Photoluminescence investigations on a native donor in ZnO
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0891-EE08-02.1
Photoluminescence investigations on a native donor in ZnO B. K. Meyer, S. Lautenschläger, S. Graubner, C. Neumann, and J. Sann I. Physikalisches Institut, Justus-Liebig-Universität, Heinrich-Buff-Ring 16, 35392 Gießen, Germany ABSTRACT We report on the optical and electrical properties of ZnO epitaxial films with different Zn/O ratios. In the Zn-rich films the dominant radiative recombination is the bound exciton line of I3a at 3.366 eV. By tuning towards oxygen-rich conditions the I3a recombination is reduced in intensity and eventually disappears and the I8 neutral donor bound exciton lines remains. Identical behavior is found for a Zn-rich film when annealed in an oxygen atmosphere at 900 °C for 30 minutes. Since the carrier density decreases about one order of magnitude from Zn-rich to oxygen-rich, and extrinsic impurities cannot account for the decrease, our experiments provide evidence for the electrical and optical activity of Zn interstitials. The localization energy of I3a is consistent with a donor binding energy of 37 meV according to Haynes rule. INTRODUCTION The shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years [1, 2]. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electronsatellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Hayne’s rule in ZnO i.e. that the donor binding energy is proportional to the localization energy of the excitons bound to the neutral donors [2]. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO [3, 4]. Interstitial zinc and oxygen vacancies were the natural choice. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. Theoretical calculations predict indeed that the oxygen vacancies are the main intrinsic defects in zinc rich ZnO whereas Zni have higher formation energies i.e. being less abundant [5]. However, for the oxygen vacancy theory predicts a negative-U-behavior with a transition from the neutral to the twofold positive charge state around EVB + 0.5 eV, i.e. the vacancy does not induce a shallow level [5]. From electrical measurements (Hall, DLTS, admittance spectroscopy) on various ZnO single crystals from different sources as well as on epitaxial films there is general consensus that apart from the shallow donor a deep donor state exists which is located around 130 ± 20 meV below the conduction band. Its concentration falls into the 1016 cm-3 range thus being relevant for the conductivity as well as for the compensation of acceptors [6-11]. The Zn interstitial is a double donor in ZnO. It should exist in three c
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