The Study On Dislocation Pattern And Hardness Of Germanium Single Crystal For Gaas Space Solar Cells

GaAs photovoltaic solar cells have been increasingly used in power supply devices in outer space by a lot of countries because of their high solar opto-electronic conversion efficiency, high performance against space radiation, excellent temperature-related characteristics and long serving life. As key component material of single junction GaAs/ Ge solar cells and GaAs/Ge matrix solar cells, the quality of germanium single crystal has a direct effect on the properties of space solar cell. With the widely use of GaAs solar cells in spacecraft and a growing investment interest in its application in civil industry in recent years, an improved quality of germanium substrate is required to meet the demand of a higher energy conversion efficiency. By the research on the dislocation pattern in the low dislocation germanium single crystal and related mechanical characteristics, this work finally obtained a large-size (4 inches and above), low dislocation germanium single crystal which can meet the requirements of space GaAs/Ge solar cells.The main contents and results are listed as follows:1. Though optimizing temperature field and crystal growth technological parameter, by Czochralski method we obtained 4-inch low dislocation density germanium single crystal (EPD<3000/cm2) and meet the requirements on germanium substrate for high conversion efficiency space GaAs/Ge solar cells.2. There are there typical dislocation patterns in germanium single crystals: dislocation array, small angle boundaries and flower-shaped structure. Inclusions are the major reason for flower-shaped structure according to the investigation by Raman, SEM, XREDS analysis, and a large number of carbon inclusions are from the furnace system while a small amount of oxygen in the single crystal derive from incomplete reduced germanium dioxide or water vapor in the argon atmosphere. Taking effective measures to strictly control the furnace system environment and avoiding pollution of carbon on the crystal can effectively reduced the amount of flower-shaped structure dislocation pattern and the dislocation density in single crystal germanium with an improved quality.3. According to the statistical analysis of longitudinal dislocation distribution in a series of germanium single crystal, a large number of dislocations grow at the beginning position of shoulder (Φ8mm) due to the effect of gravity when the growing of single crystals is finished. While this dislocation will not spread to larger extent in the whole process of crystal growing.4. Vickers hardness testing has been performed on germanium thin polished slices for the first time and it is found that hardness is anisotropic. Under the same load, the hardness of (111) surface is higher than that of (100) surface; the hardness of germanium single crystal used for substrate of GaAs/Ge (EPD<3000/cm2) is higher than that of the same size germanium single crystal used for infrared optical technology (EPD>10000/cm2) with the same manufacturing process.