Polarization dependent Raman spectroscopy characterization of kesterite Cu 2 ZnSnS 4 single crystals
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Polarization dependent Raman spectroscopy characterization of kesterite Cu2ZnSnS4 single crystals D. O. Dumcenco1, Y. P. Wang1, S. Levcenco2, K. K. Tiong3 and Y. S. Huang1 1
Department of Electronic Engineering, National Taiwan University of Science and Technology,
Taipei 106, Taiwan 2
Helmholtz Zentrum Berlin f¨ur Materialien and Energie GmbH, D-14109 Berlin, Germany
3
Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
ABSTRACT The vibrational properties of kesterite Cu2ZnSnS4 (CZTS) single crystals were studied by polarization-dependent Raman scattering measurements. The CZTS crystals grown by chemical vapor transport technique using iodine trichloride as a transport agent consist of several mirrorlike planes. The detailed analysis of the experimental spectra obtained from different planes allows determining the symmetry assignment of the observed Raman-active modes. The wavenumber values of Raman-active modes are compared with the results of recent theoretical calculations. The presented data are useful for examination of CZTS absorber films applied for solar cells to clarify the existence of structural or phase inhomogeneities. INTRODUCTION The quaternary compound Cu2ZnSnS4 (CZTS) semiconductor has attracted great interest due to its potential applications for sustainable thin-film solar cell devices [1-3]. CZTS has large direct band gap (Eg ~ 1.5 eV), high absorption coefficients (>104 cm-1), intrinsic p-type conductivity and low thermal conductivity [4-6]. Solar cells based on selenium-containing Cu2ZnSn(S,Se)4 compound have demonstrated efficiencies above 10% [7]. Comparatively to the other important solar cell materials (i.e. In, Ga, etc.) [8], the composition of naturally abundant and inexpensive elements such as Zn and Sn, makes CZTS particularly attractive candidate for large-scale commercial application [9]. From theoretical point of view, recent detailed theoretical investigations provided some vibrational properties of CZTS [10,11]. Experimentally, some infrared (IR) [12] and Raman [1114] spectroscopy experiments on CZTS thin films were also performed for the analysis of the crystal structure. However, there are still little detailed experimental data on vibrational properties of CZTS single crystals [15]. The detailed experimental results of polarization-dependent backscattering Raman spectroscopy on CZTS single crystals are presented. The CZTS crystals grown by chemical vapor transport (CVT) technique using iodine trichloride (ICl3) as a transport agent are consisted of several mirror-like planes. The observed modes detected from (100), (001), (110) and (112) planes were classified by applying the selection rules to the Raman active modes. Also, experimental results have been compared with the data of recent theoretical calculations [10,11].
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EXPERIMENT Using CVT technique with ICl3 as a transport agent, single crystals of CZTS with welldefined faces were grown [6]. From energy dispersive X-ray analysis, the average atomic ratio of Cu:Zn:Sn:S was fo
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