Homoepitaxial growth of high-quality BaSO₄:Mn⁶⁺ using low-temperature liquid phase epitaxy

Single-crystalline host materials doped with 3d1 transition-metal ions are of high interest for applications as tunable lasers. The Mn6+ ion exhibits broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown with sufficient optical quality. The active material has to be free of defects and inclusions larger than 1/20 of the optical wavelength. In addition, the surface of the active layer and its interface to the substrate must be optically flat to receive low-loss guiding properties. The structure of the barite-phase BaSO4 contains tetrahedra which are replaced partly by the dopant complexes. The BaSO4:Mn6+ growth temperature is limited by the phase transition above 1010°C and especially the noticeable reduction of Mn6+ to Mn5+/Mn4+ above 600°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 grew undoped BaSO4 substrate crystals of 10 x 5 x 4 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as the solvent. Subsequently, the growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained flat layers free of inclusions with step heights of 1.4 nm, equal to 2 unit cells, and step distances of about 200 nm. Finally, layers of BaSO4:Mn6+ were grown on c-oriented faces with thicknesses up to 150 μm, at growth rates of 3 μm/h and growth temperatures of 500–550°C. The Mn6+ concentration in the doped layer was up to 1 mol% with respect to S6+. Absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its hexavalent oxidation state. Room-temperature broadband luminescence in the wavelength range 850-1600 nm was observed.