1. Introduction

Since a silicon on insulator (SOI) substrate has high index contrast between the silicon core (n=3.45) and the oxide buffer layer (n=1.45) and it can be utilized to fabricate ultra compact optical circuits at the low-loss fiber communication wavelengths, integrations of functional photonic devices such as lasers, optical amplifiers, electro-absorption modulators etc. on SOI platforms are very attractive. Room-temperature continuous-wave (RT-CW) operations of semiconductor lasers fabricated on a silicon substrate by heteroepitaxial growth [1] and direct wafer-bonding [2] have been reported, however, it is difficult to couple light outputs to a silicon waveguide because of their thick cladding layers. Since membrane-based DFB lasers are very attractive for low threshold operation as well as good single-mode lasing properties [3–6], we have been investigating the possibility of direct wafer-bonding on an SOI substrate and obtained high quality membrane structure [7]. Recently, Park et al. reported lasing of an Al-GaInAs membrane laser on an SOI waveguide (silicon evanescent laser) using low temperature oxygen plasma-assisted wafer bonding [8].

In this paper, we would like to report a demonstration of GaInAsP/InP buried heterostructure membrane distributed feedback (BH-DFB) laser directly bonded on an SOI substrate using the direct bonding method.

2. Device structure and fabrication process

The device structure of GaInAsP/InP membrane DFB laser on an SOI substrate is shown in Fig. 1. A single-quantum-well, consisting of a Ga_0.22In_0.78As_0.81P_0.19 1% compressively-strained well (7 nm thick) sandwiched by 2-step optical confinement layers of Ga_0.22In_0.78As_0.48P_0.52 (λ_g=1.2 μm, 25 nm thick) / Ga_0.14In_0.86As_0.31P_0.69 (λ_g=1.1 μm, 50 nm thick), was grown on a (100) n-InP substrate as an initial wafer by OMVPE technique. The DFB structures were fabricated on this initial wafer by the previously reported processes [3–6].

 

Fig. 1. Illustration of membrane DFB laser on SOI substrate.

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Figure 2(a) shows a cross-sectional scanning electron microscope (SEM) view of etched and embedded active regions after the OMVPE regrowth. The typical wire width (W) of the DFB structure (period Λ =300 nm) was measured to be around 145 nm. In this experiment, a thick cladding layer was not grown because an efficient optical coupling into silicon waveguide on SOI structure was the primary concern. We calculated the coupling coefficients between the membrane laser and the silicon waveguide. The coupling coefficient is maintained around 70 % even at SOI thickness of 60 nm. Therefore, we selected 60nm SOI thickness to fabricate membrane DFB laser.

The SOI substrate used in this study has a 60-nm-thick silicon layer bonded on a 2-μm-thick SiO₂ layer. The SOI substrate was first cleaned with H₂SO₄:H₂O₂, followed by a HF dip, then the surface turned to hydrophobic. After this cleaning process, the SOI substrate was dipped in a H₂SO₄:H₂O₂:H₂O solution at 50 °C to make the surface hydrophilic, which is important for self-adhesion at RT. After cleaning the GaInAsP/InP wafer with a HCl:CH₃COOH solution, its surface turned to hydrophilic. The hydrophilic surfaces were contacted in deionized water. The substrates adhered to each other at RT after N₂ drying and their surfaces were placed in contact under a pressure of 4 kg/cm². The typical size of the GaInAsP/InP and SOI substrate were 5 × 5 mm² and 10 × 10 mm², respectively. Then the substrates were loaded into a furnace and heated in a H₂ atmosphere at 450 °C for 60 minutes. A weight of 17.5 g was applied to the wafers during the thermal annealing. Finally the n-InP substrate, GaInAs etch-stop layer and InP etch-stop layer were selectively etched to leave a thin active layer on the SOI substrate by HCl:H₂O, H₂SO₄:H₂O₂:H₂O and HCl:H₂O, respectively. Figure 2(b) shows a cross-sectional SEM view of the GaInAsP/Si interface bonded at 450 °C. A smooth interface between GaInAsP membrane layer and the SOI substrate is observed.

 

Fig. 2. Cross-sectional SEM view of (a) DFB structure before bonding and (b) the bonding interface between GaInAsP membrane and SOI.

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3. Lasing characteristics

The device was optically pumped from the top by 980 nm wavelength CW laser diode with a single-mode fiber connected to a micro-PL measurement setup. The spot size of the pump light was focused by using a cylindrical lens to a spot size of 4 μm × 176 μm. The output power was coupled into a multi-mode fiber connected to an optical spectrum analyzer. The coupling loss to the optical fiber of output power was estimated to be about 10 dB. Figure 3 shows a fiber-coupled laser mode power as a function of the pump power measured on the surface of the focusing plane. A threshold pump power P _th of 2.8 mW was obtained. Since the actual device size was 2 μm (width) × 120 μm (length), an effective threshold pump power is calculated to be 0.96 mW.

Figure 4 shows a lasing spectrum measured at a pump power of 13 mW (5.7P _th) A clear single-longitudinal mode operation was observed as shown in Fig. 4. A single-mode spectrum observed at an emission wavelength of 1579 nm with a sub-mode suppression ratio (SMSR) of 28 dB at 5.7P_th. The calculated spectra of the fundamental, the first-order and the second-order transverse modes at a bias level of 0.99 times the threshold is also shown in Fig. 4 where facet reflection was neglected. This spectrum is calculated from the equivalent refractive indices derived from the laser structure (stripe width, layer thickness and composition, active and groove width, DFB pitch). Equivalent refractive indices in the active and the groove regions used in this calculation are 2.705 and 2.598 for the fundamental-mode, 2.655 and 2.570 for the first-order mode and 2.582 and 2.566 for the second-order mode, respectively. This calculation shows that this DFB laser oscillates in the first-order mode. The reason for the first-order mode oscillation is that there was no gain in the fundamental-mode because the designed DFB period was too long. The stopband width of the first-order mode was observed to be 20 nm (from 1559 nm to 1579 nm). An index-coupling coefficient of the grating structure was estimated to be 860 cm^-1. To our knowledge, this is the first demonstration of membrane DFB lasers on SOI substrates.

 

Fig. 3. Light output against pump power.

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Fig. 4. Lasing spectrum of membrane DFB laser and simulated resonant spectra for 0th, 1st and 2nd order transverse modes.

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4. Conclusion

We demonstrated a RT-CW operation with a stable single-mode property of 1.5 μm-wavelength membrane BH-DFB laser directly bonded on an SOI substrate. The threshold pump power as low as 2.8 mW and the SMSR of 28dB were achieved with the stripe width of 2 μm and the cavity length of 120 μm. Even though a realization of injection type membrane BH-DFB lasers is strongly required for low-power consumption operation, development of this technology reveals the way to the integration of III-V light sources on high density passive SOI photonic integrated circuits.