Analysis of enhanced light emission from highly strained germanium microbridges

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Tensile strain is a widely discussed means for inducing a direct bandgap in Ge for the realization of a semiconductor laser compatible with Si microelectronics. We present a top-down fabrication approach for creating high uniaxial tensile stress in suspended Ge structures, which enhances—by a factor of more than 20—the strain induced by thermal mismatch of Ge layers grown on silicon or silicon-on-insulator substrates. Strain values up to 3.1% are measured using Raman spectroscopy, in excellent agreement with simulations using a biaxial thermal strain of 0.15%. As expected from the high value of strain, a 210 meV peak energy shift in the emission with respect to bulk Ge and a strong increase (×25) in the integrated photoluminescence intensity are observed. Although 3.1% uniaxial strain does not transform Ge into a direct-gap material, our model calculation predicts an optical gain of 460 cm−1 for 1 × 1019 cm−3 n-doped structures at an electron–hole injection density of 3 × 1019 cm−3.

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Figure 1: Characterization of prestrained Ge layers and fabrication of suspended constricted structures.
Figure 2: Raman spectroscopy and FEM investigation of the strain distribution in the suspended constricted Ge structures.
Figure 3: Raman spectra and EFs collected on structures with different dimensions.
Figure 4: µPL spectra measured at room temperature for different suspended constricted Ge structures and excitation positions.
Figure 5: Gain simulation for 3.1% strained Ge structures, doped and undoped.


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The authors acknowledge hospitality from the IR beamline of the SLS, where some of the photoluminescence experiments were performed. Part of this work is supported by the Swiss National Science Foundation (SNF project no. 130181). The authors also acknowledge support from the CARIPLO foundation regarding the project NANOGAP.

Author information

All authors contributed extensively to the work presented in this Article. M.J.S. performed Raman measurements and carried out finite element simulations. R.G. and R.A.M. fabricated the Ge/SOI samples, and G.S. fabricated the Ge/Si samples. R.G. performed the optical measurements. R.G. and J.F. performed the gain simulation. J.Fr., D.C. and G.I. performed the sample growth and XRD analysis. H.S., R.S. and J.F. collaborated on the design of the experiment. M.J.S., R.G., R.A.M. and H.S. wrote the draft manuscript. H.S. supervised the experiments and coordinated data interpretation.

Correspondence to H. Sigg.

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Süess, M., Geiger, R., Minamisawa, R. et al. Analysis of enhanced light emission from highly strained germanium microbridges. Nature Photon 7, 466–472 (2013) doi:10.1038/nphoton.2013.67

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