Search

The influence of oxygen and nitrogen impurities on the material properties of a-Si:H and μc-Si:H films and on the corresponding solar cell performances was studied. For intentional contamination of the i-layer the impurities were inserted by two contamination sources: (i) directly into the plasma through a leak at the chamber wall or (ii) into the gas supply line. The critical oxygen and nitrogen concentrations for silicon solar cells were determined as the lowest concentration of these impurities in the i-layer causing a deterioration of the cell performance. Similar critical concentrations for a-Si:H and μc-Si:H cells in the range of 4–6 × 1018 cm− 3 for nitrogen and 1–5 × 1019 cm− 3 for oxygen by applying a chamber leak are observed. Similar increase of conductivity with increasing impurity concentration in the a-Si:H and μc-Si:H films is found. A more detailed study shows that the critical oxygen concentration depends on the contamination source and the deposition parameters. For a-Si:H cells, the application of the gas pipe leak leads to an increased critical oxygen concentration to 2 × 1020 cm− 3. Such an effect was not observed for nitrogen. For μc-Si:H, a new deposition regime with reduced discharge power was found where the application of the gas pipe leak can also result in an increase of the oxygen concentration to 1 × 1020 cm− 3.

Microcrystalline Si:Cl:H films prepared by plasma deposition using mixtures of silicon tetrachloride and hydrogen as process gases were studied by effusion of implanted helium. The temperature of maximum (low temperature) effusion rate is used as a measure of material density and microstructure. The results suggest that chlorine incorporation at concentrations exceeding a few percent results in a void-rich structure. High p-type conductivities obtained for boron-doped microcrystalline Si:Cl:H are explained by the presence of rather dense material with high crystalline fraction.

The use of chlorosilanes (silicontetrachloride and dichlorosilane) for rf plasma-deposition of microcrystalline silicon is explored. Deposition conditions similar to those employed for high rate deposition of efficient microcrystalline silicon solar cells (frequency: 13.56 MHz, high rf power, high pressure and high hydrogen dilution of process gas) were used and high deposition rates were achieved. Silane-based materials, deposited under the same conditions, were studied for comparison. Lowest porosity is obtained for both chlorinated and unchlorinated material at a crystallinity (measured by Raman scattering) near 70%. Infrared absorption measurements indicate the presence of more void-related microstructure in chlorosilane-based material than in silane-based. Conductivity measurements give a Fermi level near midgap for an undoped chlorinated material while our silane-based samples show unintentional doping effects. By doping of chlorinated material with boron, high conductivities are obtained. Chlorinated material was successfully incorporated as p-layer in microcrystalline Si solar cells.

The effusion of implanted helium was studied for doped and undoped radio frequency plasma-deposited a-Si:H films as well as for Si:H films deposited by other techniques. The results suggest that the observed helium diffusion and precipitation effects give information on the material microstructure.

The structural and optoelectronic properties of μc-Si 1− x Ge x :H alloys prepared by plasma enhanced chemical vapor deposition (PECVD) were investigated as a function of the deposition parameters germane concentration and hydrogen dilution. With decreasing hydrogen dilution we found a transition from microcrystalline to amorphous growth, which shifts to higher hydrogen dilution if the germanium concentration is enhanced. Additionally, we observed different built-in coefficients of germanium in the amorphous and crystalline phase. We correlate these structural properties with the optoelectronic properties. While the dark conductivity is only weakly affected by the germanium concentration, it decreases strongly upon a decrease of hydrogen dilution. Although the optical absorption increases with rising germanium the photoconductivity decreases indicating an increase of the defect density.

The dependence of hydrogen effusion and diffusion on hydrogen concentration was studied for crystalline and amorphous Si:H samples which were hydrogenated by hydrogen ion implantation. The results are compared to data for a-Si:H and μc-Si:H films grown with various H concentrations. Although general trends of the H concentration dependence of H stability are similar, there are differences between c-Si:H and a-Si:H. These differences involve larger hydrogen-generated microstructural effects in c-Si:H and a hydrogen diffusion coefficient in compact material increasing in a-Si:H and decreasing in c-Si:H with increasing H concentration. A different flexibility of the atomic network in amorphous and crystalline Si could be the origin. Microcrystalline Si:H films show a similar concentration dependence of H diffusion as a-Si:H.

Hydrogen diffusion and solubility effects in hydrogenated crystalline and amorphous silicon prepared by hydrogen implantation with approximately equal hydrogen concentrations are compared. Hydrogen diffusion is found to be limited by hydrogen solubility in both cases. Between 300 and 450°C, an agreement of the hydrogen diffusion coefficients for the two Si:H materials approximately within a factor of ten is observed. With increasing hydrogen concentration, hydrogen diffusion increases for amorphous Si:H but decreases for crystalline Si:H. This latter decrease is attributed to a hydrogen related microstructure which appears in crystalline Si:H at about a factor of ten smaller hydrogen concentration than in amorphous Si:H.

Both lithium and hydrogen diffusion processes in a-Si:H are found to be trap-limited. The time and concentration dependence of H diffusion is not observed for Li. While hydrogen probably diffuses in a neutral charge state and is trapped by intrinsic (Si bonding) sites, lithium diffuses largely in a positive charge state and is trapped mainly by extrinsic doping-related negatively charged ions or by oxygen.

Using secondary ion mass spectroscopy (SIMS), the stability of lithium doping by in-diffusion and ion implantation is studied for doped and undoped a-Si:H films. The results show a higher stability of implanted compared to in-diffused samples attributed to interaction of Li with implantation defects. Furthermore, for highly boron-doped material the in-diffusion is greatly enhanced while the out-diffusion is strongly inhibited. These latter effects are attributed to field enhanced diffusion and ion pairing.

Hydrogen diffusion in the a-Si-Ge alloy system was studied by SIMS profiling of deuterium and hydrogen interdiffusion and by hydrogen evolution. Hydrogen diffusion coefficients close to that of amorphous germanium are obtained throughout the alloy system. It is concluded that hydrogen diffuses predominantly via weak or broken Ge-Ge bonds.