Optically-Pumped Lasing in GeSn Slab Waveguides on Silicon

Introduction

Group IV GeSn alloy semiconductors have emerged as a promising material for efficient, CMOS-compatible light sources to enable functional silicon photonic chips. While germanium (Ge) is an indirect bandgap semiconductor, the addition of tin (Sn) can effectively reduce the energy difference between the direct and indirect conduction bands. At a sufficiently high Sn content of around 7%, the bandgap becomes direct, enabling efficient light emission and lasing.

Significant progress has been made over the past two decades in growing high-quality GeSn alloys with high Sn content using low-temperature epitaxial techniques. This has led to the realization of various optically-pumped GeSn lasers as well as recent demonstrations of electrically-injected GeSn LEDs and lasers. However, there is still room for improvement in laser performance metrics like lasing threshold and maximum operating temperature.

One key factor impacting laser performance is surface recombination caused by defect states on the surfaces and sidewalls of the laser cavity, introduced during the etching process to define the cavity. This surface recombination leads to additional optical losses, increasing lasing thresholds and limiting the maximum operating temperature.

This article covers an optically-pumped GeSn slab waveguide laser design that incorporates a silicon ridge structure on the GeSn active layer surface to enhance optical confinement and mitigate surface recombination effects.

Device Structure

The device structure is depicted in Figure 1(a). A 550 nm thick GeSn active layer with 10% Sn content is grown on a silicon substrate via a 450 nm strain-relaxed Ge virtual substrate. A 100 nm amorphous silicon layer is deposited and patterned into a 200 μm wide ridge waveguide structure.

Figure 1: (a) Schematic of the GeSn ridge slab laser. Inset shows an optical image of the fabricated device. (b) Simulated quasi-TE mode energy distribution.

Simulations in Figure 1(b) show that the lower refractive index of the silicon ridge compared to GeSn and Ge enables good optical confinement in the slab waveguide, with a 66.8% confinement factor in the GeSn active region. Importantly, the mode is kept away from the silicon ridge sidewalls, suppressing surface recombination at those defect sites.

Device Characterization

Figure 2 shows the emission spectra from the device at 40K under optical pumping with a 1064 nm pulsed laser. At lower pump powers, a broad spontaneous emission peak is observed at 2238 nm, corresponding well to the calculated direct bandgap transition energy from the Γ conduction band to the heavy-hole valence band.

Figure 2: Emission spectra at 40K under varying pump powers. Inset shows the calculated GeSn band structure.

As the pump power increases above 60.9 kW/cm2, the emission narrows and intensifies, indicating lasing action. The temperature-dependent emission spectra in Figure 3 show that lasing is sustained up to 90K, with a threshold of 170 kW/cm2 at that temperature, as indicated by the onset of nonlinearity in the light-in/light-out curve (inset).

Figure 3: Temperature-dependent emission spectra. Inset shows light-in/light-out curves at different temperatures.

While room temperature operation has not yet been achieved, this GeSn slab waveguide laser design has enabled lasing up to 90K with a relatively low threshold of 170 kW/cm2 by mitigating surface recombination through the silicon ridge structure.

Summary

This article covered an optically-pumped GeSn slab waveguide laser integrated on silicon that utilizes a silicon ridge design to improve optical confinement and suppress surface recombination effects. The GeSn active region design and material properties were discussed, along with simulation results confirming the optical mode confinement. Experimental emission spectra demonstrating lasing action up to 90K were presented and analyzed. This laser design represents important progress towards realizing efficient, integrated group IV laser sources for silicon photonics.

Reference

[1]   Y.-P. Huang, B.-R. Wu, Y.-T. Jheng, and G.-E. Chang, "Optically-Pumped Lasing in GeSn Slab Waveguide on Silicon," Graduate Institute of Opto-Mechatronics and Advanced Institute of Manufacturing with High-Tech Innovations, National Chung Cheng University, Chiayi County, Taiwan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.