Influence of active layer thickness and annealing in zinc oxide TFT grown by atomic layer deposition

We utilized atomic layer deposition (ALD) for the growth of the ZnO channel layers in the oxide thin-film-transistors (TFTs) with a bottom-gate structure using a SiO₂/p-Si substrate. For fundamental study, the effect of the channel thickness and thermal treatment on the TFT performance was investigated. The growth modes for the ALD-grown ZnO layer changed from island growth to layer-by-layer growth at thicknesses of > 7.5 nm with highly resistive properties. A channel thickness of 17 nm resulted in good TFT behavior with an on/off current ratio of >10⁶ and a field effect mobility of 2.9 without the need for thermal annealing. However, further increases in the channel thickness resulted in a deterioration of the TFT performance, or no saturation. The ALD-grown ZnO layers showed reduced electrical resistivity and carrier density after thermal treatment in oxygen.