Manipulating Quantum Dot Nanostructures for Photonic and Photovoltaic Applications.

Abstract

Semiconductor quantum dots are of recent interest for use in various optoelectronic devices such as solar cells, lasers, and quantum computing. For example, embedding quantum dots within an optical nano-cavity is expected to greatly enhance performance of micro-lasers and quantum gates due to their non-linear optical response. For solar cells, quantum dots can be used to create an intermediate energy state within the band gap of the bulk material as originally proposed by Luque and Marti, increasing the thermodynamic efficiency limit to >63%, well beyond that of current devices. These device applications require selecting an appropriate material system, properly preparing the starting growth surface prior to quantum dot growth, and understanding the resulting structural, compositional, and optoelectronic properties of the dots. This work is presented in two parts, each containing multiple related studies on quantum dot nanostructures and the background information necessary for understanding the analysis presented. Part I describes the effects of lateral patterning on the size and composition of InAs quantum dots and advances the current understanding of the effects of lateral separation on dot size and composition. Increasing the pattern spacing results in an increase in quantum dot dimensions, even doubling their size, an increase in wetting layer thickness, and increased dissolution during capping. The In diffusion length during quantum dot nucleation and dissolution upon capping can be determined via patterning to be approximately 0.5 μm and >1.0 μm, respectively. Part II describes the effects of growth conditions and GaAs capping on size, shape, and segregation of Sb in type-II band offset GaSb quantum dots using various analysis techniques capable of analyzing the morphology, composition, and optical properties of uncapped and buried nanostructures. In particular, three-dimensional analysis of the morphology and composition of buried structures shows that approximately 70% of GaSb dots disintegrate into clusters of small islands with about 1/3 the diameter of their precursor, significantly altering their optoelectronic properties. A detailed analysis of the quantum dot nanostructures is presented in both parts, and examples of devices fabricated through collaborations provide a better understanding of how quantum dots can be properly tailored for specific device applications.