1. Introduction

The integrated silicon photonics can afford complex higher functionality, higher speed interconnect and parallel many-core computation with low cost, and an electrical silicon light source (laser) is a key element for that [1], but the fabrication of an efficient laser on silicon platform is still challenging due to silicon’s indirect bandgap. It has been shown that heterogeneous integration of III-V materials and silicon is a practical way to solve this problem [2–4]. Among the several types of III-V/Si hybrid lasers, silicon evanescent lasers based on silicon-on-insulator (SOI) platform which evanescently couple the optical mode to a silicon waveguide show great potential for industrial scale fabrication. Meanwhile, quite a few of bonding methods have been used to develop silicon evanescent lasers, such as direct wafer bonding [5–7], BCB bonding [8, 9], and selective-area metal bonding (SAMB) [10–12].

Based on the existing bonding technology, an increasing attention has been paid to the single mode laser and multi-wavelength lasers, one of the fundamental components in wavelength division multiplexed (WDM) optical link on chip or on board. For III-V/Si hybrid lasers, some structures have been adopted for the single longitudinal mode laser such as distributed feedback (DFB) [13, 14], distributed Bragg reflection (DBR) resonator [15], ring resonator [16], slotted feedback structure [17], and resonant grating cavity mirrors [18]. Among them, the DFB laser is considered a most convenient method because of its stable and highly reliable single-mode operation. However, if to realize different channel wavelengths by adjusting the grating periods, the gratings should be fabricated in a 0.1 nm precision, which is extremely difficult even for electron-beam lithography. Here, we fix the DFB grating period on silicon waveguide but vary the width of silicon waveguide to realize the single longitudinal mode and then the varying-wavelength laser array. As shown in earlier reports [19], the variation in the ridge width would lead to a slight difference in the effective index and then in the lasing wavelength. Such width-varying DFB waveguides can be made by standard photolithography and holographic lithography. This paper reports a detailed study of distributed feedback InGaAsP-Si evanescent laser based on selective area metal bonding (SAMB) method. Compared to the other bonding methods to developing silicon evanescent lasers, the bonding step is performed after all the silicon components and III-V ones are finished in parallel, avoiding the crossover of the silicon and III-V process flow.

2. Device design and fabrication

The schematic structure of the InGaAsP-Si hybrid laser is shown in Fig. 1. A buried heterojunction InGaAsP quantum well epitaxial structure is flip-chip bonded onto a patterned SOI wafer. The III-V structure is grown on the p⁺-type InP substrate by two-step metal organic chemical vapor deposition (MOCVD). In the first growth step, a p-InP buffer layer and an i-InP layer are grown on the substrate, followed by a lower separate confinement heterostructure (SCH) layer, a multi-quantum well (MQW) active region, and an upper SCH layer. Their doping levels and thicknesses are shown in Table 1. Then a ridge stripe of 3.0 μm width in the InGaAsP layers down to the p-InP layer is formed by photolithography and wet chemical etching. Then a 500 nm thick n-InP layer is grown as the current injection layer. After that, He⁺ implantation is carried out into the p-InP layer to create highly resistive regions on both sides of the buried stripe. To further improve the current confinement, a SiO₂ layer is deposited on the n-InP layer with a 20 μm wide window opened on top of the buried stripe. After depositing AuGeNi/Au contact on the surface, a 4.0 μm width optical coupling window is opened right above the active area. Then, after substrate thinning, an AuZn alloy layer is thermally evaporated on the back side of the p-InP substrate and the wafer is annealed at 420 °C for 35 s to form ohmic contact.

 

Fig. 1 Schematic drawing of the InGaAsP-Si hybrid laser (not to scale).

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Table 1. Structure of first epitaxial layers

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In the SOI region, the silicon waveguide has a height of 0.5 μm and width of 3.0 μm, formed on an undoped SOI substrate using photolithography and HBr/He/O₂ based inductively coupled plasma (ICP) etching. Then gratings of 237 nm period are formed by holographic lithography and a second ICP dry etch. The resulting gratings have a 25 nm etching depth and 80% duty cycle as shown in Fig. 2. Then metal lift-off technology is adopted to selectively deposit the bonding metal on each side of the silicon waveguide. The bonding metal is Indium, which is widely used in metal bonding and packing due to its low melting point and good fluidness. Finally, the III-V wafer is diced into bars with a length of 300 μm and flip-chip bonded to the SOI wafer by a Finetech Lamda A6 bonder with alignment accuracy ± 0.5 μm. The bonding process is done at 200 °C for 5 mins under a pressure of about 2 MPa in N₂ atmosphere. This structure results in that the fundamental transverse mode exists mainly in the silicon waveguide. The effective refractive indicies of the unetched and the etched regions are estimated using the finite differential in time domain (FDTD) solutions (Lumerical Solution Inc., Canada), resulting in a grating coupling constant κ of ~137 cm⁻¹.

 

Fig. 2 (a) Calculated TE fundamental mode profile. (b) Near field image of lasing mode. (c) Typical curves of voltage and light output power versus pulsed injection current of the hybrid laser at room temperature. (d) Scanning electronic microscopic image of the DFB silicon waveguide.

