Effects of thermal annealing on the energy gap and atomic structure of GaAsSbN

In this dissertation, the effects of thermal annealing on the energy gap and atomic structure of GaAsSbN, grown by gas source molecular beam epitaxy, have been investigated. From the measurement of photoluminescence and optical absorption, significant blue-shifts in energy gap, resulting from annealing with a temperature higher than 750C, were observed. The energy gap of the annealed GaAsSbN follows the band anticrossing model (BAC) reported in literature. Furthermore, the energy gap reduction can be independently controlled by Sb and N compositions. On the ground of this finding, we proposed a double BAC model, in which we suggest that N and Sb compositions control the energy of conduction and valence band respectively. Besides, the origin of the blue shift induced by thermal annealing is investigated. We believe N pairs NN1 responsible for the low energy gap of the as-grown GaAsSbN. Thermal annealing dissociates the pair into isolated N atoms, resulting in the blue-shift in energy gap. From the Kramers-Kronig modulus of photoreflectance measurement, we resolve multi-peaks relevant to different N pairs and isolated N. When the annealing temperature increases, the peaks gradually merge to that of isolated N. To further support the proposed N pairs dissociation theory, we performed N K-edge X-ray absorption near-edge spectroscopy (XANES) using the beam line 20A of National Synchrotron Radiation Research Center to study the short range structure of GaAsSbN. We use valence force field model to generate supercells with 216 atoms containing different atomic structures, including isolated N, NN1 pair, N-HBC complex, and so on. FEFF9 code, purchased from University of Washington, was then used to simulate the XANES spectra of N-centered cluster of 381 atoms, developed from supercells with different atomic structures. By comparing the experimental results with the simulated XANES, we conclude that NN1 pair existed in the as grown GaAsSbN sample. After thermal annealing at 850 C for 5 min, NN pairs transform into isolated N-HBC complex.