Ordered nanocrystalline ZnO films for high speed and transparent thin film transistors

Thin films of spontaneously ordered and closely packed 20-50nm diameter nanocolumns of ZnO were used to fabricate high speed and transparent thin film transistors (TFT). The use of nanocrystalline ZnO (nc-ZnO) helps to achieve the intrinsic electronic properties of single crystals while providing substrate agnostic thin films that can be grown on non-planar surfaces of rigid or flexible substrates. We have developed low temperature pulsed laser deposition (PLD) techniques for both doped and undoped nc-ZnO films and demonstrated microwave transistor operation (f_max=10GHz). Unlike in amorphous TFTs, the operation of nc-ZnO transistors relies on field effect charge control at vertical grain boundaries between nanocolumns and can produce very high on/off ratios (>;10¹²), very high current densities and near ideal subthreshold voltage swings (~85mV/decade). Using a combination of doped and undoped nc-ZnO films, we have fabricated the first indium-free transparent TFTs with excellent performance and transparency.