Structural peculiarities and point defects of bulk-ZnO single crystals

ZnO single crystals are related to promising direct wide band gap semiconductor materials belonging to the AIIBVI type of compounds with wurtzite structure. “Unintentional” n-type conductivity in ZnO may be caused by zinc and oxygen vacancies, and interstitial zinc atoms. To date, the comprehensive structural investigation and analysis of point defects in ZnO is absent in literature. Green, light green and almost colorless ZnO single crystals grown by the hydrothermal method in concentrated alkali solutions 4М(КОН) + 1М(LiOH) + 0.1M(NH4OH) on monohedral seeds [0 0 0 1] at crystallization temperatures in the range of 330-350 °C and pressures in the range of 30-50 MPa have been firstly investigated by neutron diffraction. It was revealed the presence of additional reflections (∼12-∼16%) for all the crystals caused by kinetic growth effects that give grounds to assign them to the space group P3 rather than to P63mc. Analysis of the refined compositions together with the color of ZnO crystals does not rule out the relationship between the color and vacancies in the zinc and oxygen positions whose concentration decreases with the discoloration of the samples. The analysis of the photoinduced variation of the total and on-axis transmittance versus the CW laser intensity showed that the colored samples have profound deep defects related to oxygen vacancies.