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In this paper, a III-V/Silicon hybrid single mode laser operating at a long wavelength for photonic integration circuit is presented. The InGaAlAs gain structure is bonded onto a patterned silicon-on insulator wafer directly. The novel mode selected mechanism based on a slotted silicon waveguide is applied, which only need standard photolithography in the whole technological process. The side mode suppression ratio of larger than 20dB is obtained from experiments.
©2013 Optical Society of America
1. Introduction
The hybrid silicon platform(HSP) is a promising approach to enable robust active components on a complementary metal-oxide semiconductor (CMOS)- compatible Si platform. Many approaches to light emission in silicon have been demonstrated including Raman lasers [1], nano-patterning [2], nanocrystalline-Si structures [3, 4], micro-disk laser [5, 6], distributed feedback(DFB) laser [7, 8], silicon evanescent laser [9]. An electrically pumped light source on silicon is a key element needed for photonic integrated circuits on silicon. For the electrically pumped on-chip light source, direct III-V/ silicon-on insulator (SOI) wafer bonding and flip-chip bonding are developed quickly [8, 10, 11]. In the case of the latter approach, two devices of different dimensions and/or materials have to be aligned to sub-micron precision to enable efficient coupling. For the direct III-V/SOI wafer bonding, though a chemically ultraclean and atom-scale smooth surface are needed, a large scale devices can be fabricated with a single bonding step. The on-chip light source fabricated on HSP employs a ring, DFB, or DBR (distributed Bragg reflection) resonator [10], which is defined on SOI or III-V material. Normally, high-resolution CMOS process is rare in the academic research environment. So the expensive, time consuming or not optimal for high volume manufacturing technique must be used, such as e-beam, holographic, regrowth, focused ion beam, deep ultraviolet and so on. If only standard 1:1 photolithography is used in the whole technological process, development cycle and cost may be reduced greatly. In fact, for avoiding the high-resolution process, complex regrowth steps and expensive electron-beam lithography, a single-mode laser has been presented in theory, which relies on slots on one end of the laser to provide the reñection [12] and these slots can be fabricated through standard low cost semiconductor process. However, for III-V/SOI hybrid architecture, the slotted hybrid laser is demonstrated for the first time in this work. In this paper, the design and the fabrication of the slotted feedback single longitudinal mode hybrid laser are presented and the whole process is standard, low cost and simple. At the temperature of around 210 K, continuous wave lasing of single longitudinal mode with the side mode suppression ratio (SMSR) of larger than 20dB is realized.
2. Design and simulation
The schematic structure of the slotted III-V/Si hybrid laser is shown in Fig. 1(a) . The III-V epitaxial structure of the laser is shown in Table 1 , which is a 12μm-wide III-V ridge. The active layer consists of eight AlGaInAs quantum wells which have an emission peak around 1535 nm at room temperature and around 1490 nm at the temperature of about 210K. The SOI material with a 340nm-thick top silicon and 2μm-thick buried oxide (BOX) is used. The evanescent strip waveguide is formed on the top silicon as shown in Fig. 1(b), whose width is about 1~3μm according to coupling efficiency and the width of the trenches surrounding the waveguide is 3μm. There are multi-section uniformly distributed slots on the core of the evanescent silicon waveguide, which act as a feedback cavity of the laser. The action of these multi-section uniformly distributed slots is like a DFB or DBR. However, the period of DFB or DBR is round sub-wavelength. And the width and period of slots is larger than 1μm, which can be defined by a standard photolithography. In the simulation, the facet at the end of the slot region is antireñection coated (no reñection is assumed in the following simulation).Under the effective index approximation, the three-dimensional (3-D) ridge waveguide is reduced to a two-dimensional (2-D) structure as shown in Fig. 1(c), where AlGaInAs multi-quantum wells region has an average refractive index of 3.517 with thickness of 0.146μm . The P contact and InP cladding of 1.6μm has an effective index of 3.16 in the 2-D situation. The contact layer to bonding buffer layer has an effective index of 3.16 with thickness of 0.15μm. For SOI part, the index of Si and SiO2 is 3.47 and 1.47, respectively. For different waveguide widths, the effective index of top Si 2D waveguide layer varies. The slot parameters such as the slot width, spacing, depth and slot number are important parameters which affect the laser performance and have to be optimized in the design. In the simulation, the scattering matrix method with a perfect matched layer absorbed boundary condition is used [12]. The fixed slot depth and slot number are 320nm and 60, respectively.
