In situ study of the thermal decomposition of B₂H₆ by combining spectroscopic ellipsometry and Kelvin probe measurements

In amorphous silicon (a-Si:H) p-i-n solar cells preparation, B ₂H₆ is added to the SiH₄ discharge in order to deposit the p layer. However, chemical vapor deposition (CVD) of the (B ₂H₆,SiH₄) mixture, in the absence of plasma excitation, can contribute to poor p/i interface characteristics. The thermal decomposition of B₂H₆ at 250 °C is studied in situ by combining spectroscopic ellipsometry and Kelvin-probe measurements. The advantage of correlating both techniques is underlined. Both diagnostics provide complementary information. Spectroscopic ellipsometry is highly sensitive upon film deposition and surface morphology modifications. On the other hand, Kelvin-probe measurements may be directly related to Fermi-level shifts. The thermal decompositon of (B₂H₆,H₂) and (B ₂H₆,SiH₄) mixtures is evidenced, even at low total pressure (30 mTorr). The boron incorporation is found to be little affected by the diluent gas. In contrast, the CVD consequences strongly increase as a function of the gas pressure. The surface sensitivity of the CVD process is illustrated whatever the total pressure. At low pressure, the variation of the Kelvin-probe measurements may be described by a model taking into account adsorption of species acting as electron acceptors. At high pressure, when a (B₂H₆,SiH₄) mixture is used, the CVD leads to the growth of a highly doped amorphous silicon layer with a very low deposition rate (2-3 Å min^-1), whatever the substrate. The implications of the CVD process for interface formation in a-Si:H devices have been explored. In particular, the use of a multichamber deposition system can result in an improvement of the p/i interface behavior.