ZnO:HCl single crystals: Thermodynamic analysis of CVT system, feature of growth and characterization

• The low diffusion growth rate is limited by the low temperature dependence on pressure.
• At temperatures lower than 1050 °C, mass transport rate is limited by kinetics.
• ZnO:HCl crystals are free from voids and have a low density of dislocations.
• Properties of ZnO:HCl materials are caused by Cl-containing stable defects.
• Activation energy of Cl donors is in the 77–103 meV range.

The full thermodynamic analysis of using HCl as a chemical vapor transport (CVT) agent (TA) for ZnO single crystals growth in closed growth chambers, including 16 chemical species, is carried out for wide temperature and loaded TA pressure ranges. The influence of the growth temperature, of the TA density and of the undercooling on the rate of ZnO mass transport was investigated theoretically and experimentally. It is shown that the mass transport is diffusion-limited at about 1050 °C, and it is limited by kinetics of the CVT reaction at lower temperatures. It is experimentally shown that using HCl favors obtaining void-free n-ZnO crystals with controllable electrical parameters, it reduces the effect of adhesiveness to the walls of the growth chamber. The characterization by the photoluminescence spectra, the transmission spectra and the electrical properties in the wide temperature range allowed analyzing energy spectra of Cl-containing stable defects in ZnO and electrical activity of Cl donors. Some methods of activation energy correction for Cl-containing centers are discussed.

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