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Date of Award

2-2012

Document Type

Campus Access

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemical Engineering

First Advisor

Dimitrios Maroudas

Second Advisor

T. J. Mountziaris

Third Advisor

Michael A. Henson

Subject Categories

Chemical Engineering | Materials Science and Engineering | Nanoscience and Nanotechnology

Abstract

The enhanced functionality and tunability of electronic nanomaterials enables the development of next-generation photovoltaic, optoelectronic, and electronic devices, as well as biomolecular tags. Design and efficient synthesis of such semiconductor nanomaterials require a fundamental understanding of the underlying process-structure/composition-property-function relationships. To this end, this thesis focuses on a systematic, comprehensive analysis of the physical and chemical phenomena that determine the composition and properties of semiconductor nanomaterials. Through synergistic combination of computational modeling and experimental studies, the thesis addresses the thermodynamics and kinetics that are relevant during synthesis and processing and their resulting impact on the properties of silicon thin films and ternary quantum dots (TQDs) of compound semiconductors.

The thesis presents a computational study of the growth mechanisms of plasma deposited a-Si:H thin films based on kinetic Monte Carlo (KMC) simulations according to a transition probability database constructed by first-principles density functional theory (DFT) calculations. Based on the results, a comprehensive model is proposed for a-Si:H thin-film growth by plasma deposition under conditions that make the silyl (SiH 3 ) radical the dominant deposition precursor. It is found that the relative roles of surface coordination defects are crucial in determining the surface composition of plasma deposited a-Si:H films and should be properly accounted for. The KMC predictions for the temperature dependence (over the range from 300 K to 700 K) of the surface concentration of SiH x (s) (x = 1,2,3) surface hydride species, the surface hydrogen content, and the surface dangling-bond coverage are in agreement with experimental measurements.

In addition, the thesis details a systematic analysis of equilibrium compositional distribution in TQDs and their effects on the electronic and optoelectronic properties. Formation of hetero-nanostructures, such as core/shell-like structures, through atomic-scale assembly driven by equilibrium surface segregation is studied as a function of nanocrystal size, composition, and temperatures for TQD morphologies that include faceted equilibrium nanocrystal shapes for ZnSe 1-x Tex and Inx Ga1-x As TQDs; the results are based on coupled compositional, structural, and volume relaxation of the nanocrystals according to Monte Carlo and conjugate-gradient methods employing a DFT-parameterized description of interatomic interactions. A phenomenological species transport theory also is developed that explains the concentration profiles due to surface-segregation-induced ordering of constituent and dopant atoms in the dilute limit. The nm-scale diffusion lengths in nanocrystals introduce an interesting interplay between the kinetic and thermodynamic stability of interfaces. The thermodynamic stability of such interfaces in ZnSe 1-x Sx TQDs are investigated based on DFT calculations combined with X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectra of TQDs that are synthesized and annealed using colloidal methods. The results demonstrate the possibility of compositional redistribution that causes degradation over time of core/shell TQD electronic properties, with far reaching implications for the use of such nanostructures in devices. Electronic structure calculations of ZnSe 1-x Sx (type-I) and ZnSe1-x Te x (type-II) TQDs elucidate the impact of composition and compositional distribution on the electron density distribution, density of states, and band gap of the TQDs. The resulting relationships with respect to the distributions in the TQDs of constituent/dopant/impurity atoms (core/shell vs. alloyed TQDs) provide an interpretation for the key features observed in the PL spectra, as well as useful guidelines for improving the design and device performance of TQDs.

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