Optical, electronic, and piezoelectric properties of nitride semiconductors

This dissertation contains basically three parts. The first part consists of chapter two and three. In the second chapter, we describe the “new” strain-dependent semi-empirical pseudopotential method we used to carry out our calculations. In the third chapter, we introduce a new mechanism to explain the large band-gap reduction and other electronic properties of (Ga_{1− y}In_y)(As _1−xN_x) and the related Ga(As_1−xN _x) alloys using large supercell (512 atoms) strain-dependent pseudopotential technique. In addition to the previously reported quantum couplings between electronic states with different reciprocal k-points, another mechanism is found to participate in the strong decrease of the band-gap of (Ga_1−yIn_y)(As _1−xN_x) alloys with respect to GaAs. This work was done in collaboration with L. Bellaiche.; The second part focuses on using density functional theory (DFT) with Vanderbilt ultra soft-pseudopotentials and the modern theory of polarization to study the piezoelectricity of ordered (Ga_0.5In_0.5)N alloys exhibiting an alternation of Ga and In planes along the c-axis. The microscopic origins of the downward deviation of piezoelectricity from the previously assumed linear behavior with composition are revealed and discussed. This work was done in collaboration with L. Bellaiche.; The third part describes an ab initio method that we develop to calculate the piezoelectric coefficients of A _1−xB_xC semiconductor alloys. Using this approach, we have computed the e₃₃ piezoelectric coefficients of (0001) ordered and disordered wurtzite cation-mixed (Ga_1−xIn _x)N alloys, where the disordered alloy is represented by a 864-atom supercell in which cation sites are randomly occupied by Ga and In atoms and anion sites with N atoms. We find that the predicted e₃₃ coefficients of the (0001) ordered alloys are in very good agreement with those determined directly from local-density-approximation (LDA) calculations and significantly deviate from the linear composition interpolation between the parent binaries. On the other hand, the piezoelectric response of the disordered alloys with composition only slightly deviates from a linear compositional behavior and is found to be best described by a third-order polynomium. The intrinsic contribution of the individual atomic responses to this piezoelectric behavior will be revealed and explained. The discrepancy between the results of our method and recent ab initio calculations is discussed. This work was done in collaboration with L. Bellaiche and Su-Huai Wei.