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The aim of this dissertation is to develop oxide semiconductors by radio-frequency
sputtering for thin-film transistor (TFT) applications. A variety of oxide semiconductors
are used as the TFT channel layer, including indium gallium oxide (IGO), zinc tin
oxide (ZTO), and indium gallium zinc oxide (IGZO). The variety of materials used underscores
the abundance of materials options available within this nascent technology,
with each material exhibiting unique chemical, mechanical, and electrical properties. The
influence of several deposition parameters is explored; oxygen partial pressure of the deposition
ambient is found to have a profound effect on the electrical characteristics of
each material. With optimized deposition conditions, TFTs based on these materials exhibit
excellent electrical properties, even when annealed at low-temperature (175 °C).
Specifically, ZTO-based TFTs which are subjected to a 175 °C post-deposition anneal
exhibit a channel mobility near 9 cm²V⁻¹s⁻¹. However, advancement of this technology
also requires research in integration-related issues. Therefore, the effect of channel
layer passivation and of TFT stability is evaluated. Passivation of the oxide semiconductor
surface is required for circuits which employ multiple levels of interconnect and for
mechanical/chemical protection of devices. Here, successful passivation of IGO, ZTO,
and IGZO-based TFTs is demonstrated using SU-8, a negative tone epoxy-based photoresist. To appraise TFT stability, a constant voltage bias stress test of 1000 minutes is utilized, where the drain current, ID, is monitored throughout the duration of testing and the turn-on voltage, Von, is evaluated before and after stressing. TFT stability is found to be correlated to the turn-on voltage of a device and to the thickness of the semiconductor
layer. IGZO-based TFTs with excellent stability are demonstrated, exhibiting almost no decrease in ID or any shift in Von throughout the duration of bias stress testing. |
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