Heterogeneous growth of microcrystalline silicon germanium

Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10⁻⁵/Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E_a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm² has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 Å/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V_oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).