Characterization of ZnO Nanostructures Grown by Pulsed Laser Deposition
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1174-V07-09
Characterization of ZnO Nanostructures Grown by Pulsed Laser Deposition Christian Weigand1, Matt Bergren3, Cecile Ladam2, Per Erik Vullum2, John C. Walmsley2, Ragnar Fagerberg2, Tom Furtak3, Reuben Collins3, Jostein Grepstad1 and Helge Weman1 1
Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway 2 SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway 3 Department of Physics, Colorado School of Mines, Golden, CO 80401, U.S.A.
ABSTRACT ZnO nanostructures were grown by pulsed laser deposition on c-plane sapphire substrates. The as-grown nanostructures were examined by scanning electron microscopy and transmission electron microscopy ZnO nanowires were grown using a gold catalyst, at a high substrate temperature of 800°C and an ambient gas pressure of 0.5 mbar (5% oxygen, 95% argon). Changing the gas composition to pure oxygen led to the growth of stacking fault-free ZnO nanosheets with their growth direction inclined to the [0001] direction. Similar nanosheets with stacking faults were found when lowering the growth temperature to 600°C for a 5% oxygen – 95% argon ambient gas composition and the same overall pressure. A growth mechanism for these ZnO nanosheets is proposed.
INTRODUCTION ZnO has been subject to extensive research in recent decades. Due to its wide band gap of 3.37 eV and its large exciton binding energy of 60 meV at room temperature, it is especially interesting for applications in short wavelength light emitting devices and solar cells [1]. ZnO is abundant in nature, and thus a suitable material for low-cost devices. It can be grown by different deposition techniques, such as metal-organic chemical vapor deposition [2], wet chemical synthesis, and different chemical and physical vapor deposition techniques, including pulsed laser deposition (PLD) [3-5]. The latter technique allows for controlled growth of various materials, preserving the stoichiometry of the source material upon transport from target to substrate. In PLD, ambient gases such as oxygen and argon can be introduced in order to aid the growth of metal oxides. In this work, ZnO nanorods and nanosheets were grown on sapphire substrates at different temperatures and ambient gas compositions from a ZnO target. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the growth mechanisms of these nanostructures for different growth conditions.
EXPERIMENT The ZnO nanostructures were grown on c-plane sapphire substrates using a KrF excimer laser at 248 nm wavelength (Lambda Physik LPX Pro 210i). A raster-scanned ZnO target (American Elements, 99.999%) was ablated with a laser fluency of approximately 1 J/cm² at a
pulse repetition rate of 10 Hz and a target-to-substrate distance of 45 mm. The base pressure of the vacuum chamber before deposition was less than 2 x 10-7 mbar. The substrate temperature was varied between 600 °C and 800 °C, and ambient background gases (oxygen and argon) were introduced into the
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