H passivation of Li on Zn-site in ZnO: Positron annihilation spectroscopy and secondary ion mass spectrometry

The interaction of hydrogen (H) with lithium (Li) and zinc vacancies (V${}_{\mathrm{Zn}}$) in hydrothermally grown $n$-type zinc oxide (ZnO) has been investigated by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. Li on Zn-site (Li${}_{\mathrm{Zn}}$) is found to be the dominant trap for migrating H atoms, while the trapping efficiency of V${}_{\mathrm{Zn}}$ is considerably smaller. After hydrogenation, where the Li${}_{\mathrm{Zn}}$ acceptor is passivated via formation of neutral Li${}_{\mathrm{Zn}}$-H pairs, V${}_{\mathrm{Zn}}$ occurs as the prime PAS signature and with a concentration similar to that observed in nonhydrogenated Li-poor samples. Despite a low efficiency as an H trap, the apparent concentration of V${}_{\mathrm{Zn}}$ in Li-poor samples decreases after hydrogenation, as detected by PAS, and evidence for formation of the neutral V${}_{\mathrm{Zn}}$H${}_2$ complex is presented.

Figure 1

Li concentration vs depth profiles for samples A-1, A-2, B-1, and B-2 as measured by SIMS. The as-grown HT sample A and the post-treated HT sample B contain $2\times {10}^{17}$ and $3\times {10}^{15}$ Li$/$cm${}^3$, respectively. After implantation and diffusion, Li has been rearranged in the peak region but still reaches about 1–$2\times {10}^{17}{\mathrm{cm}}^{-3}$ for A-2 and $6\times {10}^{14}{\mathrm{cm}}^{-3}$ for B-2 at depths exceeding $\sim$1 and $\sim$2 $\mu$m, respectively.

Figure 2

The measured $S$ parameter plotted vs the positron implantation energy (0.5$-$38 keV) for the A-1, A-2, B-1, and B-2 samples. Positrons implanted with an energy $\le$5 keV may reach the surface by diffusion and annihilate, leading to the increased $S$-parameter values observed for low energies. The observed peaks in $S$-parameter value at $\sim$10 keV for samples A-2 and B-2 are caused by the end-of-range defects induced by the H implantation.

Figure 3

The normalized $W$ parameter vs the $S$ parameter obtained for positron implantation energies ranging from 25 to 38 keV for the Li-rich (A) and the Li-poor (B) HT samples before and after hydrogenation. The black circle represents previously obtained experimental values of the $W$ and $S$ parameters for Li${}_{\mathrm{Zn}}$ in Li-rich ZnO.[16, 23]