Hydrothermal growth and characterization of bulk Ga-doped and Ga/N-codoped ZnO crystals

Bulk ZnO crystals were grown by the hydrothermal technique with Ga 2 O 3 or GaN added to the solution in an attempt to dope with Ga, or co-dope with Ga and N, respectively. Adding Ga 2 O 3 alone to the growth solution significantly reduces the ZnO growth rate; however, the resulting crystal is highly conductive, with a resistivity approaching 0.01 Ω cm. In contrast, the addition of GaN had less effect on the growth of ZnO, but the crystal was of poor quality with a higher resistivity, about 0.1 Ω cm. Photoluminescence spectra at 4 K show Ga 0 -bound-exciton peak energies of 3.3604 and 3.3609 eV for the Ga- and Ga/N-doped crystals, respectively; these energies differ slightly from the literature value of 3.3598 eV, evidently due to compressive strain. Other peaks at 3.307, 3.290, 3.236, and 3.20 eV were found in the Ga/N-codoped ZnO after the crystal was annealed at 600°C in air. The 3.307 eV peak is the so-called A line, and likely arises from recombination of a free electron with a neutral N-related acceptor.