Nanometer-scale structural and electronic properties of low dimensional heterostructures.

摘要

Mercury cadmium telluride (MCT) based heterostructures and InAs/GaAs quantum dots have enabled significant advances in optoelectronic devices such as light emitters and detectors. In both cases, the atomic-scale structural and electronic properties of the heterostructure interfaces remain the least understood aspect of the devices. Further advances will require an improved understanding of issues such as interface abruptness, alloy non-uniformities, and local band-offsets. In this dissertation, the nanometer-scale structural and electronic properties of II-VI substrates and InAs/GaAs dots are investigated using a combination of cross-sectional scanning tunneling microscopy (XSTM) and variable separation scanning tunneling spectroscopy (STS).;The influence of crystal orientation and thickness on the cleavage of CdTe and Cd1--xZnxTe substrates, as well as the influence of In doping and annealing on the substrate resistivity are explored. The flattest cleaves were obtained for 900 microm thick (111) CdTe and Cd 1--xZnxTe wafers cleaved along [110]. Furthermore, after In-doping (n ∼ 2.2 x 1017 cm -3) and post-growth annealing (T = 750 °C) in a Cd-rich environment, the CdTe substrate resistivity was reduced to 0.04 O-cm, and XSTM measurements were performed.;The influence of surrounding In0.2Ga0.8As alloy layers on the size and distribution of InAs/GaAs dots, as well as the thickness of the surrounding wetting layer (WL) are examined. XSTM images reveal that the surrounding alloy layers promoted a 38% (71%) increase in average dot diameter (height), and a three-fold increase in WL thickness. A strain-based mechanism for dot formation and collapse in the absence and presence of alloy buffer and capping layers is proposed.;The origins of electronic states in individual, uncoupled dots and the surrounding WL are investigated using a combination of XSTM and STS. Room temperature STS spectra reveal a gradient in the effective bandgap within the dots with smallest values near the dot core and top surfaces. The variations in effective bandgap are apparently dominated by indium composition gradients, with minimal effects due to dot shape and strain. Indium composition gradients also dominate the effective bandgap variations in the WL.