INTRINSIC DEFECTS AND SODIUM ACCEPTORS DOPING IN ZNO
Parmar, Narendra Singh
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Zinc oxide (ZnO), with a direct band gap of 3.37 eV at room temperature, attracts considerable attention because of its promising applications for blue-UV light-emitting diodes, diode lasers and spintronics.A problem with ZnO is that it can easily be made n-type, but it is difficult to dope p-type in a reliable and in a controlled way. In this work, Cermet ZnO single crystals were doped with sodium.Sodium acceptors were activated by air annealing. Photoluminescence (PL) measurements showdonor acceptor pair (DAP) emission in the range ~405 - 415 nm. Hall measurements show the p-type conductivity in one ZnO sample while in others it shows the indeterminate type of conduction. PL measurements and low temperature Hall measurements suggest that the sodium acceptor activation energy is ~0.34eV. Using infrared absorption spectroscopy we have observed hydrogen local vibrational modes (LVMs)in Na-doped ZnO bulk single crystals. When annealing in hydrogen, we find an absorption peak at 3303.7 cm-1 at 9 K, the LVM frequency is in good agreement with theoretical predictions for the sodium-hydrogen complex, with hydrogen in an anti-bonding configuration. With deuterium substituting the hydrogen, we find an additional peak at 2466 cm-1. The isotopic frequency ratio is r = 1.339, similar to values found in many hydrogen/deuterium-related complexes. To confirm that sodium acceptors are passivated by hydrogen, PL measurements were performed on sodium doped and hydrogen annealed sample. No DAP emission was observed in the range ~ 410-415 nm, consistent with hydrogen passivation. The sample was then oxygen annealed in steps of 500°C, 700°C, 800°C for 45 min. each, and PL measurements were taken after every annealing step. No DAP emission was detected in these spectra. After oxygen annealing at 900°C for 45 min, the DAP emission at ~ 405 -415 nm was recovered while the LVM at 3303.6 cm-1 disappeared. RT Hall measurement show semi-insulating behavior of the sample, consistent with activation of the sodium acceptors.