The microstructures of two sets of hydrogenated amorphous silicon–germanium (a-Si1−xGex:H) alloys prepared by the plasma-enhanced, chemical-vapor-deposition technique with and without hydrogen dilution of the source gases (silane and germane) have been analyzed by small-angle x-ray scattering (SAXS), infrared vibrational spectroscopy, and flotation density measurements. Optoelectronic properties of codeposited films have also been characterized. Hydrogen dilution suppresses dihydride/polyhydride formation, reduces bonded H content, and reduces the SAXS-detected microstructure for x≳0. Studies of anisotropy in the SAXS intensity indicate an increased amount of oriented microstructure as Ge is added, consistent with a trend toward columnarlike growth in both undiluted and hydrogen-diluted films, but the diluted films have a significantly reduced degree of such oriented microstructure. The improvement in the microstructure of a-Si1−xGex:H films by H2 dilution correlates with concomitant improvement of optoelectronic properties. The modification of microstructure due to H2 dilution of the source gases is discussed in terms of growth mechanisms of alloy films.
Polycrystalline silicon thin films have been deposited at fast growth rates (50 Å/s) by hotwire chemical vapour deposition (HW-CVD) from SiH4/H2 gas mixtures at low substrate temperature (400–500°C). The surface morphology of these films consists of 0.5 – 2.0μm dendritic grains as seen by electron microscopy. The films have a columnar morphology with grains starting from the substrate either on glass or c-Si. Even the 150 nm thick initial layer is polycrystalline. The preferential crystalline orientation of the poly-Si film is apparently not governed by the radiative source but strongly depends on the type and orientation of the substrate. A strong hydrogen dilution (>90%) of silane is essential to obtain poly-Si films with optimal crystalline structure.
Highly photoconductive amorphous silicon-germanium alloy (a-SiGe:H) films have been developed by plasma-enhanced chemical-vapor deposition using helium dilution (HeD) of the process gases (silane and germane). On comparison with high-quality a-SiGe:H alloys prepared under hydrogen dilution it has been observed that HeD films have higher deposition rates as well as higher mobility lifetime products ημτ throughout the alloy range; however, midgap defect densities and Urbach energy values of the two types of materials are nearly the same. Improvement in ημτ values of HeD films are found to be consistent with the reduction of microstructural defects in the films. Device potential of the helium-diluted a-SiGe:H films has also been investigated.
Commonly, the germane fraction (f = flowrateofGeH4/flowrateofSiH4 + GeH4) is changed to vary the optical
gap (Eopt) of amorphous silicon germanium alloy (a-SiGe:H) films. We
report that for a particular f, the change of deposition conditions, the
flow rate of diluent gas (H2) and the radiofrequency (rf) power density can
vary the optical gap (1.67-1.40 eV), the germanium content (41.3-22.5 at%)
and the microstructural defect density (0.92-0.42), the mobility
lifetime product (ηµτ; 6.81×10-6-1.46×10-8
cm2 V-1) in a wide range. Initially, with the increase of the
H2 flow rate up to 20 SCCM for the rf power density of 60 mW cm-2 and
up to 30 SCCM for the rf power density of 30 mW cm-2, the microstructural
defects decreases, although, the Ge content of a-SiGe:H films increases. The
microstructural defects of a-SiGe:H films becomes a direct function of the
midgap defect density and a indirect function of ηµτ. Moreover, the
nature of diluent gas is also important. We present that in a wide range of
optical gap (1.74-1.36 eV) the defect density is lower, and ηµτ
is higher for the optimized He diluted film compared to those of the optimized
H2 diluted films.
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