Si-H Vibration Only at 2000 CM -1 in Fully Polycrystalline Silicon Films Made by HWCVD
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Si-H VIBRATION ONLY AT 2000 CM-1 IN FULLY POLYCRYSTALLINE SILICON FILMS MADE BY HWCVD J.K. RATH AND R.E.I. SCHROPP, Utrecht University, Debye Institute, Section Interface Physics, P.O.Box 80000, 3508 TA Utrecht, The Netherlands ABSTRACT The Si-H vibration in IR spectra of our device quality poly-Si films grown by hot-wire chemical vapour deposition (HWCVD) made at low wire temperature (Tw=1800 oC) is at 2000 cm-1 whereas in a poly-Si film made at high wire temperature (Tw=1900 oC) both 2000 cm-1 vibrations as well as 2100 cm-1 are observed. On the other hand, the Raman spectra (probing the upper part of the film) of Si-H stretching vibration measured for both these samples show only 2000 cm-1 mode. XTEM micrographs of these films show that whereas the low Tw film has a structure made of closely packed crystalline columns, the high Tw film has conical crystalline structures with amorphous region between them. The crystal cones meet each other towards the top of the film and form a closed structure. This is confirmed by Raman spectrum at 520 cm-1. We attribute the 2100 cm-1 mode to the Si-H bonds at the surface of the cones touching the amorphous regions. The Si-H vibration shifts to 2000 cm-1 when the crystalline cones coalesce with each other, as is the case in the upper part of both types of films. INTRODUCTION Knowledge of hydrogen bonding and transport in the polycrystalline network is essential for understanding the microstructure and grain boundary properties. Moreover, metastabilities (both light induced [1] as well as thermal [2]) have been attributed to hydrogen. In amorphous silicon, the incorporated hydrogen concentration, which depends on the growth temperature and the deposition process, determines the distribution of hydrogen at various sites such as isolated sites, voids or even bubbles [3]. At present it is not known if a similar distribution can be expected in poly-Si. Moreover, in poly-silicon, hydrogen is assumed to be predominantly accommodated in the grain boundary region rather than in the interior of the crystals owing to low solubility limit of the crystalline lattice. However, the interpretation of the hydrogenbonding configuration in poly-Si is still controversial. Various configurations have been proposed i.e., (1) Si-H bonds on different grain surfaces [4,5], i.e., (111) and (100), (2) different bonding configurations [6] (monohydride S-H or polyhydride Si-Hx) and (3) optically anisotropic Si-H monolayers [7] residing on grain boundaries. The microscopic structure of the grain boundaries has long-range order imposed by the adjacent crystallites. In this paper we present the hydrogen-bonding configuration in device-quality poly-Si made by HWCVD. Details of the structure and opto-electronic properties reveal [8] that these films in many respects differ from poly-Si or microcrystalline films made by PECVD. Most important is the observation of the stretching mode vibration at 2000 cm-1 in our HWCVD poly-Si films made in a controlled regime, instead of at 2100 cm-1 commonly reported in the liter
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