Studies of Novel Nanostructures by Cross- sectional Scanning Tunneling Microscopy
This thesis presents structural and morphological studies of semiconductor nanostructures, namely quantum dots, nanowires and a dilute ferromagnetic semiconductor. These nanostructures are investigated on the atomic scale using cross-sectional scanning tunneling microscopy (XSTM). Indium arsenide (InAs) quantum dots in both an indium phosphide (InP) and GaAs matrix are studied. The InAs/InP quantum dots are shown to be vertically aligned and I present experimental and theoretical investigations on the vertical correlation of the dots. The InAs/InP dots have a pure InAs stoichiometry, with intermixing occurring only at the outermost atomic rows, and they have a truncated pyramidal shape. In the case of InAs/GaAs quantum dots, their shape could best be approximated by an oval dot shape; they are intermixed contrarily to the InAs/InP dots and have a non-uniform size distribution. Furthermore, various defects are observed in the quantum dot structures and the surrounding material. The origins of these defects are discussed and compared to available theoretical predictions. One example of such defect is the spontaneous formation of an extra quantum dot in the grown structures. The spontaneously formed dot nucleates on top of the vertical stacks and is of the same nature as the seed dots. The formation of the extra quantum dot is attributed to phase segregation effects in the top GaInAs layer of the structure and As/P exchange reactions. XSTM results on semiconductor nanowires are also presented. Using a special embedding scheme, individual atomic positions, stacking faults and different defects inside a GaAs nanowire are imaged. Moreover, I have investigated nanowire heterostructures, with wires containing an aluminium gallium arsenide (AlGaAs) segment and surrounded by an AlGaAs shell. It is shown that one of the interfaces between the wire and an embedded AlGaAs segment is sharp while the other one is diffuse and solutions on how to achieve sharp heterostructure interfaces and smooth shell capping are proposed. Finally, the location sites of manganese (Mn) atoms in a Mn-doped gallium arsenide (GaAs) lattice are studied and the Mn atoms are found at gallium sites (substitutional Mn) in the second layer gallium. This knowledge is used to investigate the diffusion of Mn atoms in GaAs/GaMnAs superlattices and it is shown that approximately 20% of Mn is found to diffuse to the adjacent GaAs layers.
Source Type:Doctoral Dissertation
Keywords:NATURAL SCIENCES; Physics; Cross-sectional scanning tunneling microscopy; Dilute ferromagnetic semiconductors Superlattices; Semiconductor nanostructures; Fysik; Physics; Epitaxy; Quantum dots; Nanowires
Date of Publication:01/01/2006