Graphene-Quantum-Dot-Mediated Semiconductor Bonding: a Route to Optoelectronic Double Heterostructures and Wavelength-Converting Interfaces

Journal of C Carbon Research

Communication Graphene-Quantum-Dot-Mediated Semiconductor Bonding: A Route to Optoelectronic Double Heterostructures and Wavelength-Converting Interfaces

Kosuke Nishigaya, Kodai Kishibe and Katsuaki Tanabe *

Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan * Correspondence: [email protected]

Received: 22 April 2020; Accepted: 6 May 2020; Published: 9 May 2020

Abstract: A semiconductor bonding technique that is mediated by graphene quantum dots is proposed and demonstrated. The mechanical stability, electrical conductivity, and optical activity in the bonded interfaces are experimentally verified. First, the bonding scheme can be used for the formation of double heterostructures with a core material of graphene quantum dots. The Si/graphene quantum dots/Si double heterostructures fabricated in this study can constitute a new basis for next-generation nanophotonic devices with high photon and carrier confinements, earth abundance, environmental friendliness, and excellent optical and electrical controllability via silicon clads. Second, the bonding mediated by the graphene quantum dots can be used as an optical-wavelength-converting semiconductor interface, as experimentally demonstrated in this study. The proposed fabrication method simultaneously realizes bond formation and interfacial function generation and, thereby, can lead to efficient device production. Our bonding scheme might improve the performance of optoelectronic devices, for example, by allowing spectral light incidence suitable for each photovoltaic material in multijunction solar cells and by delivering preferred frequencies to the optical transceiver components in photonic integrated circuits.

Keywords: wafer bonding; interface; graphene; quantum dot; wavelength conversion; double heterostructure

1. Introduction Quantum dots, often referred to as artificial atoms, exhibit extraordinary electronic and optical properties, owing to the three-dimensional confinement of electrical carriers and the discretized density of states [1]. Hence, they enable various applications, including high-performance optoelectronic devices [2–8] and single-electron manipulation [9–12]. The graphene quantum dot (GQD) [13–15] is a quantum dot that originates from carbon, which is a low-cost, earth-abundant, and environmentally friendly material. In a manner that is similar to other quantum dots, GQD is known to exhibit various electrical and optical characteristics that are based on its size and utilized for various applications [13–17]. However, GQD materials are used independently in the devices as reported to date, and such devices suffer from considerable electrical and optical losses, due to the carrier recombination and photon scattering at the surfaces and interfaces. Double heterostructures [18,19] that permit carrier and photon confinements are employed for practical optoelectronic devices to address this issue [5,8,20]. Semiconductor wafer bonding is a useful fabrication method that is employed in various electronic and photonic applications [21–23]. The bonding technique is used to form heterostructures of dissimilar semiconductor materials with high crystalline qualities, while the conventional epitaxial growth method inevitably generates substantial levels of defect densities due to crystalline lattice mismatches.

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Thus, semiconductor bonding is a promising technique for realizing high-performance semiconductor optoelectronics. Hence, it is employed in the fabrication of a variety of devices, such as light-emitting diodes [24,25], lasers [4,26,27], photodetectors [28,29], and solar cells [27,30,31]. In the present study, we fabricate a Si/GQD/Si double heterostructure via semiconductor wafer bonding, towards the realization of high-efficiency nano-optoelectronic devices. Additionally, photonic up and down conversions by GQDs were recently reported, as a notable optical property of the GQD [32–35]. Therefore, in this context, the proposed GQD-mediated bonding scheme can also introduce wavelength-converting functionalities into semiconductor interfaces, as a concept to simultaneously provide bond formation and interfacial function generation. We previously performed wafer bonding mediated by an organic fluorescent material, 4,7-bis(4-tert-butylphenyl)-2-octylbenzotriazole [36]. However, the GQDs can exhibit more advantages in terms of structural simplicity, material cost, and environmental friendliness, as a wavelength converter. This type of a semiconductor-bonding scheme can improve the performance and structural flexibility of optoelectronic devices, such as solar cells, by allowing spectral light incidence that is suitable for each photovoltaic material, and photonic integrated circuits, by delivering the respective preferred frequency to each of the optical amplifier, modulator, waveguide, and detector materials.

