We report on the design, fabrication and performance of a hetero-integrated III-V on silicon distributed feedback lasers (DFB) at 1310 nm based on direct bonding and adiabatic coupling. The continuous wave (CW) regime is achieved up to 55 °C as well as mode-hop-free operation with side-mode suppression ratio (SMSR) above 55 dB. At room temperature, the current threshold is 36 mA and the maximum coupled power in the silicon waveguide is 22 mW.
© 2015 Optical Society of America
The need for high data transmission rates keeps increasing and this trend is unlikely to slow down in the foreseeable future. In this context, doubts are casted on the capability for current copper-based interconnects and electronics circuits to follow this tendency. Shrinking microelectronics nodes gets harder both because of the density limit and the heating managements within the circuits, which become increasingly expensive to cool. Power-efficient optical interconnects appear as a way to withstand the relentlessly growing bit stream.
The refractive index difference between silicon and its native oxide (SiO2) is high enough to tightly confine an optical mode within silicon waveguide cores, consequently a great potential arises from this material for optical applications. Moreover, the interest for silicon photonics is growing [1,2], since it would leverage the know-how acquired within mature complementary metal oxide semiconductor (CMOS) technology in processing silicon. It appears as one of the most promising technology to functionally integrate photonic integrated circuits (PICs) with electronic driving devices on reduced footprints using 3D-CMOS processes.
Although silicon is a poor light emitter due to its indirect bandgap, making lasers out of silicon has been investigated during the last ten years showing promising results. The demonstration of a continuous wave silicon laser based on Raman scattering  incites to keep looking in this direction as well as the use of strained heavily doped Ge as a gain-enabler material . However, both solutions need further development to be power-efficient.
One of the most mature wafer-level solutions to provide light to the PIC is to bond III-V-gain layers on silicon on insulator (SOI) wafers. The bonding can either be adhesive with polymers used as bonding layer , or direct using a thin silica layer to stick the III-V and the silicon wafers together via covalent SiO2-SiO2 low temperature bonding [6–9]. Light is thus generated and amplified in the III-V gain material and other optical functions, such as the laser feedback cavity, are implemented within the high-index-contrast SOI photonic circuitry. Such a design allows to easily integrate the hybrid optical source with other building blocks already demonstrated: optical resonators and filters , input/ouput (I/O) couplers [11,12], high-speed modulators [13,14], Si-Ge photodiodes , and wavelength (de)multiplexers .
One approach in III-V/Si laser architecture lies in designing the hybrid active region so that the optical mode stays mainly confined within the underlying silicon waveguide, interacting with the III-V quantum wells (QWs) only with its evanescent tail [5–7] which limits the modal gain. Though such a configuration is subject to less complexity regarding coupling, the amplification is restricted and the bonding layer thickness is critical. In our approach, we opted for a design that uses the III-V gain region as efficiently as possible by having the optical mode confined in the QWs [8, 9]. To allow for the mode to transit from the III-V to the silicon waveguide, adiabatic mode transformers were designed using the deterministic adiabaticity criterion demonstrated in .
We report herein hybrid III-V on silicon DFB lasers operating at 1310 nm based on direct bonding and adiabatic coupling between both materials. The article is organized as follows. Section 2 describes the laser architecture and the design of its different components with a numerical analysis for each of them, while the fabrication process is outlined in section 3. The results, revealing the level of maturity of the heterogeneous III-V/Si DFB lasers in the O-Band, are given in section 4 focusing on the DFB laser static and thermal features.
2. Laser architecture and design
2.1 Overall structure
Schematic longitudinal and transversal cross-section views of the device are represented on Fig. 1. The III-V and the silicon waveguide are separated by a SiO2 gap of 75nm. The quarter-wave shifted (QWSh) DFB grating is etched along the 500-nm-thick silicon waveguide underneath the III-V active layers. The active region is 700-µm-long and consists of InGaAsP multiple QWs (MQWs) exhibiting maximum gain centered on 1310 nm, surrounded by p- and n-doped InP layers. The silicon rib is 200-nm-thick and needs to be narrow enough to confine the light mostly in the III-V QWs. The mode overlap with the MQWs and the barriers is calculated to be 0.35. In order to maximize the modal gain, carrier injection is concentrated in the center of the III-V active waveguide thanks to H+ “resistive” doping on the sides of the 5-µm-wide InP ridge. At both terminations of the grating, the silicon waveguide is widened adiabatically, enabling light to be coupled into the silicon with more than 90% efficiency, allowing as much optical power to be produced at both outputs of the laser. Those 100-µm-long mode transformers are a key point in the design of the device. Laser light emission is collected with a fiber positioned on the top of a waveguide-to-fiber surface grating coupler.
