High growth rate deposition of hydrogenated amorphous silicon-germanium films and devices using ECR-PECVD

Hydrogenated amorphous silicon germanium (a-SiGe:H) films and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with small amount of H₂, was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, and quantum efficiency were characterized during the process of pursuing good materials. High-quality material was successfully fabricated with the ECR-PECVD technique at high growth rates. The device we made with 1.47 eV band gap has a fill factor of 64.5%. With the graded band gap and graded doping techniques, 70% fill factor was achieved when the band gap was graded from 1.75 to 1.47 eV. We also got 68% fill factor with the band gap graded from 1.75 to 1.38 eV.