Germanium under High Tensile Stress: Nonlinear Dependence of Direct Band Gap vs Strain

• DOI: 10.1021/acsphotonics.6b00429
• Publication Date: 2016-10-04T17:46:25Z
• Authors: Kevin Guilloy; Nicolas Pauc; Alban Gassenq; Yann-Michel Niquet; Jose-Maria Escalante; Ivan Duchemin; Samuel Tardif; Guilherme Osvaldo...; Denis Rouchon; Julie Widiez; Jean-Michel Hartmann; Richard Geiger; Thomas Zabel; Hans Sigg; Jerome Faist; Alexei Chelnokov; Vincent Reboud; Vincent Calvo
• Keywords: [PHYS]Physics [physics]; Condensed Matter - Materials Science; 621; Materials Science (cond-mat.mtrl-sci); FOS: Physical sciences; Deformation Potentials; Electrooptical Properties; 02 engineering and technology; Electroreflectance; 530; 7. Clean energy; electromodulation; germanium; band gap; tight-binding; tensile strain; Absorption-Edge; Hydrostatic-Pressure; Uniaxial-Stress; 0210 nano-technology

Germanium is a strong candidate as a laser source for silicon photonics. It is widely accepted that the band structure of germanium can be altered by tensile strain so as to reduce the energy difference between its direct and indirect band gaps. However, the conventional deformation potential model most widely adopted to describe this transformation happens to have been investigated only up to 1 % uniaxially loaded strains. In this work, we use a micro-bridge geometry to uniaxially stress germanium along [100] up to $\varepsilon_{100}$=3.3 % longitudinal strain and then perform electro-absorption spectroscopy. We accurately measure the energy gap between the conduction band at the $��$ point and the light- and heavy-hole valence bands. While the experimental results agree with the conventional linear deformation potential theory up to 2 % strain, a significantly nonlinear behavior is observed at higher strains. We measure the deformation potential of germanium to be a = -9.1 $\pm$ 0.3 eV and b = -2.32 $\pm$ 0.06 eV and introduce a second order deformation potential. The experimental results are found to be well described by tight-binding simulations. These new high strain coefficients will be suitable for the design of future CMOS-compatible lasers and opto-electronic devices based on highly strained germanium.