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III-V/Si hybrid photonic devices by direct fusion bonding

III-V/Si hybrid photonic devices by direct fusion bonding III-V/Si hybrid photonic devices by direct fusion bonding SUBJECT AREAS: 1 1 1,2 Katsuaki Tanabe , Katsuyuki Watanabe & Yasuhiko Arakawa OPTICAL MATERIALS AND DEVICES 1 2 ELECTRONIC MATERIALS AND Institute for Nano Quantum Information Electronics, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan, Institute DEVICES of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan. APPLIED PHYSICS LIGHT SOURCES Monolithic integration of III-V compound semiconductors on silicon is highly sought after for high-speed, low-power-consumption silicon photonics and low-cost, light-weight photovoltaics. Here we present a GaAs/Si direct fusion bonding technique to provide highly conductive and transparent heterojunctions by Received heterointerfacial band engineering in relation to doping concentrations. Metal- and oxide-free GaAs/Si 26 January 2012 ohmic heterojunctions have been formed at 3006C; sufficiently low to inhibit active material degradation. We have demonstrated 1.3 mm InAs/GaAs quantum dot lasers on Si substrates with the lowest threshold Accepted current density of any laser on Si to date, and AlGaAs/Si dual-junction solar cells, by p-GaAs/p-Si and 16 March 2012 p-GaAs/n-Si bonding, respectively. Our direct semiconductor bonding technique opens up a new pathway for realizing ultrahigh efficiency multijunction solar cells with ideal bandgap combinations that are free Published from lattice-match restrictions required in conventional heteroepitaxy, as well as enabling the creation of 2 April 2012 novel high performance and practical optoelectronic devices by III-V/Si hybrid integration. 1–9 10–16 II-V semiconductor compound light sources integrated onto Si chips or waveguides are promising for Correspondence and 17,18 the realization of photonic integrated circuits utilizing well-established complementary metal-oxide-semi- requests for materials I conductor (CMOS) fabrication technologies. Such III-V/Si hybrid devices would compensate for the poor should be addressed to ability of silicon to act as a light source due to its low radiative recombination rate stemming from indirect energy 19,20 K.T. ([email protected] bandgaps. For solar cell applications, Si-based multijunction stacking would provide high efficiency , low cost, tokyo.ac.jp) mechanical robustness and light weight cells, relative to conventional Si and III–V multijunction cells. Additionally, heterojunction tunnel field-effect transistors consisting of low bandgap III–V semiconductors and Si are promising for the realization of high-density, low-power-consumption very-large-scale-integration 21,22 (VLSI) by enhanced drive current relative to conventional Si-only transistors . For III–V/Si hybrid integration, direct epitaxial growth of III–V compounds on Si substrates would be the most desirable approach, but hetero- epitaxy typically introduces a substantial crystalline defect density due to the large lattice mismatch and the polar- 23–25 nonpolar nature of the III–V/IV semiconductor system that can adversely affect device performance . 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, have a performance close to those obtained by homoepitaxy by confining the defect network needed for lattice mismatch accommodation to the bonded interfaces . It is known that ohmic Si/Si junctions of the same polarity, i.e. p-Si/p-Si and n-Si/n-Si, can be 26,27 relatively easily obtained by direct wafer bonding even at room temperature . It is, however, not the case for compound semiconductors such as GaAs and InP, and ohmic p-type/p-type or n-type/n-type junction formation 28–33 has required high temperature bonding above 600uC which would severely degrade the device materials. For GaAs/Si bonding that would be particularly attractive for fabrication of high performance photonic and pho- tovoltaic devices, no successfully direct-bonded ohmic junctions have been reported, and bonding even at 700uC 27,34 was reported to have failed . Although an alternative method to prepare ohmic heterointerfaces uses metal 5,35,36 bonding agents , the bonding metal layers would shadow light as well as cause photon absorption loss and, therefore, is not ideal for optoelectronic applications. In this work, we have succeeded in preparing ohmic GaAs/Si heterojunctions, to realize both optical transparency and electrical conductivity, by direct bonding at 300uCin ambient air