatments, but the bonding of GaN to C-face SiC is less dependent on surface preparation. If a native oxide is present when the bonded interface is prepared, the current through the interface is decreased, which is attributed to an energy barrier due to the presence of charged interface states. TEM images indicate 10nm spaced dislocations at the interface for the GaN/SiC (Si-face), and ~6nm for the GaN/SiC (C-face), which form to accommodate the lattice mismatch (3.4%) and twist (1~2°) and tilt misfit (0.2° for Si-face SiC and 3° for C-face SiC). In some regions (~30%) an amorphous oxide layer forms at the interface, which is attributed to inadequate surface preparation prior to bonding. The strain of the GaN film with a Ga/C interface was ~0.1%, tensile strain, and that of GaN with a Ga/Si interface was ~0.2%, tensile strain. Our analysis indicates that the GaN/SiC thermal misfit dominates the strain of the GaN after bonding. The electrical characteristics of n-p GaN/SiC heterojunctions display diode ideality factors, saturation currents, energy barrier heights, and band offsets of 1.5 ± 0.1, 10⁻¹³ A/cm₂, 0.75 ± 0.10 eV, and ΔE[subscript c]= 0.87 ± 0.10 eV for the Ga/Si interface and 1.2 ± 0.1, 10⁻¹⁶ A/cm² , 0.56 ± 0.10 eV, and ΔE[subscript c]= 0.46 ± 0.10 eV for the Ga/C interface.
- FINAL Microstructure-Property Study of Cu1-xTax Thin Films(2007-07-26) Jackson, Shannon Marie; J.M. Rigsbee, Committee Chair; Zlatko Sitar, Committee Member; Afsaneh Rabiei, Committee MemberA series of Cu1-xTax (x = 0.05 to 0.5) thin films have been produced by DC magnetron sputter deposition using co-deposition (alloy) and sequential (layered) deposition modes at ambient temperature. The nanoscale microstructures of these non-equilibrium "alloy" films have been investigated chemically and structurally using x-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The mechanical properties were measured via nanotriboligical and nanoindentation studies of selected samples to determine the affect of Ta composition and nanolayer thickness.
- Interfacing Epitaxial Oxides to Gallium Nitride(2008-08-19) Losego, Mark Daniel; Jon-Paul Maria, Committee Chair; Mark Johnson, Committee Member; Zlatko Sitar, Committee Member; Gregory Parsons, Committee MemberMolecular beam epitaxy (MBE) is lauded for its ability to control thin film material structures at the atomic level. Controlling the chemistry and structure of epitaxial interfaces at the atomic level can improve performance of microelectronics and cultivate the development of novel device structures. This thesis explores the utility of MBE for designing interfaces between oxide epilayers and the wide band gap semiconductor gallium nitride (GaN). The allure of wide gap semiconductor microelectronics (like GaN, 3.4 eV) is their ability to operate at higher frequencies, higher powers, and higher temperatures than current semiconductor platforms. Heterostructures between ferroelectric oxides and GaN are also of interest for studying the interaction between GaN's fixed polarization and the ferroelectric's switchable polarization. Two major obstacles to successful integration of oxides with GaN are: (1) interfacial trap states; and (2) small electronic band offsets across the oxide / nitride interface due to the semiconductor's large band gap. For this thesis, epitaxial rocksalt oxide interfacial layers (˜8 eV band gap) are investigated as possible solutions to overcoming the challenges facing oxide integration with GaN. The cubic close-packed structure of rocksalt oxides forms a suitable epitaxial interface with the hexagonal close-packed wurtzite lattice of GaN. Three rocksalt oxide compounds are investigated in this thesis: MgO, CaO, and YbO. All are found to have a (111) MO || (0001) GaN; <1`10> MO || <11`20> GaN epitaxial relationship. Development of the epilayer microstructure is dominated by the high-energy polar growth surface (drives 3D nucleation) and the interfacial symmetry, which permits the formation of twin boundaries. Using STEM, strain relief for these ionicly bonded epilayers is observed to occur through disorder within the initial monolayer of growth. All rocksalt oxides demonstrate chemical stability with GaN to >1000°C. Concurrent MBE deposition of MgO and CaO is known to form complete solid solutions. By controlling the composition of these alloys, the oxide's lattice parameter can be engineered to match GaN and reduce interfacial state density. Compositional control is a universal challenge to oxide MBE, and the MgO-CaO system (MCO) is further complicated by magnesium's high volatility and the lack of a thermodynamically stable phase. Through a detailed investigation of MgO's deposition rate and subsequent impact on MCO composition, the process space for achieving lattice-matched compositions to GaN are fully mapped. Lattice-matched compositions are demonstrated to have the narrowest off-axis rocking curve widths ever reported for an epitaxial oxide deposited directly on GaN (0.7° in f-circle for 200 reflection). Epitaxial deposition of the ferroelectric (Ba,Sr)TiO3 by hot RF sputtering on GaN surfaces is also demonstrated. Simple MOS capacitors are fabricated from epitaxial rocksalt oxides and (Ba,Sr)TiO3 layers deposited on n-GaN substrates. Current-voltage