Group I elements in ZnO
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0891-EE10-24.1
Group I elements in ZnO B.K. Meyer, N. Volbers, A. Zeuner, S. Lautenschläger, J. Sann, A.Hoffmann1 and U. Haboeck1 I. Physikalisches Institut, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany 1 Institut für Festkörperphysik, Technische Universität Berlin, Hardenberg Str. 36, D-10623 Berlin, Germany ABSTRACT In this report we focus on the lithium doping of ZnO epitaxial films grown on GaN templates and ZnO substrates. We compare the results with diffusion studies of Li into ZnO single-crystal substrates. The diffused and in-situ doped layers were studied using mass spectroscopy, low temperature photoluminescence and Raman spectroscopy. Li is known to produce a deep acceptor state which takes part in a shallow donor to deep acceptor recombination in the visible spectral region. We will demonstrate that also shallow Li acceptors are introduced depending on the growth/diffusion temperature. The shallow Li-related acceptor has a binding energy around 300 meV. A donor-acceptor pair recombination with its zero phonon line at 3.05 eV followed by LO phonon replica is observed. The intensity ratio of the zero phonon line compared to the replica indicates weak electron phonon coupling, hence small lattice relaxation in contrast to the recombination with the deep Li acceptor. INTRODUCTION First principles calculations predict shallow levels for the group-I elements Li: 0.09 eV and Na: 0.17 eV [1] with almost no relaxation around the LiZn acceptor [2], and a small outward relaxation for the surrounding oxygen atoms in the case of NaZn. This is in contrast to the EPR investigations reported by Schirmer in 1968 [3] on ZnO:Li. The interpretation of the hyperfine data showed that the hole is primarily located on one neighbouring oxygen atom along the c-axis and induces a substantial distortion. It is the reason why LiZn is a deep acceptor with a binding energy of 800 meV. NaZn behaves similar, the binding energy is 600 meV [4]. Both defects are responsible for broad strong phonon coupled luminescence bands in the visible spectral range with maxima at 2.1 eV (Li) and 2.17 eV (Na). The acceptors show up in shallow donor to deep acceptor recombinations as demonstrated by optically detected magnetic resonance experiments [5, 6]. For the conversion from n-type to high resistive ZnO:Li Zwingel [7] showed that the conductivity decreased in inverse proportion to the square of the Li concentration. Only 1% of the total Li concentration was effective in the compensation of the donors already present in the crystal. Zwingel assumed equal concentrations for the interstitial Li donors and LiZn acceptors and postulated the existence of another Li acceptor, needed to model the reaction kinetics. It should arise from the pairing of two Li centres. Up to now this defect has not shown up in the radiative recombinations of ZnO:Li.
0891-EE10-24.2
EXPERIMENTAL DETAILS
logPL-intensity (arb. units)
T=2K HeCd 325 nm
I7 I6
PL-intensity (arb. units)
ZnO epitaxial films doped with Li were grown by CVD
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