Aluminum Migration and Intrinsic Defect Interaction in Single-Crystal Zinc Oxide

Vacancy-mediated migration of Al in single-crystal zinc oxide (ZnO) is investigated using secondary-ion mass spectrometry (SIMS) combined with hybrid density-functional theory (DFT) calculations. A thin film of Al-doped ZnO is deposited by sputtering onto the single-crystal bulk material and heat treated at temperatures in the range of 900 degrees C-1300 degrees C. The migration of Al is found to be Zn-vacancy mediated. In order to elucidate the physical processes involved, an alternative model based on reactive diffusion is developed. The model includes the time evolution of the concentration of Al atoms on the Zn site (Al-Zn), Zn vacancies (v(Zn)), and a complex between the two, where the influence of the charge state of vZn on its formation energy is incorporated through the free carrier concentration. The modeling results exhibit close agreement with the experimental data and the Al(Zn)v(Zn) complex is found to diffuse with an activation energy of 2.6 eV and a preexponential factor of 4 x 10(-2) cm(2) s(-1). The model is supported by the results from hybrid DFT calculations combined with thermodynamical modeling, which also suggest that a complex between AlZn and vZn is promoted in n-doped material. The charge state of this complex is effectively -1, and it thus acts as a compensating acceptor, limiting full utilization of the shallow AlZn donor. Furthermore, the DFT calculations also predict a high formation energy for both substitutional Al on the O site (Al-O) and interstitial Al (Al-i), and are therefore of minor importance for Al migration in ZnO. The close coupling between the hybrid DFT calculations and the developed diffusion model enable benchmarking of the accuracy of several parameters extracted from the DFT calculations. Furthermore, since the diffusion model hinges strongly on defect concentrations, it couples directly to results from measurements by other experimental techniques than those used in this paper and provides an opportunity for independent verification of the estimated values by future studies.