Doctor of Philosophy, Case Western Reserve University, 2021, Physics

The ever-increasing number of applications requiring semiconductor materials at their core is driving the need to understand certain oxide and nitride materials. In this thesis, we investigate two of such classes. The first of those is the class of wide band-gap oxides and includes materials like β-〖Ga〗_2 O_3 and the 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 alloy system. β-〖Ga〗_2 O_3 is the most stable of the five phases in which 〖Ga〗_2 O_3 is found to exist. With a significantly high experimentally measured band gap of 4.5-4.9 eV, it is touted to be an excellent material for high-power electronics and UV transparent optoelectronic applications. Using first-principles calculations, we study this material and present the electronic band structure calculations using the quasiparticle self-consistent GW method. Next, we extend this study to the alloy system 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 in which 〖Ga〗_2 O_3 is alloyed with an even higher band-gap material, 〖Al〗_2 O_3. We study the system in both the phases, α and β, present the electronic band structures for varying compositions of Al ranging from 0% to 100%, and predict the most favorable composition and phase for such an alloy to exist. The second class of materials in this thesis is the alloy system formed by the combination of group III- and II-IV nitrides, GaN and 〖ZnGeN〗_2, respectively. In particular, we study the vibrational properties of 〖ZnGeGa〗_2 N_4. 〖ZnGeGa〗_2 N_4, at 50% composition, is an octet-preserving and lowest energy superlattice of half a cell of 〖ZnGeN〗_2 and half GaN along the b-axis of 〖ZnGeN〗_2 in the 〖Pbn2〗_1 structure. Using Density Functional perturbation theory implemented in ABINIT, the phonon modes at the zone center, Γ allow us to calculate longitudinal optical-transverse optical splittings using Born effective charges. In addition, the IR and Raman spectra along with the phonon density of states, and the phonon band structure are presented. Lastly, we study the transition metal (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, Case Western Reserve University, 2016, Physics

Our main motivation for studying V$_2$O$_5$ is that it is a layered material and therefore potentially could have interesting different properties when made in atomically thin form (monolayer) compared to its bulk form. In that sense it is similar to graphene and transition metal dichalcogenides(TMDC) which have recently attracted great attention since it became possible to make atomically flat 2D materials of them via exfoliation. Exfoliation of V$_2$O$_5$ has not yet been successful to monolayer thicknesses, but efforts to do so are currently on-going and the goal of our work is to predict and explore if it would have interesting properties in monolayer form. However, V$_2$O$_5$ is very different from graphene and TMDC because it has a different crystal structure (orthorhombic instead of hexagonal) and is an oxide with transition metal atoms with $d$-like conduction bands in which strongly correlated effects could occur. An additional interest stems from the fact that its crystal structure contains 1D chains in the layers, so it may exhibit also 1D physical aspects. Besides this motivation in terms of 2D physics, V$_2$O$_5$ has numerous potential applications in catalysis, Li-ion batteries, electrochromic behavior, electro mechanical phenomena etc. and can exhibit various nanoforms. In this dissertation we first focus on the phonons or vibrational modes. Important changes in the vibrational modes were known to occur in TMDCs in monolayer form and our goal was to see if this is also true in V$_2$O$_5$. We first reproduced the phonon spectra (Raman and Infrared) in bulk V$_2$O$_5$ and reanalyzed the existing experimental literature to demonstrate the accuracy of our computational approach. We then focused on the changes induced between bulk and monolayer V$_2$O$_5$ and explained them in terms of a detailed analysis of the force constants. An important conclusion is that the difference in screening in 2D from 3D materials leads to changes in the long-range dipola (open full item for complete abstract)

Committee: Walter Lambrecht Prof. (Advisor) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2014, Physics

In this dissertation, we present band structure studies of three types of materials, which are wurtzite GaAs, atomically thin MoS2, and highly correlated rare-earth nitrides, by using the quasiparticle self-consistent GW (QSGW) method. First, we report results for wurtzite GaAs, which is found to be stable and coexists with the zinc-blende crystal structure (more stable in bulk) in nanowires. We provide detailed band structure parameters, such as effective mass parameters, band gap, crystal field splitting, spin-orbit splitting, etc., of wurtzite GaAs. This information on the bulk band structure is needed for the study of the nanowire specific electronic states, which can be obtained within the envelope function or effective mass type theories. The strain effects on the band structure parameters are also studied, because a nanowire could be under strain due to surface tension and tension caused by the matching of the lattice constant between wurtzite and zinc-blende sections in the wire. The band structure parameters of more well known zinc-blende GaAs was also calculated in order to test the validity of our QSGW calculations. We also present the band structure of 4H GaAs as a guideline of how the band parameters might change when there is mixing between hexagonal structure (wurtzite) and cubic structure (zinc blende) in the same nanowire. Second, we present the results of bulk and atomically thin MoS2. Our QSGW results confirm the transition of the band gap nature from indirect gap to direct gap when the form of MoS2 changes from bulk to monolayer. However, the QSGW significantly overestimates the direct gaps at the K point of monolayer and bilayer MoS2 due to the very strong excitonic effect in this two dimensional material, which is not taken into account in the QSGW method. Therefore, we also estimated the exitonic effect by using the Mott-Wannier effective mass theory, and obtained a large ground state exciton binding energy for both monolayer and bilayer. Our (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor); Philip Taylor (Committee Member); Jie Shan (Committee Member); Clemens Burda (Committee Member) Subjects: Physics