Ballistic transport phenomena in Al-Ge-Al nanowire heterostructures

Abstract

So far, continuous miniaturization of classical planar electronic devices has been the main driving force behind the advancement of modern integrated circuit technology. However, due to physical limits and dramatic repercussions of short channel effects, scaling becomes increasingly difficult. Hence, a shift towards the adoption of new materials and novel design architectures is predicted to insure further improvement of integration densities, power dissipation and performance. Semiconductor nanowires (NWs) are predicted to be one of the most promising building blocks for future ultra-scaled high-speed microelectronics. Out of the wide range of NWs, germanium (Ge) occupies an exceptional position, because it combines a high carrier mobility, enabling high performance devices, with a more than five times larger exciton Bohr radius compared to silicon (Si). Hence, Ge is especially interesting for the development of novel quantum devices. Dedicated to the small feature sizes required, until now it was not possible to show ballistic transport in group IV semiconductor NW based devices at temperatures above 20mK. The scope of the diploma thesis at hand was to synthesize axial Al-Ge-Al NW heterostructures with abrupt interfaces and monocrystalline aluminum (Al) leads. This was achieved by using a thermally initiated exchange reaction between vapor-liquid-solid (VLS) grown single-crystalline Ge NWs and Al contact pads. Applying rapid thermal annealing (RTA) for the formation of Al-Ge-Al NW heterostructures is one of the key advantages of the fabrication strategy, because it enables the formation of a Ge nanodot without requiring precise lithographic alignment of the contacts, which is one of the most challenging issues of fabricating nanodot based devices. The capability to control the size of the Ge segment connected by two Schottky tunnel barriers was achieved by fine tuning of the process parameters. Thus, it was possible to fabricate Al-Ge-Al NW heterostructures featuring ultrashort Ge segments down to 10 nm, which can be operated as back-gated field-effect transistors (FETs). Based on NW heterostructures with ultrasmall Ge segments, a systematic investigation of ballistic transport phenomena was carried out by electrical characterizations at room temperature as well as cryogenic temperatures down to 5K. In order to allow interpretation and to gain a better insight into the measurement results of the conducted transport measurements, the diameter dependence of quantum confinement effects in Ge NWs was investigated by simulations based on the 2D Schrödinger equation. By comparing the experimental data and simulation results, evidence of ballistic transport for Al-Ge-Al NW heterostructures with Ge segment lengths varying between 10nm and 30nm at room-temperature is presented.