Microcrystalline silicon–germanium alloys for solar cell application: Growth and material properties

Microcrystalline silicon–germanium (μc-Si_1?xGe_x:H) alloy films have been grown by 100-MHz glow-discharge of a SiH₄/GeH₄/H₂ gas mixture. Alloys over a full range of compositions were prepared to gain a comprehensive understanding of their growth and material properties. With increasing GeH₄ concentration in the gas-phase, we observed a preferential Ge incorporation behavior in the solid. Growth rate studies revealed that the Ge incorporation efficiency from source gas to solid is about five times greater than for Si at growth temperature of 200 °C, which accounts for the variation of alloy composition. With increasing Ge incorporation in the solid, on the other hand, we find a monotonic decrease in photoconductivity, followed by an electrical transition from weak n-type to strong p-type conduction at x > 0.7. At x ≈ 0.4, however, we obtained relatively high photoconductivity gains by a factor of 20 and strong infrared response in the solar cell structure. The Ge incorporation behavior and its effect on charge carrier transport are discussed.