ABSTRACT
Tunnel field-effect transistors (TFETs) are a promising candidate for low-power applications owing to their steep subthreshold swing of sub-60 mV per decade. For silicon- or germanium-based TFETs, the drive current is low due to the indirect band-to-band tunneling (BTBT) process. Direct-bandgap germanium-tin (GeSn) can boost the TFET performance since phonon participation is not required during the tunneling process. Esaki diodes with negative differential resistance (NDR) are used to characterize the BTBT properties and calibrate the tunneling rates for TFET applications. This work demonstrates high-performance GeSn Esaki diodes with clear NDR at room temperature with very high peak-to-valley current ratios of 15-53 from 300 K to 4 K. A record-high peak current density of 545 kA cm-2 at 4 K is also reported for the tensile-strained Ge0.925 Sn0.075 device (strain ≈0.6 %). By applying tensile stresses to n-GeSn epitaxial films, the direct BTBT process dominates, leading to high tunneling rates. Hall measurements further confirm that more electrons populate in the direct Γ valley in the tensile-strained n-GeSn epitaxial films.
ABSTRACT
A demonstration of 2D hole gases in GeSn/Ge heterostructures with a mobility as high as 20 000 cm2 V-1 s-1 is given. Both the Shubnikov-de Haas oscillations and integer quantum Hall effect are observed, indicating high sample quality. The Rashba spin-orbit coupling (SOC) is investigated via magneto-transport. Further, a transition from weak localization to weak anti-localization is observed, which shows the tunability of the SOC strength by gating. The magneto-transport data are fitted to the Hikami-Larkin-Nagaoka formula. The phase-coherence and spin-relaxation times, as well as spin-splitting energy and Rashba coefficient of the k-cubic term, are extracted. The analysis reveals that the effects of strain and confinement potential at a high fraction of Sn suppress the Rashba SOC caused by the GeSn/Ge heterostructures.
