Materials Science and Engineering B88 (2002) 255–258

Temperature and excitation dependence of the luminescencespectra of InAs quantum dots

M. Hjiri, F. Hassen, H. Maaref *Laboratoire de Physique des Semi-conducteurs et des Composants Electroniques (LA-MA-06), Faculte des Sciences de Monastir,

A�enue de l’En�ironnement, 5000 Monastir, Tunisia

Abstract

Temperature dependence of the photoluminescence (PL) spectra of InAs/GaAs self organised quantum dots (QD’s) have beenstudied. By deconvoluting these spectra using multiple gaussian fits, we have observed, at low excitation density, the thermallyactivated quenching of the QD’s ground state. Our study, at high excitation density, shows a decrease of the FWHM of the QDPL band versus temperature. This effect is correlated to the thermally carriers injection from narrower QD’s to larger onesthrough the wetting layer. Finally we have shown that the temperature effects can be compensated by the excitation density.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Quantum dots; Thermal injection; Excitation density

www.elsevier.com/locate/mseb

1. Introduction

Semiconductor quantum dots (QD’s) structure, withatomic-like states, are in an area of intense investiga-tion because this feature is expected to lead to im-proved optoelectronic devices such as QD’s diode laserswhich have narrow line-width, high modulation band-width because of the high differential gain and lowthreshold current density [1]. Many studies of the elec-tronic and optical properties of these nano-structureshave been reported [2–4]. Several of them have beeninterested in the photoluminescence (PL) band tempera-ture dependence [5,6].

In this work, we report on the PL band of QD’s andits FWHM temperature dependence. The carriers’transfer, thermally injected, from narrow QD’s to largeones is occur through the wetting layer. We will showin this paper the interrelation between the excitationdensity and the temperature effects on the QD’s lu-minescence bands.

2. Experimental results and discussion

The structures studied in this work were grown, by amolecular beam epitaxy (MBE), on (001)-orientedGaAs substrate followed by an undoped GaAs bufferlayer at 580 °C. The two monolayers InAs self or-ganised islands were grown at 520 °C. The active layeris embedded between two 200 A� thick GaAs layers.About 300 A� GaAs cap layer was grown at 580 °C.

The PL spectra were obtained with Ar+ pumpedSapphire–Titanium laser. The luminescence signal fromthe sample, which is mounted in a closed cycle Hecryostat, was analysed by a double-grating spectrome-ter and detected by a photomultiplier. The spectra wererecorded using the standard lock-in technique.

The spectrum from the 2-monolayer (ML) sample(Fig. 1), seen at 12 K and under an excitation density of65 mW mm−2, consists of four luminescence linesassociated, respectively, to the band to band (b�b)emission from the GaAs substrate at 1.51 eV, to theradiative transition involving the carbon acceptor im-purity in GaAs (Ec�A) at 1.49 eV, to the wetting layer(WL) luminescence band at 1.408 eV and to the emis-sion from QD’s at 1.24 eV.

Fig. 2 shows a structured broad PL band associatedto the InAs QD’s at different temperatures and under a

* Corresponding author. Tel.: +216-3-500-274; fax: +216-3-500-278.

E-mail address: hassen.maaref@fsm.rnu.tn (H. Maaref).

0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 21 -5107 (01 )00880 -7

M. Hjiri et al. / Materials Science and Engineering B88 (2002) 255–258256

Fig. 1. About 12 K PL spectra 2-ML InAs QD’s obtained at 65 mWmm−2.

power excitation of 65 mW mm−2. This luminescencepersists up to 200 K.

A deconvolution of the luminescence spectra fordifferent temperatures (Fig. 2) is used and shows that(i) for the low temperatures, the spectrum can be wellfitted by four gaussians; (ii) by increasing the tempera-ture, the number of gaussians used to fit the experimen-tal results decreases from four to one. According to ourprevious work [7], the luminescence at low temperaturesresults from the radiative recombination on the ground(G1, G2) and excited (E1, E2) states of two QD’s sizedistributions. This result is supported by AFM mor-phological characterisation carried out on the sameuncapped structures.

A detailed study of the PL intensity behaviour versustemperature has been carried out at low excitationdensity (this excitation density is used to probe onlylocalised states in large QD’s). When measuring the PLintensities, the detuning of the laser with respect to thebarrier and wetting layer band gaps has been keptconstant in order to prevent any resonant effects. Atlower temperatures (up to 80 K) the PL intensityincreases under a constant flux of electron–hole pairsgenerated in the sample and then monotonically de-creases at higher temperatures. At lower temperature,the QD PL intensity increase with temperature indicatesthat the carriers captured from the reservoir (barrier orWL) are stronger than the carriers activation in QD’s.However, the carriers activation are stronger than car-rier captures at higher temperature. Therefore, using anintensity versus temperature Arrhenius plot, the ther-mal activation energy EA associated to the ground statefor the large QD distribution can be deduced. FromFig. 3, a value of 178�5 meV is obtained. The energyEA+ ground state PL energy (1.224 eV) line is veryclose to the WL transition energy (1.408 eV, Fig. 1).Then we believe that the wetting layer may act as abarrier to thermal activation processes.

