Transparent Transistors Based on Semiconducting Oxides
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TRANSPARENT TRANSISTORS BASED ON SEMICONDUCTING OXIDES Y.W. Kwon, Y. Li, Y.W. Heo, M. Jones, Vijay, B.S. Jeong, J. Zhou, S. Li1, P. Holloway, D.P. Norton University of Florida, Dept. of Materials Science and Engineering, Gainesville, FL 32611, U.S.A. 1 University of Florida, Dept. of Electrical and Computer Engineering, Gainesville, FL 32611, U.S.A. ABSTRACT The synthesis and properties of oxide-based thin film transistors (TFTs) is reported using pulsed laser deposition. The field effect transistors use ZnO as the channel material. Low leakage current density is achieved with amorphous (CeTb)MgAl11O19 (CTMA) serving as the gate oxide, whose dielectric strength is measured to be > 5MV/cm for structures fabricated on Indium Tin oxide (ITO) substrates. Capacitance-voltage properties show that n-type active layers are realized with undoped ZnO. Charge densities in undoped ZnO are measured to be 1018 to 1019 / cm3 using Hall measurement and CV plots. Current-voltage measurements for TFT operation are reported. Channel materials on patterned substrates show high conductance and modulation of channel conductance. C-V measurements with MOS structure using doped ZnO and ZnxMg1-xO will also be described. The properties of depletion mode TFTs fabricated with doped and undoped oxide channel will be discussed in detail. INTRODUCTION Thin films of transparent conducting oxides (TCOs) have been widely investigated for various applications such as light emitting devices[1,2], semiconductor devices [3,4], and electrodes for flat-panel displays and solar cell [5-7]. The prerequisites of TCOs are a wide optical band gap (>3eV), high electrical conductivity, and optical transmittance (>80%) in the visible region [8]. ZnO is an n-type II-VI compound semiconductor with a direct band gap of 3.35 eV. The crystal structure of ZnO is a hexagonal (wurtzite) with a = 3.25 Å and c = 5.12 Å. Electron doping via defects originates from Zn interstitials in the ZnO lattice[9]. N-type doping is caused by the intrinsic defect levels approximately 0.05eV below the conduction band. The Hall mobility of ZnO single crystals at room temperature is on the order of 200 cm2V-1s-1 [10]. Furthermore, p-type ZnO was recently reported by molecular beam epitaxy [11], radio-frequency (rf) sputtering [12], and hybrid beam deposition [13]. By using these characteristics, transparent field effect transistors (TFETs) based on ZnO as an active channel layer is being explored [4,14,15]. In this paper, we investigate the fabrication of ZnO-based thin film transistors using (Ce,Tb)MgAl11O19 (CTMA) and HfO2 for the gate oxide, and report the electric field modulation of channel conductance. EXPERIMENTAL DETAILS Figure 1 shows a schematic cross-section and top view of the field effect transistor (FET) using undoped-ZnO, 2% P-doped ZnO, and 2% P-doped Zn0.9Mg0.1O as an active channel layer. ZnO-based transparent thin film transistors were fabricated with a top gate electrode. Using the
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patterned substrates of ITO coated glass, polycrystalline channel mat
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