Effect of high temperature treatments on defect centers and impurities in hydrothermally grown ZnO

Deep level transient spectroscopy (DLTS) has been employed to study electron traps in hydrothermally grown n-type ZnO samples after thermal treatments up to 1500 degrees C. Schottky barrier contacts were formed by e-beam evaporation of Pd, followed by DLTS and secondary ion mass spectrometry (SIMS) measurements in order to investigate possible correlations between electron traps in the upper part of the band gap and the concentration of the most prominent impurities. The DLTS results show three different levels having energy positions of E(c) - 0.19 eV, E(c) - 0.31 eV, and E(c) - 0.58 eV (E(c) denotes the conduction band edge). The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Fe, Mn, and Ni with concentrations up to similar to 5 x 10(17) cm(-3). A decrease in the E(c) - 0.31 eV level is observed after temperature treatments above 1300 degrees C, and in the same temperature range the Li concentration drops from similar to 10(17) to similar to 10(15) cm(-3). However, based on absolute concentration values an association between Li and the E(c) - 0.31 eV level can be ruled out. In contrast, the E(c) - 0.19 eV level, which is not stable above 1300 degrees C, may be associated with Li but further experimental data are needed to substantiate this assignment. The E(c) - 0.58 eV level occurred in selected samples and is presumably impurity-related but no correlation was found with the main impurities detected by SIMS. Except for Li, the concentration of all the impurities remained essentially constant as a function of heat treatment temperature. (C) 2009 Elsevier B.V. All rights reserved.