ALD W-Doped SnO${}_2$ TFTs for Indium-Free BEOL Electronics

Abstract

This work reports back-end-of-line (BEOL) compatible, thin-film transistors (TFTs) with sub-10 nm tungsten-doped tin oxide (TWO) channels deposited by atomic layer deposition (ALD) at 150 ${}^{\circ }$C. TFTs with undoped SnO${}_{\mathrm{x}}$, undoped WO${}_{\mathrm{x}}$, and W-doped SnO${}_{\mathrm{x}}$ channels with W concentrations of 5% and 10% were investigated. TFT with 10% W doping exhibited the best electrostatic control and overall device performance. Post-fabrication O${}_2$ annealing at 300 ${}^{\circ }$C for 5 minutes significantly enhanced device characteristics, reducing the subthreshold swing (SS) by nearly 2$\times$, increasing the I${}_{\mathrm{on}}$/I${}_{\mathrm{off}}$ ratio from $10^7$ to $10^9$, decreasing hysteresis by nearly 3$\times$ and positive bias stress-induced threshold shift by over 2$\times$ to a low value of 93 mV at a stress field of 4 MV/cm. Kinetic Monte Carlo simulations using Ginestra${}^{\mathrm{TM}}$ support the experimental observations and attribute the bias instability to charge trapping in the gate dielectric and at the interface. This work demonstrates low-temperature ALD-grown TWO TFTs as a promising indium-free platform for BEOL and monolithic 3D integration.