We fabricated the two color surface emitting device of a coupled cavity structure which is applicable to terahertz light source. GaAs/AlGaAs vertical multilayer cavity structures were grown on a (001) and (113)B GaAs substrates and the coupled multilayer cavity structure was fabricated by wafer-bonding of them. The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-color emission of cavity-mode lights. The bonding position was optimized for the equivalent intensity of two-color emission. We formed a current injection structure, and two-color emission was observed by current injection, although lasing was not observed

Isu, Toshiro

Kitada, Takahiro

Kumagai, Naoto

Lu, Xiangmeng

Ota, Hiroto

Tokushima University Institutional Repository

Japanese Journal of Applied Physics REGULAR PAPERFabrication of two - color surface emitting device of a coupledvertical cavity structure with InAs QDs formed by wafer - bondingHiroto Ota , Xiangmeng Lu1, Naoto Kumagai1, Takahiro Kitada1, and Toshiro Isu1Center for Frontier of Engineering, Graduate School of Advanced Technology and Science,1Institute of Technology and Science, Tokushima University, Minami-Jyosanjima-Cho, Tokushima770-8506, JapanWe fabricated the two color surface emitting device of a coupled cavity structure which is applicable toterahertz light source. GaAs/AlGaAs vertical multilayer cavity structures were grown on a (001) and (113)BGaAs substrates and the coupled multilayer cavity structure was fabricated by wafer-bonding of them.The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-coloremission of cavity-mode lights. The bonding position was optimized for the equivalent intensity of two-color emission. We formed a current injection structure, and two-color emission was observed by currentinjection, although lasing was not observed.1. IntroductionTerahertz light sources based on semiconductor materials have been studied for a wide rangeof possible application fields including wireless communication, spectroscopy, and transmis-sion imaging.1–5) Several types of semiconductor devices, such as quantum cascade lasers(QCLs),6–9) resonant tunneling diodes (RTDs),10,11) and photo-mixers,12,13) have been studiedand developed as continuous-wave (CW) terahertz light sources. Although there has beensignificant progress on terahertz QCLs, there still remain various problems in each type ofdevice, such as insucient emission power, for higher frequency operation of RTDs, or lowoperation temperatures of QCLs.Ecient wavelength conversion based on dierence frequency generation (DFG) in groupIII-V compounds semiconductors is attractive for terahertz light sources operating at roomtemperature because of the large second-order nonlinearity. Terahertz DFG was demonstratedusing a GaP crystal excited by two individual lasers,14–16) and DFG from QCL was alsoreported recently.17)We have proposed a terahertz emission device of a coupled multilayer cavity structureE-mail address: c501538015@tokushima-u.ac.jp1/13The final authenticated version is available online at: https://doi.org/10.7567/JJAP.55.04EH09Jpn. J. Appl. Phys. REGULAR PAPERutilizing DFG of the two cavity modes.18,19) A GaAs=AlAs coupled multilayer cavity struc-ture, which had two equivalent cavity layers coupled by an intermediate distributed Braggreflector (DBR) multilayer, was grown on a (113)B GaAs substrate by molecular beam epi-taxy (MBE). Epitaxial growth on a non-(001) substrate is essential for terahertz DFG of thetwo cavity modes because the frequency mixing through the second-order nonlinear pro-cess is forbidden on a (001) substrate owing to crystal symmetry.20) Strong sum-frequencygeneration (SFG) in the near-infrared region21–23) and DFG in the terahertz region24–27) weresuccessfully demonstrated by simultaneous excitation of the two cavity modes in the cou-pled cavity structure using femtosecond laser pulses with a wide spectral width of 35 nm.From the viewpoint of device applications, the two modes should be generated inside of thecoupled cavity structure by current injection, which enable terahertz emission through theDFG process without external light sources. Figure 1 shows the schematic structure of THzwave emitting device. Recently, we