Room‐Temperature Negative Differential Resistance and High Tunneling Current Density in GeSn Esaki Diodes

Abstract

distribution. Novel devices with different operational principles for a sharper SS than 60 mV per decade have been extensively investigated, such as impact ionization MOS (I-MOS) transistors, [1] negative-capacitance field-effect transistors (NC-FETs), [2] or tunnel field-effect transistors (TFETs). [3] While an SS of sub-60 mV per decade has been demonstrated for those devices, the issues of the required high gate biases for I-MOS transistors or the hysteresis for NC-FETs are critical issues for the reliability of integrated circuits. For TFETs, whose device structures are similar to conventional CMOS transistors, a steep SS is achieved by band-toband tunneling (BTBT) processes between the source and channel regions. The major issue for TFETs is their low drive current. For group-IV materials such as silicon (Si) or germanium (Ge), the device performance of TFETs is poor due to their large bandgap energies and effective masses of carriers, and the requirement of phonon participation for the momentum reservation in the BTBT process. [4,5] For III-V materials, while the tunneling current is high because of their direct-bandgap characteristics, [6,7] the poor quality of oxide interface could lead to the instability of device operations. Moreover, the compatibility with the Si-based VLSI technology is still a critical issue. [8] Recently, germanium-tin (GeSn) has attracted much attention for electronic, optoelectronic, and spintronic device applications owing to its direct-bandgap characteristics, [9] high carrier mobility, [10] strong spin-orbit coupling (SOC) effects, [11] and compatibility with the VLSI technology. Simulations on GeSn TFETs showed strong current enhancement than Si-or Gebased devices owing to the direct BTBT process and the smaller bandgap and effective carrier masses in GeSn. [12,13] To justify the BTBT process and calibrate the tunneling rates, Esaki diodes with negative differential resistance (NDR) are used to characterize the peak current density. Thus far, there is no NDR demonstrated in any GeSn-based Esaki diodes at room temperature. To observe NDR, the Fermi levels must be higher or lower than the conduction band minimum or the valence band maximum at the n-type or p-type regions, respectively. If the doping levels or the activation rates of dopants are not high enough, NDR will not be observed. Moreover, if there exists a large amount of defect states in the bandgap, electrons can tunnel across the junction via those states with the assistance of thermal processes, Tunnel field-effect transistors (TFETs) are a promising candidate for lowpower 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 calibr...

Recent progress in undoped group-IV heterostructures for quantum technologies

“…In 2021, Tai et al demonstrated even larger SOC strengths in GeSn [30] than in Ge [26]. With its direct-bandgap characteristics [31] and high carrier mobility [32,33], GeSn is a promising enabler to accomplish a full-functional chip including spintronic, photonic, and electronic devices compatible with the Si CMOS platform.…”