Effects of Deposition Temperature on the Device Characteristics of Oxide Thin-Film Transistors Using In-Ga-Zn-O Active Channels Prepared by Atomic-Layer Deposition.

We demonstrated the physical and electrical properties of the In-Ga-Zn-O (IGZO) thin films prepared by atomic-layer deposition (ALD) method and investigated the effects of the ALD temperature. The film composition (atomic ratio of In:Ga:Zn) and film density were examined to be 1:1:3 and 5.9 g/cm, respectively, for all the temperature conditions. The optical band gaps decreased from 3.81 to 3.21 eV when the ALD temperature increased from 130 to 170 °C. The amounts of oxygen-related defects such as oxygen vacancies increased with increasing the ALD temperature. It was found from the in situ temperature-dependent electrical conductivity measurements that the electronic natures including the defect structures and conduction mechanism of the IGZO thin films prepared at different temperatures showed marked variations. The carrier mobilities in the saturation regions (μ's) for the fabricated thin film transistors (TFTs) using the IGZO channel layers were estimated to be 6.1 to 14.8 cm V s with increasing the ALD temperature from 130 to 170 °C. Among the devices, when the ALD temperature was controlled to be 150 °C, the IGZO TFTs showed the best performance, which resulted from the fact that the amounts of oxygen vacancies and interstitial defects could be appropriately modulated at this condition. Consequently, the μ, subthreshold swing, and on/off ratio for the TFT using the IGZO channel prepared at 150 °C showed 10.4 cm V s, 90 mV/dec, and 2 × 10, respectively. The threshold voltage shifts of this device could also be effectively reduced to be 0.6 and -3.2 V under the positive-bias and negative-bias-illumination stress conditions. These obtained characteristics can be comparable to those for the sputter-deposited IGZO TFTs.