CuPt Ordering Signatures of Phonons in GaInP 2
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ABSTRACT We present a comprehensive overview of the large spectroscopic evidence on the
influence of ordering on the Raman and infrared spectra of partially ordered GaInP 2. Our phonon mode calculation within the adiabatic bond charge model describes the appearance of new phonons and also the optical anisotropy. INTRODUCTION During the ten years following the first observation of spontaneous ordering in Ga 0 52InO048P (written as GaInP 2 for simplicity) alloys, most of the studies on optical properties were focused on analyzing the influence of ordering on the electronic states.1 The orderinginduced changes in lattice dynamics were found less sensitive and thus the studies on the optical properties of phonons were scarce. Besides the effects of ordering on the lineshape of the Raman peaks,2 3 only the emergence of a peak in the frequency range of acoustic phonons was reported.3 In the last few years, however, the growth of strongly ordered single variant GaInP 2 samples has made it possible to obtain a remarkable amount of experimental evidence for ordering effects on the phonon spectra.4 -1 However, a definitive picture of the influence of ordering on the vibrational properties has not yet been achieved. Two main difficulties prevent a reliable phonon mode assignment. The
first is that, despite the recent improvement in sample quality, ordering is still imperfect. A partially ordered alloy is characterized by an order parameter ri denoting the distribution of Ga an In in the respective /I I II planes. il values of up to 0.6 have been reported. As a consequence, ordering effects can be masked by the optical response of the corresponding random alloy. The second difficulty is associated with the experimental configuration. Thin epilayers of partially ordered compound are grown on (001) substrates. The growth direction does not coincide with any principal axis of the ordered alloy, which imposes severe limitations in performing measurements in geometries where the phonon wave vector is along one of the principal axes. As a result, experimental results do not always reveal all the clues and often it is not possible to make a complete assignment of the detected modes without the help of a theoretical model. Up to now, two theoretical approaches have been considered. The first one uses a phenomenological valence overlap shell model. 5 It gives a rather detailed outline of the anisotropy in the zone center and attempts to elucidate the experimental results, but it needs an a priori assignment of the experimental phonons in order to adjust the model parameters. The second one is based on an ab initio calculation.12 Using a first-principles density functional linear-response theory, a detailed study of phonons is performed in the random and ordered alloys. But unfortunately the authors do not give a complete account of the anisotropy of the zone
center phonons and do not discuss the experimental results in detail. 223 Mat. Res. Soc. Symp. Proc. Vol. 583 ©2000 Materials Research Society
SYMMETRY CONSIDERATIONS The coor
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