fabricated GaAs=AlAs coupled multilayer cavities withInAs quantum dots (QDs) on a (001) GaAs substrate by MBE, and two-color emission byoptical pumping was successfully demonstrated at room temperature under CW operation.28)We have shown equivalent two-color emission intensities when the optical thicknesses of twocavity layers were exactly same as each other.29) When we grow the coupled cavity structurewith InAs QDs on the (113)B substrate, terahertz DFG should be generated without externallights. However, terahertz emission will be weakened because the radiated terahertz fields inthe two cavity layers largely cancel each other out when both cavity layers have the samenonlinear susceptibility. Such cancellation can be significantly eliminated when we introducesusceptibility inversion in the middle of the structure. This situation is realized that the cou-pled cavity structure is fabricated by direct wafer-bonding of two single-cavity structures. Infact, the signal enhancement due to the susceptibility inversion was confirmed by the simu-lation and experimental observation of terahertz DFG for the wafer-bonded coupled cavitysample fabricated using two (113)B epitaxial wafers.26)In this paper, we fabricated a GaAs=AlGaAs coupled multilayer cavity structure withInAs QDs by the surface-activated bonding method30,31) of two cavity structures grown indi-vidually. We found that the wafer-bonding position aected optical property of the bondedcoupled cavity, and realized that two-color emission of equivalent intensity from them. Wefabricated a current injection structure of the bonded epitaxial wafer and observed two-colorspontaneous emission by current injection.2/13Jpn. J. Appl. Phys. REGULAR PAPER2. A coupled multilayer cavity structure formed by wafer-bonding2.1 Fabrication of a coupled multilayer cavity structure formed by wafer-bondingA coupled cavity structure with InAs QDs formed by wafer-bonding at the middle of centerDBR shows in Fig. 2 inset. Two single multilayer cavity structures were grown on (001)-and (113)B-oriented undoped GaAs substrates, by MBE. Each structure contains a double -wavelength-thick (2  729nm) cavity layer. On the (113)B substrate, the bottom(28-period)and top(6.25-period) GaAs/Al0:9Ga0:1As =4-DBR multilayer were undoped. On the (001)substrate, we grew at first an AlAs/Al0:5Ga0:5As (5nm/98nm) etch-stopper structure. Thisstructure is used for the selective removal of the (001) GaAs substrate after the wafer-bonding.The 2 cavity of the (001) epi-wafer consists of three layer of self - assembled InAs QDsembedded in In0:15Ga0:85As wells and GaAs spacer layers. InAs QDs were expected to applyto equivalent gain for two modes, because QDs has wide gain range. The 2 cavity of the(001) has a lateral thickness variation by without substrate rotation during the growth. Thebottom (28-period) and the top (6.25-period) GaAs/Al0:9Ga0:1As =4-DBR multilayers weredoped by Be and Si, respectively. These GaAs=Al0:9Ga0:1As DBR layer had graded structuresat the interfaces between GaAs and Al0:9Ga0:1As layers, in order to decrease resistance ofthe DBR layers. The two epitaxial wafers were directly bonded by a conventional surface-activated bonding method. In this method, two wafer surface were conducted treatment by Arbeam. Both epitaxial wafer’s surface had GaAs layer which has =8 thickness. After bonding,the (001) GaAs substrate was completely removed by mechanical polishing and selective wetetching using a citric acid-based etchant and BHF etchant.2.2 Consideration for bonding positionFigure 2 shows the surface emission spectrum observed at an excitation power of 398 mWat room temperature. The excitation source for PL measurements was a multimode semicon-ductor laser with a nominal wavelength of 920nm, which was operated in cw mode. Thissemiconductor laser’s nominal wavelength was located outside the high reflection band. Thelaser beam was focused on the sample surface with a diameter of about 250 m. Two peakswere observed at 1284.7 nm and 1296.7 nm. The intensity of the short wavelength peak wassmaller than the long one.We calculated the internal electric fields.When the optical thicknesses of two cavity layerswere exactly same as each other, the ratio