Infrared optical properties of MgxZn1-xO thin films (0&lex&le1): Long-wavelength optical phonons and dielectric constants

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作者：
Bundesmann, C. [1 ,4 ]
Rahm, A. [1 ]
Lorenz, M. [1 ]
Grundmann, M. [1 ]
Schubert, M. [1 ,2 ,3 ]
机构：
[1] Institut für Experimentelle Physik II, Universität Leipzig, Linństrasse 5, D-04103 Leipzig, Germany
[2] Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511
[3] Center for Materials Research and Analysis, University of Nebraska-Lincoln, Lincoln, NE 68588-0511
[4] Leibniz-Institut für Oberflächenmodifizierung e.V., Permoserstraße 15, D-04303 Leipzig, Germany
来源：
Journal of Applied Physics | 2006年 / 99卷 / 11期
关键词：
Infrared spectroscopic ellipsometry in the spectral range from ω=360 cm-1 to ω=1500 cm-1 and Raman scattering spectroscopy are applied to study the long-wavelength optical phonon modes and dielectric constants of Mgx Zn1-x O thin films in the composition range 0&lex&le1. The samples were grown by pulsed laser deposition on sapphire substrates. X-ray diffraction measurements of the thin film samples reveal the hexagonal wurtzite crystal structure for x&le0.53 and the cubic rocksalt crystal structure for x0.67. A systematic variation of the phonon mode frequencies with Mg-mole fraction x is found for both hexagonal and cubic Mgx Zn1-x O thin films. The modified random isodisplacement model matches the observed composition dependence of the phonon mode frequencies for the hexagonal structure thin films [J. Chen and W. Z. Shen; Appl; Phys; Lett; 83; 2154; (2003); whereas a simple linear approximation scheme is sufficient for the cubic structure part. We observe a discontinuous behavior of the transverse optical phonon modes (decrease); and the static and high-frequency dielectric constants (increase) within the phase transition composition region from the wurtzite structure part to the rocksalt structure part. On the contrary; the longitudinal phonon mode parameters increase almost linearly; and upon phase transition the splitting between the transverse and longitudinal modes increases. We associate this discontinuous behavior with the change of the nearest-neighbor coordination number from fourfold (wurtzite structure) to sixfold (rocksalt structure) in our samples and the associated increase in bond ionicity from ZnO to MgO. Accordingly; we propose that the reduced exciton mass parameter should approximately double upon changing from wurtzite to rocksalt crystal structure. © 2006 American Institute of Physics;
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