Structure-property Correlations in Semiconductor Alloys and Nanostructures

Abstract

Advanced growth techniques have resulted in successful fabrication of various semiconductor alloys and nanostructures, such as quantum wells (QWs) and quantum dots (QDs). Due to their tunable effective band gap and carrier localization/confinement, these nanostructures enable the improvement and development of a variety of optoelectronic devices, such as lasers, solar cells, and photodetectors. However, the influences of the dopant distribution, local compositions, and topologies of the nanostructures on the electronic and optical properties have not been fully investigated. In this dissertation, we investigate the influences of topologies and local compositions on the band structure and carrier localization/confinement of GaAs/GaAsNBi QWs and InAs/GaAs QD layers using nanoscale experimental techniques in conjunction with Schrödinger-Poisson simulations in nextnano. Furthermore, we probe the arsenic distribution and its influences on dopant activation in polycrystalline CdTeSe:As. Using a combination of capacitance-voltage measurements, Schrödinger-Poisson simulations, and photoluminescence (PL), we identify an N-dependent increase in conduction band offsets CBO. Furthermore, we show that N mainly influences the conduction band and confined electron states, with relatively small effects on the valence band and confined hole states. This work provides important insights for tailoring CBO and confined electron energies for improving infrared optoelectronic devices. We also investigated the effect of the position of Si dopants within InAs/GaAs SK-QD layers on free carrier concentrations. Using nanostructure models of local In and Si compositions from local-electrode atom-probe (LEAP) tomography as input into self-consistent Schrödinger-Poisson simulations based on the effective mass approximation, we compute the dopant ionization, electron density, and energy band diagrams for SK-QD layers. Although dopants are provided in both layers, the ionized donors primarily reside outside of the QDs, providing extra electrons that are contained within the QDs. Indeed, due to the quantum confinement-induced enhancement of the donor ionization energy within the QDs, a lower fraction of dopants within the QDs are ionized. These findings suggest a pathway towards the development of 3D modulation-doped nanostructures. Subsequently, we have investigated the origins of photoluminescence from QD layers prepared by alternating depositions of sub-monolayers and a few monolayers of size-mismatched species, termed sub-monolayer (SML) epitaxy, in comparison with their SK-QD counterparts. Using measured nanostructure sizes and local In-compositions from LEAP tomography as input into self-consistent Schrödinger-Poisson simulations, we compute the 3D confinement energies, probability densities, and photoluminescence (PL) spectra for both InAs/GaAs SML- and SK-QD layers. A comparison of the computed and measured PL spectra suggests one-dimensional electron confinement, with significant 3D hole localization in the SML-QD layers that contribute to their enhanced PL efficiency in comparison to their SK-QD counterparts. Finally, we examined the arsenic distribution and its influence on dopant activation in poly-crystalline CdTe1-xSex solar cells absorbers prepared by vapor transport deposition (VTD) followed by post-growth annealing. For as-deposited (as-dep) CdTe:As, local-electrode atom probe tomography (LEAP) reveals non-uniform distributions metallic arsenic clusters in the top "doped" layers. Following post-growth annealing, secondary ion mass spectrometry (SIMS) suggests that arsenic has diffused into the entire CdTe layer, while LEAP reveals the dissolution of clusters, with nearly uniform distribution of atomic arsenic in the CdTe. Since the concentration of arsenic is 1 × 1018 cm-3 but the hole density ranges from 7.5 to 9.5 × 1015 cm-3, we hypothesize that a large fraction of the arsenic has incorporated into interstitial sites.