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In-situ growth of CdTe quantum dots on ZnSe


Isabella Gierz, Florian Maier, Christian Kumpf

Semiconductor heterostructures are multilayer systems consisting of different materials with usually have a lattice mismatch of only a few percent between substrate and epilayer. In case of a small lattice mismatch epitaxial layers can be grown, at higher mismatch nanometer-sized islands form spontaneously during growth. Within these islands electrons are confined in all three dimensions. This makes those systems promising candidates for quantum dot applications. In this project we are investigating CdSe/ZnSe quantum dots with a lattice mismatch of approx. 7% using different methods. While so far the focus lay on x-ray diffraction methods and atomic force microscopy (AFM) we will now move to electron microscopy methods (LEEM, PEEM).



Fig. 1: AFM image of CdSe quantum dots on ZnSe.

Fig. 2: Typical reciprocal space map in the vicinity of a (220) Bragg reflection.

The samples are produced in three steps. At first a few layers of CdSe are grown under MBE conditions on a clean ZnSe(001) surface. Afterwards a thin amorphous layer of Te is depositen and in the last step the sample is annealed. The Te desorps again and CdSe quantum dots are formed on the ZnSe surface. A typical AFM image of such a sample is shown in Fig. 1. In order to learn more about the internal structure of the dots we so far successfully applied a method based on iso-strain x-ray scattering [I. Kegel et al., Phys. Rev. Lett. 85, 8 (2000), Phys. Rev B, 63, 035318 (2001)]. Together with reciprocal space mapping (and corresponding modeling of the data) this allows to determine height, shape, and gradient of relaxation of the islands as well as intermixing of components. Figure 2 shows a typical reciprocal space map which was measured at the BW2 beamline of the Hamburger Synchrotronstrahlungs-labor.

An intriguing open question is the role of the amorphous Te layer in the sample preparation process. Many questions could be answered by an in-situ method which allows to monitor the annealing process in which the island are formed. We plan to apply low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) to investigate this point. A joint project with the SMART-team at the BESSY synchrotron in Berlin is planned as well as in-house experiments using out own LEEM/PEEM microscope which will be built within 2009 at our laboratory.


Publications:

C. Kumpf, A. Stahl, I. Gierz, C. Schumacher, S. Mahapatra, F. Lochner, K. Brunner, G. Schmidt, L.W. Molenkamp, and E. Umbach, Structure and relaxation effects in thin semiconducting films and quantum dots, phys. stat. sol. (c) 4(9), 3150-3160 (2007).

Collaborations:

K. Brunner, C. Schumacher, F. Reinert, Univ. Würzburg T. Schmidt and the SMART-team, Fritz-Haber-Institut, Berlin


Address Director
Institute of Bio- and Nanosystems (IBN)
Institute 3: Institute of Thin Films and Interfaces 3 (IBN3)
Forschungszentrum Jülich GmbH
D-52425 Jülich 		Prof. Dr. Stefan Tautz
Tel.: ( 49) 24 61/61-4561
Fax: ( 49) 24 61/61-3907
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