Study Of The Polarization Characteristics Of Wide Bandgap Group III Nitride

The GaN based wide bandgap III-nitride shows important applications in high-energy, high-frequency, high-power and optoelectronic devices. Because of the asymmetry of crystal structure, strong self-polarization effect exists in materials. In the GaN-based devices, the polarization plays an important roles. Since, the focus of this thesis is to study the polarization characteristics of wide bandgap III-nitride and the application of polarization in wide bandgap III-nitride based devices.The first principles and theoretical models were introduced. The band structure and density of states of GaN and AlGaN were studied by CASTEP based the first principles. The energy levels of the valence electrons and the relationship between composition of Al and the band gap of AlGaN were analysis. The density of states and population of charges were calculated to investigate the influence of polarization effects in GaN/ Al N superlattice by DOML based first principles. Due to the polarization, the 3d electrons of Ga atoms transition from bottom of valence band to top of valence band and conduct band. At the junction of the heterojunction, there are positive and negative charge accumulation.The polarization effects in AlGaN/GaN heterostructures was studied by solving the self-consistent Schrodinger equation and Poisson’s equation. Without impurity doping, two-dimensional electron gas(~1019cm-3) were got by polarization induced. The curve between Al composition and AlGaN film thickness versus the concentration of polarization-induced two-dimensional electron gas was calculated. We also investigated polarization-induced to enhanced the doping efficiency of wide bandgap III-nitride. Without impurity doping, along [0001] crystal direction, linearly increasing the Al composition of AlGaN film, we obtained n-type AlGaN film with three-dimensional electron gas( ~1019cm-3). the high Al composition AlGaN based PN junction was fabricated on Si substrate by polarization-induced doping without any dopant.Finally, the influence of polarization effects on AlGaN-based UV LED luminous efficiency was investigated by Silvaco software ATLAS semiconductor device simulator. The polarization electric field generated by the polarization effect bend the energy band, resulting in electron-hole wave functions separated spatially, and overflow current increases.The optical property and injection efficiency of N-face AlGaN based UV-LED has been studied and compared with Ga-face AlGaN based UV-LED. Staircase electron injector is introduced in the N-face AlGaN based UV-LED. The EL spectra, power-current performance curves, energy band diagrams, carrier concentration and radiative recombination rate are numerically calculated. The results indicate that N-face UV-LED has better optical performance than Ga-face UV-LED, and the injection efficiency is enhanced owing to that the staircase electron injector is available for UV-LED.