Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

Contactless, nondestructive measurements of minority carrier lifetime by transient microwave reflectance (TMR) and photoluminescence are used to study the carrier dynamics of several ternary materials: InGaAs, GaAsSb, and InAsSb. As contactless measurements, TMR and photoluminescence can determine quality of as-grown wafers. The minority carrier lifetime is inversely proportional to the diffusion component of the dark current and can be used as an indicator of device performance, without the need for full device fabrication. The ability to yield useful information about wafer quality without the time and cost used for fabrication allows for quick feedback to growers. GaAsSb and InGaAs lattice-matched to InP are candidates for short-wave infrared (SWIR) detection at 1.5 μm, a wavelength used for eye safety and optical communication. The high speed or low signal applications at this wavelength benefit from the use of separate absorber, charge, and multiplier (SACM) avalanche photodiodes (APDs). In these devices, the absorber is optimized for detection at the wavelength of interest, and the multiplier is optimized for gain through impact ionization. InGaAs-based SACM APDs are a mature technology and are available commercially. The multipliers paired with InGaAs, however, typically have high noise. Research into low-noise multipliers has resulted in the demonstration of AlGaAsSb as a low noise material. When AlGaAsSb is paired with InGaAs, the grading material AlInGaAs creates a conduction band offset with AlGaAsSb, limiting bandwidth. GaAsSb lattice-matched to InP has similar properties to InGaAs and could be implemented without a conduction band offset due to the grading material being AlGaAsSb. When a GaAsSb/AlGaAsSb SACM APD was demonstrated, it was found to have higher dark current than commercial InGaAs-based devices. Because these materials are so similar, this was unexpected. As mentioned, the diffusion component of the dark current is inversely proportio (open full item for complete abstract)

Committee: Sanjay Krishna (Advisor); Steve Ringel (Committee Member); Preston Webster (Committee Member); Anant Agarwal (Committee Member); Shamsul Arafin (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy (PhD), Ohio University, 2014, Physics and Astronomy (Arts and Sciences)

Urbach tails are the exponential band tails observed universally in impure crystals and disordered systems. Evidence has been provided that the topological origin of the Urbach tails in amorphous materials are filaments formed by short or long bonds[20]. One aspect of my work focuses on the size effects and choice of Hamiltonian with respect to the structure of the Urbach tails. The dynamical properties of filaments have been studied by performing Molecular Dynamics simulation under constant temperature. The response of filaments under external pressure has also been explored. The second portion of this dissertation is about carbon in two-dimensional sp2 phases. Carbon has shown itself to be the most flexible of atoms, crystallizing in divergent phases such as diamond and graphite, and being the constituent of the entire zoo of (locally) graphitic balls, tubes, capsules and possibly negative curvature analogs of fullerenes, the Schwartzites. In this part, we explore topological disorder in three-coordinated networks including odd-membered rings in amorphous graphene, as seen in some experimental studies. We start with the Wooten-Weaire-Winer models due to Kumar and Thorpe, and then carry out ab-initio studies of the topological disorder. The structural, electronic and vibrational characteristics are explored. We show that topological disorder qualitatively changes the electronic structure near the Fermi level. The existence of pentagonal rings also leads to substantial puckering in an accurate density functional simulation. The vibrational modes and spectra have proven to be interesting, and we present evidence that one might detect the presence of amorphous graphene from a vibrational signature. We also explore the energy landscape of amorphous graphene and report the eigenstates near the Fermi level.

Committee: David Drabold (Advisor); Gang Chen (Committee Member); Eric Stinaff (Committee Member); Jeffrey Rack (Committee Member) Subjects: Condensed Matter Physics; Physics