Effect of Channel Thickness on Amorphous In─Ga─Zn─O‐Thin‐Film Transistors

Thin‐film transistors based on amorphous oxide semiconductors demand thinner channels for next‐generation display and memory applications. However, channel‐thickness scaling causes a positive threshold‐voltage shift and degrades field‐effect mobility. Here, we report film densification in sputter‐deposited, hydrogen‐free amorphous indium–gallium–zinc oxide as the channel thickness decreases, with the film density exceeding that of single‐crystal InGaZnO ₄ . Ab initio molecular dynamics simulations reveal a change in polyhedral connectivity from corner‐sharing to edge‐sharing. Fourier‐transform infrared spectroscopy and chemical etch‐rate measurements support this structural change, showing a shift of the M─O vibrational band peak to lower wavenumber and an increased etch rate. We propose that the resulting reduction in the M─O─M bond angle induces strain. In the proposed model, this strained local structure can enhance the formation of interstitial oxygen and lead to the formation of an oxygen Frenkel defect pair. This defect pair deepens the donor level and broadens the tail states below the conduction‐band minimum, thereby degrading thin‐film transistor performance. The degradation is suppressed in the In‐rich composition. This result may be understood in terms of the stabilization of interstitial oxygen as an electrically inactive peroxide species identified by the experimental results.