for p-GaAs/p-Si homopolarity-junctions and p-GaAs/n-Si tunnel-junctions by applying heavy, degenerating doping at the GaAs and Si surfaces to be bonded to enhance the GaAs/Si interfacial conductivity. Results GaAs/Si direct fusion bonding. We investigated experimentally the bonding of GaAs and Si wafers with varying doping concentrations and bonding temperature, and we characterized the GaAs/Si heterointerfacial electrical 1 1 1 conductivities. The doping concentrations of the p-GaAs wafers, p -GaAs layers, p -Si, and n -Si wafers were SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 1 www.nature.com/scientificreports 18 23 19 23 19 23 19 23 1 1 9310 cm Zn, 5310 cm Zn, 3310 cm B, and 3310 cm non-ohmic behaviour is seen for all p-GaAs/p -Si and p-GaAs/n -Si As, respectively. Current-voltage (I-V) curves, under the pairs including those annealed at 500uC. In contrast to the non-ohmic 1 1 measurement configuration shown schematically in Fig. 1a, for the I–V characteristics for the p-GaAs/p -Si and p-GaAs/n -Si pairs, the 1 1 1 1 bonded GaAs/Si wafer pairs are shown in Fig. 1b and 1c. Rectified, p -GaAs/p -Si and p -GaAs/n -Si pairs exhibit ohmic I–V curves as Figure 1 | GaAs/Si direct wafer bonding. (a) Configuration schematic of the I–V measurement for the direct-bonded GaAs/Si heterointerfacial electrical characteristics. A positive bias voltage was applied from the GaAs side. (b, c) I–V characteristics of the direct-bonded GaAs/Si heterointerfaces with varying doping concentrations and bonding temperature. (d, e) Calculated profiles of the conduction and valence band edges across the (d) p-GaAs/p-Si and (e) p-GaAs/n-Si heterointerfaces with varying doping concentrations. The inset in e shows a closeup around the origin. (f) Cross-sectional 1 1 transmission electron microscope image of a direct-bonded p -GaAs/p -Si heterointerface. (g–i) Selected-area diffraction patterns at the same heterointerface as of f for regions around 70 nm in radius centred (g) 80 nm above the interface, (h) at the interface, and (i) 80 nm below the interface, identified as single-crystal GaAs, a mixture of single-crystal GaAs and Si, and single-crystal Si, respectively. SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 2 www.nature.com/scientificreports seen in Fig. 1b and 1c. A cross-sectional trans- double-hetero InAs/GaAs quantum dot laser structure was grown 1 1 mission electron microscope image at a direct-bonded p -GaAs/p - on a GaAs substrate by molecular beam epitaxy and layer-transferred 1 1 1 Si heterointerface is shown in Fig. 1f. An amorphous layer at the onto a p -Si substrate by means of p -GaAs/p -Si direct bonding at GaAs/Si interface with a thickness of around 2 nm can be seen in 300uC and subsequent removal of the GaAs substrate. The finished the image. Even if this interfacial layer is an oxide, this thickness is device consists of a 3.9-mm-thick III-V semiconductor double-hetero sufficiently thin to provide ohmic interfacial conductivity by induc- laser structure on top of a Si substrate, as shown in Fig. 2a and 2b. ing a tunnelling current or by oxide breakdown by the applied Fig. 2c shows the light-current characteristics of the fabricated device 37,38 voltage . Selected-area diffraction patterns at and around the under 500 Hz, 400 ns pulsed pumping at room temperature. The GaAs/Si heterointerface, shown in Fig. 1g–i, verify that the regions clear kink in the light-current curve indicates the lasing turn-on with immediately above and below the amorphous layer are single- a threshold current density of 205 A cm ; the lowest threshold crystalline GaAs and Si, respectively. The images indicate that both current density, to the best of our knowledge, of any kind of laser the GaAs and Si materials remain single crystals during our bonding on Si. The inset of Fig. 2c shows the I–V characteristics of the laser. process with no threading dislocation generation observed around the The resistivity in the linear I–V region at higher voltages is around 0.1 2 1 vicinity of the bonded heterointerface. This is in contrast to interfaces V cm , which is the same order of magnitude as the bonded p -GaAs/ 23–25 1 in the cases of lattice-mismatched heteroepitaxy . p -Si bare wafer heterointerface shown in Fig. 1b. Fig. 2d and 2e show the electroluminescence spectra at current densities of 140 III–V quantum dot lasers on Si substrates . As a demonstration of and 380 A cm , corresponding to spontaneous and lasing emis- our GaAs/Si direct bonding technique applied to optoelectronic