During my Ph.D. studies at the Institute of Physics Polish Academy of Sciences in Warsaw we have been able to obtain semiconductor self-assembled quantum dots (QDs) of CdTe and CdMnTe in ZnTe matrix. These structures were grown by molecular beam epitaxy using Stranski-Krastanov growth mode. My primary responsibility was to characterize the optical properties of these QDs both on an ensemble and single dot levels.
Major results of my Ph.D. thesis include:

1. Single dot spectroscopy of CdTe quantum dots.
2. Redistribution of carriers between self - assembled quantum dots.
3. Optical characterization of CdTe/ZnTe quantum wells. (still to come)
4. Three - dimensional quantum dot crystal formation in CdTe/ZnTe quantum dot system.


1. Single dot spectroscopy of CdTe QDs.

High resolution photoluminescence is an extremely powerful method for unraveling the optical properties of single QDs. Out of several approaches to isolate individual emitters we have chosen to evaporate thin metal films with small apertures on top of QD sample. Apertures were made using electron beam lithography and chemical etching. A diameter of apertures ranged from 3 microns down to 100 nm. Atomic force microscopy images of two masks with apertures of 100 nm and 700 nm are shown in Fig. 1.

The measurements were carried out at the University of Nijmegen within collaboration with Fabio Pulizzi and Peter Christianen. The experimental setup allowed to collect images as well as spectra of QDs within the apertures. Fig. 2 displays three images showing metal masks with and without apertures. Fig. 3 shows spectrally resolved emission of the smallest aperture at B=0T and B=3T (unpolarized).

By analyzing single dot emission as a function of magnetic field we obtained excitonic Zeeman splitting as well as diamagnetic shifts which are invertly proportional to the exciton size in the QD. Examples of magnetic field behavior for symmetric and elongated single CdTe QDs are shown in Fig. 4 together with energy dependence of the exciton effective g-factor (estimated from the Zeeman splitting). We found that the g-factor is uniform over the ensemble.

Publications:
S. Mackowski, J. Wróbel, K. Fronc, J. Kossut, F. Pulizzi, P.C.M. Christianen, J.C. Maan, G. Karczewski, "Exciton Spectroscopy of Single CdTe and CdMnTe Quantum Dots", physica status solidi b 229, 493 (2002)
S. Mackowski, "CdTe/ZnTe Quantum Dots – Growth and Optical Properties", Thin Solid Films, 412, 96 (2002)


2. Redistribution of carriers between self-assembled QDs.

The morphology of self-assembled QDs made of II-VI compounds depends on the amount of material deposited during the growth. As schematically displayed in Fig. 5, in the first growth stage 2D platelets with small QDs are formed. With adding more material the platelets form a uniform 2D wetting layer. At the end the structure consists only of large quanutm dots.
We have shown that the morphology strongly determines the optical properties of QDs.

The PL spectrum of a QD structure with 2 monolayers (MLs) of CdTe features two lines, the high energy one is due to the wetting layer, whereas the low energy one is associated with exciton emission of QDs themselves. On the other hand, for the sample with 4 ML of CdTe, the PL shows just a single emisson line attributable to QDs (Fig. 6). Consequently, the optical propeorties of the latter are determined by the behavior of isolated single QDs: the energy dependence on the temperature follows the band gap shrinkage of CdTe. Also, the decay time of the exciton recombination shows no dependence on temperature as well as on the excitation power (see Fig. 6).
Completely different behavior is observed (see Fig. 7), when similar experiments are performed for the sample with the wetting layer. At elevated temperatures (T>35K) the carriers confined in smaller QDs are able to escape into the wetting layer and then to find other QDs with presumably lower emission energy. In other words, carriers are thermally redistributed within the ensemble of QDs. The redistribution process strongly affects the power dependence of both the PL emission and decay times of the exciton recombination. However, The redistribution determines the optical properties of the QD ensemble below the temperatures, where non-radiative recombination in the wetting layer are negligible (T<100).
Using a simple model that assumes all the effects are due to redistribution of carriers bettwen the QDs we have been able to fit the temperature dependencies of the linewidth and the emission energy  measured experimentally for several constant excitation powers.

