Highly stable amorphous zinc tin oxynitride thin film transistors under positive bias stress

Abstract

The stability of amorphous zinc tin oxynitride thin film transistors (a-ZTON TFTs) under positive bias stress (PBS) is investigated. Thin films are deposited by remote plasma reactive sputtering and are annealed at 300 °C in air for 1 hour, after which films are confirmed to be highly amorphous by transmission electron microscopy. Typical a-ZTON TFTs exhibit a threshold voltage of 2.5 V, a field effect mobility of 3.3 cm2 V –1 s –1 , a sub-threshold slope of 0.55 V dec–1 and a switching ratio over 106 . Using a thermalization energy analysis, the threshold voltage shift under PBS is analysed. A maximum energy barrier to defect conversion up to 0.91 eV is found, which is significantly greater than that the ~ 0.75 eV energy barrier for amorphous indium gallium zinc oxide (a-IGZO) and amorphous zinc tin oxide (a-ZTO) TFTs previously reported. The improved stability of these oxynitride TFTs over amorphous oxide TFTs is explained by the elimination of less stable oxygen vacancies due to the passivation of oxygen vacancies with nitrogen. The higher attempt-to-escape frequency of 108 to 109 s –1 in a-ZTON TFTs compared with 107 s –1 in amorphous oxide semiconductor TFTs, on the other hand, is attributed to the high homogeneity of the amorphous film leading to strong carrier localization in the band tails.