Colour centres investigation in pure and doped yttrium aluminium garnet epitaxial films

Epitaxial films with thickness of 10-250 mu m of yttrium aluminium garnet ( YAG) doped with Cr were grown by liquid phase epitaxy technique on YAG: Nd substrates. Go-doping with Mg2+ was used to force the Cr4+ valent state for mation. Dependence of absorption spectra of obtained films on melt-solution composition, growth conditions and thermal treatment in reducing and oxidi zing atmospheres is studied. The absorption being characteristic for YAG:Cr 4+ crystals is found in co-doped films grown at higher temperatures (1000-1 100 degrees C). The chromium entering in the tetravalent state is confirmed by the annealing experiments. A very intensive absorption band in UV regio n with maximum at 275 nm was found both in co-doped and YAG: Mg2+ epifilms caused probably by oxygen vacancies compensating the excess charge of Mg2+. Its intensity correlates with Cr4+ content in the film in the following wa y: it decreases with Cr4+ entering in the film.