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An InAs/GaAs quantum dot laser on a Si rib structure has been demonstrated. The double heterostructure laser structure grown on a GaAs substrate is layer-transferred onto a patterned Si substrate by GaAs/Si direct wafer bonding without oxide or metal mediation. This Fabry-Perot laser operates with current injection through the GaAs/Si rib interface and exhibits InAs quantum dot ground state lasing at 1.28 μm at room temperature, with a threshold current density of 480 A cm−2.
©2012 Optical Society of America
1. Introduction
III-V semiconductor compound light sources integrated onto Si chips or waveguides are promising candidates for the realization of photonic integrated circuits that utilize well-established complementary metal-oxide-semiconductor (CMOS) fabrication technologies [1,2]. Such III-V/Si hybrid devices would compensate for the poor ability of silicon to act as a light source, owing to its low radiative recombination rate, which stems from indirect energy bandgaps. In particular, III-V quantum dot (QD) lasers yield low lasing threshold currents and high temperature stability [3], which can minimize and address the problem of thermal accumulation, which therefore makes them suitable for high-density integration.
For III-V/Si hybrid integration, the direct epitaxial growth of III-V compounds on Si substrates would be the most desirable approach [4,5]; however, heteroepitaxy typically introduces a substantial crystalline defect density owing to the large lattice mismatch and the polar-nonpolar nature of the III-V/IV semiconductor system, which can adversely affect the devices' performance [6,7]. Wafer bonding, on the other hand, is not subject to the lattice matching limitations associated with epitaxial growth, and heterostructure devices fabricated via wafer bonding can, in principle, offer a level of performance close to those obtained by homoepitaxy by confining the defect network that is needed for lattice mismatch accommodation to the bonded interfaces [8,9]. In particular, semiconductor-semiconductor direct bonding without an oxide or metal bonding agent offers advantages in terms of both optical transparency and electrical conductivity. For evanescent III-V/Si hybrid laser structures [10,11], vertical current injection across direct-bonded III-V/Si heterointerfaces through the Si waveguide ribs prevents the carriers from spreading toward the laser stripe edges, as seen in lateral current injection, as well as makes the fabrication process much simpler [12,13]. In the present work, we have fabricated Fabry-Perot InAs/GaAs QD lasers on Si rib structures in order to produce lateral current confinement as well as to mimic Si waveguides, by means of direct GaAs/Si wafer bonding with no mediating agent. Our device exhibits room-temperature lasing at the 1.3-μm optical communication O-band, with a threshold current density of 480 A cm−2. This demonstration is a significant step towards the realization of direct-bonded evanescent III-V QD/Si hybrid lasers.
2. Experimental
Figure 1 shows a schematic flow diagram of our fabrication process for InAs/GaAs QD lasers on Si rib structures. A double heterostructure InAs/GaAs QD laser structure was grown on a GaAs substrate and layer-transferred onto a Si rib structure patterned on a Si substrate by means of GaAs/Si rib direct bonding and subsequent removal of the GaAs substrate.
Fig. 1 Schematic flow diagram of the fabrication process for the InAs/GaAs QD lasers on Si rib structures.
2.1 Epitaxial growth of InAs/GaAs QD laser structures on GaAs substrates
The InAs/GaAs QD laser structure was grown on a GaAs (100) substrate by molecular beam epitaxy [14]. The laser structure consisted of a GaAs layer embedded with eight layers of self-assembled InAs QDs with a per-layer density of 6 × 1010 cm−2. The GaAs layer was clad with p- and n-type Al0.4Ga0.6As layers. An Al0.7Ga0.3As etch-stop layer with a thickness of 1 μm was grown between the GaAs substrate and the lower Al0.4Ga0.6As clad. Figure 2 shows an atomic force microscope image of the as-grown InAs QDs, which are seen to be uniformly sized, coalescence-free, high-density QDs. Figure 3 shows a room-temperature photoluminescence spectrum of the as-grown laser structure, which exhibits a peak associated with the ground state emission of the InAs QDs at 1.25 μm, with a full width at half maximum of 28 meV.
2.2 Layer transfer and device fabrication
First, the laser wafer and an epi-ready p-type Si (100) wafer doped with boron with a doping concentration of 3 × 1019 cm−3 were coated with a photoresist in order to protect the bonding surface from particles generated in the dicing process. The laser and Si wafers were then diced into ~1 cm2-area dies.
Next, Si rib stripes were formed on the Si wafer by photolithography and wet chemical etching, as follows. First, the photoresist on the Si die surface was removed with acetone at room temperature. A 100-nm-thick SiO2 layer was then deposited by magnetron sputtering as an etch mask onto the Si die. Next, the SiO2 layer was photolithographically patterned into a ~3-μm-wide, 500-μm-pitched stripe by using an etch with a buffered HF solution at room temperature. Si ribs were then formed by etching the parts that were not covered by the masking SiO2 layer to a depth of around 400 nm with KOH aq. (30 wt%) for 2 min at 80 °C.
