On-chip optical interconnection using integrated germanium light emitters and photodetectors.

Germanium (Ge) is an attractive material for monolithic light sources and photodetectors, but it is not easy to integrate Ge light sources and photodetectors because their optimum device structures differ. In this study, we developed a monolithically integrated Ge light emitting diode (LED) that enables current injection at high density and a Ge photodiode (PD) having low dark current, and we fabricated an on-chip optical interconnection system consisting of the Ge LED, Ge PD, and Si waveguide. We investigated the properties of the fabricated Ge LED and PD and demonstrated on-chip optical interconnection.

Enhanced room-temperature electroluminescence from a germanium waveguide on a silicon-on-insulator diode with a silicon nitride stressor.

Germanium (Ge) is an attractive material for monolithic light sources on a silicon chip. Introduction of tensile strain using a silicon nitride (SiNx) stressor is a promising means for Ge-based light sources due to the enhancement of direct band gap recombination. We propose a device structure that enables current injection from a silicon-on-insulator (SOI) diode to a Ge waveguide with a SiNx stressor formed by a simple fabrication process. Direct-band-gap electroluminescence and direct-band-gap shrinkage due to the applied SiNx stressor was confirmed. Intensity of electroluminescence from the Ge waveguide with the SiNx stressor was about three times higher than that corresponding to the device without the SiNx stressor.

Analysis of stress distribution in microfabricated germanium with external stressors for enhancement of light emission.

In the field of silicon photonics, germanium (Ge) is an attractive material for monolithic light sources. Tensile strain is a promising means for Ge based light sources due to enhancing direct band gap recombination. We investigated strain engineering in Ge using silicon nitride (SiNx) stressors. We found that microfabricated Ge greatly improves the tensile strain because SiNx on the Ge sidewalls causes a large tensile strain in the direction perpendicular to the substrate. Tensile strain equivalent to an in-plane biaxial tensile strain of 0.8% at maximum was applied, and the PL emission intensity was improved more than five times at the maximum.

Comparison of uniaxial and polyaxial suspended germanium bridges in terms of mechanical stress and thermal management towards a CMOS compatible light source.

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 thermal management. In this paper we compare uniaxial bridges with polyaxial bridges in terms of mechanical stress and thermal management using finite element modelling (FEM). The stress simulations reveal that polyaxial bridges suffer from extremely large corner stresses which prevent larger strain from being introduced compared with uniaxial bridges. Thermal simulations however reveal that they are much less thermally sensitive than uniaxial bridges which may indicate lower optical losses. Bridges were fabricated and 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.

Germanium vertically light-emitting micro-gears generating orbital angular momentum.

Germanium (Ge) is capturing researchers' interest as a possible optical gain medium implementable on complementary metal-oxide-semiconductor (CMOS) chips. Band-gap engineering techniques, relying mainly on tensile strain, are required to overcome the indirect band-gap nature of bulk Ge and promote electron injection into the direct-gap valley. We used Ge on silicon on insulator (Ge-on-SOI) wafers with a high-crystalline-quality Ge layer to fabricate Ge micro-gears on silicon (Si) pillars. Micro-gears are created by etching a periodic grating-like pattern on the circumference of a conventional micro-disk, resulting in a gear shape. Thermal built-in stresses within the SiO2 layers that encapsulate the micro-gears were used to impose tensile strain on Ge. Biaxial tensile strain values ranging from 0.3-0.5% are estimated based on Raman spectroscopy measurements and finite-element method (FEM) simulations. Multiple sharp-peak resonances within the Ge direct-gap were detected at room temperature by photo-luminescence (PL) measurements. By investigating the micro-gears spectrum using finite-difference time-domain (FDTD) simulations, we identified vertically emitted optical modes with non-zero orbital angular momentum (OAM). To our best knowledge, this is the first demonstration of OAM generation within a Ge light source.

Enhanced light emission from improved homogeneity in biaxially suspended Germanium membranes from curvature optimization.

A silicon compatible light source is crucial to develop a fully monolithic silicon photonics platform. Strain engineering in suspended Germanium membranes has offered a potential route for such a light source. However, biaxial structures have suffered from poor optical properties due to unfavorable strain distributions. Using a novel geometric approach and finite element modelling (FEM) structures with improved strain homogeneity were designed and fabricated. Micro-Raman (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 Germanium towards developing a practical on chip light source.

Optical net gain measurement in n-type doped germanium waveguides under optical pumping for silicon monolithic laser.

Silicon (Si) monolithic lasers are key devices in large-scale, high-density photonic integrated circuits. Germanium (Ge) is promising as an active layer due to the complementary metal-oxide semiconductor process compatibility with Si. A net optical gain from Ge is essential to demonstrate lasing operation. We fabricated Ge waveguides and investigated the n-type doping effect on the net optical gain. The estimated net gain of the n-Ge waveguide increased from -2200 to -500/cm, namely reducing loss, under optically pumped condition.