2. Materials and Methods We used a commercially available aqueous solution of colloidal GQDs with a concentration of 0.1 w/v % (GS Alliance Co., Ltd., Kawanishi, Japan) in the study. Silicon, which is a versatile semiconductor, was adopted as the cladding material. We investigate two approaches as the methods to introduce interfacial GQDs: (1) bond formation via directly coating the colloidal GQD solution on the semiconductor wafers and (2) bonding via GQDs that are embedded in a hydrogel material. We performed all of the experimental processes in the study in a non-cleanroom, regular experimental 3 room with a particle density of approximately five-million m− , which we measured with a regular particle counter. We used epi-ready-grade, single-side-polished p-type Si wafers (thickness: 280 µm, crystalline plane orientation: <100>, dopant: boron, doping concentration: ~1 1019 cm 3) for most × − of the experiments in this study, unless noted otherwise. The surfaces of the polished sides of the Si wafers were first coated with a photoresist film to protect the Si surfaces from scratches and particulates during the process of being diced into approximately 1-cm2 pieces. The diced wafers were submerged in acetone for five min. to remove the photoresist film and degrease the Si surfaces. Some of the Si pieces were then subjected to a wet hydrofluoric (HF) treatment (10% aq, 1 min.) to remove the SiO2 native oxide layer formed on the Si surfaces. In some experiments, we used a hydrogel material as an adhesive and viscous organic matrix to embed the GQDs. In this case, a 2, 4, or 6 w/v % polyacrylamide (PAM) aqueous solution was prepared via mixing PAM powder with deionized water and adequately stirring in order to prevent the aggregation of the adhesive PAM particles. Equal volumes of GQD aq. and PAM aq. were mixed, which resulted in a 0.05 w/v % GQD hydrogel (PAM: 1, 2, or 3 w/v %). The bare GQD aq. or prepared hydrogel containing GQDs was then coated onto the polished-side surface of a Si piece. The GQD-coated Si piece was then bonded to the surface of the polished side of a bare Si piece under a uniaxial pressure of 0.1 MPaG in ambient air for 3 h. The bonding experiments were performed at various temperatures in the range of 20–500 ◦C, and only at room temperature for the GQD-only and hydrogel-mediated cases, respectively. The heating and cooling rates were approximately 10 ◦C/min. After the bonding, detachment normal stresses were measured for the bonded samples in order to represent bonded interfacial mechanical strengths. We connected an outer surface of the bonded sample to a digital spring weight scaler via a solid wire that was firmly attached to the sample surface while using household adhesive glue. Subsequently, we pulled the scaler outward in a direction normal to the sample die until the bonded sample was debonbed, while the weight scaler recorded the maximum force at the point of delamination. For electrical measurements, an Au–Ge–Ni alloy (80:10:10 wt %) and pure Au layers with thicknesses of 30 and 150 nm, respectively, were sequentially C 2020, 6, 28 3 of 10 depositedC 2020, 6, x via FOR thermal PEER REVIEW evaporation on both of the outer surfaces of the bonded Si pieces, as3 of ohmic 11 electrodes. In this manner, Au/Au–Ge–Ni/Si contacts were formed and covered the entire Si surfaces deposited via thermal evaporation on both of the outer surfaces of the bonded Si pieces, as ohmic ofelectrodes. the bonded In samples.this manner, We Au/Au– did notGe–Ni/Si apply any contacts annealing were formed for the and contacts covered to the prevent entire theSi surfaces potential heatingof the e ffbondedects to thesamples. bonded We interfacial did not apply characteristics. any annealing Subsequently, for the contacts we measured to prevent the current–voltagethe potential characteristicsheating effects across to the the bonded bonded interfacial interfaces. characte For opticalristics. measurements Subsequently, of we GQDs, measured a sample the containingcurrent– 3 wvoltage/v % of PAMcharacteristics between glassacross plates the bonded was prepared interfaces. in theFor same optical manner measurements as above, albeitof GQDs, with a nosample surface pretreatment.containing 3 The w/vphotoluminescence % of PAM between spectraglass plates from was the bondedprepared glass in the samples same manner were then as measuredabove, albeit with a Siwith charge-coupled no surface pretreatment. device array The spectrometer photoluminesce undernce the spectra irradiation from the of bonded a 380-nm-peaked glass samples ultraviolet were lamp.then In measured this study, with for a the Si charge-coupled purpose of simplicity, device wearray used spectrometer Si wafers asunder representative the irradiation semiconductor of a 380- materials.nm-peaked However, ultraviolet the proposedlamp. In methodthis study, can for be easilythe purpose extended of tosimplicity, other semiconductors, we used Si wafers given as that numerousrepresentative wafer-bonding semiconductor experimental materials. demonstrations However, the proposed between dissimilarmethod can materials be easily are extended reported to to dateother [4,26 semiconductors,–31]. given that numerous wafer-bonding experimental demonstrations between dissimilar materials are reported to date [4,26–31]. 3. Results and Discussion 3. Results and Discussion 3.1. Bonding via Bare GQDs 3.1. Bonding via Bare GQDs First, we present the results of the bonding approach: (1) namely, bond formation via directly coatingFirst, the colloidalwe present GQD the solutionresults of ontothe bonding the semiconductor approach: (1) wafers. namely, Figure bond 1formationa shows avia plane-view directly scanningcoating electronthe colloidal microscope GQD solution image onto of the the GQDs semiconductor on a Si wafer. wafers. The Figure samples 1a shows for observation a plane-view were lightlyscanning coated electron with microscope Au via sputtering image of tothe prevent GQDs on electric a Si wafer. charge-up The samples during for electron observation microscopy. were It waslightly observed coated thatwith theAuGQDs, via sputtering with diameters to prevent and electric an areal charge-up density during of approximately electron microscopy. 10–20 nm andIt was10 observed2 that the GQDs, with diameters and an areal density of approximately 10–20 nm and 8 8 10 cm− , respectively, were uniformly dispersed on the Si wafer before bonding. Figure1b shows ×× 1010 cm–2, respectively, were uniformly dispersed on the Si wafer before bonding. Figure 1b shows a cross-sectional image, which was taken by a regular scanning electron microscope detecting secondary a cross-sectional image, which was taken by a regular scanning electron microscope detecting electrons, of the bonded interface, for a sample bonded at 300 ◦C with HF surface pretreatment. The Si secondary electrons, of the bonded interface, for a sample bonded at 300 °C with HF surface wafers are uniformly and firmly intact, with sufficient endurance during the cleaving process, as shown pretreatment. The Si wafers are uniformly and firmly intact, with sufficient endurance during the in the figure. Figure1c shows a magnified view of the bonded interface of Figure1b. An ensemble of cleaving process, as shown in the figure. Figure 1c shows a magnified view of the bonded interface GQDs is stably confined at the interface of the bonded pair Si wafers, thereby forming a Si/GQDs/Si of Figure 1b. An ensemble of GQDs is stably confined at the interface of the bonded pair Si wafers, doublethereby heterostructure, forming a Si/GQDs/Si as shown double in the heterostructure, figure. as shown in the figure.

