Enhanced SnO₂ FETs via selective area fluorine doping

In this study, a selective fluorine (F) doping process was developed to enhance the performance of field-effect transistors (FETs) with SnO₂ channels deposited by thermal atomic layer deposition. The fluorination process was applied selectively to the source (S) and drain (D) regions of the SnO₂ FETs. Fluorination of the S/D regions before the annealing step of the SnO₂ channel resulted in a significant improvement in specific contact resistivity (ρ_c), reducing it from 1.1 × 10⁻² to 3.1 × 10⁻³ Ω cm² - more than a threefold enhancement compared to SnO₂ FETs without S/D fluorination. However, in this case, the SnO₂ channel could not be modulated, likely due to the diffusion of F ions into the channel during the annealing process. When fluorination was applied to the annealed SnO₂ channel, an on/off ratio of 2.4 × 10⁸ was achieved. However, ρ_c was 9.1 × 10⁻³ Ω cm², showing only a 15 % improvement compared to untreated SnO₂ FETs. To address this limitation, a two-step approach was developed for SnO₂ FETs, in which F-doped S/D regions were formed prior to depositing the SnO₂ channel. This approach yielded stable channel modulation with an on/off ratio > 10⁸, along with a significant enhancement in field-effect mobility, increasing from 10.4 to 25.3 cm²/V·s. Furthermore, the entire process was conducted at 400 °C or below, demonstrating its potential for enabling high-performance oxide transistors compatible with low temperature processes.