Monolithic Integration of 940 nm VCSELs on Bulk Ge Substrates: A Breakthrough for Mass Production

Introduction

Vertical cavity surface emitting lasers (VCSELs) have gained significant traction as compact, low-cost, and scalable near-infrared illumination sources, driven by their integral roles in optical interconnects and 3D sensing. However, the prevailing use of 4- to 6-inch GaAs wafers in VCSEL production has hindered scalability, as the VCSEL market is expected to grow from $1.6 billion in 2022 to nearly $3.9 billion by 2027.

The Solution: Bulk Ge Substrates

Bulk Ge substrates emerge as a promising solution to address the limitations of GaAs wafers for VCSEL production. Ge offers several advantages, including:

1. Suitable lattice constant (5.658 Å, between GaAs and AlAs), resulting in reduced strain and enhanced post-growth stability.

2. Reduced threading dislocations and enhanced mechanical resilience.

3. Availability in sizes up to 12 inches, enabling cost-effective mass production.

The Breakthrough

In a groundbreaking study, researchers at the University of British Columbia (UBC) and their collaborators successfully demonstrated the monolithic integration of full 940 nm VCSELs on 4-inch bulk Ge substrates. This work represents the second successful Ge-VCSEL technology reported worldwide and the first with key technological details disclosed.

Experiment Design and Epitaxy Growth

The researchers designed and grew a full VCSEL structure, targeting 940 nm operation temperature, on Ge and GaAs wafers under optimized growth conditions, as shown in Figure 1.

Figure 1. Schematic of the full VCSEL structures grown on 4-inch bulk Ge substrates with GaAs/InGaAs/InGaP nucleation and transition layers.

Results and Discussion

1.Surface and Cross-section Imaging Atomic force microscopy (AFM) images revealed that the Ge wafer had a smoother surface with no cracks or antiphase domains (APDs) and 40% less roughness than the GaAs wafer, indicating better surface quality. Scanning electron microscope images of the cross-sections showed near-identical epitaxy layer smoothness and thickness.

Figure 2. AFM images and roughness measurement on 10 × 10 μm surfaces of full VCSELs on (a) Ge substrates and (b) GaAs substrates.

2.Wafer Bow-warp Maps The bow-warp maps revealed that the Ge wafer had a 72% lower average bow-warp value (10.28 μm) than the GaAs wafer (36.59 μm), resulting from the smaller lattice mismatches between Ge and AlxGa1-xAs DBRs.

Figure 3. Bow-warp mapping on 4-inch (a) Ge wafers and (b) GaAs wafers after full VCSEL growth.

3.Electrical and Optical Performance Despite limited process recipe tuning, the Ge-VCSELs achieved successful lasing with a lower threshold current density of 2.33 kA/cm² than the GaAs counterpart (2.58 kA/cm²) and the IQE Ge-VCSELs (2.39 kA/cm²). Furthermore, the Ge-VCSELs obtained a 19% higher maximum slope efficiency of 0.37 mW-kA⁻¹-cm² than the GaAs-VCSEL.

Figure 4. (a) Room temperature L-J-V curves for 940-nm VCSELs grown on GaAs (red line) and Ge (blue line) substrates. (b) Optical power and optical differential efficiency (slope efficiency) as a function of bias currents.

Conclusion

This study successfully demonstrated the feasibility and revealed key details of fabricating VCSELs on Ge substrates for mass production. The Ge-VCSEL exhibited superior surface quality, reduced bow-warp, lower threshold current density, and higher maximum slope efficiency compared to the GaAs counterpart. This breakthrough paves the way for integrating various III-V based structures on bulk Ge substrates, enabling a wide range of optical and electrical applications and addressing the scalability challenges of VCSEL production.

Reference

[2] Z. Wan, Y.-C. Yang, W.-H. Chen, C.-C. Chiu, Y. Zhao, M. Feifel, L. Chrostowski, D. Lackner, C.-H. Wu, and G. Xia, "Monolithically Integrated 940 nm VCSELs on Bulk Ge Substrates," Department of Materials Engineering, The University of British Columbia, Vancouver, Canada; Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan; Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan; Graduate School of Advanced Technology, National Taiwan University, Taipei, Taiwan; Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany; Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver, Canada, 2024.