Effect of the oxygen dependent device parameters on the electrical properties of a-Si-Zn-Sn-O thin film transistors

Abstract

The effect of partial pressure of oxygen on the amorphous Si-Zn-Sn-O thin film transistor is investigated. Sput-tered amorphous films were deposited at various oxygen partial pressures (O2/(Ar + O2) = 0, 0.07, and 0.11). The parameters, such as threshold voltage, field effect mobility, subthreshold slope, and ON current for the channel deposited at PO2 = 0 are obtained as-6.47 V, 23.98 cm2V-1s-1, 663 mV/dec, and 3.11 x 10-4 A, respectively. From the transfer curve (ID-VGS) of transistors fabricated at various PO2, it is observed that while the threshold voltage shifts from-6.47 to 0.21 V, the current decreases from 3.11 x 10-4 to 2.46 x 10-4 A with increasing PO2. The increment in current at lower PO2 is due to the introduction of excess charge carriers as a result of elevated oxygen vacancies. It is further confirmed by the transmission line measurement that the channel resistance monotonously decreases as a function of oxygen flow rate. Moreover, the contact resistance increases from 0.96 to 1.95 k Omega, with respect to the increased oxygen content. The increment in contact resistance is a result of the formation of an oxide layer (TiOx) at the electrode and channel interface. The thickness of the oxide layer increases from 0.86 to 2.81 mu m with increasing oxygen partial pressure, suggesting that the effective channel length reduces with increasing oxygen content in the channel layer.