Post-annealing effect of low temperature atomic layer deposited Al2O3 on the top gate IGZO TFT

Electronical properties of top gate amorphous InGaZnO ₄ thin film transistors (TFTs) could be controlled by post-annealing treatment, which has a great impact on the Al ₂O ₃ insulator. To investigate the effect of post-annealing on Al ₂O ₃, Al/Al ₂O ₃/p-Si MOS capacitoras with Al ₂O ₃ films treated under various post-deposition annealing (PDA) temperature were employed to analysis the change of electrical properties, surface morphology, and chemical components by electrical voltage scanning, atomic force microscope (AFM), and x-ray photoelectron spectroscopy (XPS) technologies. After PDA treatment, the top gate TFTs had a mobility about 7 cm ² V ⁻¹ s ⁻¹ and the minimum subthreshold swing (SS) about 0.11 V/dec, and the threshold voltage (V _th) shifted from positive direction to negative direction as the post-annealing temperature increased. Electrical properties of MOS capacitors revealed the existence of positive fixed charges and the variation of trap state density with increasing PDA temperature, and further explained the change of negative bias stress (NBS) stability in TFT. AFM results clarified the increased leakage current, degraded SS, and NBS stability in MOS capacitors and TFTs, respectively. XPS results not only illuminated the origin of fixed charges and the trap density variation with PDA temperatures of Al ₂O ₃ films, but also showed the O and H diffusion from Al ₂O ₃ into IGZO during post-annealing process, which led to the deviation of V _th, the change of current density, and the negative V _th shift after positive bias stress in TFTs.