sion, respectively. Room temperature lasing at the 1.3 mm optical devices, we have fabricated semiconductor lasers using self- communication band, associated with the ground state transition assembled InAs quantum dots embedded in GaAs (InAs/GaAs of the InAs quantum dots, is observed. Additionally, an onset of quantum dot lasers ) on Si substrates and operated by current room temperature continuous-wave lasing has been observed in a injection through direct-bonded GaAs/Si heterointerfaces. A same type of sample (see Supplementary Information). Figure 2 | InAs/GaAs quantum dot laser on Si substrate. (a) Cross-sectional schematic diagram of the fabricated InAs/GaAs quantum dot laser on a Si substrate. The thickness and doping concentration of each layer are indicated. The abbreviations QD and ND stand for quantum dot and non-doped, respectively. (b) Cross-sectional transmission electron microscope image of the laser. The upper inset shows a detailed image of the InAs/GaAs quantum dot layers. The lower inset shows an atomic force microscope image of the as-grown InAs/GaAs quantum dots. (c) Light-current characteristics of the laser for pulsed electrical pumping at room temperature. The I–V characteristics of the laser are shown in the inset. (d, e) Electroluminescence spectra of the laser at current densities of 140 (below the lasing threshold) and 380 (above the lasing threshold) A cm , respectively. SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 3 www.nature.com/scientificreports III–V/Si multijunction solar cells . We have also fabricated AlGaAs/ optoelectronic devices that will enable both optical and electrical Si dual-junction solar cells using the direct bonding technique. An interconnections. Al Ga As subcell was grown on a GaAs substrate by molecular We have fabricated hundreds of lasers in a single wafer bonding 0.1 0.9 step demonstrating the advantage of this approach for high volume, beam epitaxy and layer-transferred onto a Si subcell by means of the 18,40 1 1 p -GaAs/n -Si direct bonding at 300uC and subsequent removal of low cost integration over the conventional pick-and-place scheme . Evanescent optical coupling to underneath waveguides to fabricate the GaAs substrate. Fig. 3a and 3b show a cross-sectional schematic 10–16,41 diagram and scanning electron microscope image of the fabricated so-called hybrid Si lasers could be realized by preparing rib structures on commercially available silicon-on-insulator substrates AlGaAs/Si dual-junction solar cell, respectively. Both the Al Ga As 0.1 0.9 in advance of wafer bonding. In contrast to oxide-mediated bonding and Si subcells had n-on-p structures, and the bonding of the p - 10–15 used for hybrid laser fabrication to date , conductive wafer-bonded GaAs/n -Si heterointerface acts as a tunnel junction to switch the heterointerfaces enable vertical carrier injection that prevents cur- polarity. The light I–V and power-voltage characteristics of the solar rent spreading towards cavity stripe edges. Therefore, direct-bonded cell under a 600 nm-peaked halogen white light source of a one-sun hybrid lasers have the advantages of higher quantum efficiencies and intensity (100 mW cm ) are shown in the inset of Fig. 3b. The device simpler fabrication without mesa etching or ion implantation for performance parameters for this solar cell are J 5 27.9 mA cm , sc carrier confinement that was required in the fabrication of earlier V 5 1.55 V, FF 5 0.58, and g 5 25.2%, where J , V , FF and g are oc sc oc lateral-current-injection III-V/Si hybrid lasers . the short-circuit current, open-circuit voltage, fill factor and energy The low FF seen in Fig. 3b is likely due to the large series resistance. conversion efficiency, respectively. However, the wafer-bonded GaAs/Si heterojunction interfacial res- istance with exactly same doping concentrations in GaAs and Si to Discussion those used for the bonding surfaces in the dual-junction solar cell The electrical conductivity dependence on bonding temperature seen in Fig. 1c is far lower than the total series resistance of the dual- seen in Fig. 1b and 1c does not show monotonic behavior, attri- junction solar cell estimated from the light I–V characteristics. We buted to the trade-off between conductivity increase and decrease therefore attribute the low FF principally to insufficient optimization by formation of covalent bonds and thermal expansion mismatch of our front metal contact grids. Very high efficiency, over 30% under between GaAs and Si at higher temperature, respectively. Interfacial 