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3. Experimental results for the single longitudinal mode laser

Figure 2(a) illustrates the calculated TE fundamental mode profile by COMSOL Multiphysics using the device dimensions stated above. From calculation we can see a majority of the optical mode of the hybrid waveguide lies in the silicon waveguide. Noticeably, the higher the confinement factor in silicon, the higher the lasing threshold. Figure 2(b) shows the near field image from an end facet of the hybrid laser under pulsed current taken by an Olympus infrared microscope. We can see the lasing mode gained in the InGaAsP MQW structure has been coupled into and propagates in the silicon waveguide. The slight difference between the calculated mode and the measured one lies in that gain in MQWs is not taken into account in calculation and perhaps that the focus plane of the microscope is not strictly on the end facet of the hybrid laser.

Then the laser is tested at room temperature with a pulsed current at a repetition rate of 1 kHz and 0.2% duty cycle. The output of the laser is measured at one facet of the device using a large-area photodetector positioned in close proximity to the facet, so that all the emitted optical power from one facet is collected. Figure 2(c) shows the light output power and voltage versus pulsed current. The series resistance and threshold current density of the hybrid lasers are 10.6 ohms and 5.56 kA/cm², respectively. Considering two facets of the laser, the power slope efficiency is about 0.006 W/A. Figure 2(d) is the Scanning electronic microscopic image of the silicon waveguide with DFB gratings.

Figure 3(a) shows the measured single longitudinal mode lasing spectrum of the silicon evanescent laser with 100 nm span driven by 75 mA pulsed current at room temperature. The spectrum was collected by a confocal μ-Raman spectrometer (HR 800, Horiba Jobin Yvon, France). The laser diode has a lasing peak of 1546.1 nm and a side-mode suppression ratio (SMSR) of ~20 dB. Furthermore, the cleaved InGaAsP gain structure before bonded to SOI wafer has a spectrum as shown in Fig. 3(b) under the same test conditions. We can see that it acts as a Fabry-Perot laser with the working wavelength around 1562 nm before bonding. Compared to Fig. 3(a), we can conclude that the Bragg gratings in silicon waveguide have played the role of the distributed reflector in the InGaAsP-Si hybrid laser. Although no λ/4 phase shift is designed in the middle of the grating, the mode degeneracy inside the DFB gratings is broken perhaps by different reflections of the two ends of the hybrid laser.

 

Fig. 3 (a) The lasing spectrum with 20 dB side mode extinction ratio at 75 mA pulsed current at room temperature. (b) The lasing spectrum of III-V laser before bonding under the same test conditions.

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4. Four-λ InGaAsP-Si hybrid lasers with varying waveguide width

To demonstrate the high scalability of this hybrid laser method for WDM optical system, we achieve a four channel laser array just by varying the width of silicon waveguide. The effective refractive index of the III-V/Si hybrid waveguide as a function of the silicon waveguide width is shown in Fig. 4, calculated by FDTD. It shows that the effective refractive index increases with the silicon waveguide width, and increases more rapidly below 2.5 μm. To guarantee the coupling efficiency, the widths of the silicon waveguide are chosen 3.0, 2.6, 2.3 and 2.1 μm which corresponds to the calculated effective indexes of 3.262, 3.259, 3.255 and 3.251, respectively and then to the lasing wavelengths as indicated in Fig. 4. The wavelength spacing is 1.4, 1.9 and 1.9 nm estimated by Δλ = 2(Δη_effΛ), where Λ = 237 nm is the Bragg gratings period. Different optical communication band can be obtained by just tuning the grating period. The silicon waveguides of different widths can be fabricated in the first photolithography and ICP dry etching step. The lasing spectra of the devices at about 80 mA are shown in Fig. 5(a). The lasing wavelengths are 1546.1, 1544.4, 1542.1, and 1539.9 nm respectively with a SMSR around 20 dB. There is some difference in wavelength between the measured values and the calculated values because there exists fabrication and calculation errors. The fabrication tolerance of the silicon waveguide width and grating constant are 10 nm and 0.2 nm respectively, which leads to a wavelength error of 1 nm in total. The wavelength spacing between two adjacent channels are 1.7, 2.3, 2.2 nm, respectively, where the resolution of the spectrometer is 0.1 nm. Because the effective refractive index of the hybrid waveguide is not linearly varied with the silicon waveguide width, a fixed wavelength spacing can be obtained by nonlinearly varying the silicon waveguide width. Furthermore, narrower wavelength spacing can be achieved by reducing the silicon waveguide width interval. Figure 5(b) shows the imaged output facet of the four InGaAsP/Si evanescent laser array operating simultaneously. The devices are driven at an identical voltage. More channel hybrid lasers with different silicon waveguide width can be fabricated to meet the needs of DWDM systems using this approach.

 

Fig. 4 Effective refractive index of hybrid waveguide versus silicon waveguide width. The four marked points are the designed parameters for the four channels with the grating period 237 nm.

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Fig. 5 (a) Spectra of the four lasing channels. (b) Infrared microscope image of the four lasing lasers.

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5. Summary

4-λ InGaAsP-Si distributed feedback evanescent lasers are fabricated by varying silicon waveguide width. It can be easily scaled up to 8-λ lasers by increasing the silicon waveguide width number and reducing the silicon waveguide width difference. Only photolithography and holographic lithography are used for the fabrication of silicon waveguides. The simplicity and flexibility of the fabrication process and high yield based on selective area metal bonding provides an energy efficient way to fabricate multi-wavelength laser array for WDM systems on chip or board.