We show that a significant enhancement of the direct band gap photoluminescence can be achieved at room temperature in bulk Ge and Ge-on-insulator heavily n-doped by gas immersion laser doping. The photoluminescence signal from bulk Ge and Ge-on-insulator increases with the donor concentration. An enhancement factor of 20 as compared to the undoped material is achieved near the 1550 nm wavelength for active dopant concentrations around 5 ϫ 10 19 cm −3. These results are supported by calculations of the Ge spontaneous emission spectrum taking into account the doping effect on the electron distribution in the direct and indirect conduction band valleys.
We have investigated the quality factors of silicon-based photonic crystal nanocavities using the photoluminescence of a single layer of Ge/Si self-assembled islands as an internal source. We focus on membrane-type L3 elongated cavities with or without their lateral edge air holes shifted in position. The photoluminescence measurements are performed at room temperature. We show that the quality factor of the fundamental mode observed at a normalized frequency u = a/lambda~_ 0.25 is strongly dependent on the incident pump power. This dependence is associated with the free-carrier absorption of the photogenerated carriers. The slope of the quality factor vs. incident pump power gives access to the carrier recombination dynamics in these Si-based nanocavities. The measurements indicate that the carrier dynamics is controlled by nonradiative recombination associated with surface recombinations. A surface recombination velocity of 4.8 x 10(4) cm/s is deduced from the experiments. The spectral red-shift of the cavity modes as a function of incident pump power is correlated to the temperature rise due to thermo-optic effects. The measured temperature rise, which can reach 190 K, is correlated to the value estimated by a thermal analysis.
Germanium on insulator on silicon substrates can be obtained by the growth of a SiGe layer on silicon on insulator followed by a condensation at high temperature and a Ge epitaxial growth. We show that these substrates can be used for photonic devices. Two-dimensional photonic crystals with defect cavities have been fabricated. The emission at room temperature of condensed germanium can be spectrally controlled by varying the lattice parameter of the photonic crystals. Resonant emission is obtained between 1400 and 1700 nm when modifying the lattice periodicity between 400 and 480 nm for L3 cavities in a triangular lattice. Quality factors of 540 are obtained for the fundamental mode of the L3 cavity around 1600 nm. The experimental radiation pattern of the defect cavities is compared to the one calculated by a finite-difference time-domain method. A specificity of the germanium-on-insulator photonic crystals is that the optical sources are distributed within the whole material, by opposition to photonic crystals with a single quantum dot layer internal source.
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