Optical properties of Microcrystalline Silicon determined by Spectroscopic Ellipsometry and Photothermal Deflection Spec
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Optical properties of Microcrystalline Silicon determined by Spectroscopic Ellipsometry and Photothermal Deflection Spectroscopy Kyung Hoon Jun1, Helmut Stiebig2, Reinhard Carius2 1 Department of Electrical Engineering & Computer Science, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea 2 Institute of Photovoltaics, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
ABSTRACT The effect of the microstructure and bonded hydrogen on the optical properties of microcrystalline films (µc-Si:H) was investigated by Spectroscopic Ellipsometry (SE) and Photothermal Deflection Spectroscopy (PDS). On samples with a high crystalline volume fraction we studied the reason for a large deviation of absorption coefficient in the energy range between 1.6 eV and 3.2 eV from the value predicted by effective medium theory. This enhancement can be attributed to scattering by the inhomogeneity of µc-Si:H, which is investigated by the introduction of the dense medium radiative transfer formalism to an optical scattering simulation. Further, we suggest strain as a reason for the enhanced absorption in highly crystalline µc-Si:H.
INTRODUCTION The recent development of intrinsic microcrystalline silicon (µc-Si:H) led to a successful incorporation of this material as an absorber layer in thin-film solar cells. However, the optical properties remain yet to be better understood [1, 2]. The silane concentration SC (gas flow ratio SC = [SiH4]/[SiH4+H2]) is the most common parameter to control the microstructure of the material prepared by plasma enhanced chemical vapour deposition (PECVD). At low SC the sample is highly crystalline and at high SC the amorphous part dominates since the crystalline growth is suppressed. The crystalline volume fraction (fc) decreases with increasing SC. However, near the transition to the amorphous growth regime the highest solar cell efficiencies were achieved [1]. The effect of the microstructure on the optical properties is an important issue for both, an insitu characterization of material growth and clarification of electronic properties. The optical characteristics of structurally inhomogeneous semiconductors such as µc-Si:H has been interpreted simply as an effective medium of the two-phase (or three-phase) materials [3, 4] although such a microstructure may cause internal light scattering [5]. This paper presents studies of Spectroscopic Ellipsometry (SE) and Photothermal Deflections Spectroscopy (PDS) of intrinsic µc-Si:H at different SC. As hydrogen may play a key role for structural relaxation and passivation of defects, the results of hydrogenated and dehydrogenated µc-Si:H samples are compared. At low SC special attention is paid to the energy range at 1.6 eV < E < 3.2 eV, where the absorption coefficient of µc-Si:H silicon is higher than for c-Si.
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