Molecular epitaxy of pseudomorphic Ge_1-ySn_y (y = 0.06-0.17) structures and devices on Si/Ge at ultra-low temperatures via reactions of Ge₄H₁₀ and SnD₄

A low-pressure CVD technique was specifically developed to prepare a new class of pseudomorphic Ge_1-ySn_y layers, with an Sn content up to 17% on Ge-buffered Si(100) wafers. The growth is conducted via reactions of SnD₄ and the recently deployed Ge₄H₁₀ custom precursor, whose large molecular weight and enhanced reactivity enables depositions at unprecedented ultra-low temperatures (150 °C-200 °C), and at pressures akin to those typically employed in solid/gas-source MBE. The thicknesses of the layers far exceed the critical limits predicted by thermodynamic considerations and are either comparable to, or larger than, those observed for MBE-grown samples. This is validated by modeling of the thickness versus the composition for the fully strained and partially relaxed alloys produced in this work relative to the MBE and CVD-grown analogs reported in the literature. Furthermore, the practical relevance of the technique was demonstrated by creating highly doped n-type alloys, which were then used as active layers to fabricate degenerate pn junctions. It was also found that the strained films gradually relax with increasing thickness, providing new types of strain-free material with enhanced optical quality relative to those produced by standard CVD methods, as evidenced by the photoluminescence studies. The strain relaxation mechanism appears to be similar to that observed in CVD-grown samples, with no sign of epitaxial breakdown or precipitous degradation of the bulk crystallinity or surface morphology, in spite of the low growth temperatures employed. Finally, we note that this method represents the first example of a chemically driven route that delivers materials with the desirable properties afforded by MBE, while offering the potential for those practical applications inherent to large-scale CVD.