Analysis on Contact Resistance and Effective Channel Length of Thin Film Transistors Using Composition-Modified In–Ga–Zn-O Active Channels Prepared with Atomic Layer Deposition and Various Electrode Materials

To verify the effects of process conditions for the TFTs employing In–Ga–Zn-O (IGZO) channels prepared by atomic layer deposition (ALD), such as cationic composition of channel and/or source/drain (S/D) electrodes, the device characteristics including the field-effect mobility (μFE), the contact resistance (RC), and the channel length deviation (ΔL) were systematically investigated for the ALD IGZO thin film transistors (TFTs) fabricated with controlling the ALD subcyclic ratios and with variations in S/D electrodes of indium–tin-oxide (ITO) and molybdenum (Mo). The ALD temperature was fixed at 200 °C. When the subcyclic ratios of precursors (triethyl indium: In–Ga bimetallic: diethyl zinc) were modulated to 0:2:2, 1:2:2, and 2:2:2, the cationic compositions (In:Ga:Zn) were determined to be 1.0:2.8:5.4, 1.0:1.1:2.3, and 1.0:0.7:1.5, respectively. As the the indium (In) contents within the IGZO channel increased from 5% to 14%, the μFE's obtained from the TFTs using ITO and Mo S/D showed dramatic improvement from 16.1 to 36.9 cm2/(V s) and 2.8 to 20.1 cm2/(V s), respectively. For both sets of devices, the ΔL's were estimated to be sensitively varied with channel compositions, and the RC markedly decreased owing to the reduction of barrier height formed at electrode/channel interfaces with increasing the In contents. The types of S/D electrodes also have a critical effect on the device performance. The devices using Mo S/D exhibited marked degradations in device characteristics rather than the devices using ITO S/D, which was suggested to originate from the additional formation of MoO3 layer at Mo/IGZO interfaces during the ALD process for the preparation of IGZO channel layers. Based on these findings, the intrinsic values of μFE for the ALD IGZO TFTs could be determined with considerations of RC and ΔL, especially for the devices with shorter channel lengths.