We present finite-element calculations of the electrostatics of NWFETs and numerical simulations of band
bending, charge distributions, and dopant ion diffusion in NWs. For NWFETs, we find that the
semiconducting nature and finite length of the NW warrant sizeable corrections to capacitance calculations
using the standard analytical formula and simulations that assume a metallic NW. We thus provide a
comprehensive set of correction factors to these approximations. We also present a possible mechanism for
explaining non-uniform dopant distributions involving electrodiffusion of charged dopant ions at high
temperatures. We find that changes in the internal NW electrostatics due to non-uniform dopant
distributions can have significant effects on the free carrier concentration and therefore conductivity of
The direct gap of the In<sub>1-x</sub>Ga<sub>x</sub>N alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells. A number of measurements of the intrinsic properties of InN and In-rich In<sub>1-x</sub>Ga<sub>x</sub>N alloys (0 < x < 0.63) are presented and discussed here. To evaluate the suitability of In<sub>1-x</sub>Ga<sub>x</sub>N as a material for space applications, extensive radiation damage testing with electron, proton, and alpha particle radiation has been performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In<sub>1-x</sub>Ga<sub>x</sub>N retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. Results are evaluated in terms of the positions of the valence and conduction band edges with respect to the average energy level of broken-bond defects (Fermi level stabilization energy <i>E<sub>FS</sub></i>). Measurements of the surface electron concentration as a function of <i>x</i> are also discussed in terms of the relative position of <i>E<sub>FS</sub></i>. The main outstanding challenges in the photovoltaic applications of In<sub>1-x</sub>Ga<sub>x</sub>N alloys, which include developing methods to achieve p-type doping and improving the structural quality of heteroepitaxial films, are also discussed.
We have studied the effects of composition and hydrostatic pressure on the direct optical transitions at the Γ point of the Brillouin zone in MBE-grown ZnO<sub>x</sub>Se<sub>1-x</sub> and ion-implantation-synthesized Zn<sub>1-y</sub>Mn<sub>y</sub>O<sub>x</sub>Te<sub>1-x</sub> alloys. We observe a large O-induced band-gap reduction and a change in the pressure dependence of the fundamental band gap of the II-O-VI alloys. The effects are similar to those previously observed and extensively studied in highly mismatched III-N-V alloys. Our results are well explained in terms of the band anticrossing model that considers an anticrossing interaction between the highly localized oxygen states and the extended states of the conduction band of II-VI compounds. The O-induced modification of the conduction band structure offers an interesting possibility of using small amounts of O to engineer the optoelectronics properties of group II-O-VI alloys.