Theoretical Study of Material and Device Properties of Group-III Nitrides

Group-III nitride semiconductors, including AlN, GaN, InN, and their alloys, are ideal materials for solid state lighting applications. Current research focuses on improving the efficiency by improvements in materials quality and novel device designs, for instance based on nonpolar and semipolar growth. The motivation for our work is to assist and guide the experimental development of high-performance solid state optoelectronic devices by performing computational studies. Our investigations range from basic structural and electronic properties of nitrides to the effects of device design on efficiency of light emission.;In the area of fundamental properties, we performed a systematic study of strain effects on the electronic band structures of the group-III-nitrides (AlN, GaN and InN) using density-functional theory with an advanced hybrid functional as well as using the quasiparticle GW method. We present a complete set of deformation potentials that allows us to predict the band positions of group-III nitrides and their alloys (InGaN and AlGaN) under realistic strain conditions. We then employed the resulting first-principles deformation potentials to predict the effects of strain on transition energies and valence-band structures of c-plane, nonpolar, and semipolar InGaN alloy layers grown on GaN substrates, with particular attention to the role of strain in the polarized light emission.;We also investigated the role of native defects in the optical properties of GaN and AlN, again using hybrid density-functional calculations. We established that complexes between Mg and nitrogen vacancies lead to the broad red luminescence that has often been observed in GaN. We find that isolated nitrogen vacancies can give rise to broad emission peaked at 2.18 eV. We show that isolated aluminum vacancies lead to an absorption peak at 3.43 eV and an emission peak at 2.73 eV. We also find that the complexes can give rise to absorption peaked at 3.97 eV and emission peaked at 3.24 eV. These results indicate that Al vacancies and their complexes with oxygen lead to distinct UV absorption lines that have hampered development of UV-transparent AlN substrates. We also investigate the optical excitation of the bulk AlN and the nitrogen vacancies in AlN using ab initio many-body perturbation theory. Our calculations show that many-body effect strongly affects the absorption spectrum of AlN and a localized exciton is formed around the vacancy.;Finally, we have carried out device simulations based on k.p theory and Schrödinger-Poisson solvers to investigate the role of strain, polarization and doping in performance of nitride-based LED devices. We elucidate the mechanisms by which modification of GaN barriers by Mg doping benefits hole transport, and we demonstrate how the strain-induced polarization field affects wavefunction overlap in the quantum wells of the c-plane and nonpolar plane LEDs and is related to the droop problem. We show that the emission wavelength shift under current injection is also related with quantum well potential profile and the polarization field inside quantum wells. These simulations shed light on the improvement of nitride-based LED efficiency by device design.