Properties of thin film III-V/IV semiconductor alloys and nanostructures

Abstract

A large amount of research and development has been devoted to engineering materials for the next generation of semiconductor devices with high performance, energy efficiency, and economic viability. To this end, significant efforts have been made to grow semiconductor thin films with the desired properties onto lattice constants with viable, cost effective substrates. Comparatively less effort has been made to explore III-V/IV heterovalent nanostructures and alloys, which may exhibit properties not available in existing materials. The investigation of these structures, grown using MOCVD, is the goal of this thesis and is motivated by two factors: one, that III-V/IV nanostructures should be good thermoelectrics based on the "phonon glass electron crystal" concept, and two, that (GaAs)₁-x(Ge₂)x alloys were observed to exhibit near-infrared room temperature luminescence, a result that can have significant implications for low bandgap optical devices. A survey of various growth conditions was conducted for the growth of the model GaAs/Ge system using MOCVD to gain insight in the epitaxy involving heterovalent materials and to identify structures suitable for investigation for their thermoelectric and optical properties. A significant decrease in the thermal conductivities of GaAs/Ge nanostructures and alloys relative to bulk GaAs and bulk Ge was observed. This reduction can be attributed to the presence of the heterovalent interfaces. The electron mobilities of the structures were determined to be comparable to bulk Ge, indicating minimal disruption to electron transport by the interfaces. A further reduction in thermal conductivity was observed in an (In₀.₁Ga₀.₉As)₀.₈₄(Si0₀.₁Ge₀.₉)₀.₁₆ alloy; the alloy had a thermal conductivity of 4.3 W/m-K, comparable to some state-of-the-art thermoelectric materials. Room temperature photoluminescence measurements of various compositions of (GaAs)₁-x(Ge₂)x alloys revealed a maximum energy transition of 0.8 eV. This bandgap narrowing is the result of composition fluctuations; the fluctuations create regions of lower bandgap, resulting in a weak dependence on luminescence as a function of Ge composition as well as lower bandgap than the homogeneous alloy with the same composition. As silicon was added to the (GaAs)₁-x(Ge₂)x alloy, the bandgap increased despite the composition fluctuations. Based on the results from this work III-V/IV nanostructures show promise for thermoelectric and optical applications.