Low temperature epitaxial growth of SiGe absorber for thin film heterojunction solar cells

Silicon germanium alloys are of great interest for thin film solar cells, thanks to their adjustable bandgaps and stronger absorption compared to Si. SiGe alloys are here epitaxially grown on (100) c-Si substrates at 175°C in standard RF-PECVD reactor from H2/SiH4/GeH4 precursors. A linear correlation is found between the GeH4/(SiH4+ GeH4) gas ratio and Ge content in the layer, with epitaxy being maintained up to 35% Ge. The alloy composition deduced from spectroscopic ellipsometry (SE) shows excellent agreement with Raman spectroscopy, glow discharge optical emission spectroscopy (GDOES) and SIMS-MCs+ analysis. Epitaxial SiGe strain and defects, arising from lattice mismatch, are studied as function of Ge atomic percentage by Raman and TEM analysis. SiGe electrical quality is investigated by making heterojunctions solar cells stacks, consisting of (p++)c-Si wafer/epi-Si0.63Ge0.27/(n)aSi:H, and comparing their characteristics to pure silicon equivalent devices. EQE confirmed short circuit current of 18.8 mA/cm2 with a thin 1.9 µm absorber and flat interfaces, along with a fill factor of 77.5% and an open circuit voltage of 416 mV, resulted in a conversion efficiency of 6.1 %.