Immediately before bonding, the photoresist on the laser die was removed with acetone at room temperature. The native oxide on the laser die and the masking SiO2 layer on the Si rib die were then removed by dipping both the dies in HF aq. (20 vol%) for 30 s at room temperature. The two die pieces were then brought into contact with each other with their (011) edges aligned—an arrangement that facilitates cleavage during laser fabrication—and were then annealed at 300 °C in ambient air for 3 h under a uniaxial pressure of 0.1 MPa.
After bonding, the GaAs substrate was removed at room temperature by selective chemical etching with H3PO4-H2O2 (3:7 vol.), followed by 50% citric acid-H2O2 (4:1 vol.); the edges of the laser die were coated with a photoresist to prevent undercutting of the QD laser structure. The compositions of the H3PO4-H2O2 and citric acid-H2O2 solutions were chosen so as to maximize the etching rate of GaAs and the etching selectivity between GaAs and AlGaAs, respectively [15]. The Al0.7Ga0.3As etch-stop layer was then removed with HCl aq. (conc.) at room temperature. Figure 4 shows a cross-sectional scanning microscope image of a laser structure bonded to a Si rib.
Fig. 4 Scanning electron microscope image of a laser structure grown on a GaAs substrate direct-bonded to a Si rib.
Following the wafer bonding and layer transfer, broad-area Fabry-Perot lasers with cleaved facets and a cavity length of 2.2 mm were formed by applying Au/AuGeNi electrodes by means of electron-beam evaporation to the top (100-μm-wide stripes) and bottom of the structure. A high-reflection coating was not applied to the cleaved edges.
3. Results and discussion
Figure 5 shows the light-current characteristics of the fabricated device under 500 Hz, 400 ns pulsed pumping at room temperature. The current densities were determined simply on the basis of the area of the front electrode, without taking into account the current spreading in the lateral direction in the core region owing to the relatively high resistance in the AlGaAs clads or the lateral current confinement by the Si rib. The clear kink in the light-current curve indicates the lasing turn-on, with a threshold current density of 480 A cm−2. It should be noted that our laser facets were processed without high-reflection coating, which would decrease the threshold current density even further. Figure 6 shows the DC current-voltage characteristics of the laser. In this current-voltage curve, there is no kink in addition to that for a standard p-n diode, which indicates the ohmic characteristics of the bonded GaAs/Si rib interface. This ohmic result is consistent with our previous work reported in Reference 9, where we investigated in depth the electrical characteristics across the direct-bonded GaAs/Si heterointerfaces accounting for band alignments in relation to doping concentrations by using planer GaAs and Si dies. Figure 7 shows the electroluminescence spectrum at a current density of 680 A cm−2, which corresponds to a lasing emission spectrum. Room temperature lasing at 1.28 μm, which is associated with the ground state transition of the InAs QDs, is observed. This result is a strong indication towards the realization of waveguide-coupled, direct-bonded III-V QD/Si evanescent hybrid lasers [13].
Fig. 5 Light-current characteristics of the fabricated InAs/GaAs QD laser on a Si rib at room temperature.
Fig. 6 DC current-voltage characteristics of the InAs/GaAs QD laser on a Si rib at room temperature.
We have fabricated hundreds of lasers in a single wafer-bonding step, thus demonstrating the advantages this approach offers for high volume, low cost integration, as compared to the conventional pick-and-place scheme [16,17]. Our laser fabrication yield however is not very high at this point presumably due mainly to the mechanical instability of the III-V thin films bonded onto the Si ribs with very small contacting fraction (n.b., ~3-μm-wide, 500-μm-pitched rib stripes), which can be improved for example by installing supporting bumps parallel to the Si ribs. Evanescent optical coupling to underlying waveguides in order to fabricate so-called hybrid Si lasers [10,11,13] can be realized simply by replacing the Si substrate with a commercially available SOI (silicon-on-insulator) substrate, because we have already established the feasibility of waveguide rib fabrication and bonding with the ribs. In contrast to the oxide-mediated bonding that has been used for hybrid laser fabrication until now [10,11], conductive wafer-bonded heterointerfaces enable vertical carrier injection that prevents current from spreading towards the cavity stripe edges. Therefore, direct-bonded hybrid lasers offer the advantages of higher quantum efficiencies and simpler fabrication, without mesa etching or ion implantation for carrier confinement that was required in the fabrication of earlier lateral-current-injection III-V/Si hybrid lasers [13].
4. Conclusions
In summary, we have demonstrated InAs/GaAs QD lasers on Si rib structures by using a GaAs/Si direct bonding technique. Our device realized electrical carrier injection across the bonded III-V laser/Si rib heterointerface. The device exhibited room-temperature lasing at 1.3 μm, which is the optical communication O-band. This work is a significant step toward the realization of direct-bonded III-V QD/Si evanescent hybrid lasers.
Acknowledgments
The authors would like to thank Kenichi Nishi and Mitsuru Sugawara of QD Laser, Inc. for carrying out the laser structure growth and Masatoshi Kitamura of Kobe University for his advice on the Si rib fabrication. This work was supported by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through the Project for Developing Innovation Systems, and Intel Corporation.
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