Characterization of Compositional Gradients in Amorphous Semiconductor thin Films By Real Time Spectroscopic Ellipsometr
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ABSTRACT A new approach has been developed for the analysis of real time spectroscopic ellipsometry (RTSE) data that has important applications in characterizing compositionally-graded layers in amorphous semiconductor devices. The new RTSE data analysis approach determines (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the dielectric function of the near-surface layer (top 20 A) of the growing film, all without any knowledge of the deposition history. We tested this new approach by characterizing a-Sil-xCx:H (x z 0.09) alloy films with continuously graded void volume fractions. The results reported here are unique in that they represent the first successful optical analysis of amorphous semiconductor structures having continuously varying properties with depth into the film.
INTRODUCTION The performance of hydrogenated amorphous silicon (a-Si:H)-based electronic devices is strongly affected by the optical gap profile throughout the thickness of the structure. ] For example, the gap profile of the i-type layer near the top junction of the heterojunction p-i-n solar cell (superstrate configuration) is particularly important. Buffer layers of a-Sil-xCx:H having graded alloy content x, have been used between the a-Sil-xCx:H:B p-type layer and the a-Si:H itype layer in attempts to improve the efficiency.2 In addition, grading the alloy composition throughout the i-layer may also improve efficiencies. Another important interface that affects the performance of solar cells in the superstrate configuration is the TCO/p-layer interface. As concluded from previous studies, minimum H2 diluent gas must be used in the preparation of the a-Sil-xCx:H:B p-layer, in order to avoid the detrimental effects of atomic or ionic hydrogen species in generating metallic Sn at the TCO/p-layer interface. 3 H2 dilution of gas sources has a number of advantages, however, including the ability to (i) use a lower substrate temperature and retain high quality material and (ii) achieve wider gap, higher stability material. 4 Thus it may be possible to start with no hydrogen dilution at the TCO/p-layer interface and increase the hydrogen dilution ratio, R ([H2]/([SiH4]+[CH4]}), to the optimum value for the i-layer at the end of the p-layer deposition. Figure 1 shows a hypothetical schematic of the top junction of a solar cell that has been prepared using this approach. In this study we have developed new techniques of data analysis so that RTSE can be applied to characterize the optical properties and microstructure of such graded layers. With this capability, the performance of solar cells can be correlated with graded layer properties in future research.
15 Mat. Res. Soc. Symp. Proc. Vol. 377 © 1995 Materials Research Society
NEW DATA ANALYSIS APPROACH In order to simulate the physical structure of a multilayered film, the conventional method of RTSE data analysis uses a multilayer optical model in which each layer is homogeneous with a fixed dielectric function. Mathematical inversion and least-
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