measurements reveal that BST epilayers have 5 orders of magnitude higher current leakage than rocksalt epilayers. This higher leakage is attributed to the smaller band offset expected at this interface; modeling confirms that electronic transport occurs by Schottky emission. In contrast, current transport across the rocksalt oxide ⁄ GaN interface occurs by Frenkel-Poole emission and can be reduced with pre-deposition surface treatments. Finally, through this work, it is realized that the integration of oxides with III-nitrides requires an appreciation of many different fields of research including materials science, surface science, and electrical engineering. By recognizing the importance that each of these fields play in designing oxide ⁄ III-nitride interfaces, this thesis has the opportunity to explore other related phenomena including accessing metastable phases through MBE (ytterbium monoxide), spinodal decomposition in metastable alloys (MCO), how polar surfaces grown by MBE compensate their bound surface charge, room temperature epitaxy, and the use of surface modification to achieve selective epitaxial deposition (SeEDed growth).
- Processing Science of Barium Titanate(2009-04-22) Aygun, Seymen Murat; Gregory Parsons, Committee Member; Zlatko Sitar, Committee Member; Jon-Paul Maria, Committee Chair; Yuntian Zhu, Committee MemberBarium titanate and barium strontium titanate thin films were deposited on base metal foils via chemical solution deposition and radio frequency magnetron sputtering. The films were processed at elevated temperatures for densification and crystallization. Two unifying research goals underpin all experiments: 1) To improve our fundamental understanding of complex oxide processing science, and 2) to translate those improvements into materials with superior structural and electrical properties. The relationships linking dielectric response, grain size, and thermal budget for sputtered barium strontium titanate were illustrated. (Ba0.6Sr0.4)TiO3 films were sputtered on nickel foils at temperatures ranging between 100-400 °C. After the top electrode deposition, the films were co-fired at 900 °C for densification and crystallization. The dielectric properties were observed to improve with increasing sputter temperature reaching a permittivity of 1800, a tunability of 10:1, and a loss tangent of less than 0.015 for the sample sputtered at 400 °C. The data can be understood using a brick wall model incorporating a high permittivity grain interior with low permittivity grain boundary. However, this high permittivity value was achieved at a grain size of 80 nm, which is typically associated with strong suppression of the dielectric response. These results clearly show that conventional models that parameterize permittivity with crystal diameter or film thickness alone are insufficiently sophisticated. Better models are needed that incorporate the influence of microstructure and crystal structure. This thesis next explores the ability to tune microstructure and properties of chemically solution deposited BaTiO3 thin films by modulation of heat treatment thermal profiles and firing atmosphere composition. Barium titanate films were deposited on copper foils using hybrid-chelate chemistries. An in-situ gas analysis process was developed to probe the organic removal and the barium titanate phase formation. The exhaust gases emitted during the firing of barium titanate films were monitored using a residual gas analyzer (RGA) to investigate the effects of ramp rate and oxygen partial pressure. The dielectric properties including capacitor yield were correlated to the RGA data and microstructure. This information was used to tailor a thermal profile to obtain the optimum dielectric response. A ramp rate of 20 °C/min and a pO2 of 10-13 atm resulted in a permittivity of 1500, a loss tangent of 0.035 and a 90 % capacitor yield in 0.5 mm dot capacitors. Yield values above 90% represent a significant advantage over preexisting reports and can be attributed to an improved ability to control final porosity. Finally, the dramatic enhancement in film density was demonstrated by understanding the processing science relationships between organic removal, crystallization, and densification in chemical solution deposition. The in situ gas analysis was used to develop an each-layer-fired approach that provides for effective organic removal, thus pore elimination, larger grain sizes, and superior densification. The combination of large grain size and high density enabled reproducing bulk-like dielectric properties in a thin film. A room temperature permittivity of 3000, a 5 μF/cm2 capacitance density, and a dielectric tunability of 15:1 were achieved. By combining the data sets generated in this thesis with those of comparable literature reports, we were able to broadly rationalize scaling effects in polycrystalline thin films. We show that the same models successfully applied to bulk ceramic systems are appropriate for thin films, and that models involving parasitic interfacial layers are not needed. Developing better models for scaling effects were made possible solely by advancing our ability to synthesize materials thus eliminating artifacts and extrinsic effects.