Fig. 1 (a) Three-dimensional schematic structure, (b) Si strip waveguide and (c) y-z cross section of the slotted single mode hybrid laser.
Table 1. Layer structure
The contour plots of the calculated amplitude reñection are given in Figs. 2(a) -2(b), which shows that the slot width and period affect the reñection strongly. It is obvious that a number of local maximum values of amplitude reflection appear with the period and width of slot varying, which locates at different wavelengths. As the slot width of 1100nm and the period of 5700nm are selected, the peak wavelength of 1490nm of amplitude reñection can be obtained. Due to the different reflection amplitudes with wavelength varying, single mode can be realized under special injection currents. In fact, the slotted hybrid laser is based on Bragg’s condition with high-order grating [12]. For the chosen parameters of slots, the slot section acts as a 26th-order grating. We are interested in the slot width around 1 μm that can be easily fabricated by standard photolithography.
Fig. 2 Contour plot of simulated amplitude reñection versus (a) slot period and wavelength with slot width of 1100nm and (b) slot width and wavelength with slot period of 5700nm.
3. Fabrication and results
The slotted III-V/Si hybrid laser is fabricated using an AlGaInAs quantum well epitaxial structure as shown in Table 1 that is bonded to a Si strip waveguide with multi-section uniformly distributed slots. The patterned top silicon waveguide structure is formed on the (100) surface of a silicon-on-insulator substrate with a 2 μm thick BOX using standard projection photolithography and plasma reactive ion etching. The patterned top strip silicon waveguide structure is fabricated with a height of 0.34 μm and width of 3μm. The fundamental optical mode confinement factors in the silicon waveguide, III-V layer and the multiple quantum wells structure are 0.47, 0.53 and 0.0596, respectively.
The period of slots is 4-10μm and the width of slot is 1-2μm. The quantum well active layer is bounded by the separate confinement hetero-structure (SCH) on the side of P-type layer. The super lattice region is used to inhibit the propagation of defects from the bonded layer into the quantum well region. This III-V structure is then transferred to the patterned SOI wafer through direct wafer bonding at low temperature of 350 °C. The low-temperature bonding process starts from rigorous wafer surface clean in H2SO4:H2O (3:1) and NH4OH (39%) solutions for SOI and InP wafers, respectively. Upon native oxide removal in the HF solution, the SOI wafer experiences an O2 plasma surface activation for 30 seconds, followed by the same treatment for the InP wafer as well. Wafers are then cleaned again with deionized water spray rinse which also serves as the final surface activation step to terminate the surface with –OH groups. The samples are annealed at 350 °C with an applied pressure of 2MPa for 10 hours. After InP substrate removal with HCl, 12 µm wide mesas are formed using photolithography and by Br2/HBr, HCl/H2O and H3PO4/H2O2 selective wet etching of InGaAs contact layer, InP cladding layer and the quantum well layers, respectively. Then a layer of SiO2 insulator with the thickness of 200nm is deposited and through one time photolithography, p-type and n-type contact window are etched using HF. After that, Ti/Au are deposited onto the whole wafer. Through photolithography and wet etching p-type and n-type electrode pads are formed. 6 µm wide Ti/Au p-type current channels are formed in the center of the ridge waveguide. After that, the wafer is diced without any polishing and 1000 µm long laser cavity is formed.
A cross-sectional SEM (Scanning Electron Micrograph) image of the final fabricated hybrid laser is shown in Fig. 3(a) . A nice bonding between SOI with waveguide and III-V material can be found. The electrode pads distribution is shown in Fig. 3(b).
Fig. 3 (a) SEM of the cross-section of III–V material bonded on a Si strip waveguide. (b) The electrode pads distribution.
