III-V lasers are monolithically integrated with silicon photonic devices

Recently, Professor Eric Tournié’s team from the University of Montpellier in France published a report entitled “Unlocking the monolithic integration scenario: optical coupling between GaSb diode lasers epitaxially grown on” in Light: Science & Applications patterned Si substrates and passive SiN waveguides”. Dr Andres Remis of the University of Montpellier is the first author and Professor Eric Tournié is the corresponding author. In addition, the Department of Electrical and Information Engineering at the Polytechnic University of Bari, Italy, the Centre for Advanced Photonics and Process Analytics at the University of Technology Münster, Ireland, and the National Institute of Tyndall contributed to the paper. Professor Eric Tournié has proposed a scheme for a III-V group laser grown on a patterned silicon photonics platform that enables optical coupling to passive SiN waveguides. The work has been supported and funded by the European Union’s H2020 programme, France’s ANR and France’s “Invest in the Future” program.

Research background

Silicon photonics is widely used in data communication, optical interconnect, quantum technology or on-chip optical sensors due to its mature silicon process technology, large silicon wafer size and excellent silicon optical properties. Among them, SiN-based waveguides have excellent high power processing capabilities, wider transparency, larger cross-section, and lower losses than silicon, making them a high-quality platform in silicon photonics chips. In addition, through atomic layer deposition or transfer printing of nonlinear crystals, a variety of ways can be used in this technology to manufacture highly efficient electro-optical modulators. Existing studies have shown that despite impressive progress in silicon-based photonic chips, the indirect bandgap of Si and Ge and the performance of GeSn devices are still too low, making the integration of III-V semiconductor lasers with passive silicon photonic integrated circuits (PICs) an important topic for achieving fully integrated silicon photonic chips. At present, the most mature integration strategy is heterogeneous integration, where III-V heterostructures are first grown on their natural substrates, then incorporated onto silicon-based PICs and processed into devices, a technology that can already be used to prepare commercial products and even enter the market. However, there is evidence that from the perspective of integration density and economy, the direct epitaxial III-V semiconductor laser heterostructure on Si-PIC may be superior to the traditional heterogeneous integration method on a medium- and long-term basis. In addition, heterogeneous methods require etching away the original III-V substrate, which is an unsustainable practice in the long run.

In view of these views, over the past decade, researchers have conducted extensive research on the direct growth of III-V lasers on silicon substrates, and have made great progress in the epitaxial integration of lasers in various bands from visible to mid-infrared to near-infrared. Nevertheless, all of the epitaxy lasers on silicon reported to date are discrete devices grown on planar silicon wafers rather than on PIC. This suggests that the next challenge for silicon-based integrated photonic chips is to combine epitaxial lasers and silicon photonics PICs and couple optics from active III-V structures to passive silicon-based devices.

The growth of III-V semiconductors on silicon inevitably leads to different types of structural defects that are detrimental to device performance. Although various strategies have been developed to avoid or attenuate their effects, existing strategies require growing relatively thick (1–5 μm) buffer layers under laser structures. While this approach is not a problem for discrete III-V silicon optoelectronic devices, these buffer layers prevent the device from coupling the elapsed light into the passive waveguide.

Another option is to use direct coupling, coupling the laser to a passive waveguide, or an epitaxial III-V photodetector to a silicon waveguide. However, epitaxial direct coupling methods also present challenges, as the manufacturing and processing conditions of III-V and silicon-based materials are only partially compatible, and SiN waveguides with minimal losses need to be deposition and processed at high temperatures (e.g., by liquid phase chemical vapor deposition). There is no alternative to first fabricating silicon-based photonic chip PICs, patterning PIC wafers to define the epitaxial region, and then epitaxial growth and processing of III-V laser structures. Therefore, methods for growing III-V materials on patterned wafers need to be investigated and devices fabricated from materials grown in recessed regions without compromising the quality of the PIC.

Innovative research

For decades, the monolithic integration of direct epitaxial III-V lasers and silicon photonic devices on the same silicon substrate has been considered a major barrier to achieving dense photonic chips. Existing studies have only reported discrete III-V lasers grown on planar silicon wafers. In response to this challenge, Professor Eric Tournié’s team proposed and explored solutions to overcome these challenges and demonstrated a III-V laser grown on a patterned silicon photonics platform that can achieve optical coupling to passive SiN waveguides.

Figure 1. Overview of integrated devices. a) Cross-sectional schematic of the final device: optoelectronics emitted from the active region of an epitaxial integrated III-V laser coupled into the SiN waveguide. b) Schematic diagram of silicon photonics PICs used in the experiment: 100 mm silicon wafers are first processed to form SiN waveguides covered by a SiO2 layer. The patterns are arranged in 20×20 mm² squares. The dielectric stack is then etched down onto the Si substrate to open the recessed area where the III-V laser will epitaxial growth. The wafers are then cut to obtain 20×20 mm² slices for epitaxy.

Specifically, the researchers grew a mid-infrared GaSb-based diode laser directly on a prepatterned silicon-based photonic wafer equipped with a SiO2-coated SiN waveguide, which can emit light around 2.3 μm, a wavelength of interest for trace gas sensing or lidar applications, and can demonstrate an emitted light output of more than 10 mW at room temperature, as shown in Figure 1. This overcomes the growth and device manufacturing challenges posed by the template architecture.

Figure 2. Structural properties of epitaxial III-V PIC structures on silicon. a) Picture of a 20×20 mm² grain after III-V epitaxy. The gray area corresponds to the waveguide region where the polycrystalline III-V material has been deposited on top of the dielectric material. The specular region is the concave Si region where single-crystal III-V group materials are grown. b) High-resolution diffraction scanning of laser structures. c) 20 x 20 μm² AFM image of the epitaxial laser surface.

In addition, about 10% of the light is coupled into the SiN waveguide, which is very consistent with the theoretical calculation of the direct coupling configuration. This work solves the problem of monolithic integration of III-V lasers and silicon photonic devices, paving the way for future low-cost, large-scale, fully integrated photonic chips.

Figure 3. a) SEM image showing laser ridge and its etching face and passive waveguide. b) A picture of a laser rod equipped with a SiN waveguide, ready for testing at the detection station. DL is 1.5 mm long.


Figure 4. L–I–V characteristics of DL on silicon-based PIC. a) L-I-V curves of 8 DLs in CW state at room temperature. b) For typical DL, L-I-V curves taken at different temperatures between 20 °C and 80 °C.

The article was published in Light: Science & Applications, a top international academic journal, entitled “Unlocking the monolithic integration scenario: optical coupling between GaSb diode lasers epitaxially grown on patterned Si.” substrates and passive SiN waveguides”。 (Source: LightScience Applications WeChat public account)

Related paper information:‍-023-0‍1185-4

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