Publications:
S. Mackowski, F.V. Kyrychenko, W. Heiss, G. Prechtl, G. Karczewski, J. Kossut, "Impact of Carrier Redistribution on the Photoluminescence of CdTe Self-Assembled Quantum Dot Ensembles", Physical Review B 69, 205325 (2004)
S. Mackowski, "CdTe/ZnTe Quantum Dots – Growth and Optical Properties", Thin Solid Films, 412, 96 (2002)
S. Mackowski, F. Kyrychenko, G. Karczewski, J. Kossut, W. Heiss, G. Prechtl, "Thermal Carrier Escape and Capture in CdTe Quantum Dots", physica status solidi b 224, 465 (2001)


4. Three - dimensional quantum dot crystal formation in CdTe/ZnTe QD system.

Since the formation of self-assembled QDs is driven by the elastic strain, a single dot covered with a barrier material produces a strain field on the surface. This strain field could, under appropriate conditions, influence the position of QDs formed above this QD. We have grown multilayer CdTe/ZnTe QD structures with  thickness of the ZnTe spacer layer varying between 3ML and 75ML. In Fig. 8 a cross-sectional image from transmission electron microscopy of sample with 50ML thick spacer is shown. In this case we observe no correlation between spatial positions of QDs in neighboring layers.

With decreasing the thickness of the spacer the positions of QDs in subsequent layers become correlated, as displayed in Fig. 9 for the structure with 15 ML thick ZnTe spacer layer. The angle of this correlation is about 40 degrees. The extension of correlated region is equal to 6 - 7 layers in growth direction and 5 - 6 islands in the layer. A driving force for the vertical alignment is an elastic anisotropy of the matrix material, which in the case of II–VI semiconductors is considerably larger than for III–V compound. In the particular of ZnTe it has been predicted that the elastic energy density exhibits four minima in <110> axis.
Lateral positions of QDs within the layers has been studied using small angle X-ray scattering. A two-dimensional image of scattering intensity along the growth direction obtained for the sampel with 10ML ZnTe spacer is given in Fig. 10 together with extracted profiles along two directions (as displayed).
The analysis of the angular dependence of the scattering intensity reveals four unamboigous maxima (Fig. 11). This implies that a three-dimensional arrangement of QD positions is achived through elastic stran field created by the dots. The observed arrangement is in perfect agreement with theoretical predictions for ZnTe.
Publications:
S. Mackowski, E. Sobczak, R. Nietubyc, G. Goerigk, S. Kret, P. Dluzewski, A. Szczepanska, E. Janik, J. Kossut, G. Karczewski, “Three-dimensional Quantum Dot "Crystal" Formation in CdTe/ZnTe Superlattices", physica status solidi b 229, 445 (2002)
S. Mackowski, G. Karczewski, J. Kossut, G. Sek, J. Misiewicz, G. Prechtl, W. Heiss, “Optical Properties of CdTe/ZnTe Quantum Dot Superlattices", Physica E 12, 503 (2002)
S. Mackowski, G. Karczewski, T. Wojtowicz, J. Kossut, A. Szczepanska, S. Kret, P. Dluzewski, G. Prechtl, W. Heiss, "Structural and Optical Evidence of Island Correlation in CdTe/ZnTe Superlattices", Applied Physics Letters 78, 3884 (2001)
M. Godlewski, S. Mackowski, G. Karczewski, E.M. Goldys, M.R. Phillips, "Scanning Cathodoluminescence and Electron Microscopy of Self-Organized CdTe Quantum Dots", Semiconductor Science and Technology 16, 493 (2001)
S. Kret, P. Dluzewski, A. Szczepanska, S. Mackowski, T. Wojtowicz, G. Karczewski, J. Kossut, P. Dluzewski, P. Traczykowski, P. Ruterana, "Electron Microscopy Study of ZnTe/CdTe Superlattices with High Density of Quantum Dots", Materials Research Society Symposium Proceedings 642, J3.8.1 (2001)
R. Nietubyc, E. Sobczak, J.B. Pelka, S. Mackowski, E. Janik, G. Karczewski, ”Anomalous Small Angle X-Ray Scattering Study of CdTe Quantum Dots in ZnTe", Journal of Alloys and Compounds 328, 206 (2001)