Layer growth of high-quality BaSO₄:Mn⁶⁺ using liquid phase epitaxy

Single-crystalline host materials doped with transition-metal ions are of high interest for applications as tunable lasers. Mn6+ ions exhibit broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown in sufficient quality. The active material has to be free of inclusions or defects larger than λ/10, with λ, the wavelength of the porpagating beam. The interface between active layer and substrate must be optically flat to receive low-loss guiding properties. Finally, in the case of homo-epitaxy of BaSO4, the doped layer has to be arranged on the substrate (001) direction, because … . The growth temperature of BaSO4:Mn6+ is limited by the decomposition of BaSO4 at 1590°C, its phase transition above 1010°C, and especially the chemical reduction of the manganese dopant from Mn6+ to Mn5+ above 620°C. Therefore, the growth of BaSO4:Mn6+ from a solution at lower temperatures is the most suitable method. Liquid phase growth is close to the thermodynamic equilibrium and has enabled us to grow high-quality layers. First, we prepared undoped BaSO4 crystals of 10 x 5 x 1 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as solvent. Subsequently, growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained crystalline layers free of inclusions, grown in the Frank-Van der Merwe mode (layer-by-layer growth). Finally, layers of BaSO4:Mn6+ were fabricated with thicknesses up to 150 μm, at growth rates of 3 μm/h and temperatures of 500–580°C. The thickness was controllable with a precision of 0.1 μm. The Mn6+ concentration in the doped layer was up to 1 mol.% with respect to S6+. In collaboration with the University of Hamburg, absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its sextavalent oxidation state. Room-temperature luminescence in the wavelength range 850-1600 nm was observed.