Spray Deposited Lithium-doped Zinc Oxide Thin-film Transistors with Electron Mobility Exceeding 50 cm2/Vs

Adamopoulos, George and Bradley, Donal and Anthopoulos, Thomas (2011) Spray Deposited Lithium-doped Zinc Oxide Thin-film Transistors with Electron Mobility Exceeding 50 cm2/Vs. In: MRS 2010 Fall Meeting, 2010-11-30.

Abstract

The high optical transparency and excellent charge transport characteristics combined with their excellent chemical stability and mechanical tolerance make oxide semiconductors attractive for applications in large area opto-/electronics and particularly thin-film transistors (TFTs). However, the vast majority of high performance oxide-based transistors reported to date are fabricated using sophisticated deposition methods that are usually incompatible with large area processing and hence potentially expensive. Here, we show an alternative processing method based on spray pyrolysis and soluble precursor molecules for the deposition of high-performance doped oxide semiconductors onto large area substrates under atmospheric conditions. Using this simple technique we are able to realise lithium-doped zinc oxide n-channel TFTs that are characterised by field-effect mobilities of >50 cm2/Vs, channel current on/off modulation ratio >10^6 and almost hysteresis-free operation. The physical properties of Li-ZnO films were investigated using a range of characterisation techniques namely AFM, XRD, Raman and Photoluminescence spectroscopy, spectroscopic ellipsometry and FTIR. Structural studies show that Li doping can lead to either interstitial or substitutional doping depending on the doping level. Interstitial doping was observed for Li concentration <1% and found to yield TFTs of high mobility. The latter was attributed to an increase in the average crystal size of Li-ZnO films. For Li doping concentration >1% (stoichiometry of precursor solution) it was shown that substitutional doping of Zn by Li occurs resulting to a drastic reduction in the average crystal size and interplanar spacing accompanied by a significant reduction in the electron field-effect mobility. The present results demonstrate that spray pyrolysis is a versatile tool for the deposition of oxide semiconductors onto large area substrates and provides a new route for the rapid development of materials far beyond those accessible by traditional deposition methods.