We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states.Peer reviewe

García Martínez, Jorge Manuel

Holtz, P. O.

Monemar, B.

Paskov, P. P.

Petroff, Pierre M.

Schoenfeld, W. V.

Applied Physics Letters

Digital.CSIC

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 6 7 AUGUST 2000Photoluminescence up-conversion in InAsÕGaAs self-assembledquantum dotsP. P. Paskov,a) P. O. Holtz, and B. MonemarDepartment of Physics and Measurement Technology, Linko¨ping University, S-581 83 Linko¨ping, SwedenJ. M. Garcia, W. V. Schoenfeld, and P. M. PetroffMaterials Department, University of California, Santa Barbara, California 93106~Received 1 May 2000; accepted for publication 10 June 2000!We report up-converted photoluminescence in a structure with InAs quantum dots embedded inGaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both thedots and the wetting layer. The intensity of the up-converted luminescence is found to increasesuperlinearly with the excitation density. The results suggest that the observed effect is due to atwo-step two-photon absorption process involving quantum dot states. © 2000 American Instituteof Physics. @S0003-6951~00!02232-4#Photoluminescence ~PL! up-conversion in semiconduc-tor heterojunctions ~HJs! and quantum wells ~QWs!, i.e., theobservation of an emission at energies higher than that of theexcitation energy, has attracted much attention in the last fewyears.1–8 Up-conversion is a well-known phenomenon innonlinear optics, but processes like second-harmonic genera-tion and two-photon absorption ~TPA! occur primary at highexcitation intensities ~>kW/cm2! and are usually quite inef-ficient in continuous-wave ~cw! experiments.9 On the otherhand, a highly efficient PL up-conversion at extremely lowexcitation intensity ~0.1–10 W/cm2! has been observed insemiconductor heterostructures.1–8 In these experiments, thecarriers photoexcited in the narrow gap material are redistrib-uted into the wide gap material where they undergo recom-bination and give rise to an up-converted PL ~UPL!. For themechanisms that up convert the carriers, Auger process1–3and two-step, two-photon absorption ~TS-TPA! have beensuggested.4–8 In the Auger process, the energy released bythe electron-hole recombination is transferred to anotherelectron or hole. The excited carriers are ejected into thebarrier and after relaxation to the band edges they recombineradiatively to give UPL. The presence of a heteroboundarylifts the k-conservation requirement in the direction perpen-dicular to the interface and allows Auger recombinationwithout a thermal threshold. On the other hand, the TS-TPAmust be distinguished from the purely TPA in nonlinear op-tics, because the intermediate state is real and relaxation ofthe excitation to lower-lying real states may occur before thesecond photon is absorbed. The up-converted band-to-bandluminescence has also been previously observed in bulksemiconductors.10,11 The effect has been explained by a TS-TPA process involving a deep energy level as an intermedi-ate state. Recently, PL up-conversion has been reported forInP/GaInP self-assembled quantum dots ~QDs!12 and InP andCdSe colloidal QDs.13 In Ref. 12, QD emission in the pres-ence of electrical current flowing through the sample hasbeen observed with an optical excitation below the groundstate. The electrical current provides electrons, some ofwhich are trapped into the QDs, while the holes are opticallya!Electronic mail: plapa@ifm.liu.se8120003-6951/2000/77(6)/812/3/$17.00Downloaded 16 Jun 2010 to 161.111.235.252. Redistribution subject texcited via deep energy levels localized close to the QDs. Inthe case of colloidal QDs, the PL up-conversion has beenexplained by a microscopic mechanism that involves the sur-face states near the valence band and conduction bandedges.13Here we report on PL up-conversion in InAs/GaAs self-assembled QDs. We find an efficient PL from the GaAs bar-rier when the excitation energy is scanned within the absorp-tion band of the InAs dots. The excitation spectrum and thedensity dependence of the UPL are studied and the mecha-nism for this emission is proposed.The self-assembled InAs QD structures studied werefabricated by molecular beam epitaxy ~MBE! on ~100! GaAssubstrate.14 At the initial stage of the growth 40 periods ofGaAs/AlAs ~2 nm/2 