| AVS 57th International Symposium & Exhibition | |
| Energy Frontiers Topical Conference | Tuesday Sessions |
| Session EN+TF-TuA |
| Session: | Thin Films for Photovoltaics |
| Presenter: | P. Schäfer, Chemnitz University of Technology, Germany |
| Authors: | P. Schäfer, Chemnitz University of Technology, Germany F. Nobis, Chemnitz University of Technology, Germany O.D. Gordan, Chemnitz University of Technology, Germany H. Kupfer, Chemnitz University of Technology, Germany F. Richter, Chemnitz University of Technology, Germany D.R.T. Zahn, Chemnitz University of Technology, Germany |
| Correspondent: | Click to Email |
Amorphous hydrogenated silicon (a-Si:H) is widely used in photovoltaic applications. The high absorption renders a-Si:H technically relevant especially for thin film solar cells. Despite lower efficiency, an amorphous silicon solar panel possesses the advantage of higher absorption rate and easier processing at lower production cost.
Here the focus lies on highly (p and n) doped amorphous silicon films. The samples are prepared using d.c.-pulsed magnetron sputtering of crystalline silicon targets. A controlled hydrogen flow is added to the sputtering plasma. Hydrogen in amorphous silicon is known to saturate dangling bonds and improves the short range atomic order [1]. To probe the influence of hydrogen in the sputtering process various spectroscopic techniques were applied for sample characterisation.
Raman spectroscopy is a technique sensitive to the morphological aspects of the film. The relaxation of quasi-momentum conservation in amorphous films results in drastically different spectra of amorphous and crystalline silicon. A broad band at ~485 cm-1 appears instead of the sharp crystalline phonon feature at 520 cm-1. Its shape and asymmetry unveils further information on the short range order like the average dispersion angle from tetrahedral conformation. With the help of Fourier transformed transmission infrared spectroscopy the concentration of hydrogen in the sample is studied. Vibrational hydrogen-silicon stretching modes in the region around 2000 cm-1 are therefore assessed by a modified [2] Brodsky-Cardona-Cuomo approach [3]. Access to the optical constants n and k and therefore the complex dielectric function ε of the sputtered material is granted by variable angle spectroscopic ellipsometry. Thereby important parameters like the Tauc-Lorentz band gap which is mainly determined by interband gap defects are revealed. The combination of these spectroscopic techniques provides a detailed picture of morphological, electrical, and optical parameters of the system. An in depth discussion of the degree of structural improvement, the decrease of interband gap defects, the saturation of hydrogen content, and evolution of optical properties in correlation with the hydrogen flow will be presented.
[1] R. A. Street, “Hydrogenated Amorphous Silicon”, chapter 2.3, Cambridge University Press.
[2] A. A. Langford, M. L. Fleet, B. P. Nelson, W. A. Lanford, N. Maley: Phys. Rev. B 45 (1992) 13367.
[3] M. H. Brodsky, M. Cardona, J. J. Cuomo: Phys. Rev. B 16 (1977) 3556.