Improvement of microstructure of amorphous silicon–germanium alloys by hydrogen dilution

Abstract

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.

Microstructure of amorphous-silicon-based solar cell materials by small-angle x-ray scattering. Annual subcontract report, 6 April 1994--5 April 1995

“…6 9 film shows no tilt effect while the x d . from Eq.4 is near our detection limit of 0.01% and therefore comparable to the lowest void fractions detected in a-Si:H. This result from high x films demonstrates that the previously observed trends of systematically increased heterogeneity with x on the Si-rich side of this alloy system [ 15,16,21,26 (Fig.2)] may be avoidable.…”

“…This is documented in Fig.2 where the integrated SAXS intensity Q (Eq.4) is shown as well as the tilt ratio versus the Ge fraction. Improved Urbach energies and photoconductivies correlated with these improved microstructures [26]. Here it is useful to contrast these results from IACS samples with those from two films prepared by USSC.…”

“…This trend was demonstrated for films prepared by several groups [21]. In a recent study of films prepared at the Indian Association for the Cultivation of Science (IACS), the effect of H2 dilution on this trend was investigated and it was found that, although the columnar-like features remained, they were definitely reduced by the dilution [26]. This is documented in Fig.2 where the integrated SAXS intensity Q (Eq.4) is shown as well as the tilt ratio versus the Ge fraction.…”

“…The Q ( 4 . 4 ) is reduced by a factor 13 at a 450 tilt (vs 3.7 for the low dilution), much larger than is typically observed for any of the a-Sil,,Ge,:H films prepared earlier by USSC [23] or by other groups (factors of 2 to 5, see Fig.2) [21,26].…”

“…Measurements up to x d . 6 have been done with a-Sil,,Ge,:H alloys [15,16,26]. The observed density deficits compared to c-Si (2.329 g/cm3), pure a-Si (2.287 g/cm3) [40], c-Sil,,Ge, alloys [41], diamond (3.52 g/cd), graphite (2.26 g/cm3), or Sic (3.22 g/cm3) can be helpful in interpretation of the SAXS and in determining the average effect of H alloying.…”