Influence of Composition and Morphology on the Electronic Properties of Semiconductor Nanostructures and Alloys

Abstract

Recent advances in fabrication techniques have led to the successful nano-engineering of semiconductor heterostructures with nanometer-scale structures. Such heterostructures make it possible to tune band gap energies and control carrier confinement for a range of electronic and optoelectronic applications, including solar cells, light emitters, and quantum-computing applications. However, the nanoscale morphologies (i.e., sizes, shapes, and local compositions) of complex heterostructures and their influence on electronic characteristics are not fully understood. To address these issues, it is essential to probe materials on the nanoscale using advanced experimental and computational tools. Therefore, this dissertation focuses on investigating the effects of nanostructure morphologies on the electronic structure of epitaxially-grown semiconductor materials that employ alloying and/or low-dimensional structures (such as quantum dots). In particular, we investigate GaAsNBi and BiSbTe alloys, and InAs/GaAs and GaSb/GaAs quantum dots (QDs) using nanoscale experimental probes in conjunction with self-consistent Schrödinger-Poisson simulations using nextnano.

First, we demonstrate an approach to examine apparent stoichiometry in GaAs-based alloys and nanostructures using local electrode atom probe (LEAP) tomography, in conjunction with Rutherford Backscattering Spectrometry (RBS) and high-resolution x-ray diffraction (HRXRC). Using the LEAP conditions identified for achievement of near-stoichiometry in GaAs, we investigate local N and Bi compositions in GaAsNBi alloys and local Si concentrations in the vicinity of Si-doped InAs/GaAs QD superlattices. For the GaAsNBi alloys, LEAP-determined average Bi compositions correlate with those determined using RBS. These are the first known studies that use LEAP to directly measure N and Bi compositions for GaAsNBi films. For Si-doped InAs/GaAs QD superlattices, 3D LEAP data reveals laterally and vertically inhomogeneous Si incorporation, with clusters of Si throughout the layers. Using the local In, Ga, As, and Si compositions from 3D LEAP data as input into Schrodinger-Poisson simulations, we find that electrons are predicted to be localized near both the QDs and the Si clusters.

Furthermore, we determined the distribution of compositions within Ga(As)Sb quantum dots (QDs), clusters, and circular arrangements of smaller QDs, termed QD-rings (QDRs) using LEAP. Sizes, shapes, and compositional gradients are used as input into self-consistent Schrödinger-Poisson simulations to compute confinement energies for individual nanostructure types. The computed confinement energies and the measured photoluminescence emission energies increase from QDs to QD-rings to 2D layers, enabling direct association of nanostructure morphologies with the optical properties of the GaSb/GaAs multilayers. This is the first known work that uses measured compositional gradients as input into 8 x 8 k·p calculations for Ga(As)Sb/GaAs nanostructures, opening opportunities for tailoring emission energies for near to far-infrared optoelectronics by varying the QD morphology.

Finally, we have investigated the bulk and local electronic states in (Bi1-xSbx)2Te3 alloys using scanning tunneling spectroscopy (STS) and magnetoresistance (MR) measurements. STS reveals both the Fermi level and Dirac point located inside the bulk bandgap, indicating bulk-like insulating behavior with accessible topological surface states (TSSs). STS and reveals a transition from n-type to p-type conduction at x ≈ 0.6. We use a two-channel analysis of MR data to differentiate the charge carrier types for surface and bulk transport; we conclude that surface transport is dominated by electrons and bulk transport is dominated by holes. Prior to this work, direct detection of topological surface states in BiSbTe systems has been achieved only for T<10 K.