Properties of Sputter Deposited ZnO Films Co-doped with Lithium and Phosphorus
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Properties of Sputter Deposited ZnO Films Co-doped with Lithium and Phosphorus T. N. Oder1, A. Smith1, M. Freeman1, M. McMaster1, B. Cai2 and M. L. Nakarmi2 1 Department of Physics and Astronomy, Youngstown State University, Youngstown, OH 44555, U.S.A. 2 Department of Physics, Brooklyn College of the CUNY, Brooklyn, NY 11210, U.S.A. ABSTRACT Thin films of ZnO co-doped with lithium and phosphorus were deposited on sapphire substrates by RF magnetron sputtering. The films were sequentially deposited from ultra pure ZnO and Li3PO4 solid targets. Post deposition annealing was carried using a rapid thermal processor in O2 and N2 at temperatures ranging from 500 °C to 1000 °C for 3 min. Analyses performed using low temperature photoluminescence spectroscopy measurements reveal luminescence peaks at 3.359, 3.306, 3.245 eV for the co-doped samples. The x-ray diffraction 2ș-scans for all the films showed a single peak at about 34.4° with full width at half maximum of about 0.17°. Hall Effect measurements revealed conductivities that change from p-type to n-type over time. INTRODUCTION The properties of ZnO make it a very attractive material for fabricating optoelectronic devices for blue/UV applications. It is a wideband gap semiconductor with a direct energy band gap of 3.37 eV at 300 K or 3.437 at 4 K [1]. While it bears several similarities with gallium nitride (GaN) currently used for UV optoelectronic devices, ZnO has several advantages over GaN, making it a candidate for replacing or complimenting GaN [2]. For instance, it has a larger exciton binding energy (60 meV for ZnO, 24 meV for GaN) making it more attractive for fabricating efficient room-temperature LEDs and lasers. Successful development of ZnO-based devices, however, has been hampered by the lack of stable p-type materials. As-grown ZnO is ntype with usually very high concentration of electrons. Some of the reasons for the difficulty in p-type doping include the low solubility and self-compensation of the acceptor dopants by defects [3,4]. Native defects such as oxygen vacancies, zinc interstitials or zinc antisites were once thought to be the cause for the unintentional n-type conductivity in ZnO. However, while recent research have cast doubt on this concept, these defects nevertheless play a big role as compensating centers to p-type doping [5,6]. Furthermore, hydrogen-related defects have been suggested to be some of the hindrances to p-type doping in ZnO, especially when H substitutes for O in ZnO and act as a shallow donor [6]. Group V elements such as nitrogen (N), phosphorus (P), arsenic (As), and antimony (Sb) have been experimentally investigated for obtaining p-type ZnO by substituting oxygen. In theory, N substituting for O (NO) was regarded as the most suitable dopants for producing p-type ZnO based on the strain effects, energy levels and the similarity of the atomic radius of N with O [6,7]. However, recently revisited theoretical work and experimental investigation showed that nitrogen in ZnO is a deep acceptor with large ionization energy o
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