MBE growth of crystalline SiGe thin films for solar cell applications with precisely controlled heterojunction

Crystalline SiGe layers with Ge contents from 10 to 80 % were grown on Si (100) substrates by molecular beam epitaxy for the applications in Si-based multijunction thin film solar cells. The effects of different buffer layers on the structural and electrical properties of SiGe layers were studied in detail. It was found that the gradient SiGe buffer formed at a lower limit temperature of 400°C was effective in reducing the threading dislocations, and in turn improving the surface morphology and Hall mobility. a-Si/SiGe heterojunction solar cells were fabricated on metallic-like Si substrates to testify the suitability of SiGe layers for solar cell applications. E_ff = 0.75% and V_oc = 320 mV were achieved at a Ge content of 8%, and 0.5% and 200 mV at 57 %. Hydrogenation for SiGe layer improved V_oc to 430 mV for the same Ge content of 8 %.