Strained silicon and silicon-germanium quantum devices by chemical vapor deposition

Abstract

Strained SiGe band-to-band tunneling (BTBT) devices and strained Si two-dimensional electron gases (2DEGs) are promising for low-power and quantum computing applications. The objective of this dissertation is to pursue the fundamental understanding of BTBT in strained SiGe films and electron transport properties in strained Si.

We report the first quantitative study of BTBT in strained p+-SiGe/n+-Si heterojunctions and p<super>+</super>-SiGe/n<super>+</super>-SiGe homojunctions at forward and reverse biases. Negative differential resistance (NDR) at forward bias is clearly observed for each device, with the highest observed peak current density of 10<super>4</super> A/cm<super>2</super>. In reverse bias, a BTBT current density of 10<super>6</super> A/cm<super>2</super> is measured and a model comparison with good agreement is also presented. Furthermore, we demonstrate that the precise modeling of reverse-biased BTBT devices requires the observation of NDR in forward bias.

The surface segregation of phosphorus in relaxed SiGe films is studied with an extremely sharp phosphorus turn-off slope of 6 nm/decade reported. This enables effective Schottky gating on a depletion-mode device of a Si two-dimensional electron gas (2DEG). We also investigate the effect of surface hydrogen on phosphorus segregation. A phenomenological model for this segregation is proposed to explain the experimental results with good agreement.

A 2DEG with a record high mobility of 522,000 cm<super>2</super>/V-s in an isotopically enriched <super>28</super>Si quantum well is presented. The estimated electron dephasing time of ~ 2 &mu;s is presented. We investigate the effects of different layers in a Si 2DEG structure on electron mobility and conclude that the remote impurity charges are the dominant source for electron scattering. The reduced segregation of phosphorus enables an inverted modulation-doped Si 2DEG with extremely high mobility of 470,000 cm<super>2</super>/V-s. For the first time second subband occupancy was achieved in a Si quantum well.