Determination of the Nitrogen Acceptor Ionization Energy in Zinc Oxide by Photoluminescence Spectroscopy
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Determination of the Nitrogen Acceptor Ionization Energy in Zinc Oxide by Photoluminescence Spectroscopy Lijun Wang, N. Y. Garces, L. E. Halliburton, and N. C. Giles Physics Department, West Virginia University, Morgantown, WV 26506-6315 ABSTRACT Photoluminescence (PL) experiments performed on bulk ZnO crystals are used to establish the ionization energy of the substitutional nitrogen acceptor. The temperature dependence of the nitrogen-related electron-acceptor (e,A0) emission band has been monitored in as-grown single crystals. A lineshape analysis of this band is used to determine the acceptor ionization energy. The temperature variation of the ZnO band gap was included in our analysis and the low-temperature acceptor ionization energy for substitutional nitrogen at an oxygen site in ZnO was found to be EA = 209 ± 3 meV. Electron paramagnetic resonance and Hall-effect measurements were also used to characterize these bulk ZnO samples. INTRODUCTION ZnO has a direct band gap energy (Eg) of 3.37 eV at room temperature. Doping with nitrogen has been considered as a means to produce p-type material [1-5]. Photoluminescence (PL) studies of heavily doped films have led to assignments of N-associated donor-acceptor-pair (DAP) recombination [4,5]. Both DAP and electron-acceptor (e,A0) emission bands were observed [6] from as-grown bulk ZnO and were suggested to be due to trace amounts of N acceptors. In this latter reference, the PL band assigned as the N-related (e,A0) emission was observed above 30 K. Thermal anneals in air of as-grown bulk ZnO crystals have resulted in the activation of nitrogen acceptors (unintentionally present in trace amounts, and detected using electron paramagnetic resonance (EPR)) [7]. Also, the Hall carrier concentration was lower after the air anneals. In N2-doped bulk ZnO, an even larger nitrogen EPR signal could be detected, and PL emission bands assigned to (e,A0) and DAP transitions associated with nitrogen were observed [8]. Direct observation of the (e,A0) emission band provides a means to determine the acceptor ionization energy. Previous reports of the nitrogen acceptor in ZnO gave the ionization energy as 165 ± 40 meV [4], 195 ± 10 meV [6], or 266 meV [5]. EXPERIMENTAL DETAILS The ZnO crystals studied in the present investigation were grown by the seeded-chemical-vapor-transport (SCVT) method at Eagle-Picher (Miami, OK). The PL data were taken in near-backscattering geometry using a 0.64-m monochromator and a GaAs photomultiplier tube operating in photon-counting mode. The excitation wavelength was 325 nm (He-Cd laser) and the incident power density on the sample surface was about 0.6 W/cm2. Samples were mounted in a Janis SuperVaritemp Dewar and sample temperature was varied from 5 K to 295 K. The
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spectra have been corrected for system response using a calibrated white-light source and wavelength calibration was performed using a Hg(Ar) lamp. Data taken from four samples are described. We refer to these samples as A, B, C, and D. Sample A is undoped as-grown ZnO,
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