The output power of the laser is measured at one facet of the device using a large-area photo detector positioned in close proximity to the facet. Devices are mounted on a copper plate. The sample is put into low temperature Dewar equipment. The following results are measured at a temperature of around 210K. Infrared micrographs taken from an end facet of the devices is shown in Fig. 4(a) . Simulations are performed to check the shape of fundamental as shown in Fig. 4(b). Confinement factor in MQW region for fundamental mode are 0.0596. For 12μm wide mesa, in a normal FP laser, high-order modes might have higher confinement factor and lead to high-order mode lasing. However, the structure with slots presents much feedback for the fundamental mode than other high-order modes. And as shown in Fig. 4(a), fundamental mode is lasing at first. For a 1000 μm long device with slot width of 1100nm and slot period of 5.8μm, the threshold current is 40mA(corresponding to a current density of 333A/cm2), the maximum optical power is 0.6 mW, while the slope efficiency is 6 mW/A, as shown in Fig. 5 . At the current of 200mA, the laser reaches saturation. I-V characteristic of the laser is also shown in Fig. 5. At the current of 65mA, the voltage is 2V and 3V for 200mA. Then, the differential resistance is about 7.5Ω, which is due to a long distance lateral injection.
Fig. 4 (a) Infrared micrographs taken from an end facet of the devices. (b) The fundamental mode field distribution.
The results show that optical output power and slope efficiency is relatively low and operating at low temperature. There are several factors leading to the results. The first is high losses, which include internal loss, bonding interface non-radiative recombination loss and slots scattering loss. The reflection and transmission amplitudes calculated for 60 slots at 1490nm are 0.47 and 0.19, respectively. Accordingly, the loss introduced by the slots for a cavity length 1000μm is ~8.3cm−1. Slots need to be optimized from the number and the shape. A periodic cross-shaped structure is better. Then bonding interface needs to be improved through surface process and wafer bonding condition. The second reason is high thermal and electrical resistance. Because the Si strip waveguide is surrounded by 340nm deep and 3μm wide trenches and the bottom of trenches is BOX layer and not silicon (the thickness of top Si is also 340 nm), bad thermal conductivity is obvious and a thicker top Si layer may be needed. Differential resistance need to be reduced through optimizing material structure and electrode distribution. In addition, 12μm wide mesa is too large for current injection and 4μm-wide current injection region is enough. If the p-region on the two sides of the mesa are implanted with protons (H + ), the non-conductive p-type mesa will prevent lateral current spreading in the p-type mesa. These methods can be used to improve performances of our slotted hybrid laser. In fact, our recent experimental results have verified these points. Through fully optimizing the number of the slot and the injection current, the operating temperature of the slotted laser can reach 273 K and detailed work will be reported in a new article.
To measure the optical spectrum of the device, we coupled the hybrid laser output into a 62.5/125μm fiber connected to an ADVANTEST Q8384 optical spectrum analyzer with a resolution of 0.01nm. Typical optical spectrum of the laser in continuous wave (CW) regime is given in Fig. 6 . At currents of 90mA, 110mA and 125mA, three optical spectrums are given for comparison and the wavelength of lasing is 1487.6nm, 1489.4nm and 1492.1nm, respectively. The red shift of the peak wavelength is obvious with an increase of the injected current. For our slotted Si hybrid laser, the differential resistance is about 7.5Ω, which is due to a long distance lateral injection. Moreover, 2 μm-thick BOX layer is not conducive to the heat. Then, the hybrid laser is with high thermal and electrical resistance. At higher current level, local temperature of the chip increases fast. Our low temperature Dewar equipment can’t give enough control, which is causing the large red-shift of lasing spectrum. The SMSR of larger than 20dB is realized at the current of 110mA. From the experimental results, we find that multi-section uniformly distributed slots affect the mode output strongly. The experimental results agree well with that of calculation as shown in Fig. 2. As the current is larger than 150mA, the instability of mode can be observed from the small fluctuations in the L-I curve.
4. Summary
In a conclusion, the on-chip laser is important device for HSP. A slotted hybrid laser with evanescent silicon waveguide coupling output is designed and fabricated. The results show that multi-section uniform slots can provide a mode selected mechanism. The hybrid laser only needs standard low cost photolithography in the whole technological process. The SMSR of larger than 20dB is obtained from experiments.
Acknowledgments
This work is supported by the Chinese National Key Basic Research Special Fund (Grant No. 2012CB933501 and 2011CB922002), the NSFC (Grant Nos. 61274070, 61021003, 61234004, 61025025, 61137003 and 60838003) and the National High Technology Research and Development Program of China (Grants No. 2012AA012202).
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