Progress in the growth and characterisation of zinc germanium phosphide crystals

Zinc germanium phosphide (ZnGeP₂, ZGP) is a ternary chalcopyrite that crystallises in the tetragonal space group I-42d. ZGP is the preferred material for high power optical parametric oscillation (OPO) applications in the 3-5 μm spectral region. However, a number of unfortunate physical and chemical properties make this material extremely difficult to grow as crack-free single crystals of high quality. ZGP exists in a complex ternary phase system, contains toxic and volatile components with high vapour pressure at the melting point and is susceptible to stress-induced twinning and cracking. Poor control of the starting charge composition and temperature during growth can lead to deviations in stoichiometry, resulting in undesirable near band edge optical absorption at the pump wavelength and the formation of microprecipitates that can act as optical scattering centres at the wavelengths of interest. The crystals are prepared from high temperature melts contained in sealed ampoules using the Bridgman method. The starting charge is pre-synthesised from high purity elements and contains a slight excess of volatile components to compensate for losses during growth. A multi-zone furnace is employed to provide accurate control of the temperature profile during growth and post-growth cooling and crystallisation occurs from orientated seeds in pyrolytic boron nitride crucibles to minimise thermo-mechanical stresses. After growth, the crystals are processed into slices and annealed at 600°C for 300 hours to reduce further optical absorption at the pump wavelength. Using this method, crack-free crystals of 20 mm diameter by 100 mm in length are obtained. Detailed assessment reveals a number of defects that limit nonlinear optical conversion efficiency and beam quality including near band edge absorptions, growth striae, solute trails and micro-precipitates. The origin of these defects is discussed and methods for their elimination presented. Steady improvements in crystal quality have been achieved and crystals are now routinely produced with optical losses < 0. 1 cm^-1 after post-growth treatment. The influence of these defects on the spatial variation of optical transmission is also described.