Verlag des Forschungszentrums Jülich
JUEL-3087
Zanzig, Lars
Einsatz alternativer Depositionsmethoden zur Herstellung amorpher Siliziumlegierungen
126 S., 1995
Abstract
The preparation of undoped amorphous silicon alloys was investigated using a thermal
dissociation (hot wire) and a microwave plasma process. For this purpose two different
kinds of dissociation sources have been developed. These sources could be integrated into
an ultra high vacuum system to enable the separated dissociation of different process
gases.
The experimental set up was designed for the simultaneous operation of different
sources allowing an independent variation of many deposition parameters. A full
characterization of the film properties and in addition gas-phase analysis by mass
spectroscopy were used to optimize the growth process and to improve the understanding
of the involved growth mechanisms. The results of these alternative deposition methods
have been compared to the commonly used rf glow discharge method.
First the fundamental working of both methods has been demonstrated. Films prepared
by remote microwave plasma showed an open void network resulting in a fast
postoxidation of the samples. These structural inhomogeneities are caused by a high
amount of higher silanes and a polymerization in the gas-phase.
A distinct correlation between deposition parameters and material properties was
obtained for amorphous silicon (a-Si:H) and amorphous germanium (a-Ge:H) prepared
by the hot wire method. Dense films can be obtained using a high gas flow and low
filament temperature TFil. On the other hand, at low pressure and high TFil we observed a
microstructure with a high density of microvoids combined with poor electronic
properties. It is concluded that a high amount of reactive radicals with a low surface
mobility could be the reason for this behaviour.
Amorphous silicon deposited at high substrate temperature shows a high defect density
probably caused by dangling bonds localized at largely unhydrogenated microvoid
surfaces.
Depositing amorphous silicon germanium alloys (a-Si1-xGex:H) by direct thermal
dissociation of only one process gas (silane or germane) is interpreted as direct proof of
an interaction between hot radicals andundissociated molecules.
Taking into account the high quality especially of the a-Ge:H films we expect that the
hot wire method may become a real alternative to the glow discharge CVD.
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