1 sun, seems quite realistic simply through a contact redesign and oxide formation might also be a cause of higher interfacial resistivity would be expected based on the J and V values obtained at this sc oc at higher temperature for our wafer bonding process in ambient air. preliminary research stage. To the best of our knowledge, while there It should be also noted that the wafer bonding process basically have been two reports for all-III–V bonded multijunction solar 42,43 contains some randomness in reproducibility for the bonded inter- cells , this is the first bonded multijunction solar cell with a Si facial properties degradable for example even by a single particle subcell. Our monolithic AlGaAs/Si dual-junction solar cell (over- accidental incorporation into the interface. The conductivity en- coming a 4% lattice-mismatch between AlGaAs and Si) has demon- hancement seen in Fig. 1b and 1c can be explained through an strated a proof-of-principle for the viability of direct wafer bonding analysis of the heterojunction band offset at the GaAs/Si interfaces. for solar cell applications. This wafer-bonding interconnecting One-dimensional simulations of the heterojunction bandbending approach is extendable to ultrahigh efficiency multijunction solar (PC1D software, University of New South Wales) indicate thin- cells, such as InGaN/AlGaAs/Si/Ge four-junction solar cells, with ning of the potential barrier at the valence band edge due to the optimal subcell bandgap sequences free from the lattice-matching 18 23 change of the doping concentration in GaAs from 9310 cm to restriction required in conventional heteroepitaxy. In this work, we 19 23 5310 cm for the p-type/p-type pairs as seen in Fig. 1d, leading to adopted an etch-back method to detach the GaAs growth substrate to the interfacial electrical conductivity enhancement. On the other simplify the fabrication process. Alternatively, the incorporation of 6,44,45 46–48 hand, simulations indicate tunnel junction formation due to the an epitaxial lift-off or ion-cutting technique would enable same change in doping concentration in GaAs for the p-type/n-type the reuse of the GaAs substrates to reduce the production costs. pairs as seen in Fig. 1e. This valence-band-edge rising on the GaAs In conclusion, we have investigated GaAs/Si direct wafer bonding side enables tunnelling carrier transport, leading to higher conduc- for electrically conductive, optically transparent materials intercon- tivity and ohmic characteristics across the heterojunction interfaces. nection in conjunction with heterointerfacial energy band alignment These p-type/p-type and p-type/n-type GaAs/Si ohmic hetero- calculations in relation to doping concentrations. Heavy, degenerat- junctions are very suitable for next-generation III-V/Si hybrid ing doping at the GaAs and Si surfaces to be bonded is found to be Figure 3 | AlGaAs/Si dual-junction solar cell. (a) Cross-sectional schematic diagram of the fabricated AlGaAs/Si dual-junction solar cell. The thickness and doping concentration of each layer are indicated. The abbreviation BSF stands for back surface field. (b) Cross-sectional scanning electron microscope image of the solar cell. Inset shows the light I–V and power-voltage characteristics of the solar cell under a 600 nm-peaked halogen white light source of a one-sun intensity (100 mW cm ). SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 4 www.nature.com/scientificreports 6. Yoon, J. et al. GaAs photovoltaics and optoelectronics using releasable multilayer significant for enhancing the GaAs/Si interfacial conductivity and epitaxial assemblies. 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License: This work is licensed under a Creative Commons Acknowledgements 1 Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this The authors thank Denis Guimard and Masao Nishioka for growing the p -GaAs bonding license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ layer. This research was supported by the JSPS through the FIRST Program and Kakenhi 23760303, MEXT Japan through the Special Coordination Funds for Promoting Science How to cite this article: Tanabe, K., Watanabe, K. & Arakawa, Y. III-V/Si hybrid photonic and Technology, and Intel Corporation. devices by direct fusion bonding. Sci. Rep. 2, 349; DOI:10.1038/srep00349 (2012). SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 6 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Scientific Reports Springer Journals

III-V/Si hybrid photonic devices by direct fusion bonding

