P 16 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
STRUCTURAL CHARACTERIZATION OF THE CORE-SHELL ZnTe/ZnMgTe NANOWIRES
E. Dynowska1∗, J.Z. Domaga la1, P. Romanowski1, E. Janik1, P. Wojnar1, and W. Caliebe2
1Institute of Physics, Polish Academy of Sciences, al. Lotnik´ow 32/46, PL–02668 Warszawa, Poland
2HASYLAB at DESY, Notkestr. 85, D–22603 Hamburg, Germany Keywords: synchrotron radiation, X-ray diffraction, nanowires, MBE, II-VI compounds
∗e-mail : dynow@ifpan.edu.pl
Semiconductor nanowires (NWs) can be used to build effective nanosensors and photovoltaic devices due to their extremely large the surface-to-volume ratio. On the other hand, the surface defects, espe- cially surface states, significantly reduce the carrier lifetime and degrade the optoelectronic device per- formance. Therefore, a surface passivation is of a great importance, especially for ZnTe NWs which oxidize easily. Such passivation can be achieved by forming a shell of a large band gap material around the NW so the surface states are moved away from the charge carriers confined in the core.
In this work we present the results of structural characterization of seven samples of the core-shell ZnTe/Zn1−xMgxTe NWs grown by MBE technique on the GaAs(111) substrate. The ZnTe NWs were grown at the temperature of about 470◦C according to the procedure described in [1]. The Zn1−xMgxTe shells were produced immediately after the growth of the ZnTe cores. In order to produce such shells the substrate temperature was reduced to the value (260 – 320◦C) which stops the axial growth of NWs and forces the radial growth of Zn1−xMgxTe shell.
The X-ray measurements were performed using synchrotron radiation at the W1 beamline at DESY- HASYLAB. The monochromatic X-ray beam of wavelength λ = 1.54056 ˚A was used. Two modes of measurement were applied: symmetrical 2θ − ω scan and coplanar 2θ scan in the glancing inci- dence geometry [2]. The results of measurements in 2θ − ω mode confirmed that the crystallographic orientation of the substrate forces the orientation of NWs — three orders of reflections from (111) lattice planes from GaAs substrate and from NWs are visible. The measurements in the glancing in- cidence geometry show the relatively thin polycrys- talline layer grown on the substrate between NWs.
The peaks originate from ZnTe and Zn1−xMgxTe phases, respectively. A zoom of the symmetrical patterns in the vicinity of 333 NW peaks allows to notice their splitting. This splitting indicates the
monocrystalline character of the Zn1−xMgxTe shell with the same crystallographic orientation as that of the ZnTe core. The lattice parameters of ZnTe core and Zn1−xMgxTe shell have been calculated from the position of the peaks related to the core and shell, respectively. For all samples, the lattice parameter of the ZnTe core was larger than that for the uncovered ZnTe NWs [3]. It is the result of the strain induced in the core by the lattice mismatched shell. Schematic model of the strain induced in the core is proposed.
Acknowledgments: This work was supported by the European Community-Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastruc- ture Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science,” Contract RII3-CT-2004- 506008), the European Union within European Regional Development Fund through Innovative Economy grant (POIG.01.01.02-00-008/08) and National Centre of Sci- ence (Poland) grant DEC-2011/01/D/ST5/05039.
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