Pure and chromium doped ZnO crystals were grown in a quartz ampoule with a mixture of H2+Cl2 or H2+C+H2O, applied as transporting chemical agent. Obtained by this method polycrystals were often red and had monocrystal grains of about 6x7x5 mm3. The as-grown ZnO crystals were of n-type and had low specific resistivity (about 0.05 cm at 300 K). The content of Cr in the ZnO:Cr crystals was below 1018 cm-3, which was proved by SIMS analysis.
The absorption of the as-grown ZnO crystals exhibits a strong near-edge band which is probably due to the oxygen centers, located in the band gap. Absorption edge, caused by the interband transitions is revealed at about 3.2 eV. Oxygen annealing provokes the disappearance of the near-edge band and it proves, that the samples were grown with the oxygen deficiency. The absorption of the ZnO:Cr crystals at T=5 K reveals a band with a peak at about 0,992 eV, which is probably caused by Cr2+ center.
The luminescence spectra, excited by the 325 nm He-Cd laser, shows two major luminescence bands with peak energies at 3.2 eV and around 2.5 eV. The first band is assigned to the thermalized bound and free exciton luminescence. The latter one is also observed at the temperature of 10 K and it is probably associated with some defect centers including oxygen vacancy Vo+. Luminescence intensity in this band decreases after the annealing of the samples in the oxygen atmosphere and it confirms its assignment to the oxygen vacancy.
An extra sharp line is also observed in the luminescence spectra of the ZnO crystals at energy of about 2.25 eV. The origine of this line is unknown, since some reports associate it with donor-acceptor pair transitions related to Cu dopant. We shall present the results of high-pressure luminescence experiments in the diamond anvil cell, which provide additional information about the luminescence line origin.