Original Paper

phys. stat. sol. (a) 202, No. 14, 2609 – 2613 (2005) / DOI 10.1002/pssa.200562013

Electron–hole complexes in individual semimagnetic quantum dots P. S. Dorozhkin*, 1, A. S. Brichkin1, V. D. Kulakovskii1, A. V. Chernenko1, S. V. Zaitsev1, S. V. Ivanov2, and A. A. Toropov2 1 2

Institute of Solid State Physics, Chernogolovka, Moscow Region, 142432, Russia Ioffe Physico-Technical Institute, St. Petersburg, 194021, Russia

Received 12 June 2005, revised 2 July 2005, accepted 17 August 2005 Published online 4 November 2005 PACS 71.35.Pq, 71.35.Ji, 71.70.Gm, 73.21.La, 75.50.Pp, 78.67.Hc We compare magneto-photoluminescence spectra of individual semimagnetic quantum dots (QDs) with different strengths of exchange interaction between a localized exciton and surrounding magnetic ions. The spectra of individual QDs with strong magnetic exchange interaction show a single line corresponding to a ground-state exciton. In QDs with weak interaction, exciton and biexciton spectra are identified, both showing a fine structure due to electron – hole exchange interaction. The anticrossing of dark and bright exciton states in low-symmetry QDs is demonstrated in magnetic fields in the Faraday geometry. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Intensive optical studies of individual nanometer-sized semiconductor quantum dots (QDs) taken in recent years provided extensive knowledge about various electron–hole (e–h) complexes in a threedimensional confinement. Exciton, biexciton and trion states were carefully characterized in nonmagnetic QDs [1]. Energy structure, Coulomb correlation and exchange interaction effects, QD symmetry influence, magnetic field polarization and many other aspects were analyzed [2]. Furthermore, such complexes as multiply charged excitons and biexcitons were investigated [3]. Incorporation of magnetic impurity atoms (usually, Mn) realized in diluted magnetic semiconductor (DMS) QDs modifies properties of e–h complexes as it changes electron and hole effective g-factors, their energy and spin relaxation, etc. [4]. Up to now, studies of individual semimagnetic QDs have been limited to the ground state of a neutral exciton in samples with a large exchange interaction between charge carriers and Mn ions [5]. In this report, we demonstrate that proper reduction of exchange interaction strength through control of electron and hole wavefunctions overlapping with Mn ions gives access to various e–h complexes in individual semimagnetic QDs. The exchange interaction strength is kept strong enough to control the properties of the complexes studied. At the same time, it is decreased to such an extent that the line broadening due to interaction with Mn magnetic moment fluctuations does not exceed the exciton splitting due to electron–hole exchange interaction. The samples investigated are CdSe self-assembled quantum dots grown by molecular beam epitaxy on ZnSe and ZnMnSe cladder layers. Schematics of the two structures studied and their energy diagrams are presented in Fig. 1. In the sample no. 1 the CdSe layer of two-monolayers nominal thickness is surrounded by DMS Zn0.75Mn0.25Se barriers from both sides [6]. In the sample no. 2 the CdSe layer is separated from a DMS Zn0.89Mn0.11Se barrier by an additional non-magnetic ZnSe layer of 1.6 nm thickness [7]. In the sample no. 1 roughly half of the QD-localized electron and hole wavefunctions penetrate into the magnetic barrier, while in the sample no. 2 only the small tail of the wavefunction overlaps with *

Corresponding author: e-mail: [email protected]

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P. S. Dorozhkin et al.: Electron–hole complexes in individual semimagnetic quantum dots Fig. 1 [Schematically] Energy structure and electron / hole wavefunction of the two samples studied.

magnetic impurities. The exchange sp–d interaction between spins of a charge carrier and a magnetic ion in DMS is proportional to the squared amplitude of the electron/hole wavefunction at the point of the magnetic ion localization [8]. Thus, the exchange interaction between an exciton and Mn ions in the sample no. 1 is large due to strong overlapping of a QD exciton with Mn ions – a case of strong exchange interaction is realized. On the contrary, in the sample no. 2 the amplitude of the exciton wavefunction in the remote area of the magnetic barrier is an order of magnitude smaller – the exchange interaction is relatively weak. Photoluminescence (PL) studies were performed with λ = 355 nm optical excitation at temperature 1.6 K in a magnetic field applied normal to the sample plane ( B|| z , Faraday geometry). In order to obtain the signal from individual QDs, a periodic mesa structure was etched on the sample with a typical mesa size of 100–300 nm. Figure 2 shows a typical PL spectrum of an individual QD from the sample no. 1 in an external magnetic field. The spectrum consists of an individual line that has a substantial ( ∼ 5 meV FWHM) width at zero magnetic field. Upon applying the field the line shifts to lower energy and narrows rapidly. It becomes nearly completely σ + -polarized at B = 1.5 T. The observed line is identified as a recombination of the QD heavy hole exciton with total spin J z = | + 1Ò [5]. It corresponds to both the electron and hole occupying their ground-state spin levels. Due to the exchange interaction with Mn spins, the magnetic field shift of the QD e-h recombination energy is proportional to the total magnetic moment, M , of all Mn ions located in the volume of the exciton localization [5, 8]. The Mn exchange integral for a heavy hole is five times larger than for an electron; thus, the exchange interaction with a hole dominates in the exciton PL energy. The QDs studied have the shape of a disk ( ∼ 5 nm diameter and a few monolayers in height). As a result, the heavy hole g-factor is strongly anisotropic due to heavy hole–light hole splitting, i.e. the heavy hole angular momentum is quantized along the z -axis with no component in the x, y -plane [5]. Correspondingly, each photon emitted from the QD has an energy defined by the instantaneous value of the projection of M onto the z -axis, M z . As a Mn magnetic moment experiences thermal statistical fluctuations, a time-integrated PL spectrum of an individual QD is broadened. In Faraday geometry, its

Fig. 2 Magnetic field dependence of photoluminescence line of individual QD in the sample with strong magnetic exchange interaction.

