Electrical and Material Properties of Strained Silicon/Relaxed Silicon Germanium Heterostructures for Single-Electron Quantum Dot Application

Abstract

A single-electron quantum dot device is an ideal environment to demonstrate the concept of a spin-based quantum bit, a promising candidate to realize a quantum computer. Two-dimensional electron gases (2DEGs) in silicon/silicon germanium heterostructures have been considered as a potential platform to fabricate single-electron quantum dots for spin manipulations because silicon has an inherently longer spin coherence time. Then two different types of silicon 2DEGs, modulation-doped 2DEG and enhancement-mode undoped 2DEG, are discussed. The efforts to improve both 2DEGs into a better material system for quantum computing application are the main focus of this thesis.

A severe leakage issue of the Schottky gating on a modulation-doped 2DEG is resolved by successful suppression of phosphorus surface segregation. A high breakdown voltage is thus achieved in a Schottky gated modulation-doped 2DEG without significant gate leakage current. Implant isolation as an alternative for lateral electrical isolation in a modulation-doped 2DEG at 4.2 K is also successfully demonstrated. It preserves surface planarization and prevents the leakage issues through the corners of etched mesas. The best implant conditions for effective isolation and better thermal stability are examined and determined. The quality of these doped 2DEGs is verified to be unaffected by the implant isolation process.

The transport property of an enhancement-mode 2DEG is significant for a spin-based quantum bit. Various mobility-limiting factors in our undoped 2DEGs grown by RTCVD are identified. Efforts to alleviate these scattering mechanisms lead to mobility as high as 400,000 cm2/Vs and the critical density as low as 3.2x1010 cm-2 at 4.2 K. A tunable screening effect on remote charges at silicon/oxide interface is found to greatly improve the transport properties of thin-cap enhancement-mode 2DEGs, which compensates the detrimental influences from the remote charges at the interface, and thus remains the capability for a sharp electron patterning from the top gates. In addition, theoretical and experimental work on the effect of the regrowth interface in undoped 2DEGs is demonstrated as well.