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${}_{\mathrm{Zn}}^{-}$), so-called substitutional Li, is identified and thus enables a quantitative determination of the content of Li${}_{\mathrm{Zn}}$. In hydrothermally grown samples with a total Li concentration of $~$2$\times {10}^{17}\hspace{0.5em}{\mathrm{cm}}^{-3}$,Li${}_{\mathrm{Zn}}$ 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$\times {10}^{19}\hspace{0.5em}{\mathrm{cm}}^{-3}$, 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)$\times {10}^{18}\hspace{0.5em}{\mathrm{cm}}^{-3}$, the samples did not exhibit any detectable p-type conductivity. The presence of Li${}_{\mathrm{Zn}}$ 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.

Figure 1

Li concentration versus depth profiles for samples HT-1, HT-2, MG-1, and MG-2 as measured by SIMS.

Figure 2

Positron lifetime spectra measured for an electron-irradiated VP-ZnO sample,[27] a typical spectrum for HT-ZnO (as well as for Li-enriched MG-ZnO samples) and the bulk lifetime spectrum as measured in the VP-ZnO reference sample.

Figure 3

$W$-parameter versus $S$-parameter as measured in the Li-rich (HT-1 and MG-2) and the Li-lean (HT-2) samples. The data points related to surface annihilation are removed for clarity. In addition, values obtained from a ZnO sample irradiated with oxygen ions, containing two different concentrations of ${\mathrm{V}}_{\mathrm{Zn}}$, are shown for comparison (Ref. 27).

Figure 4

Coincidence Doppler broadening measurements for as-grown HT-ZnO (HT-1), as-grown MG-ZnO, and Li-doped MG-ZnO as compared to the theoretical values obtained for ${\mathrm{V}}_{\mathrm{Zn}}$, Li${}_{\mathrm{Zn}}$, and the Li${}_{\mathrm{Zn}}$–H–Li${}_{\mathrm{Zn}}$ complex. The experimental data obtained for a ZnO sample irradiated with oxygen ions with high concentration of ${\mathrm{V}}_{\mathrm{Zn}}$ is shown for comparison (Ref. 27).