Zinc tin oxide thin film transistors produced by a high rate reactive sputtering: Effect of tin composition and annealing temperatures

Abstract

Amorphous zinc tin oxides (a‐ZTO), which are stoichiometrically close to the Zn2SnO4 and ZnSnO3 phases, have been deposited using remote‐plasma reactive sputtering, and incorporated as the channel layers in thin film transistors (TFTs). The influence of tin composition and annealing temperatures on the structural and phase evolutions of the thin films, and the electrical performances of the TFTs are investigated. Zn2SnO4 exhibited randomly oriented polycrystalline peaks at annealing temperatures ≥700 °C, while ZnSnO3 decomposed into Zn2SnO4 and SnO2 at 950 °C. TFTs employing a Zn2SnO4 channel, after a post‐deposition annealing at 500 °C, exhibited a field effect mobility ∼14 cm2 V−1 s−1 and a sub‐threshold slope ∼0.6 V dec−1. When the tin content was increased in the channel, as in ZnSnO3, TFTs exhibited an increase in field effect mobility ∼20 cm2 V−1 s−1, but with a slight deterioration of sub‐threshold slope to ∼0.8 V dec−1. When the post‐deposition annealing temperature was reduced to 300 °C, a mobility as high as ∼10 cm2 V−1 s−1 was still achieved, however, a significant shoulder in the IDS–VGS curve, together with a higher off‐state current was observed. TFT characteristics are explained by the sub‐bandgap defect states measured by photothermal deflection spectroscopy and the extracted Urbach energies.

Optimisation of amorphous zinc tin oxide thin film transistors by remote-plasma reactive sputtering

“…62 Using the photothermal deflection method, the Urbach energies of 98 and 102 meV are obtained for annealed films of ZTO (10% Sn) and ZTO (33% Sn), respectively. 63 These values are also very similar to previously reported Urbach energies $110 meV for ZTO 64 and $110-160 meV for IGZO. 60 The slightly lower Urbach energy of ZTO (10% Sn) comparing to ZTO (33% Sn) agrees with a slight reduction in SS of ZTO (10% Sn) comparing to ZTO (33% Sn).…”

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

“…However, the oxygen vacancy related deep states defect density is higher in 33ZTO TFT than the 50ZTO TFT which is consistent with the XPS analysis results. We have previously reported the direct measurement of the density of sub-bandgap states by photothermal deflection spectroscopy (PDS) and photoconductive laser spectroscopy, which are very sensitive optical absorption methods 53 , 54 . These measurements shows that the 33ZTO has a higher density of deep states than the 50ZTO.…”

Tail-states induced appearance of conductor‑like behaviour in zinc‑tin‑oxide photo‑thinfilm transistors under sub-bandgap light excitation

“…However, the oxygen vacancy related deep states defect density is higher in 33ZTO TFT than the 50ZTO TFT which is consistent with the XPS analysis results. We have previously reported the direct measurement of the density of sub-gap states by photothermal deflection spectroscopy (PDS) and photoconductive laser spectroscopy, which are very sensitive optical absorption methods 53,54 . These measurements shows that the 33ZTO has a higher density of deep states than the 50ZTO.…”