Subgap states in aluminium- and hydrogen-doped zinc-oxide thin-film transistors

This study presents the Al2O3-induced hydrogen and aluminium doping effects on the electronic structures of atomic-layer-deposited ZnO films via low-temperature measurements and ab initio density functional theory (DFT) calculations. Bottom-gate ZnO thin-film transistors (TFTs) show n-type enhancement-mode transfer characteristics. However, when equipped with a top Al2O3 layer, the TFTs exhibit conductive transfer characteristics: the electron current increases significantly, and the threshold voltage clearly shifts to a depletion mode. Film analyses using time-of-flight secondary ion mass spectrometry (SIMS) depth profiles, ultraviolet photoelectron spectroscopy (UPS), and Hall measurements show that Al2O3-induced hydrogen and aluminium ions diffuse into the ZnO film, resulting in electron doping. In addition, temperature-dependent current-voltage measurements show that adding the top Al2O3 layer causes the tail-type energy-state distribution to change to a subgap-type distribution. Indeed, ab initio density functional theory (DFT) calculations reveal that the subgap states in doped ZnO films are formed through hybridization between the Zn(3d), Al(2p), and H(1s) bands. Thus, we conclude that Al2O3-induced aluminium and hydrogen ion doping leads to conductive changes in the ZnO films with substantial electronic structural changes in the subgap-type band structure.