nm! short period smoothing superlatticewas grown at the substrate temperature of 630 °C, followedby the deposition of a 200-nm-thick GaAs buffer layer atTg5580 °C with substrate rotation. The dots were formed bydepositing 1.7 monolayers of InAs at Tg5530 °C. A growthinterruption of 30 s was used to narrow down the size distri-bution. Then the dots were covered at 530 °C with a thinGaAs cap of a thickness 1 nm, followed by a second 30 sgrowth interruption. Atomic force microscopy ~AFM! aftersuch a growth procedure, revealed well defined islands oflateral dimension ’60 nm and height ’1.5 nm. A final 50-nm-thick GaAs capping layer was grown at a temperature of580 °C to further protect the QDs layer. The PL and PLexcitation ~PLE! measurements were done at low tempera-ture (T52 K) using a cw Ti-sapphire laser. The lumines-cence was dispersed by an 0.85 m double monochromator~SPEX 1404! and detected with a cooled InGaAsP/InP pho-tomultiplier tube ~Hamamatsu R5509-42! using a conven-tional lock-in techniques. The detection system provided aspectral resolution of 0.2 nm.Figure 1~a! shows the PL spectrum of the sample with anexcitation energy above the GaAs band gap (Eexc51.63 eV). The excitation intensity is Iexc580 W/cm2. Thespectrum involves both the QD emission and the emission ofthe GaAs barrier. A weak PL from the InAs wetting layer~WL! is also observed at EWL51.423 eV. The QD emissioncan be deconvoluted into three Gaussians with a full width athalf maximum ~FWHM! of 23–26 meV and peak energies© 2000 American Institute of Physicso AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp813Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000 Paskov et al.centered at 1.303, 1.348, and 1.385 eV. The peak at E051.303 eV originates from the ground-state emission, whilethe peaks at E151.348 eV and E251.385 eV correspond tothe electron-hole recombination at the first and the secondexcited states of QDs, respectively. The excited-state natureof the high energy peaks is unambiguously confirmed bymagneto-PL measurements where a splitting between thestates with different angular momentum has been observed.15The GaAs peaks at 1.515 and 1.490 eV are attributed tofree-exciton ~FE! and conduction-band-to-acceptor ~FB! re-combination, respectively. The small feature at 1.453 eV canbe assigned as a LO-phonon replica of the FB peak. The factthat the QD emission dominates over the GaAs PL impliesan efficient capture of the photoexcited carriers by the WLand afterward by QDs. However, the luminescence of QDscan be observed even at excitation below the WL emissionpeak. The PL spectrum at Eexc51.348 eV is shown in Fig.1~b!. The same excitation intensity as above band gap exci-tation is used. Now, the carriers are generated directly intothe first excited state of QDs and after relaxation recombinefrom the ground state. This resonant excitation selects onlythe dots of an appropriate size; namely, the dots with anexcited state energy that matches the excitation energy. As aresult, the QD emission intensity decreases more than oneorder of magnitude in comparison with the above band gapexcitation and the width of PL peak is reduced to 18 meV.The sharp line at 33.8 meV below laser excitation is due tothe Raman mode of a bulk GaAs phonon. More interestingly,the PL from the barrier can also be detected although theexcitation energy is well below the GaAs band gap. The rightpart of the spectrum in Fig. 1~b! clearly reveals the FB peakat 1.49 eV and its phonon replica, implying an up conversionof the exciting photon energy. We note that a RG850 long-pass filter was used after the laser to prevent the directlyexcitation of GaAs with the short-wavelength luminescenceof the Ti:sapphire laser. The spectrally integrated intensity ofthe UPL is about 2% of the total PL output. On the otherhand, the ratio of the intensities of the FB emission excitedbelow and above the band gap is of the order of 0.2, indicat-ing a high efficiency of the up-conversion process.FIG. 1. Photoluminescence spectra of the sample excited above the GaAsband gap ~a! and resonantly with the first excited state of QDs ~b!. Theexcitation intensity is Iexc580 W/cm2.Downloaded 16 Jun 2010 to 161.111.235.252. Redistribution subject tThe UPL is observed at different excitation energies inthe spectral range of the QDs