Temperature and excitation power dependencies of the photoluminescence of planar and vertically self-organized Si0.70Ge0.30/Si strained superlattices

We have studied the correlation between the morphological characteristics and the photoluminescence (PL) behavior of nominally 20-nm Si/7-nm Si 0.70 Ge 0.30 superlattices (SLs) showing planar SiGe layers (#1) and vertical self-aligned undulations (#2). At 8 K and low PL excitation density, no-phonon (NP) peaks were centered at 919 and 934 meV, for SLs #1 and #2, respectively. Increasing excitation power resulted in a high-energy side broadening and a strong blue-shift of sample #2 peaks, which were interpreted by in-plane localization and spreading of the hole wave function within the thickness undulations. Up to intermediate temperature values (∼ 120 K), carrier diffusion and localization at the crest (∼ 11 nm) of the undulations enhanced PL efficiency. Room temperature SiGe PL was observed for both samples with an intensity exponential decay above 150 K. Activation energies of 260 meV (#1) and 172 meV (#2) were derived from Arrhenius plots. The energy difference can be explained by the lateral channel formed by the troughs (∼ 2 nm) in the long scale (∼ 120 nm) waviness of sample #2 that assists hole thermoionic emission in the Si barriers.

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