Anomalous process temperature scaling behavior of sol-gel ZrOx gate dielectrics: Mobility enhancement in ZnO TFTs

Solution-processed transparent conductive oxides have emerged as an attractive material system for realization of large-area and flexible electronic systems, where the scalability of solution-processing offers potential advantages [1]. Recently, the performance of solution-processed ZnO thin-film transistors (TFTs) deposited via spray pyrolysis at 400°C has become comparable to sputtering with a competitive mobility of 85 cm²/Vs on a sol-gel ZrO_x gate dielectric [2]. Similar results such as these have been obtained in other TCO material systems by exploiting the commonly observed yet unexplained phenomena of mobility enhancement due to high-k dielectrics. [3] While many of these results have been obtained at process temperatures of >500°C, there is particular interest in lowering the process temperatures to expand the range of compatible substrates. We have investigated the impact of lowering the deposition temperature of sol-gel ZrO_x dielectrics [4] on the electrical performance of ZnO TFTs processed at a low temperature of 250°C using a spray pyrolysis technique [2]. Surprisingly, and potentially of tremendous advantage, transistor performance improved dramatically as the processing temperature of the ZrO_x was reduced.