ZnO-based MESFET Devices
- PDF / 1,632,508 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 20 Downloads / 186 Views
1201-H01-01
ZnO-based MESFET Devices M. Grundmann, H. Frenzel, A. Lajn, H. von Wenckstern, F. Schein, and M. Lorenz Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, 04103 Leipzig, Germany
ABSTRACT We present transistors and inverters based on the MESFET principle. The channel consists of thin ZnO:Mg thin films on sapphire, deposited with pulsed laser deposition. The ohmic source and drain contacts are formed with sputtered gold. The Schottky gate electrode is formed by metal oxides providing high barrier height and a reliable contact. The voltage swing of our inverters is superior to any other reported oxide devices. Annealing studies show that our devices withstand temperatures up to 150°C, partly improving during annealing. INTRODUCTION Thin film field-effect transistors (TFT) and circuits based on them have many applications, the most prominent probably in flat panel display. Amorphous and poly- or microcrystalline silicon enables a viable technology. The typical performance values are a channel mobility below 1 cm2/Vs and an on/off current ratio above 106 [1, 2]. Devices based on oxides provide a viable alternative, offering higher performance, lower fabrication temperatures, transparency and flexible substrates. Most oxide-based TFTs are based on the MISFET principle, i.e. a gate electrode with an insulating dielectric layer, either as bottom or top gate (Fig. 1a, b). While this strategy has been very successful in silicon technology due to the low density of defects at the Si/SiO2 interface and the low leakage current through SiO2, typical thickness of the dielectric in oxide MISFETs is 50-200 nm. Thus a fairly larger portion of the gate voltage drops over the insulator. Here, we show MESFETs, i.e. TFTs with a Schottky diode as gate electrode [3] (Fig. 1c). The high Schottky barrier height of typically 0.95 eV for Ag and 0.90 eV for Pt [4] provides rather low reverse currents of 5 nA/cm2 (at −1V) and low voltage operation of the devices. Based on the MESFETs, we report inverters with the highest reported gain in oxide electronics.
Fig. 1: Schemes of various thin film field-effect transistor geometries: (a) bottom gate MISFET, (b) top gate MISFET, and (c) MESFET. S: source, D: drain, G: gate, dark grey: dielectric, light grey: channel, white: substrate.
EXPERIMENTAL The channel of the TFTs is formed by ZnO or Mg0.003Zn0.997O. The latter offers improved reproducibility. For the devices reported here, the material has been deposited by pulsed laser deposition (PLD) on sapphire at T=650 °C and oxygen pressure of p=0.02 mbar. The typical surface morphology of our ZnO channel layers on sapphire is shown in Fig. 2. We note that our process also yields working transistors on glass substrate [5]. The ohmic contacts to the channel are formed by sputtered gold. The Schottky gate is fabricated by reactive sputtering in an Ar/O2 plasma. Both sputter processes are in principle suitable for room temperature, large area and mass production. As metals in the reactive sputtering process for the Scho
Data Loading...
