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Pronounced enhancement of the photoluminescence intensity in tensile-strained and heavily n-doped (4×1019 at.cm−3) Ge/Si epilayers

In recent years, tensile-strained and n-doped Ge has emerged as a potential candidate for the realization of optoelectronic devices that are compatible with the mainstream silicon technology. Compared to Si, pure Ge displays unique optical properties, the direct (Γ) valley of its conduction band is only 140 meV above the indirect (L) valleys at room temperature while it is larger than 2000 meVin Si. It has been shown that under application of a tensile strain, the Ge direct band gap reduces faster than the indirect one and with a tensile strain of ~1.9%, Ge can become a direct band gap semiconductor [1]. On the other hand, n-type doping in Ge leads to a more efficient population of the zone center Γ valley and thus enhances radiative recombination at the Γ valley [2].