Reconfigurable field-effect transistors based on aluminium-silicon-germanium heterostructures

Abstract

In today’s world, data acquisition and processing systems as well as means of communication are ubiquitous, with a general trend towards increasingly powerful systems. In order to keep up with this trend, the transistor size has been down-scaled more and more, finally reaching physical limits. The implementation of alternative device concepts and the use of novel material systems, such as silicon-germanium or high-k dielectrics, are paving newways by also diversifying the overall functionality of the related transistors. In the light of this, in this thesis, a doping-free reconfigurable field-effect transistor (RFET) is fabricated out of a silicon-germanium containing wafer stack. The corresponding transistors are fabricated in a top-down fashion from the associated silicon-germanium-oninsulator (SGOI) initial wafers. In order to form the underlying metal-semiconductormetal heterostructure, in the case of this thesis, the aluminium-SiGe-aluminium, thermally activated diffusion of the aluminium (Al) into the semiconducting nanosheet is used resulting in a reliable, reproducible and abrupt transition. The passivation layer between its top-electrode configurations and the semiconductor channel is based either on a pure silicon dioxide (SiO2) or on a combination of silicon dioxide (SiO2) and the high-k dielectric hafnium dioxide (HfO2), depending on the fabricated sample. It has been shown that with a SiO2 and the associated low trap density at the interface between the channel and the top-electrode, the fabricated nanostructures exhibit significantly lower hysteresis than those samples with only a pure high-k dielectric as the insulating layer. Therefore, an extrasilicon capping layer has been added to the SiGe layer as a sacrificial layer in the used wafer stack. The realised Schottky barrier (SB)FETs, in the case of a single top electrode over both metal-semiconductor junctions, or the fabricated RFETs with three, four or five top electrodes are measured electrically, especially an in-depth bias spectroscopy investigation, in order to determine relevant characteristic properties of the created structures and in order to make a comparison with other existing RFET devices in literature. Considering the goal of this thesis, namely the realisation of an RFET with relatively higher on-state currents of the two operation modi with a sufficient on-state current symmetry, especially regarding the Si counterparts, the electrical characterisation of the fabricated structures provides remarkably results. The sample passivated with only SiO2 exhibits a on-state current symmetry of 2.2 with an insignificant hysteresis and, compared to the Al-Si-Al, relatively higher on-state and off-state currents for both operation types. A further improvement is made by using a combination of SiO2 and the high-k dielectric HfO2,where the hole-dominated on-state and the off-state current for both operation modi is enhanced, however, by the expense of the general on-state symmetry and a slightly higher hysteresis. In respect to the threshold voltage and the sub-threshold slope, the sample with the high-k dielectric obtains improved values, accordingly. Generally speaking, the presented material system and the related RFET structures fabricated in this thesis are showing the possibilities regarding energy-efficient and adaptive circuits for a broad field of applications, for instance in the field of hardware security or artificial intelligence.