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- Genre: Doctoral Dissertation
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Description
In this dissertation, the interface chemistry and electronic structure of plasma-enhanced atomic layer deposited (PEALD) dielectrics on GaN are investigated with x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). Three interrelated issues are discussed in this study: (1) PEALD dielectric growth process optimization, (2) interface electronic structure of comparative PEALD dielectrics on GaN, and (3) interface electronic structure of PEALD dielectrics on Ga- and N-face GaN. The first study involved an in-depth case study of PEALD Al2O3 growth using dimethylaluminum isopropoxide, with a special focus on oxygen plasma effects. Saturated and self-limiting growth of Al2O3 films were obtained with an enhanced growth rate within the PEALD temperature window (25-220 ºC). The properties of Al2O3 deposited at various temperatures were characterized to better understand the relation between the growth parameters and film properties. In the second study, the interface electronic structures of PEALD dielectrics on Ga-face GaN films were measured. Five promising dielectrics (Al2O3, HfO2, SiO2, La2O3, and ZnO) with a range of band gap energies were chosen. Prior to dielectric growth, a combined wet chemical and in-situ H2/N2 plasma clean process was employed to remove the carbon contamination and prepare the surface for dielectric deposition. The surface band bending and band offsets were measured by XPS and UPS for dielectrics on GaN. The trends of the experimental band offsets on GaN were related to the dielectric band gap energies. In addition, the experimental band offsets were near the calculated values based on the charge neutrality level model. The third study focused on the effect of the polarization bound charge of the Ga- and N-face GaN on interface electronic structures. A surface pretreatment process consisting of a NH4OH wet chemical and an in-situ NH3 plasma treatment was applied to remove carbon contamination, retain monolayer oxygen coverage, and potentially passivate N-vacancy related defects. The surface band bending and polarization charge compensation of Ga- and N-face GaN were investigated. The surface band bending and band offsets were determined for Al2O3, HfO2, and SiO2 on Ga- and N-face GaN. Different dielectric thicknesses and post deposition processing were investigated to understand process related defect formation and/or reduction.
ContributorsYang, Jialing (Author) / Nemanich, Robert J (Thesis advisor) / Chen, Tingyong (Committee member) / Peng, Xihong (Committee member) / Ponce, Fernando (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2014
Description
Many species e.g. sea urchin form amorphous calcium carbonate (ACC) precursor phases that subsequently transform into crystalline CaCO3. It is certainly possible that the biogenic ACC might have more than 10 wt% Mg and ∼ 3 wt% of water. The structure of ACC and the mechanisms by which it transforms to crystalline phase are still poorly understood. In this dissertation our goal is to determine an atomic structure model that is consistent with diffraction and IR measurements of ACC. For this purpose a calcite supercell with 24 formula units, containing 120 atoms, was constructed. Various configurations with substitution of Ca by 6 Mg ions (6 wt.%) and insertion of 3-5 H2O molecules (2.25-3.75 wt.%) in the interstitial positions of the supercell, were relaxed using a robust density function code VASP. The most noticeable effects were the tilts of CO3 groups and the distortion of Ca sub-lattice, especially in the hydrated case. The distributions of Ca-Ca nearest neighbor distance and CO3 tilts were extracted from various configurations. The same methods were also applied to aragonite. Sampling from the calculated distortion distributions, we built models for amorphous calcite/aragonite of size ∼ 1700 nm3 based on a multi-scale modeling scheme. We used these models to generate diffraction patterns and profiles with our diffraction code. We found that the induced distortions were not enough to generate a diffraction profile typical of an amorphous material. We then studied the diffraction profiles from several nano-crystallites as recent studies suggest that ACC might be a random array of nanocryatallites. It was found that the generated diffraction profile from a nano-crystallite of size ∼ 2 nm3 is similar to that from the ACC.
ContributorsSinha, Sourabh (Author) / Rez, Peter (Thesis advisor) / Bearat, Hamdallah A. (Committee member) / Bennett, Peter A. (Committee member) / McCartney, Martha R. (Committee member) / Peng, Xihong (Committee member) / Arizona State University (Publisher)
Created2012
Description
Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core.
ContributorsCopple, Andrew (Author) / Peng, Xihong (Thesis advisor) / Chan, Candace (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2016