Effects of High-Energy X-ray Irradiation on the Electrical and Chemical Properties of In–Ga–Sn–O Thin Films with Al2O3 Passivation Layer for Thin-Film Transistor Applications

In this paper, the effects of X-ray irradiation on the electrical performance of the direct current magnetron sputtered indium–gallium–tin oxide (IGTO) thin-film transistors (TFTs) passivated with an aluminum oxide (Al2O3) passivation layer were investigated. The radiation stability of IGTO TFTs passivated with the Al2O3 passivation layer was evaluated depending on the thicknesses of the channel and passivation layer as well as the deposition technique of the passivation layer, which was determined to achieve both excellent electrical stability and radiation tolerance. The chemical structures and surface morphologies of the IGTO thin films having an atomic composition of In:Ga:Sn = 66.2:31.3:2.5 before and after X-ray irradiation were characterized, and the X-ray irradiation induced persistent photocurrent effect on the IGTO thin films that ionizes the oxygen vacancy to the charged state was evaluated. The mechanisms of radiation damage to the IGTO TFTs were systematically proposed in terms of the generation, ionization, and compensation of oxygen vacancies. The different barrier roles of radio-frequency (RF) sputtering- and atomic layer deposition-based Al2O3 passivation layers were observed in the IGTO TFTs under X-ray irradiation, and the proposed devices passivated with the RF sputtering-based Al2O3 layer of up to 15 nm thickness exhibited the excellent radiation hardness (shifts of threshold voltage <0.02 V) under X-ray irradiation with a high dose of 1000 Gy. Further, the IGTO TFTs passivated with RF sputtering- and ALD-based Al2O3 layers exhibited excellent long-term stability over 18 days without an aging effect, as well as enhanced electrical bias stability of the device under positive bias stress and negative bias illumination stress with increasing thickness of the passivation layer. Thus, a radiation tolerant IGTO TFTs passivated with the Al2O3 passivation layer was proposed, and the corresponding mechanisms can provide meaningful understanding of the radiation stability of the oxide TFTs for operation in harsh X-ray irradiation environments.