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Subsequent projects

Innovative Sub-Wavelength High-Contrast Gratings for Long-Wavelength Lasers

The research groups of Prof. M.-C. Amann (Walter Schottky Institut, TUM) and Prof. C. Chang-Hasnain (University of California, Berkeley) established a cooperation to develop and fabricate long-wavelength mid-infrared Vertical-Cavity Surface-Emitting Lasers (VCSELs) with sub-wavelength gratings. Due to their high and polarization-dependent reflectivity, the sub-wavelength gratings promise to be the ultimate thin mirrors for polarization-stable single-mode VCSELs and could replace the thick dielectric DBRs, currently used in the devices. The initial device shall be co-designed by both groups. The novel VCSELs designed for wavelengths above 2 µm will then be fabricated in Munich. Afterwards, the sub-wavelength gratings will be fabricated and integrated in the VCSEL in Berkley. Furthermore, the grating is expected to improve heat dissipation of the device due to smaller thickness compared to conventional DBRs.

 

Primary project: Novel Sub-Wavelength Grating High-Speed Data-Com-Lasers (VCSELs)

 

Final Report

During the cooperation between the Chair of Semiconductor Technology, Walter Schottky Institut, Technical University Munich and Department of Electrical Engineering and Computer Science, University of California, Berkeley, the feasibility and design of vertical-cavity surface-emitting laser diodes (VCSELs) with sub-wavelength gratings for the wavelength range between 2 and 3 μm were investigated.

Sub-wavelength gratings with high refractive index contrast to the surrounding media (high contrast grating (HCG)) can provide high broadband reflectivity and therefore are a valid candidate to replace conventional multi-layer distributed Bragg reflectors (DBRs) as mirror for VCSELs. Further advantage of the HCG is polarization stability and enhanced single mode operation, which is of great interest for long wavelength VCSEL applications, for example gas spectroscopy.

The Group of Prof. Connie Chang-Hasnain (University of California, Berkeley) designed the necessary sub-wavelength high-contrast gratings. First, the concept used in a previous BaCaTeC project (Novel Sub-Wavelength Grating High-Speed Data-Com-Lasers (VCSELs)) with evaporated grating materials was contemplated. However, this idea was discarded in favor of a more innovative and ambitious approach where a free-hanging membrane with etched sub-wavelength grating is fabricated. The Group of Prof. Amann (Technical University Munich) evaluated and adapted their existing VCSEL concepts to facilitate the desired grating. As the substrate for the laser diodes InP was selected due to its superior thermal performance and relative ease of fabrication (for example, compared to GaSb). After careful consideration, the recently developed low-optical-loss concept was chosen for the VCSEL.

The initial devices were then co-designed by both groups for the wavelength of 2.1, 2.3 and 2.5 μm. The aim was to create, as far as possible, a universal design for multiple devices, therefore only the active region and cavity length differ for various wavelengths.

The final device consists of the undoped epitaxial DBR (materials InP and InGaAs), followed by the n-InGaAs bottom contact layer and the InP spreading layers. As the active region type-I or type-II InP-based quantum wells are used and the current aperture is achieved using buried-tunnel-junction. The tunnel junction is then epitaxially overgrown to create the top current spreader and the necessary layers for the grating fabrication: sacrificial n-InGaAs layer, which also acts as the top contact layer, and the actual InP grating layer. Furthermore, a fabrication procedure was developed to ensure highest possible yield, despite the need to transfer the samples between the two groups.

Altogether novel and sophisticated concepts for both VCSEL and HCG were implemented in the device design, which considerably increased the complexity of the project. Unfortunately, it was not possible to finish the fabrication of the aforementioned devices in the timeframe of this project. Due to unforeseen technological failures only the basis of the device could be fabricated and further fabrication hat to be put on hold. Nevertheless, unique state-of-the-art VCSELs were designed during this cooperation and solid foundation for further collaborations was created for the two groups.

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