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Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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III-V/Si hybrid photonic devices by direct fusion bonding SUBJECT AREAS: 1 1 1,2 Katsuaki Tanabe , Katsuyuki Watanabe & Yasuhiko Arakawa OPTICAL MATERIALS AND DEVICES 1 2 ELECTRONIC MATERIALS AND Institute for Nano Quantum Information Electronics, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan, Institute DEVICES of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan. APPLIED PHYSICS LIGHT SOURCES Monolithic integration of III-V compound semiconductors on silicon is highly sought after for high-speed, low-power-consumption silicon photonics and low-cost, light-weight photovoltaics. Here we present a GaAs/Si direct fusion bonding technique to provide highly conductive and transparent heterojunctions by Received heterointerfacial band engineering in relation to doping concentrations. Metal- and oxide-free GaAs/Si 26 January 2012 ohmic heterojunctions have been formed at 3006C; sufficiently low to inhibit active material degradation. We have demonstrated 1.3 mm InAs/GaAs quantum dot lasers on Si substrates with the lowest threshold Accepted current density of any laser on Si to date, and AlGaAs/Si dual-junction solar cells, by p-GaAs/p-Si and 16 March 2012 p-GaAs/n-Si bonding, respectively. Our direct semiconductor bonding technique opens up a new pathway for realizing ultrahigh efficiency multijunction solar cells with ideal bandgap combinations that are free Published from lattice-match restrictions required in conventional heteroepitaxy, as well as enabling the creation of 2 April 2012 novel high performance and practical optoelectronic devices by III-V/Si hybrid integration. 1–9 10–16 II-V semiconductor compound light sources integrated onto Si chips or waveguides are promising for Correspondence and 17,18 the realization of photonic integrated circuits utilizing well-established complementary metal-oxide-semi- requests for materials I conductor (CMOS) fabrication technologies. Such III-V/Si hybrid devices would compensate for the poor should be addressed to ability of silicon to act as a light source due to its low radiative recombination rate stemming from indirect energy 19,20 K.T. ([email protected] bandgaps. For solar cell applications, Si-based multijunction stacking would provide high efficiency , low cost, tokyo.ac.jp) mechanical robustness and light weight cells, relative to conventional Si and III–V multijunction cells. Additionally, heterojunction tunnel field-effect transistors consisting of low bandgap III–V semiconductors and Si are promising for the realization of high-density, low-power-consumption very-large-scale-integration 21,22 (VLSI) by enhanced drive current relative to conventional Si-only transistors . For III–V/Si hybrid integration, direct epitaxial growth of III–V compounds on Si substrates would be the most desirable approach, but hetero- epitaxy typically introduces a substantial crystalline defect density due to the large lattice mismatch and the polar- 23–25 nonpolar nature of the III–V/IV semiconductor system that can adversely affect device performance . 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, have a performance close to those obtained by homoepitaxy by confining the defect network needed for lattice mismatch accommodation to the bonded interfaces . It is known that ohmic Si/Si junctions of the same polarity, i.e. p-Si/p-Si and n-Si/n-Si, can be 26,27 relatively easily obtained by direct wafer bonding even at room temperature . It is, however, not the case for compound semiconductors such as GaAs and InP, and ohmic p-type/p-type or n-type/n-type junction formation 28–33 has required high temperature bonding above 600uC which would severely degrade the device materials. For GaAs/Si bonding that would be particularly attractive for fabrication of high performance photonic and pho- tovoltaic devices, no successfully direct-bonded ohmic junctions have been reported, and bonding even at 700uC 27,34 was reported to have failed . Although an alternative method to prepare ohmic heterointerfaces uses metal 5,35,36 bonding agents , the bonding metal layers would shadow light as well as cause photon absorption loss and, therefore, is not ideal for optoelectronic applications. In this work, we have succeeded in preparing ohmic GaAs/Si heterojunctions, to realize both optical transparency and electrical conductivity, by direct bonding at 300uCin ambient