FigureFigure 1. 1.(a )(a Plane-view) Plane-view scanning scanning electron electron microscopemicroscope im imageage of of the the graphene graphene quantum quantum dots dots (GQDs) (GQDs) onon a Si a Si wafer; wafer; (b ()b Cross-sectional) Cross-sectional scanning scanning electronelectron microscope image image of of the the bonded bonded interface, interface, for for the the

samplesample with with wet wet hydroﬂuoric hydrofluoric (HF) (HF) surface surface pretreatmentpretreatment bonded bonded at at 300 300 °C◦C for for bonding bonding approach approach (1); (1); and,and, (c) ( Magniﬁedc) Magnified view view of of the the bonded bonded interface interface ofof ((b).b).

FigureFigure2 shows 2 shows typical typical current–voltage current–voltage characteristics characteristics across the bonded bonded interfaces interfaces for for various various bondingbonding conditions. conditions. TheThe ohmicohmic electricalelectrical property, which is is suitable suitable for for device device applications, applications, was was obtainedobtained for for the the bonding bonding conditions conditions withwith HFHF surfacesurface pretreatment, as as observed observed inin the the straight straight current–voltage curves. We observed that the samples bonded at 300 °C, the central bonding current–voltage curves. We observed that the samples bonded at 300 ◦C, the central bonding temperature,temperature, with with and and without without HF HF surface surface pretreatmentpretreatment exhibited exhibited interfacial interfacial electrical electrical resistivities resistivities of of approximately 1 and 20 Ω·cm2, respectively. This result indicates that the HF surface pretreatment approximately 1 and 20 Ω cm2, respectively. This result indicates that the HF surface pretreatment provides significantly higher· interfacial electrical conductivity, which can be attributed to the provides signiﬁcantly higher interfacial electrical conductivity, which can be attributed to the removal removal of the electrically insulating native oxide layer of SiO2 on the Si wafer surface via the HF C 2020, 6,C 28 2020, 6, x FOR PEER REVIEW 4 of 10 4 of 11

