Browsing by Author "Zlatko Sitar, Committee Chair"

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Aluminum Nitride Bulk Crystal Growth in a Resistively Heated Reactor
(2005-08-23) Dalmau, Rafael Federico; Jon-Paul Maria, Committee Member; Raoul Schlesser, Committee Member; Robert Nemanich, Committee Member; Zlatko Sitar, Committee Chair
A resistively heated reactor capable of temperatures in excess of 2300°C was used to grow aluminum nitride (AlN) bulk single crystals from an AlN powder source by physical vapor transport (PVT) in nitrogen atmosphere. AlN crystals were grown at elevated temperatures by two different methods. Self-seeded crystals were obtained by spontaneous nucleation on the crucible walls, while seeded growth was performed on singular and vicinal (0001) surfaces of silicon carbide (SiC) seeds. During self-seeded growth experiments a variety of crucible materials, such as boron nitride, tungsten, tantalum, rhenium, tantalum nitride, and tantalum carbide, were evaluated. These studies showed that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, influencing nucleation, coalescence, and crystal morphology. In terms of high-temperature stability and compatibility with the growth process, the best results for AlN crystal growth were obtained in crucibles made of sintered tantalum carbide or tantalum nitride. In addition, contamination from the commercially purchased AlN powder source was reduced by pre-sintering the powder prior to growth, which resulted in a drastic reduction of nearly all impurities. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction, x-ray topography, glow discharge mass spectrometry, and secondary ion mass spectrometry. Average dislocation densities were on the order of 10³ cm⁻³, with extended areas virtually free of dislocations. High resolution rocking curves routinely showed peak widths as narrow as 7 arcsec, indicating a high degree of crystalline perfection. Low-temperature partially polarized optical reflectance measurements were used to calculate the crystal-field splitting parameter of AlN, Δ[subscript cr] = -230 meV, and from this, a low-temperature (1.7 K) band gap energy of 6.096 eV was obtained for unstrained wurtzite AlN. Seeded growth of AlN bulk crystals on on-axis and off-axis (0001), Si-face SiC seeds was investigated as a means to scale up maximum single crystal size and pre-define crystal orientation. A two-step deposition process was developed for the growth of thick layers. AlN layers 0.1—3 mm thick were deposited on inch-sized seeds. X-ray diffraction analysis evidenced that the AlN grew in the direction of the seed. A one-dimensional isotropic model was formulated to calculate the thermal stress distribution in AlN/SiC heterostructures. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increasing AlN thickness, in agreement with model calculations. Crack-free AlN crystals were obtained from grown layers by evaporating the SiC seed in situ during high-temperature PVT growth. Based on these results, a reproducible seeded growth process was developed for production of crack-free AlN crystals having pre-determined polarity and orientation.
Growth of GaN from Elemental Gallium and Ammonia via a Modified Sandwich Growth Technique
(2005-01-07) Berkman, Elif; Robert M. Kolbas, Committee Member; Nadia A. El-Masry, Committee Member; Raoul Schlesser, Committee Member; Zlatko Sitar, Committee Chair
Gallium nitride (GaN) thin films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia NH3. The feasibility of the growth process was demonstrated and discussed. One of the biggest challenges of working in the Ga–NH3 system was the instability of molten Ga under NH3 atmosphere at elevated temperatures, especially between 1100–1200°C. In the first part of the study, transport of Ga species from the source-to-substrate during the GaN growth process and the influence of ammonia—liquid Ga reaction on Ga transport were investigated. Experimental results under different conditions were studied and compared to theoretical predictions to quantify the mechanism of transport in the vapor growth technique. In presence of NH3, Ga transport far exceeded the predicted upper limit for the vapor phase transport. Visual observations confirmed that a significant amount of Ga left the source in a cluster rather than atomic form. A novel Ga source design was employed in an effort to obtain a stable and high vapor phase transport of Ga species at moderate temperatures. In this design, pure N2 was flowed directly above the molten Ga source. This flow prevented the direct contact and reaction between the molten Ga and NH3 and prevented Ga spattering and GaN crust formation on the source surface. At the same time, it significantly enhanced Ga evaporation rate and enabled control of Ga transport and V/III ratio in the system. Growth characteristics were described by a mass transport model based on process parameters and experimentally verified. The results showed that the process was mass transport limited and the maximum growth rate was controlled by transport of both Ga and reactive ammonia species to the substrate surface. A growth rate of 1.4 μm/h was obtained at 1050C, 800 Torr, 3 slm of ammonia flow rate, and 1250C Ga source temperature at a 24 mm source-to-substrate distance. It was found that the process required a more effective supply of active NH3 to the substrate in order to increase the crystal quality and growth rate. The surface morphology of the deposited layers was examined by optical and scanning electron microscopies. XRD analysis was used to determine the crystallinity of deposited films and revealed a full-width at half-maximum (FWHM) of 0.6 deg. for the (0002) GaN peak. EDX analysis was employed for the chemical characterization of the samples and showed that the deposited material contained only Ga an N elements. Room temperature PL spectrum demonstrated the optical quality of the grown samples.
High-Rate Diamond Deposition by Microwave Plasma CVD
(2008-08-01) Li, Xianglin; Zlatko Sitar, Committee Chair; Ramon Collazo, Committee Member; Gerd Duscher, Committee Member; Carl Osburn, Committee Member
Polarity Control in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition
(2008-08-21) Mita, Seiji; John Muth, Committee Member; Gerd Duscher, Committee Member; Ramon Collazo, Committee Member; Zlatko Sitar, Committee Chair
Polarity control of gallium nitride (GaN) on c-plane sapphire substrate was studied via low pressure Metalorganic Chemical Vapor Deposition (MOCVD). Under mass-transport-limited growth regime with a given process supersaturation, the polarities of GaN thin films (i.e. gallium (Ga) and nitrogen (N)-polarities) depended on specific treatments of the sapphire substrate prior to GaN deposition, in addition, identical growth rates for both polar films were obtained. This ability made the fabrication of lateral polar junction (LPJ) GaN structures possible. New designs of novel device structures utilizing the resulting polarity control scheme were developed. N-polar films were consistently obtained after exposing a H2-annealed sapphire substrate to an ammonia atmosphere at temperature above 950°C. Ga-polar films were obtained either by preventing any exposure of the substrate to ammonia prior to deposition or by depositing the film on a properly annealed low temperature aluminum nitride nucleation layer (LT-AlN NL) deposited on a previously ammonia annealed sapphire substrate. As-grown Ga-polar films were generally insulating and smooth surface morphology while N-polar films exhibited n-type conductivity with carrier concentration approaching 1x1019 cm-3 and a rougher surface morphology. Following the established polarity control scheme for GaN films, LPJ structures consisting Ga-polar and N-polar domains side-by-side on a single sapphire wafer were achieved by utilizing a prior patterned AlN⁄bare sapphire template. The two regions were separated by an inversion domain boundary (IDB), which did not hinder the current flow across it, i.e. no energy barrier for the charge carriers. This in principle showed the possibility for the fabrication of lateral junctions and lateral based devices within the GaN technology exploiting polar doping selectivity. Understanding the doping selectivity of the two different polar domains allowed us to fabricate a lateral p⁄n junction in GaN by the simultaneous growth of the p- and n-type regions. Identifying the basic characteristics of a p⁄n junction demonstrated that the fabricated structure was a functional p/n diode. For GaN based junctions, these characteristics were: current rectification, electroluminescence and the photovoltaic effect under UV excitation.