Extraction of Trap Densities in ZnO Thin-Film Transistors and Dependence on Oxygen Partial Pressure During Sputtering of ZnO Films

Trap densities in the channel layers Dt of ZnO thin-film transistors have been extracted. First, the low-frequency (low-f) capacitance-voltage C-V characteristics are measured using the customized measurement system. Next, the surface potential is calculated from the low-f C-V characteristic, and the surface potential gradient is calculated by applying Gauss´s law. Finally, the spatial profile of the electric potential is calculated by applying Poisson equation and carrier density equations, and Dt is extracted. It is found that, generally, the deep states are flatly distributed in the energy gap apart from the conduction band, and the shallow states seem to be the tail states. Moreover, the dependence on the oxygen partial pressure during the sputtering of ZnO films [p(O₂)] has been analyzed. It is found that for p(O₂) = 0.50 ~ 0.75 Pa, Dt changes little, whereas for p(O₂) = 0.17 ~ 0.33 Pa, Dt increases. It is clarified that the abnormal shapes of the current-voltage and low-f C-V characteristics originate from the increase of Dt.