The microstructure of amorphous Si1−xGex:H films with x<0.40 was studied using small-angle x-ray scattering (SAXS) and the results compared with those from opto-electronic and density measurements. The SAXS, the sub-band-gap absorption determined from photothermal deflection spectroscopy, and the photo/dark conductivity ratio all show relatively sharp changes above x=0.2. A corresponding sharp change in the anisotropic character of the SAXS is consistent with a transition to a columnar-like microstructure above x=0.2. The correlated results provide strong evidence for a direct link between degraded opto-electronic properties and the increased heterogeneity associated with the microstructural transition.
Small-angle x-ray scattering (SAXS) measurements were made on a-SiGe:H alloys to study microstructure on the nanometer scale as a function of Ge content, and the results were compared with representative single-junction solar cell properties. Samples consisting of only the i-layer were used for SAXS. Above a Ge content of 20 %, a significant increase in SAXS was seen. From measurements made with the samples tilted relative to the incident x-ray beam, the increase in scattering is attributed to the appearance of elongated low density regions in the film, modeled as ellipsoidal microvoids, which are preferentially oriented perpendicular to the film surface and may be related to columnar-like microstructure. Flotation density measurements support the presence of low density regions. Initial and light-degraded measurements on corresponding solar cell structures do not show a correlation between SAXS and initial cell properties; there is, however, some evidence that the light-induced degradation is higher for cells with larger amounts of SAXS-detected microstructure and this needs further investigation.
The degree of interdiffusion at the amorphous semiconductor/bulk Al interface was studied using Auger electron spectroscopy analysis. 300-500 Å thick a-Si:H and a-Ge:H films were deposited onto high-purity Al and 5052 Al alloy substrates and subsequently annealed to various temperatures up to 500 °C for 6 hrs. The high-purity Al is used as a substrate for our small-angle x-ray scattering studies of amorphous silicon-based alloys. For all the films deposited on the pure Al, little or no interdiffusion was noted at or below anneal temperatures of 400°C. This result is contrary to those commonly found for samples produced by evaporating Al onto the previously deposited amorphous semiconductor without breaking the vacuum where interdiffusion has been noted at temperatures at or below 200°C. We suggest interdiffusion in the amorphous semiconductor/bulk Al samples is hindered by the presence of a 150-300 Å Al oxide on the Al. A large amount of interdiffusion and partial crystallization is noted in the a-Ge:H sample after an anneal of 450°C while a temperature between 450 and 500°C is required for interdiffusion and crystallization to occur in the a-Si:H sample. In the case of the a-Ge:H films deposited on the 5052 Al alloy, interdiffusion occurs after 300°C anneals due possibly to the migration of Mg and other components of the alloy into the amorphous semiconductors or structural defects in the alloy which enhance interdiffusion.
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