Chee-Wei Lee

Silicon/III-V laser with super-compact diffraction grating for WDM applications in electronic-photonic integrated circuits.

We have demonstrated a heterogeneously integrated III-V-on-Silicon laser based on an ultra-large-angle super-compact grating (SCG). The SCG enables single-wavelength operation due to its high-spectral-resolution aberration-free design, enabling wavelength division multiplexing (WDM) applications in Electronic-Photonic Integrated Circuits (EPICs). The SCG based Si/III-V laser is realized by fabricating the SCG on silicon-on-insulator (SOI) substrate. Optical gain is provided by electrically pumped heterogeneous integrated III-V material on silicon. Single-wavelength lasing at 1550 nm with an output power of over 2 mW and a lasing threshold of around 150 mA were achieved.

Generic Heterogeneously Integrated III–V Lasers-on-Chip With Metal-Coated Etched-Mirror

In this paper, electrically pumped III-V quantum-well lasers bonded on SiO2 with a metal-coated etched-mirror are reported. There are three key features for the device demonstrated: (i) The metal-coated etched-mirror ensures that the lasers can be used as on-chip light source and provides high reflectance, but requires no additional fabrication steps due to our process design, (ii) the bonded III-V on SiO2 enables high-light confinement in the active region due to high index contrast between III-V and SiO2. Moreover, it promises a flexible choice of host substrate, in which the silicon substrate could also be replaced with other materials, and (iii) the active III-V region is sufficiently close to the SiO2 interlayer, allowing the laser mode to overlap with SiO2. This facilitates effective optical coupling with in-plane passive waveguides, which can be fabricated from thin film of amorphous silicon, silicon nitride or other waveguide materials, to form a subsystem on chip through in-plane integration. The laser devices demonstrated have the lowest threshold of 50 mA, a maximum output power of 9 mW, and a differential quantum efficiency of 27.6%.

Heterogeneous Integrated III–V Laser on Thin SOI With Single-Stage Adiabatic Coupler: Device Realization and Performance Analysis

A III-V on silicon heterogeneous integrated laser with highly efficient single-stage adiabatic coupler is presented in this paper. The structure consists of an electrically pumped III-V ridge waveguide gain section on silicon, III-V/Si optical adiabatic coupler, and silicon-on-insulator (SOI) nanophotonic waveguide. The adiabatic coupler is 50-μm long and is formed by tapering the III-V ridge and the underneath thin SOI waveguide along the same direction for efficient coupling of light between III-V ridge and silicon waveguide. Fabrication details and characterizations of this heterogeneous III-V/Si Fabry-Pérot (FP) laser are presented. Experimental data show that such structure has a low taper end reflection of ~-37 dB, and a high optical coupling efficiency of ~85%. The fabricated FP laser shows a high differential quantum efficiency of 23.78% under pulse operation at room temperature. The maximal single facet emitting power is about 7.5 mW, and the side-mode suppression ratio is ~30 dB. Thermal characterization shows a length normalized thermal independence of 20.02 °C·mm/W. Since this new heterogeneously integrated III-V/Si laser structure is realized directly on thin SOI, it offers a potential solution for developing more complex, efficient, and scalable integrated on-chip subsystems for various applications.

Study of ultrathin SiO2 Interlayer wafer bonding for heterogeneous III–V/Si photonic integration

We demonstrated low-temperature bonding of III–V InP-based compound semiconductor on silicon via nano-thin SiO2 interlayer down to thickness of 20 nm, with ultra-smooth surface for heterogeneous photonic integration. The bonding is achieved with chemical cleaning of the sample surface, followed by oxygen plasma surface activation, which gives high quality bonding between the two different materials. Detail analyses on the bonded samples are carried out. From the photoluminescence and the x-ray diffraction measurements, in which no significant peak shift and peak broadening are observed, we conclude that the crystalline quality of the bonded thin film is preserved. The cross-sectional high resolution transmission electron microscopy shows that bonded III–V-SiO2–Si interface has no observable defect. These results reinforce that the proposed bonding offers a promising technology for realizing versatile heterogeneous photonics integration.