of the internal electric field intensity at the QDlayers was unity and frequency dierence became minimum. In previous report, we reportedthat the ratio of the two peak intensities by photo luminescence fitted with the ratio of the3/13Jpn. J. Appl. Phys. REGULAR PAPERinternal electric field intensity by calculated as shown in Ref. 29. Figure 3 shows the ratio ofthe electric field intensities at the QD layer versus the optical frequency dierence (brokenline). The experimental results of the peak intensity ratio of the two-colors versus the opticalfrequency dierence that depending on the wafer position are also shown in the figure (closedcircle). The ratio of the two peak intensities was not unity when frequency dierence wasminimum, although optical thicknesses of the two cavity layers are equivalent. Assuming thatthe bonding interface has absorbance layer: 2.5 nm (absorption coecient:  = 15000cm 1),two color emission ratio fit with solid line in Fig. 3.Figure 4 (a) shows the distribution of electric fields and refractive indices at the casewhen two epitaxial wafers were bonded at center of middle DBR. Position is a distance froma surface. A short wavelength’s electric field had maximum at bonding interface, while a longwavelength’s electric field had almost minimum at the bonding interface. Assuming opticalloss at the bonding interface, the short mode may have loss. Therefore, two epitaxial waferswere bonded where the distribution of electric fields of two modes had minimum. Figure 4(b) shows the distribution of electric fields and refractive indices at the just above 2 cavityon (113)B substrate. The first neighboring for =4 GaAs to a cavity layer is replaced with3=4 GaAs layer for the bonding interface. Electric fields of the both modes had minimum atnear bonding interface. In the case, the optical losses are considered negligible small.Figure 5 shows the surface emission spectrum of the sample bonded at the layer near thecavity layer. This structure contained nine layers of InAs QDs at the top cavity layer Top-and bottom-DBR layer also increased 28-pairs and 32-pairs, respectively. The laser beamdiameter was about 250 m and excitation power was 158 mW. Two mode emissions wereclearly observed at 1282.2 and 1293.9 nm. The mode frequency dierence between the twoemission modes was 2.1 THz, which was almost the same as the designed value. We observedtwo-color emission with equivalent intensities, resulting from that two epitaxial wafers werebonded at the position where the electric fields of the both modes were minimum.3. Two-color surface emitting device by a current injectionWe fabricated a device structure for current injection, using the bonding wafer which wasbonded at near (113)B side cavity as shown in Fig. 4 (b). The processes are shown below.A p-type metal (Ti/Au) electrode was deposited on the surface of the p-type DBR. Ti andAu were 5nm and 100nm, respectively. A mesa shape was formed on the substrate by wetetching. The p-type DBR was etched by phosphoric acid, the topside cavity layer was etchedby citric acid, and the n-type AlGaAs of the first layer of the middle DBR was etched diluted4/13Jpn. J. Appl. Phys. REGULAR PAPERphosphoric acid. The current confinement layer was formed by selective oxidation of an AlAalayer, which was grown just above the topside cavity layer, from the side wall of the mesastructure. An n-type metal (AuGe/Ni/Au) electrode was deposited on the GaAs surface ofthe n-type DBR. AuGe, Ni and Au were deposited 50 nm, 12.5 nm and 50 nm, respectively.Annealing was performed in N2 atmosphere at 703 K for 45 seconds. At the end, passivationlayer was coated.We investigated the optical property by current injection at room temperature. The spon-taneous emission spectrum of two color emitting device was shown in Fig. 6 left side. Theinjection current versus light output (I-L) curve was shown in Fig. 6 right side. Injection cur-rent source was pulsed operation with a duty of 0:20%(Period: 5 msec, Width: 10 sec). TheI-L measurement range was 0 mA   100 mA. In this measurement system, light output hasbias power at pulsed current 0mA. An emission image is shown in Fig. 6 inset. In emissionimage, two color emitting device operates CW