ZnO/SrTiO3 transparent field effect transistor

Thin ZnO films (thickness from 20 to 1000 nm) were deposited by pulsed laser ablation on strontium titanate substrates. The growth is carried on in different oxygen pressures (from 10-5 to 10-1 mbar), at substrate temperatures varying from 550 to 800 °C, using a stoichiometric ZnO target. By monitoring the growth with an in situ Reflection High-Energy Electron Diffraction (RHEED) system and by ex situ x-ray analysis, we found out that zinc oxide grows epitaxially on the 110 surface of SrTiO3 with high crystalline quality. All the samples were also characterized by spectroscopic ellipsometry and atomic force microscopy finding a roughness which decreases with the substrate temperature and optical properties similar to those of single crystals. Electrical properties were studied by measuring the resistivity and the Hall effect as a function of the temperature and of the magnetic field. Mobility values up to 60 cm2/Vs at room temperature and 300 cm2/Vs below 50 K were obtained. By exploiting the semiconducting properties of ZnO thin films and the dielectric properties of the SrTiO3 substrate (dielectric constant up to 10000 at low temperature), we realized field effect transistors transparent at visible wavelength. Such ZnO/SrTiO3 field effect transistors have been fabricated both using a traditional stacked structure and a side gate planar geometry. In both cases, an on/off ratio of 106 at gate electric field as low as 1 kV/cm has been obtained.