- Spectroscopy of Oxide-GaN Interfaces(2009-03-05) Craft, Hughes Spalding; Zlatko Sitar, Committee Member; Donald Brenner, Committee Member; Thomas Pearl, Committee Member; Jon-Paul Maria, Committee ChairGaN-based devices are of interest for applications requiring high-frequency, high-power operation at elevated temperatures. As in traditional, silicon-based devices, integration of semiconducting phases with insulators is critical. Additionally, applications involving the integration of GaN with polar oxides such as perovskite ferroelectrics have been proposed, due to the coupling that may be achieved between the respective polar vector. Devices utilizing such a coupling behavior would make possible two-dimensional electron gases of high charge densities that could be modulated by the oxide’s polarization. The current status of oxide-GaN research is far behind that of oxide-Si research, and large-scale realization of GaN devices will require detailed understanding of oxide-GaN interfaces. This thesis focuses on the characterization of several oxide-GaN interfaces using x-ray photoelectron spectroscopy (XPS), as well as the identification of issues relating to the GaN surface. The rocksalt oxides MgO and CaO have been proposed as candidates for GaN MOSFET gate oxides, passivating layers, and buffer layers in GaN-ferroelectric structures. Thus, knowledge of film growth modes and band alignments is critical. Utilizing in-vacuo molecular beam epitaxy (MBE) and XPS, the growth of MgO on GaN was found to occur by the Volmer-Weber mode, with coalescence occurring at ~12 nm. This coalescence behavior was not found to affect the band alignment. As measured by XPS, the valence band offset at the MgO-GaN interface is 1.2 ± 0.2 eV, leading to a conduction band offset of 3.5 eV. A similar study was undertaken for the CaO-GaN system, in which more rapid coalescence was observed, leading to the conclusion of a Stranski-Krastanov growth mode. The difference in coalescence behavior is attributed to the increased reactivity of the CaO surface. The band offsets at the CaO-GaN interface were found to be 1.0 ± 0.2 eV at the valence band, and 2.5 eV at the conduction band. The band structures measured in this thesis are considered to be sufficient for limiting leakage current by Schottky emission for high-temperature devices. Surface chemical stability of rocksalt oxides is a known issue with respect to hydroxylation through water adsorption. XPS characterization of water uptake was performed using the O 1s photoelectron line after several in-vacuo exposures, culminating in a one-hour exposure to a water/oxygen mixture at 1 x 10-6 Torr. Characterization of polycrystalline MgO showed a saturating coverage of –OH groups at approximately 1 monolayer, regardless of exposure. CaO films exhibited increased reactivity, with hydroxyl coverage increasing to 3 monolayers, in addition to a similar amount of physisorbed water, suggesting the possibility for further reaction. Complete recovery of both oxide surfaces is shown to be achievable using mild vacuum anneals. Finally, the surface of GaN has been characterized with respect to several issues encountered during these investigations. GaN surfaces are found to be significantly Ga-rich, with surface stoichiometries routinely in excess of Ga2N. Several wet chemistries for GaN preparation were evaluated for their ability to modify the electrical behavior of subsequently grown oxide films. XPS could not unambiguously identify any change in surface chemistry that promotes these effects. Finally, p-type GaN films were noted to consistently possess greater oxide contamination in the as-grown state. Typical n-type or undoped GaN were marked by submonolayer quantities of oxide surface coverage, while p-type GaN typically exhibited coverages in the 1-2 nm scale. This difference has been found to be due to the p-type dopant activation anneal, during which GaN oxidation cannot be suppressed
- Synthesis and Properties of Barium Titanate Solid Solution Thin Films on Copper Substrates(2006-09-26) Ihlefeld, Jon F.; Jon-Paul Maria, Committee Chair; Angus Kingon, Committee Member; Robert Nemanich, Committee Member; Zlatko Sitar, Committee Member; William Borland, Committee MemberBarium titanate thin films were deposited via chemical solution deposition using a hybrid-chelate chemistry directly on copper foil substrates. A process was developed to crystallize and densify the ferroelectric films at 900C by using a reductive atmosphere containing nitrogen, hydrogen, water vapor, and oxygen impurities such that film constituents were oxidized to form barium titanate and the foil substrate remained metallic. The crystallized films are polycrystalline with equiaxed morphology and average grain diameters in excess of 100 nm. The dielectric properties exhibit permittivities in excess of 1800 at room