Solution-processed Oxide Thin Film Transistors

Oxide-semiconductor material based on Zinc oxide (ZnO) has been considered as one ofthe most suitable active material of thin film transistor (TFT) for its advantage of highmobility, good visible light transmittance and low cost. Currently, the sputtering process ofZnO TFT (especially doped with In and Ga, abbreviated as IGZO) is relatively mature andguarantees high performance. However, sputtering needs high vaccum equipment, whichrequires a high process cost. In comparison to sputtering, solution-process can avoid usingvaccum equipment, and it is compatible with ink-jet printing and roll-to-roll fabricationtechniques. What’s more, it significantly reduces the process cost. Therefore, to studysolution-processed ZnO-TFT has a profound significance and it is also a hot direction incurrent field of TFT.However, solution-processed ZnO-TFT has at least two shortcomings compared tosputtering: one is the high annealing temperature to decompose the precursor to formhigh-quality film which is not compatible with most flexible substrates; the other is that thesolution process may easily introduce impurities which will cause poor performance andprocess reproducibility. Against to above shortcomings, this paper is committed to the studyof lowering the process temperature and improving the electrical performance of ZnO-TFT.To lower the process temperature of ZnO-TFT, this paper used a carbon-free aqueoussolution as ZnO precursor which may allow metal and ammine dissociate, hydroxidedehydration and condensation to form ZnO polycrystalline film under low temperatures(never higher than180°C). Together with anodizing process at room temperature and polymerpassivated process under120°C, this paper achieves to prepare ZnO-TFT at low temperatureand with low cost. The final device has a field effect mobility of0.9cm2/Vs.As undoped ZnO usually is polycrystalline, the presence of grain boundaries has a greatinfluence on the performance of TFT. To improve the electrical performance of ZnO-TFT, thispaper uses a doping method. High performance amorphous oxide TFT has been achieved bydoping the ZnO precusors with In, Ga and Sn. InZnO (IZO), InGaZnO (IGZO) and SnZnO(ZTO) have been prepared and their process parameters have also been optimized in theexperiment. It can be observed that after doping In or Sn into ZnO, the mobility of the device has greatly improved (e.g. ZTO-TFT has a mobility of37cm2/Vs). On the other hand, afterdoped with Ga, the intrinsic carrier density is suppressed and the off current is lowered, whichleads to a higher Ion/Ioff.