Zn acceptors in β-Ga2O3 crystals

Abstract

Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga2O3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the ZnGa10 acceptor) and one from Zn ions at octahedral Ga(2) sites (the ZnGa20 acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69Ga and 71Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: ZnGa10 acceptors interact equally with two Ga(2) ions and ZnGa20 acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors (ZnGa1− and ZnGa2−). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral ZnGa20 acceptors when photoinduced holes are trapped at ZnGa2− acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower ZnGa20 acceptors and produces the EPR spectrum from the more stable ZnGa10 acceptors. The ZnGa20 acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to MgGa20 acceptors, whereas the slightly deeper ZnGa10 acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.