Electrical transport in monolithic Al-Ge-Al Schottky barrier field-effect transistors

Abstract

The increasing demand for microelectronic components has prompted the exploration of new device architectures, processing technologies, and material systems to replace traditional silicon (Si) based technology. Schottky barrier field-effect transistors (SB-FETs) represent one promising alternative, offering potential advantages in scaling and integration. Recent approaches to forming metal-semiconductor heterostructures through thermally induced exchange reactions have emerged as a particularly effective method for creating metal-semiconductor heterostructures with atomically sharp interfaces, opening new possibilities for improving device properties, including higher switching speeds and better power efficiency. In recent years, germanium (Ge) has garnered significant attention due to its excellent processability, the availability of new passivation techniques, high charge carrier mobility, and strong quantum confinement effects compared to other group IV-based materials. The combination of Ge and aluminum (Al), along with their integration into Al-Ge-Al heterostructures, has facilitated the development of innovative field-effect transistor (FET) architectures. In this thesis, nanosheet structures were fabricated using a top-down approach and a Germanium-on-insulator (GOI) substrate. The Al-Ge-Al heterostructures were finally established through a thermally induced metal-semiconductor solid-state exchange process, initiated by rapid thermal annealing (RTA). The precise placement and sizing of the nanosheets, along with the well-controlled diffusion process, enabled the creation of a large array of monolithic,single-crystalline Al-Ge-Al nanosheet heterostructures. This method accommodated various Ge segment lengths, including ultra-short segments, without the limitations typically imposed by conventional lithography. The high-κ passivation of the sheet structure with alumina (Al2O3) and the vapor deposition of gold (Au) top gate structures enabled the integration of the heterostructures as channels in electrostatically gated SB-FETs. This setup facilitated a systematic investigation of charge carrier transport mechanisms across a range of Ge segment lengths from 50nm to 100μm, atvarious temperatures. The electrical characterization involved analyzing the transfer characteristics and conducting multi-variable bias spectroscopy. These techniques provided comprehensive insight into current modulation capabilities, revealing the effects of surface states and Fermi-level pinning. Furthermore, the analysis visualized the various dominant emission mechanisms at the Schottky barrier (SB), specifically thermionic emission (TE), thermionic-field emission (TFE), and field or tunnel emission (FE). This understanding enabled the determination of device resistivities and SB heights in both n-type and p-type transport regimes. The low-temperature characterization enabled the exploration of cryogenic properties, considering the charge carrier freeze-out effect and the associated reduction in scattering. Due to the different device scalings, including ultra-short Ge segment lengths approaching the mean free path of electrons, short-channel effects were also examined. These effects include punch-through, drain-induced barrier lowering (DIBL), and ballistic transport. The systematic evaluation of current transport mechanisms across different channel lengths and temperatures revealed key factors influencing device performance. These insights are essential for advancing the development of SB-FETs and exploring Ge as a fundamental material for emerging nanoelectronic and quantum devices.