First-Principles Calculations on the Electronic, Optical, and Vibrational Properties of Ultrawide-Band-Gap Semiconductor Materials

Abstract

First-principles calculations enable an unprecedented atomistic insight to experimentally-observed phenomena on a massive scale. Such calculations can intelligently guide experimental efforts to save both human time and money. The methods in this dissertation are first-principles calculations based on density functional theory (DFT), many-body perturbation theory (using the GW method), and density functional perturbation theory (DFPT). With these methods, the electronic, optical, and vibrational properties of beta-Ga2O3, h-BN, and r-GeO2 are calculated and analyzed to assess their promise for deep-ultraviolet luminescence and high-power electronic applications.

First, I investigate the near-edge electronic and optical properties of beta-Ga2O3. The fundamental band gap is indirect, but the minimum direct gap is only 29 meV higher in energy, which explains the strong near-edge absorption. Our calculations verify the anisotropy of the absorption onset and explain the range of experimentally reported band-gap values. Our results for the radiative recombination rate indicate that intrinsic light emission in the deep-UV range is possible at high excitation. I calculate the phonon dispersion, which relates to properties such as thermal transport, carrier mobility, and breakdown voltage. The large mode Grüneisen parameters reflect the anharmonicity of monoclinic beta-Ga2O3 and help explain its low thermal conductivity. I also evaluate the electron-phonon coupling matrix elements for the lowest conduction band to determine the phonon mode that limits the mobility at room temperature. I apply these matrix elements to estimate the breakdown field of beta-Ga2O3.

A second ultra-wide-band-gap material which shows promise for deep-UV luminescence applications is hexagonal boron nitride (h-BN). I examine the effects of stacking sequence and number of layers on the electronic and luminescence properties of h-BN structures. I explore the variations of the quasiparticle band gap and interband optical matrix elements for bulk, bilayer, and monolayer stacking polytypes. Although the fundamental gap for most structures is indirect, phonon-assisted transitions are strong (typically 600 times stronger than bulk Si) and enable efficient deep-UV luminescence. The polarization of the emitted light is transverse electric, which facilitates light extraction perpendicularly to the h-BN basal plane.

Rutile germanium dioxide (r-GeO2) is an ultra-wide-band-gap semiconductor that has not been explored for semiconducting applications in electronic and optoelectronic devices. Our band-structure calculations indicate a dipole-forbidden direct band gap at gamma with an energy of 4.44 eV and relatively low carrier effective masses. The first allowed optical transition at gamma occurs at 5.04 eV (in-plane). I also evaluate the optical absorption coefficient along both crystallographic directions. I theoretically predict the electron and hole mobility of r-GeO2 as a function of temperature. At 300 K, the carrier mobilities (in cm2 V-1 s-1) are muelec = 153.6 (in-plane), muelec = 74.1 (out-of-plane), muhole = 4.7 (in-plane), and muhole = 2.2 (out-of-plane). The thermal conductivities at 300 K are 37 W m-1 K-1 in-plane and 57 W m-1 K-1 out-of-plane. Last, I compare the Baliga’s figure of merit (BFOM) for high-power-electronic applications and show that the BFOM for r-GeO2 surpasses those of competing semiconductors, including beta-Ga2O3.

Our theoretical characterization of the electronic, optical, and vibrational properties of beta-Ga2O3, h-BN, and r-GeO2 highlight the viability of these ultra-wide-band-gap semiconductors for applications in deep-UV luminescence and high-power electronics and provides a path for experimental development of materials for improved performance in devices.