On-Chip Integration of Quantum Dot Lasers with Silicon Nitride Waveguides

Introduction

The monolithic integration of lasers with silicon photonics has been an active area of research for several decades, promising higher integration densities and lower production costs compared to other integration schemes. Quantum dot (QD) lasers, which utilize self-assembled InAs quantum dots as the active region, have emerged as a promising candidate for on-chip laser sources due to their superior temperature stability, low threshold current density, and high reliability.

In this article, we will explore the recent demonstration of on-chip monolithic integration of QD lasers coupled to foundry-processed silicon nitride (SiN) waveguides on a 300 mm silicon-on-insulator (SOI) wafer. This groundbreaking achievement showcases the potential for scalable production of next-generation on-chip light sources for silicon photonics.

Wafer Preparation and Fabrication

The integration process begins with a 300 mm SOI wafer containing pre-fabricated SiN and silicon waveguides, as well as other passive photonic components, processed in a silicon photonics foundry. Narrow pockets, approximately 35 μm wide, 3 mm long, and 6 μm deep, are etched into the oxide layer of the SOI wafer using dry etching techniques. These pockets serve as the spaces for direct epitaxial growth of the III-V laser stack, enabling butt-coupling to the SiN waveguides.

Figure 1: (a) 300 mm SOI wafer after in-pocket epitaxial growth and CMP. Inset shows a sample from the location marked on the wafer, after laser fabrication. (b) Angled-view SEM of finished lasers coupled to waveguides, inset shows a zoomed-in view of the etched facet at the pocket end. (c) Cross-section SEM of the in-pocket laser. (d) FIB cross-section of the laser coupled to the SiN waveguide. All scale bars are 5 μm.

The in-pocket surface is treated and grown with a GaP/GaAs template using metal-organic chemical vapor deposition (MOCVD) at NAsP III-V GmbH. Subsequently, molecular beam epitaxy (MBE) growth of the subsequent layers, including five layers of InAs QD active region, is carried out in a 300 mm reactor at IQE, Inc. The laser stack is designed to vertically align the center of the mode with the center of the mode of the SiN waveguide couplers for efficient on-chip butt-coupling.

After growth, chemical-mechanical polishing (CMP) is performed on the wafer to remove polycrystalline material and prepare the surface for device fabrication. The wafer is then diced into smaller pieces for processing at the UCSB Nanofab facility.

Ridge waveguides are fabricated along the pocket direction, with an additional dry etch at the end of the pocket to form the on-chip etched-facet mirrors. P-contacts (Pd/Ti/Pd/Au) on top of the ridges and n-contacts (Pd/Ge/Au) on one side of the waveguides are deposited, along with probe metal for direct electrical injection. The facet-etch is carefully developed to remove polycrystalline material and create a cleared pathway for direct light integration through on-chip butt-coupling.

Measurement Results

The lasing performance of the integrated QD lasers is characterized on-chip with direct electrical pumping. Scattered light is collected using an integration sphere placed in close proximity to the sample, and light-current-voltage (LIV) measurements are performed.

Figure 2: (a) Light-current measurement of an on-chip laser with an integration sphere. (b) Light-current measurement of the same laser with fiber-coupled from waveguide output. (c) Waveguide-coupled lasing spectrum. (d) Microscopic view of the in-pocket laser connected to SiN waveguide couplers through etched and passivated air-gap. (e) Zoomed-out microscopic view of on-chip etched facet lasers being probed for electrically injected testing. (f) Experimental setup of the laser characterization performed with a lensed-fiber on one side.

Clear on-chip lasing with the etched-facets is observed, as shown in Figure 2(a). To measure the coupling of laser light into the waveguides, the sample is diced into smaller sections, and facet-polishing is performed for the edge couplers. Waveguide-coupled light is collected from a lensed fiber at the edge coupler output, and the LI curve is measured, as shown in Figure 2(b).

The lasing spectrum, obtained from an optical spectrum analyzer at different injection currents, is presented in Figure 2(c). Clear lasing is observed above the threshold, with the ground state shifting to excited states as the injection current increases. The peak lasing wavelengths are around 1300 nm to 1250 nm, consistent with the photoluminescence of the InAs QDs.

Conclusion

In this article, we have explored the groundbreaking demonstration of on-chip monolithic integration of QD lasers coupled to foundry-processed SiN waveguides on a 300 mm SOI wafer. By utilizing direct in-pocket MBE growth and etched-facet mirrors, this achievement showcases the potential for scalable production of next-generation on-chip light sources for silicon photonics.

The successful on-chip butt-coupling of QD lasers to SiN waveguides, with waveguide-coupled lasing performance characterized through edge couplers, represents a significant step forward in the pursuit of complete monolithic integration of III-V gain elements with CMOS-compatible silicon photonic platforms on SOI wafers. This integration approach promises reduced costs and scalable production, paving the way for the realization of high-density, high-performance photonic integrated circuits for a wide range of applications.

Reference

[2] K. Feng et al., "On-Chip Monolithic Integration of QD Lasers Coupled to Foundry-Processed SiN Waveguides on 300 mm SOI Wafer," Dept. of Electrical and Computer Eng., University of California Santa Barbara, Santa Barbara, CA, USA, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.