Continuous-Wave InP-InGaAsP Microsquare Laser—A Comparison to Microdisk Laser

We demonstrate compact optically pumped microsquare cavity laser on InP-based multiple-quantum-wells material platform. Continuous-wave operation is achieved for microsquare cavity with footprint as small as 4 × 4 μm2. Numerical mode analysis and experimental characterization of the microsquare laser are conducted, and the results are compared with the commonly used microdisk cavity laser fabricated on the same platform. The microsquare laser shows a lower lasing threshold and infers a higher differential efficiency than the microdisk counterpart. The microsquare cavity laser has also sufficiently high quality factor, and higher pumping injection efficiency due to the more evenly distributed field profile as compared to that of the microdisk. Experimental result also shows that the microsquare laser has better temperature stability than the microdisk. These results promise a potential alternative laser structure for on-chip light source applications.

Ultra-thin oxide interlayer wafer bonding for heterogeneous III-V/Si photonics integration

We report a low-temperature (220°C) covalent bonding of InP-based epitaxy substrate to silicon substrate through a thin thermal oxide interlayer of around 20 nm. Our SiO2 interlayer is grown only on the silicon substrate, which avoids the challenge in obtaining high quality SiO2 film on III-V substrate. The 20 nm thin oxide is proved to be sufficient as the outgassing medium during the bonding process. It is found that the bonding has minimal effect on the transferred epitaxy layer. This is evident from the X-ray Diffraction and room temperature photoluminescence (PL) characterizations of the III-V sample before (as-grown) and after bonding, where no significant peak shifting or broadening is observed. The high resolution Transmission Electron Micrograph (HR-TEM) also reveals almost zero-defect atomic bonding between III-V and thermal oxide on silicon.

Comparison of III-V/Si on-chip lasers with etched facet reflectors

Electrically pumped heterogeneously integrated III-V/SiO2 semiconductor on-chip lasers with different types of etched facet reflectors are designed and fabricated and their lasing performances are characterized and compared. The III-V quantum-well-based epitaxial layers are bonded on silica-on-silicon substrates and fabricated to form Fabry-Perot lasers with dry-etched rear facets. Three types of reflectors are demonstrated, which are etched facets terminated by air, benzocyclobutene, and metal with a thin layer of SiO2 insulator in-between. The laser devices are characterized and compared, including lasing threshold, external quantum efficiency, and output power, and show the impact of different types of etched facet reflectors on lasing performance.

Generic heterogeneously integrated III-V lasers-on-chip with metal-coated etched-mirror

We demonstrate electrically-pumped III-V quantum-well lasers bonded on SiO2 with a metal-coated etched-mirror. The metal-coated etched-mirror allow the lasers to be used as on-chip laser, but our process design make sure that it requires no additional fabrication step to fabricate the metal-coated etched mirror. The bonded III-V on SiO2 also permits tight laser mode confinement in the active region due to high index contrast between III-V and SiO2. Moreover, it promises a flexible choice of host substrate, in which the silicon substrate could also be replaced with other materials. The laser devices demonstrated have the lowest threshold of 50 mA, a maximum output power of 9 mW and a differential quantum efficiency of 27.6%.

Hetero-core III-V/Si microlaser

We design and demonstrate optically pumped microlasers with a hetero-core cavity formed by III-V and silicon-on-insulator (SOI) materials. Hetero-core cavities with identical lateral dimension are fabricated. The cavity is formed by III-V layer with thickness of 210 nm on top of SOI layer with thickness of 300 nm via SiO2 interlayer wafer bonding. Continuous wave laser operation is achieved for a diameter down to 2 μm with a corresponding mode volume of 0.07λ3 and quality factor of 1.3×104. The architecture renders an alternative laser structure for heterogeneous laser-on-chip, with no dedicated vertical coupling mechanism needed between the two materials layers.

Metal–Semiconductor Square Nanocavity and Light Extraction

We propose a design of metal-semiconductor square nanocavity on silicon that could be used for on-chip nanolaser application, together with an efficient mechanism of light extraction, which is important for practical application. The analysis of the spatial mode for the square nanocavity is carried out, and hybrid plasmonic-photonic mode profile is observed in the cavity. Optimized design of the square cavity gives a mode volume of 0.96(λ/2n)3 and resonant wavelength of 1474 nm when the side length is 600 nm. The outcoupling mechanism for the nanocavity is based on a direct-joint plasmonic waveguide. The effects of width, position, and angle of the output plasmonic waveguide on the outcoupling efficiency are studied, and the difference in cavity mode with respect to circular nanocavity has contributed to the different outcoupling performances. The analysis shows that the output coupling efficiency can be tuned up to 60% by varying these parameters, and this offers an output coupling mechanism with great flexibility for ultra-compact on-chip light source.