We report on technological and magneto-optical investigations of formation of quantum dots (QDs) with single magnetic ions. Motivation of our study is to control ion-exciton interaction by controlling position of magnetic ion in QD. In particular, we studied impact of position of Mn δ-doping on optical properties of QDs.

   The first reported QDs with single magnetic ions were obtained by Mn δ-doping of ZnTe barrier, a few nm below CdTe QDs layer [1]. The method was further improved by simultaneous deposition of Mn and CdTe layer [2,3] what increased probability of good overlap of Mn and QD exciton. Within this work Mn was introduced at the bottom, in the middle and on the top of CdTe layer. 

   Quantum dots were grown using molecular beam epitaxy and a method of amorphous tellurium deposited after CdTe layer [4]. We used very thin CdTe layer (1-3 monolayers (MLs), instead of the typical 5-8 MLs [4]) in order to lower QDs density [5,6] and to study individual QDs with various sizes and emission energy, not limited to low energy tail. In order to get very low concentration of Mn ions, crucial for incorporating one Mn ion per QD on average, we have calibrated the Mn molecular beam of our effusion cell using the giant Zeeman effect in (Cd,Mn)Te and (Zn,Mn)Te [6] layers.

Fig.1. μPL lines of quantum dots with single manganese ions: excitons (X), trions (X⁺) and biexcitons (XX).

   In layers of dilute magnetic semiconductors, exciton lines split at low temperature (1.7K) and magnetic field (up to 5T) due to the giant Zeeman effect which is proportional to manganese concentration. Experimentally observed exciton splitting was fitted using modified Brillouin function [7,8] and Mn concentration was determined. Mn concentration appears to be in the range from 0.02% to 8%. The source calibration for the lowest Mn concentration was used for further growth of magnetic quantum dots.

   Low temperature μ-photoluminescence (μPL) study shows that for all samples in the series we can identify QDs with exactly one Mn ion (Fig. 1). Independently on position of Mn δ-doping we observe QDs with large exciton splitting of about 2 meV, what indicates good overlap of Mn and exciton. This surprising result doesn't indicate how to improve control of Mn position in QD. On the other hand we observe strong impact of position of Mn δ-doping on QDs density and on amount of Mn incorporated into QDs. We discuss it as a consequence of modification of surface atom mobility [9].

   The advantage of having QDs with low density is that we can study not only large QDs with emission lines in the low energy tail of QDs ensemble as it was typically studied before [3,10] but also access typical QDs having PL in the middle of spectrum. This opens a possibility to study statistics of incorporation of manganese into QDs. The analysis of μPL spectra shows that there is a clear dependence between the energy of a quantum dot and the probability of presence of manganese ions as shown on Fig. 2. The ratio of the number of quantum dots with Mn ions and all QDs within the constant area (1 μm2 given by our laser spot) decreases with the spectral energy. We interpret this effect in the following way: due to the quantum confinement large dots have lower energy than small dots hence manganese ions are more likely to arrange in dots with more space within.

Fig. 2. The probability of finding Mn in a QD. We divided spectra into 45 meV wide parts and count the ratio between QDs with Mn ions and all QDs in each part. There are no dots with energy lower than 1900 meV and higher than 2400 meV. The probability decreases with emission energy. Statistics made for 15 points on the samples.

  Large s,p-d splitting of QDs excitons with single Mn ions allow us to resolve well all six lines related to Mn spins and perform exciton decay measurements for various lines. Our results allow us to conclude on temporal evolution of exciton-magnetic ion complex.

[1] L. Maingault, L. Besombes, Y. Leger et al., Appl. Phys. Lett.  89, 193109 (2006).
[2] P. Wojnar, J. Suffczynski, K. Kowalik et al., Phys. Rev. B  75, 155301 (2007).
[3] M. Goryca, T. Kazimierczuk, M. Nawrocki et. al., Phys. Rev. Lett.  103, 087401 (2009).
[4] F. Tinjod, B. Gilles, S. Moehl et al., Appl. Phys. Lett.  82, 4340 (2003).
[5] J. Kobak, J.-G. Rousset, R. Rudniewski et al., arXiv:1210.2946 (2012).
[6] K. Gietka, J. Kobak, J.-G. Rousset et al., Acta Physica Polonica A  122, 1056 (2012).
[7] J.A. Gaj, W. Grieshaber, C. Bodin-Deshages et al., Phys. Rev. B  50, 5512 (1994).
[8] A. Twardowski, P. ́Swiderski, M. von Ortenberg et al., Solid State Commun.  50, 509 (1984).
[9] S. Kuroda, Y. Terai, K. Takita et al., J. Cryst. Growth  214-215, 140 (2000).
[10] L. Besombes, Y. Léger, L. Maingault et al., Phys. Rev. Lett.  93, 207403 (2004).

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