Zn acceptors in β-Ga₂O₃ crystals

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.