mode. We succeeded to observe the two colorspontaneous emission with nearly equivalent intensities by current injection, although lasingwas not observed.4. ConclusionsWe fabricated a GaAs/AlGaAs coupled multilayer cavities with InAs QDs formed by wafer-bonding. Topside cavity layer included InAs QDs layers as optical gain materials. We foundthat two color emission intensities were not equivalent when the bonding interface was thecenter of middle DBR. We investigated distribution of the electric fields, and optimizedbonding position to improve the optical property of the coupled cavity. Two color emissionwith equivalent intensities was demonstrated. Using the bonded wafer, we fabricated a de-vice structure for current injection. We successfully observed two color emission with nearlyequivalent intensity by current injection, although lasing was not observed.AcknowledgmentThis work was partly supported by SCOPE (Strategic Information and Communications R &D Promotion Programme ) from the Ministry of Internal Aairs and Communications, Japan.5/13Jpn. J. Appl. Phys. REGULAR PAPERReferences1) S. Komiyama, Phys. Rev. Lett. 48, 271 (1982).2) R. Ko¨hler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A.Ritchie, R. C. Iotti, and F. Rossi, Nature 417, 156 (2002).3) M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida,Appl. Phys. Lett. 85, 3974 (2004).4) M. Tonouchi, Nat. 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(Color online) The schematic structure of THz wave emitting device.8/13Jpn. J. Appl. Phys. REGULAR PAPER1260 1280 1300 1320102103104105106 Emission intensity (counts)Wavelength (nm)CW@RTIexc: 398 mW			Fig. 2. (Color online)Surface emission spectrum of the coupled cavity that bond at center of middle DBR.The emission intensity is plotted on a log scale. A couple cavity structure, which was bonded at the center ofmiddle DBR, is shown inset.9/13Jpn. J. Appl. Phys. REGULAR PAPER1 2 3 4 50.010.1110100RatioFrequency (THz)Calculated value|E|2short /|E|2longα: 0 cm-1α: 15,000 cm-1Experimental value[Wafer-bonding]  Ishort / IlongFig. 3. (Color online) The closed circle shows relationship between peak intensity ratio of the experimentaltwo color emissions on the vertical axis and the frequency dierence on the horizon axis. The broken lineshows the ratio of the simulating internal electric field intensity at the QD layers versus the optical frequencydierence. The solid line shows the same one when the bonding interface has absorbance layer : 2.5 nm(absorption coecient:  = 15000 cm 1).10/13Jpn. J. Appl. Phys. REGULAR PAPER346.7 6.8 6.9 7.0 7.1 7.2 7.302004006008007.7 7.8 7.9 8.0 8.1 8.2 8.3n|E|2Position (µm) 1284.5 nm 1296.8 nm[f = 2.215 THz]Position (µm) 1284.6 nm 1296.7 nm[f = 2.184 THz](001) epi (113)B epi (001) epi (113)B epi(a) (b)Fig. 4. (Color online) Distribution of the optical intensities for two color and refractive indices at (a) thecenter of middle DBR and (b) the just above the 2 cavity on (113)B substrate. Position is a distance from asurface.11/13Jpn. J. Appl. Phys. REGULAR PAPER1260 1280 1300 1320102103104105106 Emission intensity (counts)Wavelength (nm)Iexc: 158 mWCW@RT			Fig. 5. (Color online) Surface emission spectrum of the coupled cavity by optical pumping. The emissionintensity is plotted on a log scale. A couple cavity structure, which was bonded at the just above (113)B-sidecavity layer, is shown inset.12/13Jpn. J. Appl. Phys. REGULAR PAPER1260 1270 1280 1290 13001031041050 20 40 60 80 10002468101214Emission intensity (counts)Wavelength (nm) 1.4mAPulsed oprationLight output(µW)Pulsed current(mA)Pulsed opration10 mACW@RTFig. 6. (Color online) The emission spectrum of two color emitting device (left side). The injection currentdependences of the light output (right side). The emission image of two color emitting device which operatesCW mode (right side inset).13/13

Fabrication of two - color surface emitting device of a coupled vertical cavity structure with InAs QDs formed by wafer - bonding

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Fabrication of two - color surface emitting device of a coupled vertical cavity structure with InAs QDs formed by wafer - bonding

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