C 2020, 6, x FOR PEER REVIEW 4 of 11 solution. The higher interfacial electrical conductivity is preferable for most device applications; and of the electrically insulating native oxide layer of SiO on the Si wafer surface via the HF solution. therefore, we focused on the bonding with HF2 pretreatment. Figure 3 plots the dependence of the The highersolution. interfacial The higher electrical interfacial conductivity electrical conductivi is preferablety isfor preferable most device for most applications; device applications; and therefore, and we therefore,interfacial we focusedelectrical on theresistivity bonding onwith the HF bondinpretreatment.g temperature Figure 3 plots for thethe dependence samples withof the HF surface focused on the bonding with HF pretreatment. Figure3 plots the dependence of the interfacial electrical interfacialpretreatment. electrical It shouldresistivity be notedon the that bondin the gcurrent–voltage temperature for data the (suchsamples as thatwith inHF Figure surface 2) includes all resistivity on the bonding temperature for the samples with HF surface pretreatment. It should be noted pretreatment.series resistances It should through be noted the that sample. the current–voltage Hence, we in datadependently (such as that determined in Figure 2) the includes contact all resistance of that the current–voltage data (such as that in Figure2) includes all series resistances through the sample. seriesthe resistancesmetal electrode/semiconductor through the sample. Hence, interfaces we independently while using determined the transmission the contact resistanceline method, of and then Hence,the we metal independently electrode/semiconductor determined interfaces the contact while resistance using the oftransmission the metal line electrode method,/semiconductor and then determined the nominal resistivity at the bonded interface by subtracting it from the slope of the interfacesdetermined while usingthe nominal the transmission resistivity at line the method, bonded andinterface then by determined subtracting the it from nominal the slope resistivity of the at the current–voltage curve. A minimum interfacial resistivity is observed in terms of the bonding bondedcurrent–voltage interface by subtractingcurve. A minimum it from theinterfacial slope of resistivity the current–voltage is observed curve.in terms A minimumof the bonding interfacial temperature, as shown in Figure 3. This behavior can be attributed to the trade-off between more resistivitytemperature, is observed as shown in terms in Figure of the 3. bonding This behavior temperature, can be attributed as shown to in the Figure trade-off3. This between behavior more can be stable bond formation and GQD or Si oxidation at higher temperatures. As the highest interfacial attributedstable tobond the formation trade-oﬀ andbetween GQD moreor Si oxidation stable bond at higher formation temperatures. and GQD As ortheSi highest oxidation interfacial at higher electrical conductivity measured, we obtained an interfacial resistivity of2 0.26 Ω·cm2 for the bonding temperatures.electrical conductivity As the highest measured, interfacial we obtained electrical an conductivityinterfacial resistivity measured, of 0.26 we Ω·cm obtained for the an bonding interfacial temperaturetemperature of 300of 300°C, while°C, while the plots the inplots Figure in Figure3 show 3the show average the valuesaverage for valueseach bonding for each bonding resistivity of 0.26 Ω cm2 for the bonding temperature of 300 C, while the plots in Figure3 show the temperature.temperature. Such· Such a high a highinterfacial interfacial electrical electrical conductance conductance◦ value is consideredvalue is considered as being highly as being highly average values for each bonding temperature. Such a high interfacial electrical conductance value preferablepreferable for mostfor most optoelectronic optoelectronic device device applications, applications, such as solar such cells as solar [37,38]. cells [37,38]. is considered as being highly preferable for most optoelectronic device applications, such as solar cells [37,38].

