Toward All-MOCVD Grown InAs/GaAs Quantum Dot Laser Diodes on Silicon Substrates

Introduction

Silicon photonics (SiPh) is rapidly gaining traction for applications in optical data communications, sensing, biomedical devices, automotive, astronomy, aerospace, and artificial intelligence. A key challenge for SiPh technology is the integration of efficient light sources based on III-V semiconductor materials onto silicon substrates. Directly growing III-V materials on silicon is difficult due to lattice constant mismatch, differing thermal expansion coefficients, and lattice polarity issues which lead to defect formation.

One approach to overcome these challenges is wafer bonding of pre-grown III-V layers onto silicon. However, wafer bonding suffers from wafer size mismatch limitations and high cost. An alternative is direct epitaxial growth using defect reduction techniques like dislocation filter layers (DFLs) and quantum dot (QD) active regions which are more defect-tolerant than quantum wells.

This tutorial covers progress toward developing InAs/GaAs quantum dot laser diodes (QDLDs) directly grown on silicon substrates using an all metalorganic chemical vapor deposition (MOCVD) process. The key enabling technologies are DFL-based GaAs/Si templates and QD active regions.

GaAs Buffer on Silicon

A low-defect GaAs buffer layer is first grown on an offcut silicon substrate using DFLs to reduce threading dislocations. As shown in Figure 1, three thermal cycling annealing steps and five DFL sets composed of strained InGaAs/GaAs superlattices are employed.

Figure 1: Schematic of the GaAs buffer structure grown on silicon to achieve low defect density

The efficacy of this buffer approach is confirmed by electron channeling contrast imaging in Figure 2, revealing a threading dislocation density of only 1.5 x 10^7 cm^-2 for the 2 μm thick GaAs layer.

Figure 2: Electron channeling contrast image of the GaAs/Si template showing a low threading dislocation density of 1.5 x 10^7 cm^-2

InAs/GaAs QD Growth & QDLD Fabrication

With a low-defect GaAs/Si template, the epitaxial QDLD structure can be grown. First, the QD growth conditions are optimized on a GaAs substrate using a vertical MOCVD reactor. The active region consists of five stacked dot-in-a-well (DWELL) layers with 65 nm GaAs spacers. AlGaAs cladding layers and separate confinement heterostructure layers sandwich the active region.

Photoluminescence measurements in Figure 3 show the QDs emit around 1300 nm, well-suited for O-band telecommunications. The QD density is 5x10^10 cm^-2 with 7 nm height and 40 nm diameter based on AFM imaging.

Figure 3: Photoluminescence spectrum of the InAs/GaAs quantum dots and AFM image (inset) showing 5x10^10 cm^-2 density.

Broad-area QDLDs processed from this epitaxial material exhibit room temperature lasing up to 30 mW output power per facet under continuous wave operation, as plotted in Figure 4. The threshold current density is 175 A/cm^2, and the inset shows single mode lasing at 1253 nm wavelength.

Figure 4: Light output vs current curve for the InAs/GaAs QDLD grown on GaAs under CW operation, with the lasing spectrum at 1.1x threshold shown in the inset

Continuous wave lasing is maintained for temperatures up to 75°C for these broad-area GaAs substrate QDLDs.

QDLD on Silicon Substrates

Applying the GaAs/Si template and QD growth optimization, full QDLD structures were grown directly on silicon substrates with and without DFLs. Cross-sectional transmission electron microscopy in Figure 5 confirms successful growth of the QDLD layers on the DFL-based GaAs/Si template, including the 7-stack DWELL active region.

Figure 5: Cross-sectional TEM image of the QDLD grown on a silicon substrate using a GaAs/Si template with DFLs and a 7-stack DWELL active region

Photoluminescence measurements reveal that including DFLs boosts the QD emission intensity by 3x compared to templates without DFLs. This underscores how dislocations severely degrade QD and QDLD performance, making DFLs essential for direct QDLD growth on silicon substrates.

Conclusion

This work demonstrates key progress toward realizing InAs/GaAs QDLDs directly grown on silicon substrates using an all-MOCVD process. Low threading dislocation density GaAs/Si templates enabled by DFLs allow high-quality InAs/GaAs QDs to be grown, which serve as an effective active region for QDLDs operating up to 75°C. Integrating such QDLDs could provide efficient light sources for silicon photonic integrated circuits across a variety of applications.

Reference

[1] H. Kim, S. Lee, K.-J. Kim, H. Park, Y.-H. Ko, D.-J. Kim, D.-M. Geum, J. T. Ahn, and W. S. Han, "Toward All-MOCVD Grown InAs/GaAs Quantum Dot Laser Diode on a Si Substrate for O-Band Telecommunication," Electronics and Telecommunications Research Institute, Daejeon, Republic of Korea, and School of Electronic Engineering, Chungbuk National University, Cheongju, Republic of Korea, 2024.