ZnGeP₂ (ZGP) crystals are used as a high effective nonlinear optical medium for laser radiation frequency conversion in the middle IR. They are also promising for creation of tunable terahertz radiation sources. Application in nonlinear optical devices demands from crystals a high structural perfection and uniformity of properties. A presence of two volatile components, Zn and P, in ZnGeP₂ structure makes difficult it’s high-yield synthesis and causes deviations of the melt composition from stoichiometry during synthesis and growth, what leads to emergence of point and other growth defects.

The aim of this contribution is to present results of our studies on synthesis, growth and structural defects in ZGP crystals, revealed by X-Ray transmission topography based on Borrmann effect and other methods.

ZGP compound was synthesized from elements by two-temperature method. Vertical Bridgman method was used to grow the single crystals 24-45 mm in diameter. Typical broadening of the rocking curves (FWHM) is 13-35 seconds of arc what shows a high crystal structural perfection. At that FWHM is larger in the center of the cross-section of crystals than that on periphery, what is apparently related with concave interface during growth. Presumably, the defects are accumulated in the center because of the interface concavity as it was demonstrated by K. Sangwal and K.W. Benz.

X-Ray topography reveals edge and screw dislocations, inclusions of second phases of different types and composition. Some of the inclusions (so-called solute trails) give dislocations with powerful impurity atmospheres. But, as a rule, coherent microdefects of second phases are the primary in our ZGP. Their density is varied from 1x10² cm^-2 (1.4 x10³ cm^-3) to 5x10²cm^-2 (7x10³ cm^-3). The inclusions of vacancy type are met in 8 times more often than the inclusions of interstitial type. Such microdefects as dislocation loops amount less than 2% from the total ones. In all studied crystals growth striae, having concave shape (type I), are observed. A contrastive analysis of optical transmission microscopy and X-Ray topographs allows to assume that striae are related to variations of Zn vacancy concentration, giving a high absorption in ZGP near fundamental absorption edge (~0.65 mkm), as it was shown by American scientists. With perpendicular directions of diffraction vector g (the reflection planes are parallel to growth axis (case 1) or reflection from planes, perpendicular to growth axis (case 2)) the X-Ray topographs from the same sample are different. In the first case the striation is not seen, but rectilinear dislocations inclined or located along the growth axis and microdefects are seen. For the second case (g is parallel to growth axis) conditions of observation of growth striae are realized with maximal contrast. Here the dislocations are also seen. However, a washout of dislocation images in so- called “special position” and absence of dislocation images in case 2, seen on the topograph, obtained with perpendicular direction of diffraction vector g (case 1) are strange. This can be related to features of chalcopyrite lattice of ZGP. The center of cross cuts of single crystals contains dislocations with big edge component of the Burgers vector and their rows, they are perpendicular to the sample surface. The periphery of the cross plates has dislocations, giving a black and white contrasts, with a big screw component of the Burgers vector and crossing the samples approximately orthogonally. The dislocation density is decreased from the origin of the ingot to it’s end from 2x10³ cm^-2 to 2.5x10² cm^-2 on average.

Thermal annealing and electron irradiation allows us to decrease the optical absorption at 2.06 mkm (the main wavelength for optical parametric oscillation from ZGP) from 0.5 to 0.02 cm^-1 that allows to use our ZGP in powerful optical systems.

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1. Structural and optical properties of ZnGeP₂ single crystals
2. A High-power Generation in Mid-Infrared Region Based on ZnGeP₂ Optical Parametric Oscillation
3. Vertical Bridgman Growth of Large ZnGeP₂ Single Crystals for High Power Applications
4. Investigation of Defects in ZnGeP₂ Single Crystals by X-ray Topography on Base of Borrmann Effect
5. Computer Diagnostics of Crystal Quality
6. The Analysis of Defects in ZnGeP₂ Single Crystals from Birefringence Images
7. Quantitative Criteria of Image Quality Evaluation, Theory and Experiment