Real time spectroscopic ellipsometry characterization of structural and thermal equilibration of amorphous silicon–carbon alloy p layers in p-i-n solar cell fabrication

Abstract

Real time spectroscopic ellipsometry (RTSE) has been applied to investigate the near-surface optical changes that occur during p/i interface processing for hydrogenated amorphous silicon carbon alloy (a-Si1−xCx:H, x≈0.05) p layers prepared at ∼200 °C by plasma-enhanced chemical vapor deposition in the p-i-n solar cell configuration. Trimethylboron [B(CH3)3] was used as the p-type dopant source gas in order to avoid p-layer surface contamination that occurs when using diborane (B2H6). We have analyzed the changes in the RTSE data detected after extinguishing the plasma for a-Si1−xCx:H p-layer deposition while maintaining the p layer near its growth temperature. We have attributed these changes to: (i) structural equilibration characterized by the emission of bonded hydrogen (∼2 at. %) from the p layer into the vacuum, and (ii) thermal equilibration characterized by near-surface temperature variations (∼7 °C) due to gas composition and pressure variations within the reactor. From the RTSE data, the kinetics of H emission and the time evolution of the near-surface temperature have been determined separately. We have found that a significant fraction of the H emitted from the a-Si1−xCx:H p layer at ∼200 °C is lost within minutes of terminating the p-layer plasma. To restore the p-layer H content and improve the p/i interface characteristics in a-Si:H p-i-n solar cells, we applied a H2 plasma treatment to the p layer just prior to i-layer deposition at 200 °C. Such a treatment yielded a 0.05 V increase in the open circuit voltage of the cell.