Donor spectroscopy at large hydrostatic pressures and transport studies in compound semiconductors

In the first part of this work, the author describes studies of donors in AlSb and in GaAs at large hydrostatic pressures, two materials in which the conduction band minimum is not parabolic, but has a camel`s back shape. These donors were found to display only one or two absorption lines corresponding to ground to bound excited state transitions. It is shown that due to the non-parabolic dispersion, camel`s back donors may have as few as one bound excited state and that higher excited states are auto-ionized. Thus, it is possible that transitions to these other states may be lost in the continuum. In the second part, calculations of mobilities in GaN and other group III-Nitride based structures were performed. GaN is interesting in that the carriers in nominally undoped material are thought to originate from impurities which have an ionization energy level resonant with the conduction band, rather than located in the forbidden gap. These donors have a short range potential associated with them which can be effective in scattering electrons in certain situations. It was found that effects of these resonant donors can be seen only at high doping levels in III-Nitride materials and in Al{sub x}Ga{sub 1{minus}x}N alloys, where the defect level can be pushed into the forbidden gap. Calculations were also performed to find intrinsic mobility limits in Al{sub x}Ga{sub 1{minus}x}N/GaN modulation doped heterostructures. Theoretical predictions show that electron mobilities in these devices are capable of rivaling those found in the best Al{sub x}Ga{sub 1{minus}x}As/GaAs heterostructures structures today. However, the currently available nitride heterostructures, while displaying mobilities superior to those in bulk material, have sheet carrier concentrations too large to display true two-dimensional electron gas behavior