Large diameter germanium wafers for CPV applications

Umicore has a long tradition in supplying germanium wafers to the space solar cell market. The main product for this application up to today is a substrate with a diameter of 100 mm and a thickness in the range of 140 – 180 ¼m. With the new emerging market of high-efficiency concentrator photovoltaics (CPV) the need has arisen to investigate the use of germanium wafers with a larger diameter. Especially for small size CPV cells, increasing the wafer diameter will significantly reduce the processing cost per die, which will also contribute to lowering the overall cost/kWh of CPV technology. In this paper the development of dislocation free 150 mm germanium wafers for CPV applications is presented. Different wafer thicknesses down to 200 ¼m have been realized. The process steps starting from crystal pulling up to final epi-cleaning and drying will be addressed. First measurement results on wafer level are shown as well. Finally, the future challenges in optimizing the 150 mm wafer specifications will be reviewed.