200 mm Germanium Wafers, Ge Substrates, MOVPE, High Efficiency Space Solar Cells, Semiconductor Bonding Technology (SBT)
Summary / Abstract:
In the framework of the H2020 project ‘RadHard’ we demonstrate for the first time high efficiency triplejunction space solar cells on 200 mm Ge wafers with excellent efficiencies exceeding 30% (AM0, 25°C). Crystal growth of dislocation free germanium ingots with sufficient diameters is achieved and a pilot-line for wafer manufacturing is implemented successfully at UMICORE. The surface quality obtained on the 200 mm wafers is close to the grade of commercially available 150 mm substrates. A comprehensive growth study on different III/V materials comparing MOVPE growth on 150 and 200 mm substrates shows good to excellent workability of the 200 mm substrates at AZUR SPACE. The epitaxy process of the AZUR standard 3G30 triple-junction structure has been performed in an AIXTRON planetary reactor using an industrially feasible MOVPE recipe, whereas due to the wafer format >11% wafer area is treated in a single epitaxial run by using 200 mm instead of 150 mm wafers. For solar cell manufacturing the standard 30.18 cm² cell design is applied, using a cell arrangement that allows the assessment of the epitaxy homogeneity across the 200 mm wafer surface by means of cell performance. Engineering tests are performed on a test coupons built up with these cells verify the device quality. Finally, the growth of solar cell structures on two separate 200 mm Ge substrates and their subsequent merging by the direct semiconductor bonding technology (SBT) is demonstrated, which is known to enable ultra-high efficient tandem solar cells but is cost-intensive when using smaller wafers.