Vibrational and crystalline properties of polymorphicCuInC2(C=Se,S)chalcogenides

Abstract

This paper deals with the analysis of the vibrational and crystallographic properties of $\mathrm{Cu}\mathrm{In}{C}_{2}$ $(C=\mathrm{S},\mathrm{Se})$ chalcogenides. Experimentally, evidence on the coexistence in epitaxial layers of domains with different crystalline order---corresponding to the equilibrium chalcopyrite (CH) and to CuAu (CA)---has been obtained by cross section transmission electron microsopy (TEM) and high resolution TEM (HREM). Electron diffraction and HREM images give the crystalline relationship ${[1\overline{1}0]}_{\mathrm{CH}}\ensuremath{\Vert}{[100]}_{\mathrm{CA}}$ and ${(112)}_{\mathrm{CH}}\ensuremath{\Vert}{(011)}_{\mathrm{CA}}$, observing the existence of a ${(112)}_{\mathrm{CH}}\ensuremath{\Vert}{(001)}_{\mathrm{CA}}$ interphase between different ordered domains. The vibrational properties of these polytypes have been investigated by Raman scattering. Raman scattering, in conjunction with XRD, has allowed identifying the presence of additional bands in the Raman spectra with vibrational modes of the CA ordered phase. In order to interpret these spectra, a valence field force model has been developed to calculate the zone-center vibrational modes of the CA structure for both $\mathrm{Cu}\mathrm{In}{\mathrm{S}}_{2}$ and $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ compounds. The results of this calculation have led to the identification, in both cases, of the main additional band in the spectra with the total symmetric mode from the CuAu lattice. This identification is also supported by first-principles frozen-phonon calculations. Finally, the defect structure at the interphase boundaries between different polymorphic domains has also been investigated.