Study Of 4-inch Dislocation-free Germanium Crystal For GaAs Solar Cells

The high efficiency and good comprehensive performance of GaAs solar cells fabricated on the P type germanium single crystal substrate has made the P type germanium single crystal become the main substrate material of GaAs space solar cells. Dislocation defects in germanium could reduce the quality of epitaxial layers, lower the conversion efficiency and the service life of GaAs solar cell for space. How to lower the dislocation density in germanium has been a difficult and hot spot. At present, our technology has been unable to meet the needs of space solar cell development, and we mainly rely on imports, which has become the bottleneck of the development of space solar cell. So it has great practical application value and great strategic significance to carry out the research of 4-inch P type dislocation-free germanium crystal.Based on the conditions of temperature gradient growing 4-inch dislocation-free germanium, the paper carried on global simulation analysis using professional crystal growth simulation software to optimization of thermal field reasonably and reduce the dislocation density eventually.In this paper, the effect of heaters, insulation layer, crucible’s support bar on the temperature distribution in the crystal growth system of germanium growth system was analyzed, and the double heaters growth system in which 4-inch dislocation-free germanium can be grown had been designed and developed. And then temperature, temperature gradient and stress distribution in the crystal grown system have been obtained by numerical simulation. The outcomes by numerical simulation show that the axial temperature gradient is in the range of 0.1-0.6 K·cm-1 and the radial temperature gradient is kept in the 0.02-0.26 K·cm, and that the maximum thermal stress value in some areas of crystal exceeds barely the critical shear stress. Numerical simulation results show that by using double heater system composite insulation layer and supporting rod, the temperature gradient inside the crucible and the crystal thermal stress could be effectively reduced.Germanium crystal grown in the double heaters growth system were cut into pieces according to requirements and then dislocation corrosion measurement experiments were made which can obtain the dislocation density and longitudinal distribution of dislocation of the test crystal plate. The contrast of numerical simulation and experimentation showed the prospective results of numerical simulation about stress distribution are in agreement with those of the experiment study on the dislocation and the main results are as follows: the double heaters system can be used for growing 4-inch dislocation-free germanium;the density of dislocation is in the range of 350-480 cm-2 and etched pits of dislocation are scattered.