emission. To clarify the physi-cal origin of the up-conversion process, we study the depen-dence of the UPL on the excitation energy. The emission at1.490 eV is monitored while varying the laser energy belowthe GaAs band gap. This ‘‘up-converted’’ PL excitation~UPLE! spectrum is shown in Fig. 2~a!. For a comparison,the ‘‘normal’’ PL excitation ~PLE! spectra detected at theground state (E051.303 eV) and the first excited state (E151.348 eV) of QDs is presented in Fig. 2~b!. The UPLEspectrum shows that the UPL signal has disappeared if thelaser is tuned below QD emission. The low-energy tail of thespectrum in Fig. 2~a! represents quite well the energy distri-bution of the first excited state of the QD ensemble. There-fore, the up-conversion process must be generated by theformation of electron-hole pairs in the excited states of QDsor in the WL. Indeed, the UPLE spectrum exhibits the samefeatures as the normal PLE spectra in Fig. 2~b!. There is anenhancement of the UPLE at excitation energies of 1.385and 1.43 eV which correspond to the absorption into thesecond excited state of QDs and the WL. The absorptionpeak at the first excited state of the QDs, however, is notresolved in the UPLE spectrum. This can be explained by thefact that when exciting between 1.348 and 1.385 eV dots ofdifferent sizes contribute to the UPL signal: the large dotsare excited in their second excited state and the small dotsare excited in their first excited state. In the PLE measure-ments, the dots are selected by both the excitation energy andthe detection energy. For completeness, we checked on thepossibility that the UPL is due to an excitation process in theGaAs barrier itself. The UPLE spectrum of a referencesample with the QDs and the wetting layer etched away,measured at same experimental conditions is shown in Fig.2~a!. In fact, the UPL is also detected in the referencesample, but its intensity is more than two orders lower andno clear structure in the spectral region of the QD emission isobserved.Our results imply the existence of an effective mecha-nism that kicks out the carriers from the QDs into the barrier.FIG. 2. ~a! Up-converted PLE spectra of the QD sample ~top spectrum! andthe reference sample without QDs ~bottom spectrum!. ~b! PLE spectra of theQDs detected at the ground state and the first excited state, respectively. Thesecond spectrum is shifted for clarity.o AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp814 Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000 Paskov et al.Since the measurements are performed at low temperature(T52 K), the thermal excitation of the carriers can be ruledout. Furthermore, the low excitation intensity used(Iex,200 W/cm2) makes the contribution of direct TPA neg-ligible. As a more plausible explanation of the UPL, we con-sider the TS-TPA mechanism. The up-conversion processstarts with the creation of electron-hole pairs in the excitedstates of the QDs. In the next step, the electron-hole pairsrelax to form excitons in the ground state. These excitonshave a very limited spatial extension due to the strong con-finement in QDs and can survive long enough to be excitedby another photon. The second absorption process createselectron-hole pairs with an energy higher than the GaAsband gap, which can reach the lowest barrier states beforethe relaxation process into QDs occurs. Then the carriers aretrapped again in the QDs or recombine radiatively in thebarrier giving rise to the UPL. A similar excitonic up-conversion process has been observed in CdTe/CdMgTeQWs.4 At higher excitation energies the excited states ofQDs can also participate as an initial state for the secondexcitation process. This consistently explains the enhance-ment of the UPL when exciting at the second excited state ofQDs and at the WL. The larger number of the availableintermediate states should increase the probability for theTS-TPA.Within the above model, the number of up-convertedelectron-hole pairs is proportional to the population of theintermediate states and to the number of photons responsiblefor the second excitation process. Assuming that the popula-tion of the QD states increases linearly with the excitationintensity, a quadratic intensity dependence of the UPLshould be expected. Figure 3 shows the excitation intensitydependence of the