Stability Of Big-size ZnGeP₂ Crystal Growth Using Numerical Modeling Method

Zinc–germanium diphosphide,ZnGeP₂（ZGP）,is one of the most effective nonlinear optical materials for the mid-infrared region,used for laser frequency conversion from one spectral range to another.In this paper,we investigated the VB technique which is the most common and effective tool for growing of 60 mm ZnGeP₂ single crystals.However,it is still difficult to grow high quality big size crystals on account of the limitation of the long growth time and direct observation of growth process.We use computer modeling combined with experiment to investigate the influence of process parameters and hot-zone designs of ZnGeP₂ crystal.By using CGSim software,we establish the geometry of the furnace and the calculation equations of mathematical physics,define the material property and boundary conditions,create the calculation grids and set the process parameters.Based on the experimental date,calibration process is done by tuning the material property,boundary conditions and target temperature of thermal couple.Finally,good agreement between experimental date and simulation result is achieved in our model.In the same hot-zone design,5 different unsteady cases of crucible pulling speed which are 0.3 mm/h,0.5 mm/h,0.7 mm/h,0.9 mm/h and 1.1 mm/h are simulated considering the crystal growth experience.By analyzing the simulation results,the influence of different crucible pulling speed on crystallization speed,growth interface shape and thermal stress distribution is achieved.Finally,we get the optimal pulling speed design where we should use 0.3 mm/h when the crystal height is lower than 70 mm and use higher pulling speed between 0.5-0.9 mm/h to save growth time.Cooling stage design is an important factor of thermal stress releasing and crystal cracking during cooling process.In this paper,we compare 2 different cooling stage designs with the initial case and investigate the difference of the final thermal stress distribution and thermal stress evolution during the cooling process.To avoid crystal cracking and decrease the thermal stress,we can slow the cooling speed at the beginning stage and speed up the cooling stage when the thermal stress is low enough.Different hot-zone designs are also calculated in our work,different temperature gradient in the growth zone is achieved by adjusting the thickness of insulation in the growth zone.Four unsteady cases of different temperature gradient which are 3.5 K/cm,5 K/cm,7 K/cm and 9 K/cm is analyzed,and we investigate the influence of different temperature gradients on growth interface shape and distribution of thermal stress.Compared with the initial 5 K/cm case,the optimal simulation result is achieved in the 7K/cm case.Meanwhile,the approach of simulation combined with experimental data also can be used for other crystal growth and it provides a new way to better understanding crystal growth process and design new crystal growth process.