Precursor Cat-CVD a-Si films for the formation of high-quality poly-Si films on glass substrates by flash lamp annealing
Introduction
In recent years polycrystalline Si (poly-Si) thin-films formed by post-annealing of amorphous Si (a-Si) have attracted considerable interest as a solar cell material [1], [2], [3], [4], [5], [6], [7], [8]. This is because these films show no light-induced degradation due to the absence of a-Si in them; as a result, less Si material is required compared to a bulk crystalline Si solar cell. These advantages result in the fabrication of high-efficiency and low-cost solar cells. However, high temperature processes used to obtain micrometer-thick poly-Si films, which include epitaxial growth on a seed layer consisting of large-grained poly-Si [4], [5], hour-order furnace annealing [6], [7], and zone melting recrystallization of a-Si films [8], eliminate the possibility of using low-cost substrates. For less thermally resistive substrates such as soda lime glass, we utilize millisecond annealing to sufficiently heat micrometer-thick a-Si films and to avoid thermal damage to glass, which can be realized by flash lamp annealing (FLA). We have already investigated the FLA of 4.5 µm-thick a-Si films prepared by catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD (HWCVD), which were deposited on glass substrates with the assistance of a Cr adhesion layer inserted between the a-Si and the glass [9], and reported the fabrication of prototype thin-film solar cells using these poly-Si films [10]. Although we attributed the successful crystallization of the Cat-CVD a-Si films, without serious peeling, to their low hydrogen content as well as the highly adhesive nature of the Cr films, we were unable to determine which of these factors was more important.
In this paper, we have compared the crystallization, by FLA, of a-Si films deposited both by plasma-enhanced CVD (PECVD) and by Cat-CVD with different hydrogen contents. We have also attempted to crystallize dehydrogenated Cat-CVD a-Si films in order to further understand of effect of the film hydrogen content on the adhesion of a-Si films during FLA.
Section snippets
Experimental details
Precursor a-Si films with a hydrogen content of about 3% were deposited by Cat-CVD using deposition conditions described previously [11]. Films that were dehydrogenated were heated at 500 °C under a nitrogen atmosphere for 12 h, resulting in a film hydrogen content less than 1%. We also prepared 2.0 µm-thick PECVD a-Si films which contained 10% hydrogen; the deposition conditions used were a SiH4/H2 flow ratio of 1/5, a chamber pressure of 2 Torr, and a substrate temperature of 200 °C. These
Results and discussion
Fig. 1 shows surface images of the lamp-annealed Cat-CVD and PECVD Si films which underwent FLA without dehydrogenation. As can be seen, the 1.5 and 3.0 µm-thick a-Si films without dehydrogenation are crystallized under proper irradiance, and are accompanied by partial Si peeling. The 3.0 µm-thick a-Si films are crystallized under a lower lamp irradiance than are the 1.5 µm-thick films, because the temperature of the a-Si film achieved during FLA is determined by a balance between generated
Conclusion
We have shown that a-Si films with lower hydrogen contents show better adhesion to glass during FLA. PECVD a-Si films containing 10% hydrogen start to peel off even for a lamp irradiance lower than that needed for crystallization, and no poly-Si remains after FLA with a higher irradiance, whereas part of the Cat-CVD a-Si films adhere even after crystallization. On the other hand, dehydrogenated Cat-CVD a-Si films show no significant peeling after complete crystallization. Although the poly-Si
Acknowledgments
The authors acknowledge T. Owada and T. Yokomori of Ushio Inc. for their expert operation of and fruitful discussion on FLA. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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