temperature and zero bias with tunabilites of greater than 90% and high field loss tangents of less than 1%. A series of samples was prepared with varying grain and crystallite sizes by dividing and processing a single film over a range of temperature from 700 to 900C. This ensures that the chemical composition and film thickness is invariant for each sample. It is shown that the grain size increases with higher process temperatures and results in a concomitant increase in permittivity and tunability. These enhancements, combined with the constant paraelectric⁄ferroelectric phase transition temperature, indicated that a combination of film crystallinity and grain size is responsible for diminished performance. The phase transition temperature and temperature coefficient of capacitance modified by partially substituting zirconium, hafnium, and tin for titanium. The resulting films were single phase and the phase transition shifts were consistent with bulk materials. A reduction in permittivity was observed for increasing substituent level and was attributed to a reduction in grain size for both barium titanate zirconate and barium titanate hafnate. Processing conditions were chosen to stabilize Sn2+ during the firing process in an attempt to flux the system and increase grain size. The barium titanate stannate films had less reduction in grain size per substituent level than either zirconium or hafnium, however a similar reduction in permittivity was observed. The diminished dielectric response was explained by a defect reaction involving divalent tin and oxygen vacancies that quenched the extrinsic domain response to the dielectric constant. Defect equilibria were investigated with respect to processing atmosphere, stoichiometry, and dopant concentration. The solubility of excess barium and titanium was found to be greater in the films than is expected in the bulk, however it is unclear that equilibrium is achieved in the process. It was demonstrated that dopants could successfully eliminate the necessity of a reoxidation anneal to compensate for oxygen point defects resulting from the low pO2 atmospheres. The dopant levels necessary and insulation resistance of pure BaTiO3 were greater than expected from thermodynamic calculations. It was suggested that this is the result of a reduction in the enthalpy of reduction, stemming from an increase in grain boundary volume. Barium borate fluxes were used to improve densification and crystallinity. Barium borate additions between 0 and 3% uniformly increased grain size and density, while levels greater than 3% resulted in anomalous grain growth. Films with exaggerated grains show tetragonal peak splitting in the X-ray diffraction patterns, consistent with bulk barium titanate. In materials without exaggerated grain growth, dielectric measurements revealed permittivities in excess of 3000 at room temperature (for average grain sizes of approximately 160 nm). This value is equivalent to the finest-prepared bulk ceramics and substantially greater than any polycrystalline film ever reported. This has been attributed to in improvement in film crystallinity. These two accomplishments — tetragonal crystal symmetry and permittivities in excess of 3000 — represent dramatic breakthroughs in ferroelectric thin film technology.
- Thermionic Emission from Doped and Nanocrystalline Diamond(2003-04-21) Koeck, Franz Alexander; Robert J. Nemanich, Committee Chair; Hans D. Hallen, Committee Member; Zlatko Sitar, Committee MemberMicrowave Plasma assisted Chemical Vapor Deposition (MPCVD) has been utilized to synthesize nitrogen doped and intrinsic nanocrystalline diamond films to investigate thermionic field emission behavior. Sulfur-doped nanocrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD) have been included in the thermionic field emission measurements. The samples were imaged in UHV by photo electron emission microscopy (PEEM) using a UV Hg lamp for photoemission excitation. The same instrument was used to obtain the thermionic-field emission electron microscopy images (T-FEEM) at temperatures up to 900°C. The Raman spectra of the films showed a strong diamond peak at 1332cm-1 and weaker signal from the graphitic regions in the sample. Field emission could not be measured at room temperature, but the PEEM images showed relatively uniform emission. The PEEM images showed little change as the temperature is increased. At temperatures as low as 640°C the T-FEEM images exhibited strongly enhanced electron emission with increasing temperature. Doped and undoped nanocrystalline diamond films showed localized emission from small emission sites with a significant temperature dependence of the electron emission for the sulfur doped films at around 600°C. This thesis focuses on developing a consistent model of thermionic emission from doped and nanocrystalline diamond films.
