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

Abstract

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 electrons injection into the direct-gap valleys. We used 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. 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).

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Identifiers

ORCID for Frederic Gardes: orcid.org/0000-0003-1400-3272

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

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