a-Plane oriented gallium nitride thin films: Heteroepitaxy, quantum wells, and lateral overgrowth

Nonpolar growth is a promising means of circumventing the strong polarization-induced electric fields that exist in wurtzite nitride semiconductors. Current nitride-based devices employ heterostructures grown along the polar [0001] direction, resulting in the formation of electrostatic fields parallel to the growth direction. These internal fields spatially separate electron and hole wavefunctions in quantum wells (Quantum Confined Stark Effect), thus, red-shifting the emission and reducing carrier recombination efficiency. The potential improvements gained by eliminating polarization-induced electric field effects are the driving force behind the development of nonpolar nitride materials. This dissertation focuses on the growth and characterization of nonpolar a-plane heteroepitaxial films, quantum well structures, and lateral epitaxial overgrowth.; Initially, the growth of (112¯0) a-plane GaN films on (11¯02) r-plane sapphire and (112¯0) a-plane 6H-SiC substrates was investigated. Depositing low temperature GaN or high temperature AlN nucleation layers prior to the epitaxial GaN growth produced planar nonpolar GaN films on both sapphire and SiC (AlN only) substrates. The GaN microstructure, analyzed in conjunction with surface morphology and crystal mosaic, was dominated by ∼3 × 1010 cm−2 threading dislocations and mid-105 cm−1 stacking faults. Despite the defective microstructure, the absence of internal electric field effects was confirmed by the photoluminescence emission characteristics of GaN/AlGaN multiple quantum wells.; In order to realize the full potential of the nonpolar films, line and planar defect reduction is necessary. Since the common dislocation line direction observed for the a-GaN films is parallel to the growth direction, a-GaN is an excellent candidate for threading dislocation reduction via lateral epitaxial overgrowth (LEO). Parallel LEO stripes aligned along [1¯100]_GaN, the most favorable a-plane GaN stripe orientation, had rectangular cross-sections and “dislocation-free” overgrowth. Since the vertical sidewalls of these stripes are c-plane facets with opposing polarities, polarity dictated the lateral growth and planar fault reduction. In addition to characterizing the effectiveness of LEO, the polarity effects observed in LEO provided a basis for investigating the microstructural evolution of the heteroepitaxial growth.