Investigation of the Microstructure of Underdense Hydrogenated Amorphous Silicon Layers for Silicon Heterojunction Solar Cells by Raman Spectroscopy and Hydrogen Effusion

The application of thin underdense hydrogenated amorphous silicon (a-Si:H) films for passivation of crystalline Si (c-Si) and for avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Here, we investigate the microstructure of such underdense a-Si:H films, as used in Jülich solar cell technology. From the refractive index and hydrogen content, the density of our films is estimated to 2.1-2.2 g/cm3. In H effusion experiments (besides effusion at higher temperature) a low temperature H effusion peak near 400 °C shows up which has been attributed to the diffusion of molecular H2 through a void network. The dependence of the H effusion peaks on film thickness is similar as observed previously for low substrate temperature a-Si:H material. By applying different plasma power (0.08 W/cm2 – 0.16 W/cm2) the Si-H microstructure parameter measured by Raman decreases from about 0.5 to 0.25 for about 20 nm film thickness. The nucleation zone is estimated from the Raman results to < 10 nm. The substrate type (HF etched c-Si, c-Si with native oxide, glass) shows no influence on the Raman microstructure parameter. The fact that with such material good passivation of c-Si solar cells was achieved suggests that in the c-Si passivation process molecular hydrogen plays an important role. The second layer of dense a-Si:H as usually required for good c-Si passivation may be primarily a H2 out-diffusion barrier.