Nanometer-scale Structural and Electronic Properties of Low Dimensional Heterostructures.

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 μm thick (111) CdTe and Cd1-xZnxTe wafers cleaved along [110]. Furthermore, after In-doping (n ~ 2.2 × 1017 cm-3) and post-growth annealing (T = 750 °C) in a Cd-rich environment, the CdTe substrate resistivity was reduced to 0.04 Ω-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.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75974/1/vdasika_1.pd

Moments-based tight-binding calculations of local electronic structure in InAs/GaAs quantum dots for comparison to experimental measurements

Local electronic properties of InAs/GaAsInAs∕GaAs nanostructures are studied using a real-space moments method sp3d5s*sp3d5s* tight-binding approach. The order (N)(N) method is unique because it allows for accurate and highly resolved determination of local density of states that accounts for local strain, disorder, and defects, without diagonalization of the full tight-binding Hamiltonian. The effects of free surfaces and strain are first investigated by considering pure, cuboidal GaAs nanostructures. The quantum confinement in an embedded InAs quantum dot is then shown directly through the local densities of states projected on different atoms in the structure. The relationship between effective energy band gap and quantum dot size is mapped onto a simple equation. Finally, the real-space study is applied to quantum dot structures observed experimentally using scanning tunneling microscopy. Atomic positions are obtained from the images and used as input into the tight-binding calculations in order to study interfacial effects on the local electronic structure of real embedded quantum dots.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87778/2/053109_1.pd

Nanometer-scale studies of point defect distributions in GaMnAsGaMnAs alloys

We have investigated the concentrations and distributions of point defects in GaMnAsGaMnAs alloys grown by low-temperature molecular-beam epitaxy, using ultrahigh-vacuum cross-sectional scanning tunneling microscopy (XSTM). High-resolution constant-current XSTM reveals “A,” “M,” and “V” defects, associated with AsGaAsGa, MnGaMnGa, and VAsVAs, respectively. A and V defects are present in all low-temperature-grown layers, while M defects are predominantly located within the GaMnAsGaMnAs alloy layers. In the GaMnAsGaMnAs layers, the concentration of V defects ([V])([V]) increases with the concentration of M defects ([M])([M]), consistent with a Fermi-level-dependent vacancy formation energy. Furthermore, [M][M] is typically two to three times [A][A] and [V][V], suggesting significant compensation of the free carriers associated with MnGaMnGa. A quantitative defect pair correlation analysis reveals clustering of nearest V–V pairs and anti-clustering of nearest M–M, M–V, and M–A pairs. For all pair separations greater than 2 nm2nm, random distributions of defects are apparent.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87856/2/011911_1.pd