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Physica E 17 (2003) 180–182

www.elsevier.com/locate/physe

Optical properties of self-organized InAs nanostructures grownon InAlAs/InP(0 0 1)

M. Hjiria, F. Hassena, H. Maaref a ;∗, B. Salemb, G. Bremondb, O. Martyc,J. Braultd, M. Gendryd

aLab. de Phys. des Semiconducteurs et des, Facult�e des Sciences de Monastir, Composants Electroniques (LA-MA-06)Route de l’environnement 5000, Monastir, Tunisia

bLPM, UMR CNRS 5511, INSA de Lyon, 69621 Villeurbanne, FrancecLENAC, UCB Lyon1, 69622 Villeurbanne, France

dLEOM, UMR CNRS 5512, Ecole Centrale de Lyon, 69134 Ecully, France

Abstract

We report on photoluminescence (PL) and polarized PL studies of self-organized InAs quantum dots (QDs) and quantumwires (QWRs) grown on InAlAs lattice matched to InP(0 0 1). A strong linear polarization degree is observed for thesingle-layer QWR structures. However, the QDs are found merely isotropic ones and their luminescence is a mixture of QDsground and excited states.? 2002 Elsevier Science B.V. All rights reserved.

Keywords: Quantum dots; Quantum wires; Polarized photoluminescence; InAs

1. Introduction

The electronic band structure and optical propertiesof low-dimensional systems, particularly semicon-ductor quantum wires (QWRs) and quantum dots(QDs) have recently attracted considerable interestdue to their potential for device applications [1,2].Optical experiments [3] determining luminescencepolarization dependence on the direction of the emit-ted light relative to the wire axis have been performedon V-grooved QWRs. These experiments have shownstrong polarization anisotropy of the emitted lightwhich is taken as a signature of con>nement of

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

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

carriers in the wire. In this letter, we describe photo-luminescence (PL) and polarized photoluminescence(PPL) measurements made on samples containingself-assembled InAs QWRs and QDs grown on In-AlAs/InP(0 0 1). The PPL results have shown a highoptical anisotropy for the InAs QWRs as comparedto that of the QDs.

2. Experimental procedures

The samples investigated in this study were grownby solid-source molecular beam epitaxy (SSMBE)in a Riber 2300 Reactor. InAs self-assembled quan-tum islands (QIs) were grown on InAlAs latticematched to semi-insulating InP nominally (0 0 1)oriented substrates. A 400 nm-thick InAlAs buFer

1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S1386-9477(02)00750-6

M. Hjiri et al. / Physica E 17 (2003) 180–182 181

layer was grown at 525◦C using an arsenic pressureequal to 7 × 10−6 Torr. The InAs deposit is >xedat a 3:5 monolayers (ML) equivalent thickness justabove the 2:5 ML critical threshold for the 2D/3Dgrowth mode transition [4]. This deposit was grownat 525◦C. Samples were covered with a 300 nm-thickInAlAs cap layer. The optical properties have beeninvestigated by PL and PPL measurements. Thestructures were immersed in a closed helium cy-cle cryostat. The sample temperature can be variedfrom 10 K to room temperature. The PL was ex-cited by a tunable Titanium–Sapphire laser, the laserpolarization was selected by a linear polariser. ThePL was dispersed by a 0:6 m double monochro-mator and detected by an InGaAs photodiode. Thepolarized emission component was detected by a po-lariser oriented parallel or perpendicular to the wireaxis.

3. Results and discussion

Typical AFM images of InAs/InAlAs/InP QIs areshown in Figs. 1a and b. The shape, size and lat-eral distribution of the islands are quite diFerentdepending on the growth conditions [4]. In Fig. 1a,InAs islands appear as closely joined wires of a fewhundred nm long, with 15 nm width and 1:8 nmheight, oriented in the [1−1 0] direction. In Fig. 1b,islands appear as dots elongated in the [1−1 0] di-rection, with 55 nm length, 30 nm width and 1:5 nmheight.

200 nm

[1-10]

(a) (b)

Fig. 1. AFM images of InAs quantum wires (a) and quantum dots (b) grown on InAlAs/InP(0 0 1).

In Figs. 2a and b, the 10 K PL spectra of the pre-vious QWRs and QDs are shown. The maximum ofthe luminescence is located around 1.08 and 1:02 eVfor QWR and QD structures, respectively. In thecase of QDs structure, a resolved multiline spectrumis obtained. PL intensity transfer has been observedfrom low to high energy peaks versus the excitationdensity. So, this multiline shape could be attributedto a mixture of ground states and corresponding >rstexcited states of QDs with monolayer Juctuation ofthe height of the QD. The linear PPL has been investi-gated as a >ngerprint of one-dimensional (1D) carriercon>nement in the semiconductor QWRs. The varia-tions of the linear polarization degree P of the emittedlight for QDs and QWRs structures are representedin Figs. 2a and b (dotted line and right scale). P isde>ned by the following equation: P= I‖− I⊥I‖+ I⊥where I‖ (I⊥) is the [1−1 0] ([1 1 0]) polarized lu-minescence intensity. The QDs structure shows aweak linear polarization degree. On the contrary, thespectrum from QWRs structure shows a signi>cantintensity diFerence for the two diFerent polarizationdirections indicating an important optical anisotropy.The data have shown that the polarized PL intensityis independent on the angle of the incident light.At least, two reasons have to be taken in considera-tion to explain the appearance of a linear polarizationin the luminescence of the QWRs. The >rst onecan be due to the mixing of the light and heavyhole states in the 1D valence subbands [5]. Thesecond one is connected to the shape anisotropyeFects [6].

182 M. Hjiri et al. / Physica E 17 (2003) 180–182

0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25

Energy (eV)

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0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

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0

5

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Linear polarization degree (%

)PL

Int

ensi

ty (

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uni

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(a) (b)

Fig. 2. 10 K polarized PL spectra of the quantum wire sample (a) and the quantum dot sample (b) with the correspondent linear polarizationdegree.

4. Conclusion

In conclusion, using PL and PPL spectroscopies,we have investigated the optical properties of InAsquantum islands grown on InAlAs/InP(0 0 1). Wehave shown that the QWRs structures have a stronglinear polarization degree, however the QDs aremerely isotropic ones.

References

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