High electron mobility transistor structures in the gallium arsenide and gallium nitride material systems by organometallic vapor phase epitaxy

In recent years, significant advances in high electron mobility transistors have been achieved. Higher power densities and improved RF performance has been demonstrated in several material systems. An attractive alternative for problematic AlGaAs barriers on GaAs based structures can be realized by GaInP thin films. In addition, GaInP placed on top of GaInAs channels can provide strain compensation and increase the maximum obtainable 2DEG sheet densities. A strong amount of activity is currently taking place in AlGaN/GaN materials grown on both SiC and sapphire substrates. Higher band gaps and chemical stability of the GaN-based system show great potential for large power densities at RF frequencies. The work presented in this dissertation is directed towards OMVPE growth of transistor structures in both GaAs and GaN material systems.; Several growth obstacles had to be overcome in these two different material systems. Early efforts focused on reducing and/or eliminating unintentional charge within the buffer layers. For GaAs, substrate surface preparation, growth temperatures, and V/III ratios were optimized, while nucleation layer studies were conducted on GaN deposition on SiC and sapphire substrates. In preparation for strained layer structures on GaAs, latticed matched epitaxy was first evaluated. New techniques for multiple delta doping of the supply layer and arsenide to phosphide transition schemes are reported. Strain compensated pseudomorphic GaInP/GaInAs/GaAs structures were then investigated on vicinal and miscut GaAs substrates.; Developing a nucleation process of GaN-based materials on severely mismatched substrates proved challenging. The commonly used two-temperature process produced planar films, but were plagued with a large background of free carriers from native defects associated with merging islands. A new, single temperature method was developed which produces atomically flat nucleation layers within the first several hundred Angstroms of growth. This provides the transition from the mismatched substrate to epitaxy, and results in highly resistive GaN films. Undoped and doped AlGaN is subsequently grown on the GaN buffer and channel structures. In the case of undoped barriers, polarization effects induce the 2DEG at the heterostructure with the electrons being supplied from the surfaces with possible contribution from the GaN buffer and nucleation layers.