0.10 0.10 With HF With HF o With HF o With HF ) 300 C 100 C -2 o o 0.05) 300 C 100 C -2 0.05

0.00 0.00Without HF o 300 CWithout HF -0.05 o

Current density (A cm With300 HFC -0.05 o 500 C Current density (A cm With HF -0.10 -1.0 -0.5500 oC 0.0 0.5 1.0 -0.10 Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Figure 2. Current–voltage characteristics acrossVoltage the bonded (V) interfaces with HF surface pretreatment bonded at 100, 300, and 500 °C, and without HF surface pretreatment bonded at 300 °C for bonding Figure 2. Current–voltage characteristics across the bonded interfaces with HF surface pretreatment approach (1). bonded atFigure 100, 300,2. Current–voltage and 500 ◦C, and characteristics without HF surfaceacross the pretreatment bonded interfaces bonded atwith 300 HF◦C surface for bonding pretreatment approachbonded (1). at 100, 300, and 500 °C, and without HF surface pretreatment bonded at 300 °C for bonding approach (1). )

2 5 cm Ω 4 )

2 5 cm

3 Ω 4

2 3 1 2

Interfacial electrical resistivity ( electrical resistivity Interfacial 0 0 100 200 300 400 500 600 1 Bonding temperature (oC)

Figure 3. Dependence of the interfacial electrical resistivity on bonding temperature for the samples

with HF surface pretreatment for ( electrical resistivity Interfacial 0 bonding approach (1). 0 100 200 300 400 500 600 Bonding temperature (oC)

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Figure 3. Dependence of the interfacial electrical resistivity on bonding temperature for the samples with HF surface pretreatment for bonding approach (1). C 2020, 6, 28 5 of 10 Figure 4 shows the dependence of the interfacial mechanical bonding strength on the bonding temperatureFigure4 shows for the the samples dependence with HF of surface the interfacial pretreatment. mechanical No bonding bonding was strength formed on or the the bonding bonding temperaturestrengths were for the lower samples than with our HF detection surface pretreatment.limit with respect No bonding to the was bonded formed samples or the bondingat room strengthstemperature. were lowerFigure than 4 clearly our detection indicates limit that with th respecte increase to the in bonded bonding samples temperature at room increases temperature. the Figureinterfacial4 clearly mechanical indicates stability. that the increase The bonding in bonding temper temperatureatures at increasesor above the300 interfacial °C were mechanicalobserved to provide sufficient interfacial mechanical stability for practical device applications. With respect to the stability. The bonding temperatures at or above 300 ◦C were observed to provide suﬃcient interfacial mechanicalhighest data stability measured, for practical we obtained device an applications. interfacial mechanical With respect strength to the of highest up to data340 kPa measured, for the webonding obtained temperature an interfacial of 300 mechanical °C, while strengththe plots ofin upFigure to 340 4 show kPa forthe the average bonding values temperature for each bonding of 300 temperature. In addition, the optical property of wavelength conversion was observed as severely ◦C, while the plots in Figure4 show the average values for each bonding temperature. In addition, thedegraded optical propertyfor the bonded of wavelength glass samples conversion that were was observedprepared asfor severely the optical degraded measurement for the bonded for approach glass samples(1), and thatthis wereis presumably prepared due for the to the optical aggregation measurement of GQDs for approachduring the (1), drying and thisout isof presumablythe solution due[39]. to the aggregation of GQDs during the drying out of the solution [39].

200

150

100

50 Interfacial bonding strength (kPa) strength bonding Interfacial 0 0 100 200 300 400 500 600 Bonding temperature (oC)

Figure 4. Dependence of the interfacial mechanical bonding strength on bonding temperature for the samplesFigure 4. with Dependence HF surface of pretreatment the interfacial for mechanical bonding approach bonding (1).strength on bonding temperature for the samples with HF surface pretreatment for bonding approach (1). 3.2. Bonding via Embedding GQDs in Hydrogel

Second, we present the results for approach (2), wherein we perform bonding by embedding GQDs in a hydrogel material. Figure5 shows typical current–voltage characteristics across the bonded interfaces for various bonding conditions. In a manner that is similar to the case of the bonding approach (1), the ohmic electrical property was obtained for the bonding conditions with HF surface pretreatment. Figure6 shows the dependence of the interfacial mechanical bonding strength and electrical resistivity on the PAM concentration for the samples with HF surface pretreatment bonded at room temperature. It is noted that the employment of hydrogel can enable mechanically stable bond formation at room temperature [40–42], which was conversely diﬃcult in the case of the bonding approach (1). The hydrogen bonds stemming from PAM presumably causes adhesion to semiconductor surfaces [43,44]. Speciﬁcally, hydrogen bonds can form between the -NH2 groups of PAM and the Si surface terminated by the -OH groups due to the water contained in PAM [43]. In addition, the PAM matrix holds the GQDs and suppresses their sedimentation owing to its viscosity induced by the entanglement of PAM polymer chains. Other hydrogel materials, such as agarose and polyvinyl alcohol, may also be used as bonding agents [42]. The interfacial bonding mechanical strength was observed to gradually decrease when the PAM concentration increases, which is potentially attributed to the spatial non-uniformity of the PAM at the bonded interface due to the increased viscosity of the PAM solution, as shown in Figure6. As the highest data measured, we obtained an interfacial mechanical strength of up to 350 kPa for the case without the HF pretreatment, a PAM concentration of 3 w/v %, and bonding at room temperature. C 2020, 6, x FOR PEER REVIEW 6 of 11

