Structural and optical properties of phase transition cubic phase gallium nitride for photonic devices

Gallium nitride (GaN) semiconductors and its compounds (AlGaInN) have transformed the visible light emitting diode (LED) industry thanks to their direct bandgap across the entire visible spectrum and ultra violet. Despite its success, the conventional hexagonal-phase GaN has fundamental deficits that hinders performance. These include: internal polarization field (~MV/cm2), high acceptor activation energy (260 meV), low hole mobility (20 cm2/V), and expensive substrates (Al2O3, SiC). The metastable cubic-phase GaN offers interesting properties: no internal fields, lower acceptor energy (200 meV), and higher hole mobility (150 cm2/V), that are preferable over the conventional hexagonal GaN through the higher symmetry in the cubic-phase crystal. Due its metastability, however, cubic GaN has not been synthesized with device-worthy crystal quality as large lattice mismatch between foreign substrates and relaxation to the hexagonal phase result in highly defective and mixed phase crystals. Therefore, the superior properties of cubic GaN could not be utilized. This thesis explores the novel properties of cubic GaN grown on Si(100) via phase transition and nano patterning enabled through phase-transition modeling and cubic GaN material characterization. Crystal growth geometry of GaN in nano-patterned silicon U-shaped grooves separated by oxides are modeled through crystallographic equivalence to estimate the geometry of the structure and the required deposition height for complete cubic phase material transition. Structural characterizations, including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy show excellent crystal uniformity and predictable phase transition behavior. Raman spectroscopy and cathodoluminescence show excellent phase purity and clearly controlled phase transition. Extensive optical characterization was conducted via polarization dependent photoluminescence and time-resolved photoluminescence to extract carrier recombination and photon emission behavior. Temperature-dependent cathodoluminescence was conducted to extract the Varshni coefficients for bandgap, defect luminescence activation energies, and most importantly the internal quantum efficiency