Atomic Layer Deposition of Al-Doped ZnO Contacts for ZnO Thin-Film Transistors

We report on the source-drain contact improvement in zinc oxide thin-film transistors (ZnO-TFTs) using Al-doped ZnO (AZO) intermediary layers by a thermal atomic layer deposition (ALD) process. This plasma-free method enables optimization of the AZO atomic ratio (Al:Zn) to improve contact resistance. In this study, Al:Zn is modulated between 1.0-5.0% using ALD and confirmed by energy-dispersive X-ray spectroscopy. AZO intermediary contacts are measured electrically using both linear transfer-length method (TLM) structures and integrated at the source-drain regions of ZnO-TFTs. These measurements show that the 20 nm AZO intermediary contact reduces TLM contact resistance compared to direct contact between Al metal and ZnO. Al:Zn ratios between 1.5% and 3.0% yield TFTs with switching characteristics. ZnO-TFTs with a 2.5% AZO intermediary layer exhibit the most favorable electrical characteristics with a contact resistance of
$140~\Omega /\mu \text{m}$
, sub-threshold swing of 130 mV/dec, on/off-current ratio of
${1}.{9}\times {10} ^{{9}}$
, a threshold voltage of −6.81 V, hysteresis of 10 mV, and field-effect mobility of 44.8 cm2/(
$\text{V}\cdot \text{s}$
). These measurements suggest that 2.5% AZO is the most effective Al:Zn ratio for intermediary contact layers in ZnO-TFTs.