Microstructural transition and degraded opto-electronic properties in amorphous SiGe:H alloys

Abstract

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.

Techniques for measuring the composition of hydrogenated amorphous silicon–germanium alloys

“…Because the EPMA data is averaged from the surface to the sampling depth, the resulting O data are strongly affected by surface oxides and are larger than the SIMS data averaged from 0.25 to 0.50 µm into the film. The increase in O concentration with Ge alloying-over the entire alloy range for the EPMA data and below 50 at.% Ge for the SIMS data-is consistent with an increase in structural heterogeneity on the nanometer scale of films with increasing Ge alloying [6,8,10]. However, it is not clear why the O concentration as measured by SIMS decreases in films with greater than 50 at.% Ge.…”

“…Both processes produce a-SiGe:H alloys that have an increase in structural heterogeneity at the nanometer scale (e.g., voids or atomic clustering) with increasing Ge content. This increase occurs at ~12 at.% Ge for HWCVD-grown a-SiGe:H [8,10] and ~20 at.% for PECVD-grown a-SiGe:H [6]. To our knowledge, little work has been done in the area of compositional measurements of a-SiGe:H grown by HWCVD.…”

“…Alloying hydrogenated amorphous silicon (a-Si:H) with germanium narrows the optical bandgap of the material, allowing it to adsorb photons from less energetic portions of the solar spectrum. However, these alloys tend to be electronically [5] and structurally [6] inferior to a-Si:H. Therefore, the improvement of a-SiGe:H alloys continues to be an important aspect of a-Si:H-based solar cell research and development.…”

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

“…Considerable circumstantial evidence has been acquired suggesting that nanostructure in the form of 1 to 4 nm voids or H-rich clusters is harmful to the optoelectronic properties and device performance [ 15,16,. As will be demonstrated in this discussion of our work during the last year, device-quality a-Si:H films prepared by PECVD or hot-filament CVD have nanovoid volume fractions near or below our present detection limit of about 0.01 %.…”

“…The latter can mimic rod-like or plate-like objects through appropriate choice of major-to-minor axis ratio, while a preferred orientation will lead to readily observed [3,7,11,12,15,16, and modelled [3,28,29] anisotropies in the SAXS. In those cases where interparticle interference is evident in our data, usually in the a-SiC:H alloys, a hard-sphere pair correlation model for the structure factor (e.g., the Percus-Yevick solution [30]) can often account for the observed IN($.…”