Enhanced performances of a-IGZO TFTs with oxide passivation layers fabricated by hollow cathode assisted PLD

Abstract

Pulsed laser deposition (PLD) and two-step thermal annealing are combined to fabricate MgO, TiO2, Y2O3, Al2O3, ZrO2 and HfO2 passivation layers on a-IGZO TFTs. The influences of the passivation layers on the electrical properties are investigated. It is found that conventional PLD is sufficient to deposit high-quality Y2O3 and ZrO2 passivation layers, but hollow cathode assisted PLD (HC-PLD) should be employed to solve the oxygen deficiency in MgO, TiO2, Al2O3, and HfO2 thin films by oxygen plasma. Furthermore, two-step annealing is adopted to improve the performances of the devices: Firstly, a-IGZO thin films are pre-annealed to enhance the carrier mobility. Secondly, the passivation layers are deposited on a-IGZO and the whole structures are post-annealed to endow the good on-off performance. The sharp a-IGZO/passivation interfaces remain after thermal annealing. The Hall mobility is mainly restricted by the scattering of metal ions at the a-IGZO/passivation interface. TiO2 passivated devices exhibit the highest Hall mobility of 16.7 cm2V−1s−1, owing to the smaller cation charge. The threshold voltage stability under positive gate bias is dependent on the metal-oxygen bond strength in the passivation layers. ZrO2 passivated devices have the lowest threshold voltage shift, beneficial from the strong Zr-O bonds. The subthreshold performance and the stability under illumination are determined by the oxygen vacancies at a-IGZO/passivation interfaces and ZrO2 passivated devices exhibit the lowest subthreshold swing and the best illumination stability.