Germanium Doped SnO₂: An Exploratory Channel Material for High On–Off Current Ratio and Low Subthreshold Slope in n-Type SnO₂:Ge Thin Film Transistor

We report germanium (Ge) doping in tin oxide (SnO2), which led to achieving a record ON–OFF current ratio of ~109 and a subthreshold slope (SS) of 77 mV/decade in a bottom-gated n-type SnO2:Ge thin film transistor (TFT) with 40- $\mu $ m channel length. Ge atomic percentage control to 12.2% during cosputtering of Ge and Sn in O2 plasma was shown to reduce oxygen vacancies (from 26.13% to 12.3%), which occurred due to Ge substitution in the Sn vacant sites of SnO2 lattice, leading to rearrangement of higher formation enthalpy Ge–O bonds. Low oxygen vacancies, therefore, impacted OFF current and SS of TFT with the Ge doped channel. Furthermore, for the same percent of atomic doping with Ge, field effect mobility was increased to 14.5 cm2/V-s, and barrier height of aluminum source–drain contacts with the SnO2:Ge channel was reduced from 0.69 to 0.47 eV, which were found suitable for enhancing drive current of SnO2:Ge TFT. Physical and electrical parameters of TFT fabricated with this exploratory channel material were characterized in detail.