Identification of substitutional Li in n-type ZnO and its role as an acceptor

Monocrystalline n-type zinc oxide (ZnO) samples prepared by different techniques and containing various amounts of lithium (Li) have been studied by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. A distinct PAS signature of negatively charged Li atoms occupying a Zn-site (Li-Zn(-)), so-called substitutional Li, is identified and thus enables a quantitative determination of the content of Li-Zn. In hydrothermally grown samples with a total Li concentration of similar to 2 x 10(17) cm(-3), LiZn is found to prevail strongly, with only minor influence, by other possible configurations of Li. Also in melt grown samples doped with Li to a total concentration as high as 1.5 x 10(19) cm(-3), a considerable fraction of the Li atoms (at least 20%) is shown to reside on the Zn-site, but despite the corresponding absolute acceptor concentration of >=(2-3) x 10(18) cm(-3), the samples did not exhibit any detectable p-type conductivity. The presence of LiZn is demonstrated to account for the systematic difference in positron lifetime of 10-15 ps between Li-rich and Li-lean ZnO materials as found in the literature, but further work is needed to fully elucidate the role of residual hydrogen impurities and intrinsic open volume defects.