Siegl, Manuel; (2018) Atomic-scale investigation of point defect interactions in semiconductors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Miniaturisation of computer hardware has increased the transistor density in silicon devices significantly and is approaching the ultimate physical limit of single atom transistors. A thorough understanding of the nature of materials at the atomic scale is needed in order to increase the transistor density further and exploit more recent technology proposals. Moreover, exploring other materials with more desirable characteristics such as wide band gap semiconductors with a higher dielectric strength and optical addressability are paramount in the effort of moving to a post-silicon era. Scanning Tunnelling Microscopy (STM) has been shown to be a suitable tool for the investigation of the technologically important material surface properties at the atomic scale. In particular, Scanning Tunnelling Spectroscopy (STS) – and its spatial extension Current Imaging Tunnelling Spectroscopy (CITS) – can reveal the electronic properties of single atom point defects as well as quantum effects caused by the confinement of energetic states. Nanoscale device performance is governed by these effects. In order to control and exploit the quantum effects, they firstly need to be understood. In this thesis, three systems have been investigated with STM and STS/CITS to broaden the comprehension of confined quantum states and material surface properties. The first data chapter concentrates on the interaction of confined quantum states of dangling bonds (DB) on the Si(111)-(√ 3 × √ 3)R30◦ surface. The site dependent interaction between neighbouring bound states is investigated by changing the distance and crystallographic direction between two DB point defects, revealing a non-linear constructive interference of the bound states and an antibonding state in resonance with the CB. In the second data chapter we explore subsurface bismuth dopants in silicon, a system relevant to recent information processing proposals. Bismuth was ion-implanted in the Si(001) surface and hydrogen passivated before the STM study. The bismuth dopants form a bismuth-vacancy (Bi+V) complex, which acts as an acceptor and lowers the Fermi level. The Bi+V complex further induces in-band gap states, which appear as square-like protrusions with a round depression in the centre. Interference of these states is energy dependent and the antibonding state is found at a lower energy than the bonding state due to the acceptor-like nature of the Bi+V defect complex. The third investigated system concerns the silicon face of the wide band gap semiconductor Silicon Carbide (SiC(0001)). The influence of atomic hydrogen on the 4HSiC(0001)-3 × 3 surface was investigated and found to result in a surface etching at the lower and upper end of the passivation temperature range. The electronic structure of two different surface defects of the 3 × 3 reconstruction is presented and a new superstructure consisting of silicon atoms on top of the 4H-SiC(0001)-(√ 3 × √ 3)R30◦ surface was discovered. A Schottky barrier height study of different surface reconstructions finds a nearly optimal power device fabrication value for the (√ 3× √ 3)R30◦ prepared surface. In summary, I have found a quantum interference that results in bonding and antibonding states for DB bound states on the Si(111):B surface and Bi+V complex states in the Si(001):H surface. Additionally, a new silicon superstructure on the SiC surface and a silicon reconstruction dependent Schottky barrier height are found.

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