Structural Investigations of Zn_1-xMg_xSe Polytypes in  by Transmission Electron Microscopy and Cathodoluminescence

Zn_1-xMg_xSe is of great interest with respect to applications in semiconductor optics as a buffer material for heteroepitaxy as well as for use as an active light emitting component. Structural investigations have been performed using Transmission Electron Microscopy (TEM) and X-Ray diffraction (XRD). Luminescence properties have been characterized using photoluminescence as well as cathodoluminescence. The material shows a transition from sphalerite to wurtzite structure with increasing x. Within the transition range, near x=0.19, regular and irregular sequences of hexagonal and cubic stacking grow. Regular sequences form the polytypes 4H and 8H. The luminescence of the various crystal structures is found to be different. We used this effect to distinguish the phases by spatially and spectrally resolved cathodoluminescence measurements.

 

Fig.1    Schematic drawing of Zn_1-xMg_xSe a grown  bulk crystal. The Mg content increases in the growth direction. The [111] direction of 3C and the [001] directions of the 8H, 4H and 2H, respectively, are nearly parallel to the growth direction.

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a)                                                                    b)

Fig.2    HRTEM images of Zn_1-xMg_xSe in sphalerite (3C, x=0.15), a), and wurtzite (2H, x=0.21), b), structure. In the upper left edge of b), there is a simulated HRTEM image of 2H inserted. Note, that the x values were estimated from macroscopic measurements and do not necessarily give the local value within the imaged area. Crystal growth direction is from bottom to top or vice versa (sense of direction may not surely be determined).

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a)                                                                                                b)

c)                                                                 d)

e)

Fig.3    HRTEM images of crystal structures containing hexagonal and cubic stacking: 2H with an indicated cubic stacking sequence (x=0.21), a), interface between 2H and 4H, b), 4H polytype (x=0.195, a simulated HRTEM image of 4H is inserted), c), interface region between 4H and 8H, d), and 8H polytype together with a larger area of cubic (3C) stacking (x=0.185), e). For the local validity of the x values and the growth direction, see caption of Fig. 2.

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a)                                                b)                                               c)

Fig.4    CL and SE images of a specimen area consisting mainly of 4H and 8H. The images are -- from top -- a) CL image at 2.97 eV photon energy, b) CL image at 2.99 eV photon energy, and c) SE image, all showing the same area. The supposed growth direction is oriented perpendicular with respect to the dark and bright stripes in the CL images.

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Fig.5    CL spectra measured from the places indicated in Fig. 4. The peak at 2.97 eV is related to 8H, the peak at 2.99 eV is related to 4H.

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J. Cryst. Growth 184/185 (1998) 1015-1020

Autoren: U. Falke, A. Cichos, F. Firszt, H. Meczynska, P. Dluzewski, W. Paszkowicz, J. Lenzner, M. Hietschold

Institute of Physics, Technical University Chemnitz, D-09107 Chemnitz, Germany
Institute of Physics, N. Copernicus University, ul. Grudziadzka 5/7, 87-100 Torun, Poland
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warszawa, Poland
Faculty of Physics, University of Leipzig, D-04103 Leipzig, Germany