TY - JOUR
T1 - Infrared optical properties of Mg xZn 1-xO thin films (0≤x≤1)
T2 - Long-wavelength optical phonons and dielectric constants
AU - Bundesmann, C.
AU - Rahm, A.
AU - Lorenz, M.
AU - Grundmann, M.
AU - Schubert, M.
N1 - Funding Information:
This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant No. SCHUH1338/3-1, and within the DFG research units 404 (Gr 1011/14-1, SCHUH1338/4-1, and SCHUH1338/4-2) and 522 (Gr 1011/11-1). The authors acknowledge D. Speeman for RBS measurements. They also thank the technical help of U. Teschner (Raman), H. Hochmuth (PLD), E. M. Kaidashev (PLD), and G. Ramm (PLD).
PY - 2006/6/1
Y1 - 2006/6/1
N2 - 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≤x≤1. 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≤0.53 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.
AB - 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≤x≤1. 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≤0.53 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.
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U2 - 10.1063/1.2200447
DO - 10.1063/1.2200447
M3 - Article
AN - SCOPUS:33745253370
SN - 0021-8979
VL - 99
JO - Journal of Applied Physics
JF - Journal of Applied Physics
IS - 11
M1 - 113504
ER -