Evolution kinetics of elementary point defects in ZnO implanted with low fluences of helium at cryogenic temperature

Hydrothermally grown $n$-type ZnO samples, implanted with helium $({\text{He}}⁺)$ at a sample temperature of $\ensuremath{\sim}40$ K and fluences of $5\ifmmode\times\else\texttimes\fi{}{10}⁹$ and $5\ifmmode\times\else\texttimes\fi{}{10}¹⁰\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, have been studied in situ by capacitance voltage (CV) and junction spectroscopy measurements. The results are complemented by data from secondary ion mass spectrometry and Fourier transform infrared absorption measurements and first-principles calculations. Removal/passivation of an implantation-induced shallow donor center or alternatively growth of a deep acceptor defect are observed after annealing, monitored via charge carrier concentration $({N}_d)$ versus depth profiles extracted from CV data. Isothermal anneals in the temperature range of 290--325 K were performed to study the evolution in ${N}_d$, revealing a first-order kinetics with an activation energy, ${E}_a\ensuremath{\approx}0.7\phantom{\rule{4.pt}{0ex}}\text{eV}$ and frequency factor, ${c}₀\ensuremath{\sim}{10}⁶\phantom{\rule{4.pt}{0ex}}{\text{s}}^{\ensuremath{-}1}$. Two models are discussed in order to explain these annealing results. One relies on transition of oxygen interstitials $({\text{O}}_i)$ from a split configuration (neutral state) to an octahedral configuration (deep double acceptor state) as a key feature. The other one is based on the migration of Zn interstitials (double donor) and trapping by neutral Zn-vacancy-hydrogen complexes as the core ingredient. In particular, the latter model exhibits good quantitative agreement with the experimental data and gives an activation energy of $\ensuremath{\sim}0.75$ eV for the migration of Zn interstitials.