Improving the mechanical, thermal and optical properties of biaxial and polyaxial Germanium suspended bridges towards a CMOS compatible light source

Abstract

Germanium (Ge) is a promising candidate for a CMOS compatible laser diode. This is due to its compatibility with Silicon (Si) and its ability to be converted into a direct band gap material by applying tensile strain. In particular uniaxial suspended Ge bridges have been extensively explored due to their ability to introduce high tensile strain. There have been two recent demonstrations of low-temperature optically-pumped lasing in these bridges but no room temperature operation accredit to insufficient strain and poor ther- mal management. In this thesis the merits of using biaxial and polyaxial suspended Ge bridges to move towards room temperature operation were outlined. Uniaxial bridges were compared with polyaxial bridges in terms of mechanical stress and thermal man- agement using Finite Element Modelling (FEM). The stress simulations revealed that polyaxial bridges suffer from extremely large corner stresses which prevent larger strain from being introduced compared with uniaxial bridges. Thermal simulations however re- veal that they are much less thermally sensitive than uniaxial bridges which may indicate lower optical losses. Bridges were fabricated and micro-Raman (μ-Raman) spectroscopy was used to validate the results of the simulations. We postulate that polyaxial bridges could offer many advantages over their uniaxial counterparts as potential laser devices. Using a novel geometric approach and finite element modelling (FEM) structures with improved strain homogeneity were designed and fabricated. μ-Raman spectroscopy was used to determine central strain values. Micro-PhotoLuminescence (μ-PL) was used to study the effects of the strain profiles on light emission; we report a PL enhancement of up to 3x by optimizing curvature at a strain value of 0.5% biaxial strain. This geometric approach offers opportunity for enhancing the light emission in Ge towards developing a practical on chip light source. Finally the thesis concludes with a novel simple elliptical design for polyaxial bridges. This design further reduced the corner stresses in polyaxial bridges by 20%, even without optimization. A large 1.11% strain was achieved in a Ge on Si stack as confirmed by μ-Raman and FEM. Furthermore an optical cavity was introduced in this deign with no extra complex fabrication, this was confirmed using μ-PL and Finite Difference Time Domain (FDTD) simulations.

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ORCID for Shinichi Saito: orcid.org/0000-0003-1539-1182

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