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Germanium photonics

High quality tensile-strained n-doped germanium thin films grown on
InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is direcly observed by room temperature direct band gap photoluminescence.

Appl. Phys. Lett. 98, 091901 (2011)

Ge diode
Direct and indirect band gap room temperature
 electroluminescence of Ge diodes

We have investigated the room temperature electroluminescence of pure germanium diodes grown by metal organic chemical vapor deposition. The dependence of the optical response of the p-n diodes is studied as a function of the injected current. Both direct and indirect band gap recombinations are observed at room temperature around 1.6 and 1.8 􏰠m. The amplitude of the direct band gap recombination is equivalent to the one of the indirect band gap.

J. Appl. Phys. 108, 023105 (2010).

Ge bulge
Control of direct band gap emission of bulk
germanium by mechanical tensile strain

We have shown that the recombination energy of the direct band gap photoluminescence of germanium can be controlled by an external mechanical stress. The stress is provided by an apparatus commonly used for bulge or blister test. An energy red-shift up to 60 meV is demonstrated for the room temperature photoluminescence of a thin germanium membrane (125 nm wavelength shift from 1535 to 1660 nm). This photoluminescence shift is correlated to the in-plane tensile strain generated in the film. A biaxial tensile strain larger than 0.6 % is achieved by this method. This mechanical strain allows to approach the direct band gap condition for germanium which is of tremendous importance to achieve lasing with this material.

Appl. Phys. Lett. 96, 041909 (2010)


Two-dimensional photonic crystals with germanium on insulator obtained by a condensation method

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 have shown 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.

Optics Communications 281, 846 (2008)