1. Introduction

Monolithic devices of III-V semiconductor compound light source or lasers integrated with silicon-based waveguides are promising for realization of photonic integrated circuits utilizing well-established CMOS technologies [1–4]. Such III-V/Si hybrid devices would compensate the poor ability of silicon as light source due to its low radiative recombination rate stemming from indirect energy bandgaps. Specifically III-V quantum dot (QD) lasers yield low lasing threshold current and high temperature stability [5], which can minimize and address thermal accumulation, and therefore are suitable for high-density integration. For III-V/Si hybrid integration, direct epitaxial growth of III-V compounds on Si substrates would be the most desirable approach, but is difficult to achieve due to the large lattice mismatch and to the polar - non polar nature of the III-V/IV system. There has been only one report of QD lasers on Si substrates; Mi et al. demonstrated an InGaAs/GaAs QD laser grown directly on a Si substrate by molecular beam epitaxy (MBE) yielding ground state lasing at 1.0 μm at room temperature (RT) with a threshold current density (J_th) of 900 A/cm² [6]. This hetero-epitaxy approach, however, can introduce a substantial number of misfit dislocations that can adversely affect device performance. Also, for optical communication and interconnections, it is of first importance to achieve ground state lasing at the telecommunication band of 1.3 μm.

Wafer bonding on the other hand is not subject to lattice matching limitations associated with epitaxial growth and heterostructure devices fabricated via wafer bonding can in principle have performance close to those obtained from homoepitaxy. We previously demonstrated optically pumped GaAs photonic crystal nanocavity lasers with InAs QDs gain on Si substrates by means of GaAs/SiO₂ wafer bonding and layer transfer [7]. In this present work, we have fabricated broad-area Fabry-Perot InAs/GaAs QD lasers on Si substrates using Au-Ge-Ni-alloy-mediated GaAs/Si wafer bonding and layer transfer. Our device exhibits RT lasing at the 1.3 μm optical communication band by electrical carrier injection with J_th of 600 A/cm² and is the first QD laser on Si emitting at longer than 1.0 μm. The highly-conductive GaAs/Si heterointerface enabled vertical carrier injection, which prevents carrier spreading toward laser stripe edges seen for lateral carrier injection and makes fabrication process much simpler.

2. Experimental

2.1 Crystal growth of InAs/GaAs quantum dots

An InAs/GaAs QD laser structure including a 350-nm-thick GaAs layer embedding five layers of self-assembled InAs QDs with a density per layer of 4 × 10¹⁰ cm⁻² and 1.4-μm-thick Al_0.4Ga_0.6As clads with an Al_0.7Ga_0.3As etch stop layer underneath was grown on a (001) GaAs substrate by antimony-mediated MOCVD [8]. The inset of Fig. 1 shows an atomic force microscope image of InAs QDs we grew by this scheme. The use of antimony surfactant allows the growth of high density, coalescence-free InAs QDs in the 1.3 μm band with high optical quality [8]. Figure 1 shows a RT photoluminescence spectrum of the as-grown InAs QDs exhibiting a peak associated with the ground state emission of the QDs at 1.30 μm with a full width at half maximum of 30 meV.

 

Fig. 1 RT photoluminescence spectrum and (inset) 1 × 1 μm atomic force microscope image of the as-grown InAs QDs grown by antimony-mediated MOCVD.

