Cubic phase gallium nitride photonics integrated on silicon(100) for next-generation solid state lighting

Semiconductors made of gallium nitride (GaN) and its compounds (AlInGaN) have transformed the visible light emitting diode (LED) industry thanks to their direct bandgap across the entire visible and ultraviolet spectra. Despite its success, the conventional hexagonal-phase GaN has fundamental disadvantages in performance and cost that hinder market adoption. These include: internal polarization field ( MV/cm2), high acceptor activation energy (260 meV), low hole mobility (20 cm2/V), and expensive substrates (Al2O3, SiC). Gallium nitride also crystallizes in the cubic crystal that has a higher degree of symmetry. This leads to some advantageous properties for light emitting applications: polarization-free, lower acceptor energy (200 meV), and higher hole mobility (150 cm2/V). These advantages are critical for the development of the next-generation solid state lighting. Difficulty in its synthesis stemming from the large crystal lattice mismatch, chemical incompatibility, and phase metastability has prohibited the growth of high quality semiconductor crystals that are device-worthy. This thesis explores a method of synthesizing phase-pure, high-quality cubic GaN crystals on nanopatterned Si(100) substrates via hexagonal-to-cubic phase transition, and the thesis presents a comprehensive material characterization of the crystals. Crystal growth geometry modeling of GaN on nanopatterned Si(100) substrates is used to estimate the necessary patterning parameters to facilitate complete phase transition. The cubic GaN material is then studied using structural characterization techniques including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The carrier recombination properties are studied using photoluminescence, Raman spectroscopy, and cathodoluminescence. The cubic GaN synthesized using the phase transition method on carefully patterned Si(100) substrates is shown to be phase-pure, defect-free, and optically superior. Material properties such as internal quantum efficiency, Varshni coefficients, and defect levels are extracted from the experiments. Other work on hexagonal GaN light emitters on silicon substrates, chamber conditioning for metalorganic chemical vapor deposition of III-nitrides, and space-based laser instruments for NASA missions is also discussed. Class lab module development and outreach activities are included

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