3.2. Bonding via Embedding GQDs in Hydrogel Second, we present the results for approach (2), wherein we perform bonding by embedding GQDs in a hydrogel material. Figure 5 shows typical current–voltage characteristics across the bonded interfaces for various bonding conditions. In a manner that is similar to the case of the bonding approach (1), the ohmic electrical property was obtained for the bonding conditions with HF surface pretreatment. Figure 6 shows the dependence of the interfacial mechanical bonding strength and electrical resistivity on the PAM concentration for the samples with HF surface pretreatment bonded at room temperature. It is noted that the employment of hydrogel can enable mechanically stable bond formation at room temperature [40–42], which was conversely difficult in the case of the bonding approach (1). The hydrogen bonds stemming from PAM presumably causes adhesion to semiconductor surfaces [43,44]. Specifically, hydrogen bonds can form between the -NH2 groups of PAM and the Si surface terminated by the -OH groups due to the water contained in PAM [43]. In addition, the PAM matrix holds the GQDs and suppresses their sedimentation owing to its viscosity induced by the entanglement of PAM polymer chains. Other hydrogel materials, such as agarose and polyvinyl alcohol, may also be used as bonding agents [42]. The interfacial bonding mechanical strength was observed to gradually decrease when the PAM concentration increases, which is potentially attributed to the spatial non-uniformity of the PAM at the bonded interface due to the increased viscosity of the PAM solution, as shown in Figure 6. As the highest data measured, we obtained an interfacial mechanical strength of up to 350 kPa for the case without the HF pretreatment, a PAM concentration of 3 w/v %, and bonding at room temperature. C 2020, 6, 28 6 of 10

0.10 With HF PAM 1 w/v% ) -2 0.05 Without HF PAM 2 w/v% 0.00 With HF PAM 2 w/v% -0.05 Current density (A cm With HF PAM 3 w/v% -0.10 -0.5 0.0 0.5 Voltage (V)

Figure 5. Current–voltage characteristics across the bonded interfaces with HF surface pretreatment Figure 5. Current–voltage characteristics across the bonded interfaces with HF surface pretreatment C 2020,bonded 6, x FOR atPEER room REVIEW temperature with polyacrylamide (PAM) concentrations corresponding to 1, 2, and7 of 11 3 w/v %,bonded and without at room HF surfacetemperature pretreatment with polyacrylamide bonded at room (PAM) temperature concentrations with aPAM corresponding concentration to 1, 2, and of 2 w/v %3 w/v for %, bonding and without approach HF surface (2). pretreatment bonded at room temperature with a PAM concentration of 2 w/v % for bonding approach (2). )

200 5 2 cm Ω 4 150

3 100 2 ( resistivity ical

50 1 Interfacial bonding strength (kPa) bonding strength Interfacial

0 0 electr Interfacial 01234 PAM concentration (w/v%)

