## 3 Experimental Results and Discussion
According to the results of calculations performed in [18], melted ZnGeP\({}_{2}\) crystallizes have relatively high equilibrium concentration of vacancies [V\({}_{\text{P}}\)] in the phosphor sublattice that decreases significantly with the crystal temperature. At the same time, the concentration of vacancies [V\({}_{\text{zn}}\)] in the zinc sublattice first increases with decreasing temperature and then remains almost independent of the temperature in a certain temperature interval (\(\sim\)25 K). This means that the total germanium solvability, proportional to the concentration of vacancies in the corresponding sublattices, should significantly depend on the temperature oscillations at the crystallization front; moreover, the amplitude and time characteristics of the temperature oscillations for crystals grown by the vertical Bridgeman method are determined by the following factors [20]:  
Figure 4: Hologram of the ZnGeP\({}_{2}\) plate with thickness of 6 mm cut parallel to the growth direction (on the **right**) and restored image of the plate cross section containing a volume defects at a depth of 4.8 mm (on the **left**). Figure 3: Images of the etched cut tilted to the crystal growth axis at indicated zooming (**a**,**b**) and image of the cut tilted to the crystal growth axis reconstructed from the hologram (**c**). * supersaturation of the ZnGeP\({}_{2}\) melt by the doping component (in our case--Ge) and axial temperature gradient influencing on the rate of forming temperature inversion layers at the crystallization front;
* viscosity of the melt and radial temperature gradient influencing on the period of convectional melt flow instability;
* crystallization front speed influencing on the period of concentration instability. The above-indicated reasons for the nonstationary motion of the crystallization front lead to the formation of the single crystal material with different concentrations of point and volumetric defects and, as a consequence, to the formation of growth striations and modulation of the absorption along the crystal length [9; 19]. The growth striations are clearly seen in Figure 5 recorded by the digital holography method. The striations correspond to the crystallization isotherms and hence, are concave toward the ingot center. This testifies to the presence of the temperature dip in the ingot center in the process of its growth. These results are in good agreement with the data obtained in [9] by optical transmission microscopy of thin plates as well as with the results obtained in [21] using the X-ray transmission topography. One of the defect types in a single crystal is a dislocation. Sources of dislocation nucleation in crystals are regions of enhanced dislocation density formed on the lateral surface of the seed crystal below the seed crystal--crystal interface due to the melt inflow between a crucible and a seed crystal. The melt inflow polycrystallizes due to fast induration below the phase interface and causes the high dislocation density N\({}_{\text{d}}\)\(>\) 10\({}^{5}\) cm\({}^{-2}\)[21]. Intense dislocation chain propagates from a highly deformed area on the lateral surface of the seed deep into the growing crystal. In [20], it was shown that the deformation field around a dislocation results in the appearance of inhomogeneous density variations determined by the sum of main strains. These density variations scatter radiation transmitted through the sample under control. High strain gradients and hence, refractive index gradients in the direction normal to the dislocation lead to significant curvatures of trajectories of beams incident on the crystal in the direction parallel to the dislocation row. Figure 6b,c shows strain distributions in the material recorded by the digital holography method. They lead to refractive index variations and radiation scattering indicating the presence of dislocations. The dislocation rows consist of \(\sim\)10-50 dislocations. Figure 6d shows the image of dislocation rows recorded by the method of X-ray topography [21].