Experimental studies on germanium-tin p-channel tunneling field effect transistors

Abstract

Recent years have shown a growing interest in device concepts based on quantum mechanical tunneling. The tunneling field effect transistor (TFET) is a device that competes directly with the metal-oxide-semiconductor field effect transistor (MOSFET) in terms of speed, power and area. The drive current injection mechanism in TFETs is a band-to-band tunneling (BTBT) current and the promise of the TFET lies in its steep subtreshold current-voltage (I-V) characteristics, which is not restricted by the MOSFET’s 60 mV/dec limit at room temperature. TFETs could perform better at low supply voltages, but improvement of the drive current is necessary to outperform the MOSFET. In this work different device tuning strategies for the p-channel germanium (Ge) TFET are studied. Modifications involving the semiconductor material and doping profiles are investigated with the aim of increasing the tunneling probability and achieving high drive currents. This investigation has been conducted through designing, fabricating and characterizing the vertical TFET structures. Vertical semiconductor structures were grown by means of molecular beam epitaxy (MBE), and the vertical devices were fabricated using a gate-all-around (GAA) geometry fabrication process. It is shown that the drive current (ION) can be effectively increased by the introduction of germanium-tin (GeSn) in the channel. A successive increase in ION is seen when increasing the tin (Sn)-content, x, in a germanium-tin (Ge1-xSnx) channel from x = 0 % to x = 2 % and x = 4 %. This is due to the lowering of the bandgap, which effectively increases the tunneling probability. Furthermore, it is found that when Ge0.96Sn0.04 is confined within a 10 nm delta-layer, TFET device performance can be tuned by shifting the position of this layer at the source-channel interface. A high ION is achieved when this layer is completely inside the channel, while the leakage current (IOFF) is reduced when this layer is shifted from the channel and into the source. A complicating factor with incorporating Ge1-xSnx in the p-channel Ge TFETs is found to be the difficulty of maintaining a high epitaxial quality when increasing the Sn content. Together with the lowering of the bandgap, this is shown to degrade the IOFF and subthreshold swing (SS) of the device through increased Shockley-Read-Hall (SRH) generation and trap-assisted tunneling (TAT) currents. This further calls into question the feasibility of achieving acceptable performance with GeSn as channel material. Based on the results, some device performance strategies are discussed. Varying the source doping concentration in p-channel Ge TFETs with gate-source overlap is found to mainly influence the subthreshold characteristics of the devices. Steeper subthreshold characteristics is found with increasing source doping concentration. This correlation is believed to be a result of TAT in the source-gate overlap region. Contrary to results from published simulation studies, no effect of varying the source doping concentration on ION could be distinguished for the doping levels investigated. A MBE pre-buildup technique of antimony (Sb) is investigated as a means to achieve steep source doping profiles in vertical p-channel Ge TFETs. It is seen that for a Sb pre-buildup concentration of 1/20 monolayer (ML), both ION and SS is improved. This is explained by that the extent of the tunneling barrier into the source region is reduced, leading to an increase of the tunneling probability and improvement of the band pass filtering. The boost in ION is small, but the pre-buildup technique imposes no extra load onto the TFET fabrication process and can easily be combined with other strategies for boosting the drive current for TFETs. The results also suggests that an optimal pre-buildup doping exists. In this work also the aluminum oxide (Al2O3), which is used as gate oxide, and the Ge/Al2O3/Al system is studied. A germanium oxide (GeOx)-passivation achieved through post-plasma oxidation and a sulfur (S)-passivation achieved through an aqueous Ammonium sulfite solution treatment, are both investigated through the fabrication and electrical characterization of MOS-capacitors. For the sample passivated with GeOx, a hysteresis and a shift in the flatband voltage is explained by acceptor traps in the oxide. A general parallel shift of the capacitance-voltage (C-V)-curve towards positive gate voltages indicates fixed negative charges and an O-rich Al2O3. It is suggested that these O-rich regions could be induced by the post plasma oxidation treatment. Temperature dependent current-voltage (I-V)-characteristics indicate a Schottky emission process as the main transport mechanism through the oxide at low electric fields. The effect of S-passivation of the Ge surface is seen to reduce both the C-V hysteresis and the leakage current in the low E-field region. The measured oxide capacitances also reveal that this does not come at the expense of a thicker equivalent oxide thickness (EOT).