spectrally integrated UPL at 1.49 eV. Thedouble logarithmic plot indicates a linear regression with aslope close to 2 at low excitation intensities. When the exci-tation energy is within the QD excited states, a decrease ofthe slope at higher intensities is observed. This is probablydue to the saturation in the population of the intermediatestates for the TS-TPA process. The QD states can be occu-FIG. 3. Excitation intensity dependence of the spectrally integrated UPL atthree excitation photon energies. The lines show the linear regression in thedouble-logarithmic plot. The slope of each dependence is also indicated.Downloaded 16 Jun 2010 to 161.111.235.252. Redistribution subject tpied by a limited number of electron-hole pairs, i.e., two forthe ground state and six for the first excited states. Therefore,increasing the excitation intensity a situation can be reachedwhere all low lying states of QDs selected by the resonantexcitation are populated. It is seen from Fig. 3 that there isno change in the slope if the excitation is resonant with theWL. In this case, all dots in the ensemble are excited and thesaturation cannot occur at the intensities used in our experi-ment.It is worth noting that with above band gap excitation,the FE and FB GaAs peaks exhibit an almost linear intensitydependence. The slopes are found to be 1.1 and 0.85, respec-tively. Furthermore, the FE emission quenches at excitationintensities below 10 W/cm2. In the up-conversion process,the corresponding excitation levels are even lower because alimited number of carriers are excited into the barrier. This isthe reason that the FB peak is only observed in UPL spec-trum.The Auger recombination in QDs can also be consideredas a possible mechanism for the up-conversion. In this case,however, the UPL should exhibit a cubic dependence on theexcitation intensity, because both the electron-electron-hole~eeh! and the hole-hole-electron ~hhe! processes are requiredto excite one electron-hole pair into the barrier. The observedquadratic dependence can be explained if the Auger processis of an excitonic origin, i.e., one electron-hole pair recom-bines while a second one is excited into the barrier, but thesaturation behavior on Fig. 3 is still difficult to understand.Hence, we conclude that the Auger process is not a dominantup-conversion mechanism in the samples studied here.In summary, we report on PL up-conversion in a hetero-structure with InAs QDs embedded into GaAs. An intensePL of the GaAs barrier was observed upon a resonant exci-tation of QDs. The experimental results suggest that the UPLis due to a two-step, two-photon absorption process involv-ing the QD states.1 W. Seidel, A. Titkov, J. P. Andre, P. Voisin, and M. Voos, Phys. Rev.Lett. 73, 2356 ~1994!.2 F. A. J. M. Driessen, Appl. Phys. Lett. 67, 2813 ~1995!.3 F. A. J. M. Driessen, H. M. Cheong, A. Mascarenhas, S. K. Deb, P. R.Hageman, G. J. Bauhuis, and L. J. Giling, Phys. Rev. B 54, R5263 ~1996!.4 R. Hellmann, A. Euteneuer, S. G. Hense, J. Feldmann, P. Thomas, E. O.Go¨bel, D. R. Yakovlev, A. Waag, and G. Landwehr, Phys. Rev. B 51,18053 ~1995!.5 J. Zeman, G. Martinez, P. Y. Yu, and K. Ushida, Phys. Rev. B 55, R13428~1997!.6 Y. H. Cho, D. S. Kim, B. D. Choe, H. Lim, J. I. Lee, and D. Kim, Phys.Rev. B 56, 4375 ~1997!.7 H. M. Cheong, B. Fluegel, M. C. Hanna, and A. Mascarenhas, Phys. Rev.B 58, R4254 ~1998!.8 T. Kita, T. Nishiro, C. Geng, F. Scholz, and H. Schweizer, Phys. Rev. B59, 15358 ~1999!.9 See, e.g., Y. R. Shen, The Principles of Nonlinear Optics ~Wiley, NewYork, 1984!, and references therein.10 E. J. Johnson, J. Kafalas, R. W. Davies, and W. A. Dyes, Appl. Phys. Lett.40, 993 ~1982!.11 L. G. Quagliano and H. Nather, Appl. Phys. Lett. 45, 555 ~1984!.12 I. V. Ignatiev, I. E. Kozin, H. W. Ren, S. Sugou, and Y. Masumoto, Phys.Rev. B 60, R14001 ~1999!.13 E. Poles, D. C. Selmarten, O. I. Micic, and A. J. Nozik, Appl. Phys. Lett.75, 971 ~1999!.14 J. M. Garcia, T. Mankad, P. O. Holtz, P. J. Wellman, and P. M. Petroff,Appl. Phys. Lett. 72, 3172 ~1998!.15 P. P. Paskov, P. O. Holtz, B. Monemar, J. M. Garcia, W. V. Schoenfeld,and P. M. Petroff ~unpublished!.o AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Photoluminescence up-conversion in InAs/GaAs self-assembled quantum dots

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