2.2 Adiabatic III-V to silicon transition
In order to obtain the optical mode coupling from the III-V to the silicon at the output of the gain region, an adiabatic taper is designed in the silicon waveguide as explained below . Calculations of the effective indices are made with a two-dimensional (2D) finite elements method (FEM) solver, first for the isolated modes in the silicon waveguide and in the III-V, then for the modes of the coupled structure, referred to as supermodes. The normalized coupling constant (γ) is found as a function of the silicon waveguide width (Wrib):Eq. (1-3), an interpolation leads to Wrib = f(γ). The shape of the taper is then deduced from the adiabaticity criterion demonstrated in :
Once the shape is obtained, an optimization of the input and output width of the taper needs to be implemented. Figure 2 represents a schematic view of the taper with a picture of the calculated supermode at both ends: at the input it stays mainly confined within the III-V region showing a good overlap with the MQWs while at the output, the mode is very well confined in the silicon. Two insets are added in Fig. 2 displaying the field intensity of the even and odd supermodes at an intermediate width. The taper aims at transferring all the power in the even supermode and with only a few percent (ε) in the odd one. The taper coupling efficiency is calculated using a beam propagation method (BPM) solver leading to the coupling efficiency map shown in Fig. 3(a). It is important to have a robust design regarding parameters which depend strongly on fabrication processes. For this taper, the influence of the output width is very limited, while that of the input width is more important but remains within the process variations window. The adiabaticity of the taper is checked plotting the coupling efficiency as a function of the taper length as in Fig. 3(b). It shows that once the maximum coupling efficiency is reached, there is almost no back-coupling oscillation contrary to what is observed with directional couplers for instance. The remaining oscillations with amplitude ε are due to the coupling to the unwanted odd supermode.
2.3 Distributed-feedback cavity design
The cavity in the DFB is characterized by a grating underneath the active region where the top of the silicon waveguide is partially etched as represented on Fig. 4(a). Three parameters define the grating: the rib width, equal to the taper input width, the etching depth d and the period a. The optical mode being confined within the III-V along the cavity, the coupling strength between the grating and the mode needs to be determined to ensure a high enough cavity quality factor and achieve single-mode operation. This strength is described by the grating coupling constant (κr), calculated using the equation derived from :Fig. 4 (a).
Based on the calculated reflectivity for distributed Bragg reflector (DBR) lasers , the κrLg product is estimated to be within the 1-1.6 range for a DFB, Lg being the grating length. For a grating length of 500 µm, κr has then to fit between 20 cm−1 and 30 cm−1. To determine such κr values, the even supermode refractive index of the coupled structure is calculated using a FEM mode solver for different etching depths and silicon waveguide widths. The results are shown on Fig. 4(b). Above 70 nm of etching depth, the coupling strength evolution is insignificant. On the contrary, the silicon rib width has much more effect on κr. This trend was expected because the wider the silicon waveguide is, the stronger is the overlap for the mode with the DFB grating. For the laser described here, a width of 0.68 µm is selected which, for an 80 nm etching depth, leads to κr = 25.5 cm−1 and a κrLg value of 1.28, while the taper coupling efficiency stays between 90 and 100%. Using the same method, the grating coupling constant of the odd mode was calculated to be less than 0.2 cm−1 for these dimensions showing that the effect of the grating on this mode is much lower than on the even one.
A grating period of 202 nm was then calculated using the Bragg Law in order to operate at 1310 nm:Fig. 4(a).
3. Laser fabrication
The laser front- to back-end processing can be summarized as follows. First, a 200-mm SOI wafer is processed with four successive 193-nm-deep UV (DUV) photolithography and reactive ion etching (RIE) steps. Figure 5 illustrates the first three steps. The 2nd and 3rd levels are defined using SiO2 hard masks while photoresist is sufficient for the two other levels. First the DFB grating is etched, then the rib etching ensues. The third level corresponds to the definition of the fiber coupler grating. A last phase consists in separating all the lasers on the wafer, etching all the silicon left around the devices.
