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

• Direct epitaxial growth of Si1-XGeX on c-Si by RF-PECVD at 175 °C.
• Up to 35% Ge alloy achieved at low temperature
• Precise determination of Ge% by ellipsometry as confirmed by SIMS, Raman and GDOES.
• Transition from pseudomorphic to metamorphic growth identified by Raman and TEM.
• Solar cell with 1.9 µm Si0.73Ge0.27 absorber, 18.8 mA/cm2 and 77.5% fill factor.

Silicon germanium alloys are of great interest for thin film solar cells, thanks to their adjustable bandgap and stronger absorption when compared to Si. SiGe alloys are herein epitaxially grown on (100) c-Si substrates at 175 °C in a standard RF-PECVD reactor from H2/SiH4/GeH4 precursor gas mixture. A linear correlation is found between the GeH4/(SiH4+ GeH4) gas ratio and Ge content in the layer, with epitaxial growth being maintained up to 35% Ge. The alloy composition deduced from spectroscopic ellipsometry shows excellent agreement with Raman spectroscopy, glow discharge optical emission spectroscopy and SIMS-MCs+ analysis. Epitaxial SiGe strain and defects, arising from lattice mismatch, are studied as function of Ge atomic percentage by Raman and transmission electron microscopy. The 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. External quantum efficiency confirmed a 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, resulting in a conversion efficiency of 6.1%.