PhD defense: Steffen Zelzer

Scanning Tunnelling Microscopy of Molecular Beam Epitaxy Grown Intrinsic and Buried InAs Quantum Well Structures

In current InAs quantum well based device architectures, highly strained interfaces and a low mobility of the InAs channel are the main bottlenecks to their success. Such devices are commonly grown on {001}-oriented substrates. The use of {111}-oriented substrates might pose a route to reduce strain at interfaces and yield higher mobility in the InAs channel. This thesis presents results of scanning tunnelling microscopy and spectroscopy studies of the molecular beam epitaxy grown InAs(111)A surface and buried InAs quantum wells.

First, a surface roughness driven optimization of the molecular beam homoepitaxy of InAs(111)A towards macroscopic and atomic-scale clean surfaces are presented. Preparation of such surfaces, prior to in-situ superconductor deposition is expected to result in higher-quality superconductor-semiconductor interfaces. The electronic states of the InAs(111)A surface provide a platform for studying a plethora of many-body effects. We provide a method for band offset extraction via scanning tunneling spectroscopy and trace the origin of surface states to the

InAs(111)A-(2 × 2) reconstruction. Furthermore, we report on the behavior of the two types of electronic states on the InAs(111)A surface when encountering native step edge defects.

We then investigate state-of-the-art quantum devices, featuring a buried InAs quantum well. We present surface-averaged scanning tunneling spectroscopy data that captures the tail of the two-dimensional quantized states formed in the quantum well at the surface. Fitting a comparison to tight-binding calculations yields a realistic band offset. We further report on efforts to leverage the access to the cross-section of such samples in a specially designed auxiliary chamber.