The temperature behaviour of the QD’s PL FWHMis shown in Fig. 4 for an excitation density of 65 mWmm−2. The initial FWHM decrease of the lumines-cence band, from 30 to 110 K, is due to the fact thatwhen the temperature is risen, the carriers are injectedout of the excited states of the larger QD’s and from,both, excited and ground states of narrower ones. Thisassumption is supported by the red shift and the de-crease of the PL band width.

Indeed enhanced non radiative recombinations ofthermally injected carriers reduce the average filling ofthe QD’s levels, which lead to a decrease of the radia-tive emission from states localised in high energy levels.

It is well known that, at low temperature, the in-crease of the excitation density results in a PL satura-tion and a blue shift of the QD’s luminescence bands.

We have used this procedure on our samples for lowand high temperature regions. Fig. 5a presents the PLband, measured at 30 K with 16 mW mm−2 excitation

Fig. 2. InAs QD’s PL bands at different temperatures for 65 mWmm−2. Dashed lines are associated to the gaussian fit.

Fig. 3. Arrhenius plot at low excitation density (1 mW mm−2) ofInAs/GaAs QD’s.

M. Hjiri et al. / Materials Science and Engineering B88 (2002) 255–258 257

Fig. 4. InAs/GaAs QD’s FWHM of luminescence band vs. tempera-ture at high excitation density (65 mW mm−2), straight line and atlow excitation (1 mW mm−2), dotted line.

back of PL spectrum is due to the saturation of groundstates accompanied by the repopulation of the excitedstates.

From this result, we can conclude on the interrela-tion between the temperature and the excitation den-sity, when the temperature is increased, the carriersdensity in the excited states is decreased (elimination ofthe high energy gaussian of each distribution). Then,when the power density is increased for the same tem-perature the high energy states are repopulated (reap-pearance of the high energy gaussian).

Above 110 K, both the electron–phonon scatteringand thermal distribution become important, and conse-quently, the FWHM increases with temperature. Undervery weak excitation conditions, the temperature de-pendence of the FWHM of the PL line remains con-stant (48 meV) in the temperature range 30–110 K andthe PL spectra are well fitted by one gaussian (G1) (seedotted line Fig. 4). This result confirms that the PLexperiment has the disadvantage that higher levels asso-ciated to the excited states of QD can not be probedwithout filling all the lower lying levels.

3. Conclusion

In conclusion, InAs self-assembled QD’s have beenstudied by PL. Our study shows a decrease of theFWHM of the QD PL band versus temperature due tothe decrease of the carrier density in the excited statesof the QD’s. The QD integrated PL intensity variation

density, for those conditions, the spectrum is fitted bythree gaussians (G1, G2, and E1). When the temperatureis increased to 80 K, the PL band shows a shrinkage,which seems as a slight red shift of the high energy sideof the hole PL spectrum, and it is fitted by only twogaussians (G1, and G2,), Fig. 5b, this effects means thatfor this temperature carriers are injected from localisedexcited states to the wetting layer. By increasing theexcitation density to 80 mW mm−2 and unchangedtemperature (80 K), the PL spectrum shows a displace-ment toward higher energies (blue shift) and it is fittedby three gaussians (G1, G2, and E1), Fig. 5c. This turn

Fig. 5. PL spectra of InAs QD’s at different temperatures and excitation densities.

M. Hjiri et al. / Materials Science and Engineering B88 (2002) 255–258258

versus temperature shows that the carrier capture andthermalisation processes take place with the assistanceof the WL. At high temperature, an increase of theFWHM caused by the vibronic effects is observed. Thecompensation of the Temperature effects by those ofthe excitation density are shown.

Acknowledgements

This work has been supported by the Secretariatd’Etat a la Recherche Scientifique et a la Technologieof Tunisia (SERST) under a PNM 98 program. Theauthors wish to thank Dr R. Murray of the ImperialCollege (London) for supplying some of the samples aswell as for stimulating discussions.

References

[1] R.P. Mirin, J.P. Ibbetson, K. Nishi, A.C. Gossard, T.E. Bowers,Appl. Phys. Lett. 67 (1995) 3795.

[2] J.Y. Marzin, J.M. Gerard, A. Izrael, D. Barrier, G. Bastard, Phys.Rev. Lett. 73 (1994) 716.

[3] N.N. Ledentsov, M. Grundmann, N. Kirstaedter, O. Schmidt, R.Heitz, J. Bohrer, D. Bimberg, V.M. Ustinov, V.A. Shchukin, A.Yu Egorov, A.E. Zhukov, S. Zaitsev, P.S. Kop’ev, Z.I. Alferov,S.S. Ruvimov, A.O. Kosogov, P. Werner, U. Gosele, J. Heydenre-ich, Sol. Stat. Elect. 40 (1996) 785.

[4] P. Hawrylak, A. Wojs, Semicond. Sci. Technol. 11 (1996) 1516.[5] L. Brusaferri, S. Sanguinnetti, E. Grilli, M. Guzzi, A. Bignazzi, F.

Bogani, L. Carraresi, M. Colocci, A. Bosacchi, P. Frigeri, S.Franchi, Appl. Phys. Lett. 69 (1996) 3354.

[6] P.O. Vaccaro, M. Hirai, K. Fujita, T. Watanabe, J. Phys. D Appl.Phys. 29 (1996) 2221.

[7] M. Hjiri, F. Hassen, H. Maaref, Mater. Sci. Eng. B69–70 (2000)514.