Figure 6. Dependences of the interfacial mechanical bonding strength and electrical resistivity on PAM Figureconcentration 6. Dependences for the samples of the withinterfacial HF surface mechanical pretreatment bonding bonded strength at room and temperatureelectrical resistivity for bonding on PAMapproach concentration (2). for the samples with HF surface pretreatment bonded at room temperature for bonding approach (2). With respect to the interfacial electrical conductivity, in a manner that is similar to the case of the bondingWith respect approach to the (1), interfacial the HF surfaceelectrical pretreatment conductivity, was in observeda manner tothat reduce is similar the resistivity to the case from of theapproximately bonding approach 10 Ω cm (1),2 tothe 3 HFΩ cmsurface2 on average.pretreatme Nevertheless,nt was observed the to PAM reduce concentration the resistivity does from not · · approximatelysignificantly aff ect10 theΩ·cm interfacial2 to 3 Ω electrical·cm2 on average. conductivity, Nevertheless, as shownin the Figure PAM6. Asconcentration the highest interfacialdoes not significantlyelectrical conductivity affect the measured,interfacial weelectrical obtained conductivity, an interfacial as shown resistivity in ofFigure 0.53 Ω6. cmAs2 thefor thehighest case · interfacialwith HF pretreatment, electrical conductivity a PAM concentration measured, ofwe 1 obtainedw/v %, and an bondinginterfacial at roomresistivity temperature, of 0.53 Ω while·cm2 for the theplots case in with Figure HF6 show pretreatment, the average a PAM values concentration for each condition. of 1 w/v The %, and realization bonding of at this room level temperature, of electrical whileconductivity the plots is in considered Figure 6 show to be the preferable average for values many for optoelectronic each condition. device The applications,realization of suchthis level as solar of electricalcells [37, 38conductivity]. PAM, a hydrophilic is considered polymer to be material,preferable can for contain many water,optoelectronic and the water-originateddevice applications, ions suchact as as electrical solar cells carriers [37,38]. to PAM, induce a electricalhydrophilic conductance polymer material, [44]. Reference can contain 42 discusses water, theand influencethe water- of originatedthe presence ions of act water as electrical in the hydrogel carriers on to electronic induce electrical component conductance fabrication [44]. and Reference characteristics. 42 discusses the influenceIn contrast of tothe approach presence (1), of nowater degradation in the hydrogel was observed on electronic in the optical component property, fabrication even after and the characteristics.dry out of the samples for bonding approach (2), due to the use of the hydrogel material. This result In contrast to approach (1), no degradation was observed in the optical property, even after the dry out of the samples for bonding approach (2), due to the use of the hydrogel material. This result can be potentially attributed to the effects of the surface passivation and/or aggregation suppression of the GQDs [39,45]. Figure 7 shows the photoluminescence spectra of the bonded glass samples with and without GQDs. The mild twin peaks that were observed around 440 and 490 nm were presumably due to possible bimodal size distribution of the GQDs. The change in the spectrum of the photon count in Figure 7 verifies that the GQDs embedded in the bonded interface converts the incident light peaking at approximately 380 nm into another bundle of light peaking at approximately 450–500 nm. This type of optical wavelength conversion can be utilized in applications involving solar cells, where the interfacial GQDs absorb ultraviolet light (380 nm) and emit visible light (450– 500 nm) to fit the solar spectral irradiance to the crystalline silicon absorption spectral sensitivity. However, the proposed GQD-bonding scheme can be used, irrespective of the material species. Hence, the most suitable combination of semiconductors can be selected on demand for each application. The tunability of the absorption and emission wavelengths via the quantum-dot size is advantageous for the adjustment of each application. Moreover, the use of an ensemble of dots with a size distribution can fit some of the applications, such as photovoltaics, in response to the broad solar spectrum. In this study, we demonstrated photonic down conversion by GQD, but not up conversion. However, with down conversion, the implementation of an efficient GQD-mediated interface in a multijunction solar cell might not be trivial because it would require a relatively large separation in the bandgap energies between the top and bottom subcells, such as that in an (Al) C 2020, 6, 28 7 of 10 can be potentially attributed to the effects of the surface passivation and/or aggregation suppression of the GQDs [39,45]. Figure7 shows the photoluminescence spectra of the bonded glass samples with and without GQDs. The mild twin peaks that were observed around 440 and 490 nm were presumably due to possible bimodal size distribution of the GQDs. The change in the spectrum of the photon count in Figure7 verifies that the GQDs embedded in the bonded interface converts the incident light peaking at approximately 380 nm into another bundle of light peaking at approximately 450–500 nm. This type of optical wavelength conversion can be utilized in applications involving solar cells, where the interfacial GQDs absorb ultraviolet light (380 nm) and emit visible light (450–500 nm) to fit the solar spectral irradiance to the crystalline silicon absorption spectral sensitivity. However, the proposed GQD-bonding scheme can be used, irrespective of the material species. Hence, the most suitable combination of semiconductors can be selected on demand for each application. The tunability of the absorption and emission wavelengths via the quantum-dot size is advantageous for the adjustment of each application. Moreover, the use of an ensemble of dots with a size distribution can fit some of the applications, such as photovoltaics, in response to the broad solar spectrum. In this study, C 2020, 6, x FOR PEER REVIEW 8 of 11 we demonstrated photonic down conversion by GQD, but not up conversion. However, with down conversion, the implementation of an efficient GQD-mediated interface in a multijunction solar cell (In)GaN–Si combination. In contrast, up-converting GQDs [32–34] could be more beneficial in such might not be trivial because it would require a relatively large separation in the bandgap energies an application. For two-dimensional graphene sheets, albeit not quantum dots, we previously between the top and bottom subcells, such as that in an (Al) (In)GaN–Si combination. In contrast, developed graphene-mediated semiconductor wafer bonding to form a Si/graphene/Si double up-converting GQDs [32–34] could be more beneficial in such an application. For two-dimensional heterostructure [46]. Hence, we demonstrated the preparation of two types of graphene-based- graphene sheets, albeit not quantum dots, we previously developed graphene-mediated semiconductor material-cored double heterostructures that can provide a basis for high-performance nanophotonic wafer bonding to form a Si/graphene/Si double heterostructure [46]. Hence, we demonstrated the devices in the future. preparation of two types of graphene-based-material-cored double heterostructures that can provide a basis for high-performance nanophotonic devices in the future.

