Encapsulation of Zinc Tin Oxide Based Thin Film Transistors

In this paper, we study the effect of thin film encapsulation on transparent thin film transistors (TFTs) with zinc tin oxide channel layers deposited by oxygen plasma assisted pulsed laser deposition (PAPLD). Chemisorption and desorption of oxygen are identified as the most important physical processes governing the impact of surface passivation on the device characteristics. For an in-depth study, we apply three different methods to remove chemisorbed oxygen from the channel surface: thermally activated desorption in vacuum; light induced desorption; and chemical reaction with the highly reactive agent trimethyl aluminum (TMA) during atomic layer deposition (ALD) thin film encapsulation. In each case, the removal of chemisorbed oxygen significantly shifts the threshold voltage of the TFTs on the order of 1−2 V. On the basis of the understanding gained in these experiments, a two-step encapsulation process is proposed that enables the application of high-performance ALD grown permeation barriers layers without alteration of the device characteristics. This finding is an essential step toward active matrix OLED displays with metal oxide driving electronics.