Bulk GaN growth and Characterization
Konkapaka, P. Phanikumar
GaN is a wide bandgap semiconductor material that is used for fabrication of laser diodes, light emitting diodes, and high power, high frequency electronic devices. Due to the absence of free standing GaN substrates, these devices are fabricated on foreign substrates such as SiC, Sapphire, LiGaO2 or LiGaO3. Performance of these devices is reduced due to high dislocation densities resulting from lattice mismatch and thermal expansion coefficient mismatch between the GaN and substrate. In order to improve the performance of devices, homoepitaxial device layers grown on high quality free standing GaN substrates are needed. Bulk GaN crystals of dimensions 8.5 mm x 8.5 mm were grown at growth rates greater than 200mm/hr using gallium oxide vapor transport technique. Commercially available GaN powder and ammonia were used as the precursors for growing bulk GaN. Nitrogen was used as the carrier gas to transport the gallium containing species that was obtained from the decomposition of GaN powder. These experiments were performed in a flow over configuration where the nitrogen carrier gas was flowing over the powder transporting the growth species. Using this process, it was possible to achieve growth rates of above 200 microns/hr. The GaN layers thus obtained were characterized using X-Ray diffraction, scanning electron microscopy, and atomic force microscopy. X-ray diffraction patterns showed that the grown GaN layers are single crystals oriented along c direction. AFM studies indicated that the dominant growth mode was dislocation mediated spiral growth. Hall mobility measurements indicated a mobility of 550 cm2/V.s and a carrier concentration of 6.67 x 10 18/cm3. It was found that kinetics of decomposition of pure GaN powder without oxygen resulted in incongruent evaporation leading to the formation of the liquid gallium in the powder. A flow through configuration was also used because of its high collection efficiency of growth species. A mixture of Ga2O3 and carbon powder as well as commercial GaN powder were used as precursors of gallium suboxide in this configuration. This configuration also demonstrated growth rates that are comparable to flow over configuration.
Gallium Nitride, Wide bandgap semiconductor, Bulk growth, growth rate, characterization, vapor transport
dissertation or thesis