Overcoming Negative nFET V_TH in Ultra-Thin ITO-IGZO Hetero-Oxide Channel FET by Defect-Compensation to Achieve Record Mobility

In this work, we have successfully demonstrated, low-thermal budget amorphous oxide-based field effect transistor (FETs) with a record ON-current (I_{D, Max}) of 790 µA/µm at V_DS=1V. This device shows an enhancement-mode operation (V_TH>0), sub-threshold slope (S.S.) <90 mV/dec., and DIBL ~20mV/V at an ultra-scaled channel length (L_CH) of 50 nm. The FETs have been fabricated at low-temperature (<350°C) combining sputter deposition (channel layer) and atomic layer deposition (gate insulator HfO₂), making this fabrication approach compatible with low-thermal budget Cu interconnects for back-end-of-line (BEOL). In pure ITO channel, ON-current increases with channel thickness but the V_TH become progressively negative and the SS is degraded. However, in hetero-junction channel, the performance of our FET is comparable to emerging two-dimensional materials and superior to that of existing metal oxides. The high performance is enabled by interfacial channel defect self-compensation in an optimized InSnOx-InGaZnOx (ITO-IGZO) hetero-junction channel. Moreover, this approach overcame the fundamental issue of negative V_TH seen in n-type oxide FETs due to donor-type channel oxygen vacancy (Vo) and the limited tunability of gate metal work function. Through our ITO-IGZO channel and defect self-compensation approach, our transistor effective mobility (µ_eff) is boosted to 110 cm²/Vs which is independent of the channel thickness (T_CH) when the T_CH is scaled down to 4 nm. This unique T_CH-independent mobility behaviour is not observed for IGZO or ITO mono-channel FETs. With such enhancement, our ITO-IGZO FETs exhibit the best-in-class mobility among oxide-based FETs, and are competitive to unstrained Silicon thin film and SOI FETs, while being compatible with sub-400 °C BEOL processes.