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

Abdelrahman Z Al-Attili,Zuo Li,Daniel Burt,Yasuhiko Ishikawa,Katsuya Oda,Frederic Y Gardes,Shinichi Saito,Naoki Higashitarumizu

doi:10.1364/oe.26.034675

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 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.

Highlights

The rapid progress towards the convergence of electronics and photonics on a Complementary Metal-Oxide-Semiconductor (CMOS) platform is motivating the development of a Silicon (Si)-compatible laser source [1,2,3,4]
Ge is a group IV element that is already established within the CMOS industry, it is used in the source and drain in some MOS Field-Effect-Transistors (MOSFETs) generations [12]
Thermally-grown SiO2 layers, such as the buried-oxide layer (BOX), are expected to have higher initial compressive stresses compared to plasma-enhanced chemical-vapor deposition (PECVD) SiO2, due to higher growth temperatures [44]

Summary

Introduction

The rapid progress towards the convergence of electronics and photonics on a Complementary Metal-Oxide-Semiconductor (CMOS) platform is motivating the development of a Silicon (Si)-compatible laser source [1,2,3,4]. Ge is a group IV element that is already established within the CMOS industry, it is used in the source and drain in some MOS Field-Effect-Transistors (MOSFETs) generations [12] It has a unique band-gap that can be engineered to provide direct-gap optical gain [1,2,3,4]. Band-gap engineering of Ge relies mainly on tensile strain to minimize or even eliminate this energy difference between Γ and L CB valleys [16, 17] This leads to an enhanced probability of electrons injection into the direct Γ valleys, increasing the efficiency of radiative recombinations [15,16,17]. We demonstrate fabricating Ge micro-gears on Si pillars (Fig. 1) with an all-around SiO2 stressor for vertical emission of direct-gap resonances.

Device structure and fabrication

Strain characterization and analysis

Photo-luminescence and optical modes characteristics

Conclusion