A Spatially Resolved Study of Chemical Vapor Deposition of a-Si:H with Pure Thermal Excitation of Disilane
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A SPATIALLY RESOLVED STUDY OF CHEMICAL VAPOR DEPOSITION OF a-Si:H WITH PURE THERMAL EXCITATION OF DISILANE G. AMATO*, NOBILE***
R. SPAGNOLO*,
F. FIZZOTTI**,
C. MANFREDOTTI**,
*Istituto Elettrotecnico Nazionale Galileo Ferraris,
91,
10135 Turin,
"**Dipartimento
P. MENNA***,
G.
Strada delle Cacce
Italy di Fisica Sperimentale Universit5 di Torino,
Via P. Giuria
1, 10125 Turin, Italy Centro Ricerche Fotovoltaiche, 80055 Portici (Naples), Italy ***ENEA,
Localita Granatello, P.O.
Box 32,
ABSTRACT Optical,
electronic and structural properties of a-Si:H samples grown
by Low Pressure Chemical Vapor Deposition have been investigated by means of IR spectroscopy, Raman scattering and Photothermal Deflection Spectroscopy. Samples grown at different positions along the tube show very different properties that can be related to the amount of H and to the nature of the Si-H bonds. The most important parameters governing the thermally excited growth of a-Si:H are presented and discussed. INTRODUCTION Some of the problems related to the plasma-enhanced chemical vapor deposition (PECVD) of a-Si:H films are still unsolved. As an example, the effect of ion bombardment on the growing surface of the film is still the subject of a long-standing controversy. It is generally beliTved that ion bombardment enhances the lateral momentum of adsorbed species giving rise to a more compact material. The presence of voids in the film is usually correlated with an IR active mode at 2100 cm . A widely accepted explanation assigns this mode to interacting Si-H bonds on the inner surface of mode is pre3s ett in IR spectra, voids. As a consequence, when the 2100 cm the material shows very poor photoelectronic properties The difficulties to understand these fundamental problems, arise from the fact that the chemical and physical processes occurring in a glow discharge reaction are very complicated, so a comprehensive theoretical approach is not available to date. There is indeed a strong evidence of the difficulty of controlling the glow discharge deposition in order to avoid undue effect like grainy structures, gap narrowing and the creation of interface states at the p/i interface which reduce the solar cells efficiency. The advantage of studying purely thermal processes is that we are dealing with simpler mechanisms that allow a better insight of the reactions. Low Pressure Chemical Vapor Deposition (LPCVD) presents unique advantages from this point of view, since it is a very simple and reliable way to grow a-Si:H films.
Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
686
The advantages of LPCVD can be summarized as follows: - since the absence of the plasma allows all the reactive gaseous species to contribute to the film growth, the film properties are mostly governed by the more reactive radicals; - due to the absence of any electric field, no constraints are present for the incidence angles on the growing surface of the neutral and ionized radicals; - the LPCVD growth tube can be considered as a one-dim
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