air for p-GaAs/p-Si homopolarity-junctions and p-GaAs/n-Si tunnel-junctions by applying heavy, degenerating doping at the GaAs and Si surfaces to be bonded to enhance the GaAs/Si interfacial conductivity. Results GaAs/Si direct fusion bonding. We investigated experimentally the bonding of GaAs and Si wafers with varying doping concentrations and bonding temperature, and we characterized the GaAs/Si heterointerfacial electrical 1 1 1 conductivities. The doping concentrations of the p-GaAs wafers, p -GaAs layers, p -Si, and n -Si wafers were SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 1 www.nature.com/scientificreports 18 23 19 23 19 23 19 23 1 1 9310 cm Zn, 5310 cm Zn, 3310 cm B, and 3310 cm non-ohmic behaviour is seen for all p-GaAs/p -Si and p-GaAs/n -Si As, respectively. Current-voltage (I-V) curves, under the pairs including those annealed at 500uC. In contrast to the non-ohmic 1 1 measurement configuration shown schematically in Fig. 1a, for the I–V characteristics for the p-GaAs/p -Si and p-GaAs/n -Si pairs, the 1 1 1 1 bonded GaAs/Si wafer pairs are shown in Fig. 1b and 1c. Rectified, p -GaAs/p -Si and p -GaAs/n -Si pairs exhibit ohmic I–V curves as Figure 1 | GaAs/Si direct wafer bonding. (a) Configuration schematic of the I–V measurement for the direct-bonded GaAs/Si heterointerfacial electrical characteristics. A positive bias voltage was applied from the GaAs side. (b, c) I–V characteristics of the direct-bonded GaAs/Si heterointerfaces with varying doping concentrations and bonding temperature. (d, e) Calculated profiles of the conduction and valence band edges across the (d) p-GaAs/p-Si and (e) p-GaAs/n-Si heterointerfaces with varying doping concentrations. The inset in e shows a closeup around the origin. (f) Cross-sectional 1 1 transmission electron microscope image of a direct-bonded p -GaAs/p -Si heterointerface. (g–i) Selected-area diffraction patterns at the same heterointerface as of f for regions around 70 nm in radius centred (g) 80 nm above the interface, (h) at the interface, and (i) 80 nm below the interface, identified as single-crystal GaAs, a mixture of single-crystal GaAs and Si, and single-crystal Si, respectively. SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 2 www.nature.com/scientificreports seen in Fig. 1b and 1c. A cross-sectional trans- double-hetero InAs/GaAs quantum dot laser structure was grown 1 1 mission electron microscope image at a direct-bonded p -GaAs/p - on a GaAs substrate by molecular beam epitaxy and layer-transferred 1 1 1 Si heterointerface is shown in Fig. 1f. An amorphous layer at the onto a p -Si substrate by means of p -GaAs/p -Si direct bonding at GaAs/Si interface with a thickness of around 2 nm can be seen in 300uC and subsequent removal of the GaAs substrate. The finished the image. Even if this interfacial layer is an oxide, this thickness is device consists of a 3.9-mm-thick III-V semiconductor double-hetero sufficiently thin to provide ohmic interfacial conductivity by induc- laser structure on top of a Si substrate, as shown in Fig. 2a and 2b. ing a tunnelling current or by oxide breakdown by the applied Fig. 2c shows the light-current characteristics of the fabricated device 37,38 voltage . Selected-area diffraction patterns at and around the under 500 Hz, 400 ns pulsed pumping at room temperature. The GaAs/Si heterointerface, shown in Fig. 1g–i, verify that the regions clear kink in the light-current curve indicates the lasing turn-on with immediately above and below the amorphous layer are single- a threshold current density of 205 A cm ; the lowest threshold crystalline GaAs and Si, respectively. The images indicate that both current density, to the best of our knowledge, of any kind of laser the GaAs and Si materials remain single crystals during our bonding on Si. The inset of Fig. 2c shows the I–V characteristics of the laser. process with no threading dislocation generation observed around the The resistivity in the linear I–V region at higher voltages is around 0.1 2 1 vicinity of the bonded heterointerface. This is in contrast to interfaces V cm , which is the same order of magnitude as the bonded p -GaAs/ 23–25 1 in the cases of lattice-mismatched heteroepitaxy . p -Si bare wafer heterointerface shown in Fig. 1b. Fig. 2d and 2e show the electroluminescence spectra at current densities of 140 III–V quantum dot lasers on Si substrates . As a demonstration of and 380 A cm , corresponding to spontaneous and lasing emis- our GaAs/Si direct bonding technique applied to optoelectronic