Optical properties of Ge_1x-ySi_xSn_y alloys with y > x: Direct bandgaps beyond 1550 nm

Ge_1-x-ySi_xSn_y alloys with y > x have been grown directly on Si substrates. Room temperature photoluminescence measurements indicate that the alloys have direct bandgaps below that of pure Ge, thus representing an alternative to tensile-strained Ge and to Ge _1-ySn_y for long-wavelength applications. In comparison with binary Ge_1-ySn_y alloys, ternary Ge _1-x-ySi_xSn_y alloys have superior stability due to their increased mixing entropy. The observation of photoluminescence from these films confirms that high-quality material can be grown in spite of the large size mismatch between Si and Sn.