Raman Spectra of Gyrotropic Cadmium Diphosphide Crystals

Abstract

Samples of CdP 2 single crystals oriented relative to the crystallographic axes were prepared. Their optical quality was studied using conoscopy and atomic-force microscopy. Raman spectra of CdP 2 single crystals in polarized and unpolarized light in backscattering confi guration were obtained and analyzed.Introduction. Wide-bandgap gyrotropic cadmium diphosphide (CdP 2 ) crystals are extensively used to fabricate electronic and nonlinear-optics devices. Their operating principle is based on the utilization of nonlinear polarizability and solid-state gyrotropy. Instantaneous feedback to the radiation active fi eld, which is the functional basis of many electronic devices, is possible due to their practical lack of inertia in CdP 2 crystals [1,2]. The interest of researchers with respect to these crystals has persisted throughout the years [3][4][5][6][7]. It is well known that gyrotropic CdP 2 crystals consist of a large quantity of Cd and P atoms joined into chains. Most of them are joined into moderately ordered groupings (clusters) with slightly variable geometric parameters that depend on the synthesis technology and crystal-growth conditions. Therefore, they determine the crystal physical properties [8,9].The goal of the present work was to establish the processing parameters (temperature gradient in the crystal-growth zone, the rate of movement, and post-processing annealing) in order to grow homogeneous crystals of high optical quality and the effectiveness of using conoscopy, atomic-force microscopy, and spatially polarized Raman scattering (RS) in order to monitor the structural perfection of the crystals.Experimental. Previously synthesized polycrystalline CdP 2 that was obtained from the starting materials by a dualtemperature method was used to grow CdP 2 single crystals [10]. Ampuls with polycrystalline CdP 2 were evacuated to 10 -3 Pa, sealed, and placed into a vertical dual-temperature resistance furnace. Single crystals of CdP 2 grew from the gas phase in the conical part of the ampul, creating a small critical overcooling that provided a suffi cient transfer rate of the compound from the vaporization zone into the crystallization zone. It was found that the critical overcooling for forming CdP 2 nuclei was 3-8 K; the crystallization-zone temperature, 980-1010 K. Low supersaturations were created in large-diameter ampuls (18-20 mm) in order to control nucleus formation and limit the processing to reasonable times. This kept the growth rate rather high. Moving the ampul toward the crystallization zone, i.e., increasing the temperature gradient, at a rate of 0.6-0.8 mm/h that was approximately equal to the crystal growth rate, provided a constant vaporization-zone temperature during the whole growth process and maintained stable growth conditions. Thus, the optimum combination of the three factors, i.e., crystallization temperature, critical overcooling, and ampul movement rate, that allowed uninterrupted growth of tetragonal CdP 2 single crystals was found [10][11][12]. The grown CdP 2 crystals wer...

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