sion, respectively. Room temperature lasing at the 1.3 mm optical devices, we have fabricated semiconductor lasers using self- communication band, associated with the ground state transition assembled InAs quantum dots embedded in GaAs (InAs/GaAs of the InAs quantum dots, is observed. Additionally, an onset of quantum dot lasers ) on Si substrates and operated by current room temperature continuous-wave lasing has been observed in a injection through direct-bonded GaAs/Si heterointerfaces. A same type of sample (see Supplementary Information). Figure 2 | InAs/GaAs quantum dot laser on Si substrate. (a) Cross-sectional schematic diagram of the fabricated InAs/GaAs quantum dot laser on a Si substrate. The thickness and doping concentration of each layer are indicated. The abbreviations QD and ND stand for quantum dot and non-doped, respectively. (b) Cross-sectional transmission electron microscope image of the laser. The upper inset shows a detailed image of the InAs/GaAs quantum dot layers. The lower inset shows an atomic force microscope image of the as-grown InAs/GaAs quantum dots. (c) Light-current characteristics of the laser for pulsed electrical pumping at room temperature. The I–V characteristics of the laser are shown in the inset. (d, e) Electroluminescence spectra of the laser at current densities of 140 (below the lasing threshold) and 380 (above the lasing threshold) A cm , respectively. SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 3 www.nature.com/scientificreports III–V/Si multijunction solar cells . We have also fabricated AlGaAs/ optoelectronic devices that will enable both optical and electrical Si dual-junction solar cells using the direct bonding technique. An interconnections. Al Ga As subcell was grown on a GaAs substrate by molecular We have fabricated hundreds of lasers in a single wafer bonding 0.1 0.9 step demonstrating the advantage of this approach for high volume, beam epitaxy and layer-transferred onto a Si subcell by means of the 18,40 1 1 p -GaAs/n -Si direct bonding at 300uC and subsequent removal of low cost integration over the conventional pick-and-place scheme . Evanescent optical coupling to underneath waveguides to fabricate the GaAs substrate. Fig. 3a and 3b show a cross-sectional schematic 10–16,41 diagram and scanning electron microscope image of the fabricated so-called hybrid Si lasers could be realized by preparing rib structures on commercially available silicon-on-insulator substrates AlGaAs/Si dual-junction solar cell, respectively. Both the Al Ga As 0.1 0.9 in advance of wafer bonding. In contrast to oxide-mediated bonding and Si subcells had n-on-p structures, and the bonding of the p - 10–15 used for hybrid laser fabrication to date , conductive wafer-bonded GaAs/n -Si heterointerface acts as a tunnel junction to switch the heterointerfaces enable vertical carrier injection that prevents cur- polarity. The light I–V and power-voltage characteristics of the solar rent spreading towards cavity stripe edges. Therefore, direct-bonded cell under a 600 nm-peaked halogen white light source of a one-sun hybrid lasers have the advantages of higher quantum efficiencies and intensity (100 mW cm ) are shown in the inset of Fig. 3b. The device simpler fabrication without mesa etching or ion implantation for performance parameters for this solar cell are J 5 27.9 mA cm , sc carrier confinement that was required in the fabrication of earlier V 5 1.55 V, FF 5 0.58, and g 5 25.2%, where J , V , FF and g are oc sc oc lateral-current-injection III-V/Si hybrid lasers . the short-circuit current, open-circuit voltage, fill factor and energy The low FF seen in Fig. 3b is likely due to the large series resistance. conversion efficiency, respectively. However, the wafer-bonded GaAs/Si heterojunction interfacial res- istance with exactly same doping concentrations in GaAs and Si to Discussion those used for the bonding surfaces in the dual-junction solar cell The electrical conductivity dependence on bonding temperature seen in Fig. 1c is far lower than the total series resistance of the dual- seen in Fig. 1b and 1c does not show monotonic behavior, attri- junction solar cell estimated from the light I–V characteristics. We buted to the trade-off between conductivity increase and decrease therefore attribute the low FF principally to insufficient optimization by formation of covalent bonds and thermal expansion mismatch of our front metal contact grids. Very high efficiency, over 30% under between GaAs and Si at higher temperature, respectively. Interfacial 