Investigation of VO–Zni native donor complex in MBE grown bulk ZnO

Abstract

In this paper, we have experimentally investigated the theoretical predictions of VO–Zni to be a native donor in ZnO. Intrinsically zinc-rich n-type ZnO thin films having ND ∼ 6.23 × 1018 cm−3 grown by molecular beam epitaxy on Si (0 0 1) substrate were annealed in oxygen environment at 500–800 °C, keeping a step of 100 °C for 1 h, each. Room temperature Hall measurements demonstrated that free donor (VO–Zni) concentration decreased exponentially and Arrhenius plot yielded activation energy to be 1.2 ± 0.01 eV. This value is in agreement with theoretically reported activation energy of VO–Zni donor complex in ZnO. We argue; this observation can be explained by two-step process: (i) incoming oxygen fills VO of VO–Zni complex leaving behind Zni; (ii) Zni releases its energy and moves to a lower energy state with respect to the conduction band minima and/or occupies an inactive location. Consequently, Zni–VO complex loses its donor role in the lattice. Our experimental data supported theoretical predictions of VO–Zni to be a native donor. Results from photoluminescence spectroscopy carried out on Zn-rich ZnO additionally justify the existence of VO–Zni complex.