The patterned SOI wafer is encapsulated with 700 nm of SiO2 afterward, followed by chemical-mechanical polishing (CMP) which allows planarisation of the wafer as well as aiming at the 75 nm SiO2 gap. The III-V epitaxial structure to bond is depicted on Table 1 without the InP substrate. The MQWs are separated from the n- and p- layers by separated confinement layers (SCH) and superlattice layer are added within the n-contact to prevent the defect propagation from the bonded interface to the QWs. Both the III-V and the SOI surfaces are activated through oxygen plasma and then put in contact at room temperature. Though room temperature is sufficient to ensure bonding, a 180 minutes post-bonding annealing is performed at 200 °C to reinforce it. A picture of the bonded wafers is shown on Fig. 6(a). The subsequent steps comprise the InP substrate removal using HCl/H2O wet etching, the H+ implantation and the wafer downsizing from 8 to 3 inches.
The III-V waveguide is then defined with CH4-H2 dry etching followed by the p- and n-type contact deposition thanks to a lift-off method. Afterward, the 2-inches-wafer is covered by 3-µm-thick BCB polymers used as encapsulation layer which is open to reach the contacts and finally, TiPtAu metallic pad are deposited. An optical microscope picture displaying one of the devices at the end of the whole fabrication process is represented on Fig. 6(b).
Laser operation with a classic DFB signature was demonstrated on the whole wafer, however a statistic study will be the object of future work and we chose here to focus on both static and thermal characteristics of one particular device. The chip was mounted on a Peltier module to set its temperature from 20°C up to 80°C. The output power was measured collecting the light from the surface grating coupler with a multimode fiber (MMF). Electrical pumping is ensured applying a positive bias on the p-contact of the laser. L-I curves in CW regimes for different temperatures are presented on Fig. 7, showing lasing effect up to 55 °C.
As represented on Fig. 8(a), the room temperature (RT) threshold current is 36 mA which corresponds to a 1.03kA/cm2 current density for a 700-µm-long and 5-µm-wide active region. The maximum output power is then 2.8 mW in the waveguide. The fiber coupling losses were measured to be 6 dB. We can therefore conservatively assess an output power of 11 mW coupled into the silicon waveguide and 22 mW if we consider both outputs. The L-I slope being 0.24W/A, the resulting differential quantum efficiency of the laser is 25%. Moreover, the laser diodes are characterized by a turn-on voltage of 1.23 V and a series resistance of 10.5 Ω.
Figure 8(b) displays the laser emission spectrum at 107 mA driving current at RT. Single-mode operation is reached with more than 56 dB of side mode suppression ratio (SMSR) which is the highest value for III-V/Si DFB lasers to the best of our knowledge. As shown in Fig. 9(a), the device operates single-mode over a current range of 170 mA until the very end of the roll-off region for different stage temperatures (25 °C, 35 °C,45 °C,55 °C). Such a structure might allow competition between the even and odd mode and the fact that the spectrum is mode-hop-free proves that the taper is adiabatic and gets rid of the odd mode, as expected. Furthermore, such a spectral purity demonstrates the uniformity of the grating and the QWSh. Had there been other defects in addition to the QWSh in the grating, modal competition with other defect-modes may have been observed.
To complete the thermal analysis, measurements were implemented in pulsed regime (0.1% duty cycle and 1 ms pulse repetition) to limit heating from self-induced power dissipation in the laser. Measurements of the threshold for each temperature from 15 °C to 75 °C led to a characteristic temperature T0 of 44 °C which is coherent with the fact that the laser effect stops for temperatures higher than 60 °C. A linear fit of the peak wavelength shift versus stage temperature in pulsed regime gives (Δλ/ΔT)DC0.1%,1ms = 0.08 nm/°C as shown in Fig. 9(b). Since the peak wavelength shift versus dissipated power in CW regime is (Δλ/ΔP)CW = 3.72nm/W (Fig. 9(c)), the thermal impedance, defined as the ratio of both values, is ZT = 44.1°C/W.
This work reports on the recent breakthroughs toward the maturity of III-V on Silicon lasers emitting in the O-band for datacom applications. A 700-µm-long QWSh distributed feedback laser was demonstrated operating in the continuous-wave regime up to 55°C, with a current threshold of 36 mA at 25°C and maximum output power of 22 mW available for the Si-PIC transmitter. The investigation is extended to a thermal analysis showing a laser characteristic temperature of 44°C and a laser thermal impedance of 44.1 °C/W.
Current and future works are mainly focused on the improvement of maximum operation temperature, refining the fabrication process, and the co-integration of the laser either with a III-V on silicon electro-absorption modulator or with a SOI Mach-Zender modulator. Development of the whole process on the 200-mm-platform as well as 3D wafer-level packaging are ongoing.
The authors would like to thanks K. Ribaud and P. Grosse for their help on device characterization. This work was supported by the French national program ‘programme d’Investissements d’Avenir’, IRT Nanoelec ANR-10-AIRT-05.
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