1 sun, seems quite realistic simply through a contact redesign and oxide formation might also be a cause of higher interfacial resistivity would be expected based on the J and V values obtained at this sc oc at higher temperature for our wafer bonding process in ambient air. preliminary research stage. To the best of our knowledge, while there It should be also noted that the wafer bonding process basically have been two reports for all-III–V bonded multijunction solar 42,43 contains some randomness in reproducibility for the bonded inter- cells , this is the first bonded multijunction solar cell with a Si facial properties degradable for example even by a single particle subcell. Our monolithic AlGaAs/Si dual-junction solar cell (over- accidental incorporation into the interface. The conductivity en- coming a 4% lattice-mismatch between AlGaAs and Si) has demon- hancement seen in Fig. 1b and 1c can be explained through an strated a proof-of-principle for the viability of direct wafer bonding analysis of the heterojunction band offset at the GaAs/Si interfaces. for solar cell applications. This wafer-bonding interconnecting One-dimensional simulations of the heterojunction bandbending approach is extendable to ultrahigh efficiency multijunction solar (PC1D software, University of New South Wales) indicate thin- cells, such as InGaN/AlGaAs/Si/Ge four-junction solar cells, with ning of the potential barrier at the valence band edge due to the optimal subcell bandgap sequences free from the lattice-matching 18 23 change of the doping concentration in GaAs from 9310 cm to restriction required in conventional heteroepitaxy. In this work, we 19 23 5310 cm for the p-type/p-type pairs as seen in Fig. 1d, leading to adopted an etch-back method to detach the GaAs growth substrate to the interfacial electrical conductivity enhancement. On the other simplify the fabrication process. Alternatively, the incorporation of 6,44,45 46–48 hand, simulations indicate tunnel junction formation due to the an epitaxial lift-off or ion-cutting technique would enable same change in doping concentration in GaAs for the p-type/n-type the reuse of the GaAs substrates to reduce the production costs. pairs as seen in Fig. 1e. This valence-band-edge rising on the GaAs In conclusion, we have investigated GaAs/Si direct wafer bonding side enables tunnelling carrier transport, leading to higher conduc- for electrically conductive, optically transparent materials intercon- tivity and ohmic characteristics across the heterojunction interfaces. nection in conjunction with heterointerfacial energy band alignment These p-type/p-type and p-type/n-type GaAs/Si ohmic hetero- calculations in relation to doping concentrations. Heavy, degenerat- junctions are very suitable for next-generation III-V/Si hybrid ing doping at the GaAs and Si surfaces to be bonded is found to be Figure 3 | AlGaAs/Si dual-junction solar cell. (a) Cross-sectional schematic diagram of the fabricated AlGaAs/Si dual-junction solar cell. The thickness and doping concentration of each layer are indicated. The abbreviation BSF stands for back surface field. (b) Cross-sectional scanning electron microscope image of the solar cell. Inset shows the light I–V and power-voltage characteristics of the solar cell under a 600 nm-peaked halogen white light source of a one-sun intensity (100 mW cm ). SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 4 www.nature.com/scientificreports 6. Yoon, J. et al. GaAs photovoltaics and optoelectronics using releasable multilayer significant for enhancing the GaAs/Si interfacial conductivity and epitaxial assemblies. 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License: This work is licensed under a Creative Commons Acknowledgements 1 Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this The authors thank Denis Guimard and Masao Nishioka for growing the p -GaAs bonding license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ layer. This research was supported by the JSPS through the FIRST Program and Kakenhi 23760303, MEXT Japan through the Special Coordination Funds for Promoting Science How to cite this article: Tanabe, K., Watanabe, K. & Arakawa, Y. III-V/Si hybrid photonic and Technology, and Intel Corporation. devices by direct fusion bonding. Sci. Rep. 2, 349; DOI:10.1038/srep00349 (2012). SCIENTIFIC REPORTS | 2 : 349 | DOI: 10.1038/srep00349 6

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Published: Apr 2, 2012

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