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2.2 Layer transfer of InAs/GaAs quantum dot laser structures onto Si substrates

Then the laser structure was layer transferred onto a highly doped p-type (001) Si substrate through wafer bonding and subsequent removal of the GaAs substrate. A 30-nm-thick AuGeNi alloy (80:10:10 wt%) layer, a 150-nm-thick Au layer, and a 100-nm-thick AuGeNi alloy layer were first deposited in this order onto the bonding surface of the QD laser structure. A 100-nm-thick AuGeNi alloy layer was deposited onto the epi-ready, polished side of the Si substrate. These metal bonding layers enhance bonding strength and interfacial electrical conductivity utilizing the low eutectic point (~280 °C) of Au-Ge alloy [9]. The GaAs and Si wafers were coated with photoresist to protect the bonding surfaces from particles generated in the following dicing process because interfacial particles would degrade bonding strength. The wafers were then diced into ~1 cm² area. Then the applied photoresist was removed with acetone along with degreasing of the bonding surfaces. The two wafers were then brought into contact and annealed at 300 °C in atmosphere for 3 hours under uniaxial pressure of 0.1 MPa. Then the GaAs substrate was removed at RT by selective chemical etching with H₃PO₄ - H₂O₂ (3:7 vol.) followed by 50% citric acid - H₂O₂ (4:1 vol.) with the edges of the GaAs wafer coated with photoresist to avoid undercut of the QD laser structure. The solution compositions were chosen to maximize the etching rate of GaAs for the H₃PO₄ - H₂O₂ solution and the etching selectivity between GaAs and AlGaAs for the citric acid - H₂O₂ solution. The Al_0.7Ga_0.3As etch stop layer was then removed by HCl aq. (conc.) at RT.

2.3 Fabrication broad-area Fabry-Perot lasers

Broad-area Fabry-Perot lasers with cleaved facets were then formed by applying electrodes to the top and bottom of the structure (Fig. 2 ). Our finished device consists of a 3.45-μm-thick double-hetero laser structure bonded on a Si substrate via a 380-nm-thick metal layer. The laser cavity length and width were 2.4 mm and 100 μm, respectively. No high-reflection (HR) coating was applied to the cleaved edges. The mirror loss α_m for this cavity is calculated to be 5 cm⁻¹. Electroluminescence measurements were conducted at RT under pulsed operation.

 

Fig. 2 (a) Cross-sectional schematic of the fabricated InAs/GaAs QD laser on Si substrate. (b) Cross-sectional scanning electron microscope image of the QD laser structure layer-transferred onto a Si substrate.

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3. Results and discussion

Figure 3 shows the light-current characteristics of the fabricated device under 500 Hz, 700 ns pulsed pumping at RT. The clear kink in the light-current curve shows lasing turn-on with J_th = 600 A/cm². Note that our laser facets were processed without HR coating, which would decrease J_th further. The insets of Fig. 3 show the electroluminescence spectra at J = 0.2 and 1.0 kA/cm², corresponding to spontaneous and lasing emission, respectively. RT lasing at 1.31 μm, associated with the ground state transition of the InAs QDs, is observed. This lasing wavelength is significantly longer than the previously-reported value of 1.02 μm obtained from a QD laser directly grown on a Si substrate [6] and suitable for optical communication applications.

 

Fig. 3 Light-current characteristics of the laser (CL/CL, pulsed, RT). Insets show the electroluminescence spectra at J = 0.2 and 1.0 kA/cm².

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We have fabricated 100’s of lasers in one wafer bonding step, demonstrating the advantage of this approach for high volume, low cost integration over the conventional pick-and-place scheme [10]. InAs/GaAs QD growth on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) we adopted in this study gives further merits for large-scale, low-cost, high-throughput fabrication over those using InP substrates or MBE growth. Evanescent optical coupling to underneath waveguides to fabricate so-called “hybrid Si lasers” [1] would be realized by opening patterned arrays of optical windows in the metal bonding layer. The wafer bonding and layer transfer approach to fabricate metal/semiconductor/metal thin film structure demonstrated in this work is also applicable for development of subwavelength-scale plasmonic lasers [11,12].

4. Summary

We have fabricated an InAs/GaAs QD laser on a Si substrate through metal-mediated wafer bonding and layer transfer technique for realization of highly integrated silicon photonic circuits. Our device exhibited QD ground state lasing at 1.31 μm at RT with J_th of 600 A/cm². This work is the first demonstration of 1.3 μm QD lasers on Si substrates.