Low-temperature SiGe PECVD epitaxy: from wafer equivalent to ultra-thin crystalline solar cells on inexpensive substrates

A standard RF-PECVD reactor is used to grow epitaxial SiGe layers at 175°C on c-Si (100) substrates. By adding GeH4 to SiH4/H2 plasmas, we show for the first time epitaxial SiGe alloys with Ge atomic fraction up to 35% grown at such low temperature. Few µm thick layers are used as an absorber in the wafer equivalent approach: thin film c-Si(p++)/epi-Si1-xGex/a-Si:H(n+) heterojunction solar cells are fabricated. Structural and electronic properties of these materials are investigated using Ellipsometry, Raman, TEM, J(V) characteristics and EQE. Simulations of device structure are performed with PC1D. Stress induced lift off enables detachment and transfer of ultra-thin Si layers of 1 cm2, on inexpensive substrate. Crystal quality is not affected by the transfer process, and first proof of concept of PECVD epitaxial ultrathin transferred solar diode is obtained.