Role of postannealing temperature on the microstructure of Al2O3/ZnO thin films grown by atomic layer deposition for TFT applications

In this study, the effects of postannealing on ZnO and Al2O3 films grown for a thin‐film transistor (TFT) by the atomic layer deposition (ALD) method were examined using transmission electron microscopy (TEM) and energy‐dispersion spectroscopy. Samples were subjected to rapid thermal annealing for 30 s at 610, 615, 620, and 800 °C in a N2 atmosphere. At these temperatures, the polycrystalline ZnO layer in a TFT channel rapidly degraded to amorphous, and in the Al2O3 layer of a gate oxide layer of the TFT, a ZnAl2O4 spinel structure was formed because of Al and Zn diffusion. This spinel structure was observed to be independently nucleated, and there were no specific postgrowth crystallographic orientation relations. Additionally, in electrical properties of ZnO‐TFTs with ZnO/Al2O3 layers, typical TFT characteristics was shown until the annealing temperature increased to 600 °C, and the Vth gradually decreased from 4.2, 3 to 1.8 V. However, in the 700 °C annealed case, the TFT devices are always opened, and the ZnAl2O4 generation and the ZnO layer degradation was related to the drastic changes of these ZnO‐TFTs characteristics. During postannealing, rapid degradation of the ZnO channel layer and phase transformation by means of diffusion should be carefully considered.