With GQDs

Normalized photon count (-) count photon Normalized Without GQDs

300 400 500 600 700 Freespace wavelength (nm)

Figure 7. Photoluminescence spectra of the bonded glass samples with and without GQDs for bonding Figureapproach 7. Photoluminescence (2). spectra of the bonded glass samples with and without GQDs for bonding approach (2). 4. Conclusions 4. Conclusions In the study, we proposed and experimentally demonstrated a semiconductor bonding scheme, whichIn wasthe study, mediated we byproposed GQDs. and We investigatedexperimentally two demonstrated approaches as a methods semiconductor for introducing bonding interfacial scheme, whichGQDs, was namely mediated bond formation by GQDs. via We directly investigated coating the two colloidal approaches GQD solution as methods onto the for semiconductor introducing interfacialwafers and GQDs, bonding namely via GQDs bond embeddedformation via in adirect hydrogelly coating material. the colloidal For both GQD cases, solution we analyzed onto the the semiconductordependence of wafers the interfacial and bonding mechanical via GQDs bonding embedde strengthd in a and hydrogel electrical material. conductivity For both on cases, bonding we analyzedconditions, the anddependence obtained of those the interfacial suﬃcient mechanical for practical bonding device strength applications. and electrical The use conductivity of hydrogel onas bonding a matrix conditions, agent provided and obtained stable bond those formation sufficient atfor room practical temperature. device applications. We observed The ause clear of hydrogeloptical wavelength as a matrix conversionagent provided by GQDs stable at bond the semiconductorformation at room interface, temperature. and this We can observed lead to variousa clear optical wavelength conversion by GQDs at the semiconductor interface, and this can lead to various applications for photon management in integrated devices. The process technique simultaneously allows for bonding formation and interfacial function generation, thereby leading to efficient device production. The proposed bonding scheme can also provide a pathway to fabricate highly efficient double heterostructured optoelectronic devices, which employ GQDs as emitters or absorbers, such as light-emitting diodes [47–49], lasers, optical amplifiers, modulators, photodetectors, and solar cells.

Funding: This research was funded by the Fujikura Foundation, the Kato Foundation for Promotion of Science, the Murata Science Foundation, and the Japan Society for the Promotion of Science (JSPS). C 2020, 6, 28 8 of 10 applications for photon management in integrated devices. The process technique simultaneously allows for bonding formation and interfacial function generation, thereby leading to efficient device production. The proposed bonding scheme can also provide a pathway to fabricate highly efficient double heterostructured optoelectronic devices, which employ GQDs as emitters or absorbers, such as light-emitting diodes [47–49], lasers, optical amplifiers, modulators, photodetectors, and solar cells.

Author Contributions: Conceptualization, K.T.; methodology, K.N., K.K. and K.T.; validation, K.N.; formal analysis, K.N.; investigation, K.N. and K.T.; resources, K.T.; data curation, K.N. and K.T.; writing—original draft preparation, K.N. and K.T.; writing—review and editing, K.N., K.K. and K.T.; visualization, K.N. and K.T.; supervision, K.T.; project administration, K.T.; funding acquisition, K.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Fujikura Foundation, the Kato Foundation for Promotion of Science, the Murata Science Foundation, and the Japan Society for the Promotion of Science (JSPS). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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