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Original Paper

phys. stat. sol. (a) 202, No. 14 (2005)

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linewidth is proportional to the value of the statistical fluctuations of the QD magnetic moment ·δM ||2 Ò along the direction of heavy hole quantization z and the magnetic field B|| z (longitudinal fluctuations). An external magnetic field leads to alignment of Mn ion spins along B , i.e. to an increase of M z and to a suppression of magnetic fluctuations ·δM ||2 Ò . As a result, an individual QD PL line shifts to low energy and narrows [5]. Importantly, all the single QD PL lines observed in the sample no. 1 behave qualitatively identically to the one shown in Fig. 2. No signatures of any other e–h radiative transitions in QDs have been observed. We now move to the sample no. 2, where the exchange interaction between the exciton spin and Mn magnetic ions is relatively weak. There are several major differences of this sample compared to sample no. 1 as the magnetic part of the barrier is moved further from the QD. Firstly, exciton spin relaxation through Mn is reduced, leading to an increase of the spin relaxation time. Secondly, the magnetic polaron energy [5, 8] is greatly reduced and can be lower than or comparable to the value of the e–h exchange interaction in a QD. Finally, the influence of magnetic fluctuations on a QD PL line width is smaller, which should allow good spectral separation of different QD spectra at zero magnetic field as well as determining their fine structure. Figure 3 shows individual QD spectra of the sample no. 2 at zero magnetic field. At low optical excitations, a strong doublet is observed (in the region 2.28–2.285 meV) accompanied by a weak line at the lower-energy side. This doublet corresponds to heavy hole exciton (X) recombination, as will be discussed later. Upon increasing the excitation, another doublet appears at ∼ 20 meV below the exciton spectrum with an intensity growing superlinearly with respect to the excitation laser power. This doublet

Fig. 4 Energy diagram of exciton and biexciton transitions in QDs of different symmetry.

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P. S. Dorozhkin et al.: Electron–hole complexes in individual semimagnetic quantum dots Fig. 5 Magnetic field evolution of exciton PL spectrum in QD with weak magnetic exchange interaction.

corresponds to biexciton (XX) recombination in the same QD. This is confirmed by measurement of the linearly polarized spectra shown for the highest optical excitation in the upper part of Fig. 3. The two major lines of the exciton spectrum appear to be linearly polarized along [110] and [1 10] crystal orientations. The biexciton doublet is also linearly polarized – with the reverse energy order of the p x and p y polarizations. These lines correspond to two allowed transitions in the QD exciton and biexciton, as shown in the energy diagram of optical transitions for QDs with different symmetry in Fig. 4. If the e–h exchange interaction is neglected, the exciton state is four-fold degenerate with J z = | ± 1Ò, | ± 2Ò . Existence of the e–h exchange interaction leads to the exciton splitting into three states (| ± 1Ò, | + 2Ò + | - 2Ò, | + 2Ò - | - 2Ò ) in a QD with cylindrical symmetry [2]. The observed strong splitting of the two bright transitions composed of | ± 1Ò states indicates that the point-group symmetry is well below D2d , i.e. the QD shape is far from cylindrical. The resulting exciton and biexciton transitions at B = 0 are marked by solid arrows in Fig. 4 [2]. The observed weak unpolarized line on the low-energy side of the exciton doublet should correspond to recombination of “dark” states composed of | ± 2Ò (dashed arrow in Fig. 4). Their appearance in optical spectra indicates a marked admixture of | ± 1Ò into | ± 2Ò states and implies that the QD symmetry is very low, lower than C2 . This symmetry reduction may be partially caused by the effect of Mn moment fluctuations. It is seen in Fig. 3 that the “forbidden” transition into the dark exciton state is absent in the biexciton spectrum. The enhancement of the “forbidden” line in the exciton spectrum is connected with energy relaxation of excitons into this ground exciton state. Figure 5 shows spectra of the QD exciton in the magnetic field in Faraday geometry. As expected, the two linearly polarized components corresponding to | ± 1Ò transitions show a strong Zeeman splitting and become circularly (σ + and σ - ) polarized. The “dark” state line shifts to high energy and crosses the | + 1Ò (σ + ) component at B ∼ 5 T. It is seen that in this range of magnetic field the two states show anticrossing behavior and the “dark” component gains the oscillator strength from the bright one. The anticrossing behavior is well expected in the QDs with the symmetry below C2 and the corresponding transitions are shown on the right-hand side of Fig. 4. Similar anticrossing between dark and bright states was recently observed by Besombes et al. [9] for a semimagnetic quantum dot containing a single Mn atom. It was explained in terms of simultaneous electron and Mn spin flips that change a bright exciton into a dark exciton. In conclusion, PL spectra of individual QDs with strong exchange interaction show a single line corresponding to the ground-state exciton recombination. No fine structure of the line is observed due to the large line broadening at zero magnetic field, which is caused by thermal magnetic fluctuations. On the contrary, QDs with smaller exchange interaction demonstrate narrow PL lines revealing a fine structure. Exciton and biexciton lines are identified and are found to be split by electron–hole exchange interaction in asymmetrical QDs. Mixing of dark and bright exciton states in low-symmetry QDs is demonstrated in a magnetic field in the Faraday geometry.

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Original Paper

phys. stat. sol. (a) 202, No. 14 (2005)

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Acknowledgements Financial support of RFBR is gratefully acknowledged. The work of P.S.D. was partially supported by INTAS and INTAS young scientist grants.

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