Low-temperature liquid phase epitaxy of rare-earth-ion doped KY(WO₄)₂ thin layers

Rare-earth-ion doped KY(WO4)2 (hereafter KYW) is a promising material for novel solid-state lasers. Low laser threshold, high efficiency, high output powers, and third-order nonlinear effects have stimulated research towards miniaturized thin-film waveguide lasers and amplifiers for future photonic devices. Active films can be fabricated by liquid-phase epitaxy (LPE), pulsed laser deposition, ion implantation, and diffusion bonding. We report here on the low-temperature liquid-phase epitaxy of KYW:RE3+ single crystalline layers (RE = Tb, Dy, Yb). Undoped KYW crystals grown by a modified Czochralski method with laser-grade polished (010) faces were used as substrates. The ternary chloride eutectic NaCl-KCl-CsCl with a melting temperature of 482oC was employed as a solvent. This flux dissolves all the solute components, possesses low viscosity and volatility, is non-toxic and available in high purities. The layers were grown at start temperatures as low as 540°C, which is favorable in order to decrease the thermal stress due to the differences in thermal expansion coefficients of substrate and layer [4]. However, at high RE-dopant concentrations elastic stress due to the lattice parameter misfit between substrate and layer is often released by formation of a crack network along certain cleavage planes. Typically, the growth started with the nucleation of 3D islands, which then coalesced into a uniform epitaxial layer with areas of up to 1 cm2. X-ray diffraction measurements confirmed that the epilayers were strictly oriented in [010] direction. Elemental composition, nano-hardness, and elastic constants of the layers were determined. RE-ion incorporation into the KYW host decreases in the series Dy3+>Yb3+>Tb3+ according to the increasing misfit between the ion radii of RE and replaced Y3+ ion. The influence of solute concentration, growth temperature, and doping level was investigated in order to produce layers up to 20-µm thick and with low defect concentration, which can be suitable as optically activated waveguides.