Havryliuk Y. Phonon spectra of crystals and nanosized structures of semiconductors of quaternary chalcogenides of metals

Investigation of the phonon spectra of quarternary chalcogenides of metals, such as Cu2ZnSn(S,Se)4, which are promising for use in photovoltaic, was done. The interpretation of the peculiarities of the polarization low-temperature Raman scattering spectra of orthorhombic wurtzstannite Cu2ZnSiS4 single crystals, prepared by the chemical transport method, and the assignment of spectral bands to specific types of lattice eigenmodes was made on the basis of comparison of experimental results with theoretical calculations, based on density functional lattice dynamics theory. The transformation of the vibrational spectrum of Cu2Zn(Sn1-xGex)S4 single crystals upon variation of composition is studied experimentally by low temperature Raman scattering. It is shown that fully symmetric “breathing” modes of GeS4 and SnS4 octahedra exhibit the so-called two-mode type of behavior, despite the closeness of their frequencies (360 and 340 cm-1, respectively). This implies the existence of corresponding modes with a virtually unchanged frequency over the entire composition range (0 ≤ x ≤ 1) and their weak interaction with other modes of the system. Density functional lattice dynamics calculations confirm this experimental finding. Other vibrational modes are found to show the typical one-mode type of transformation with composition. Resonant Raman scattering experiments, which are realized for the Ge-rich alloys of Cu2Zn(Sn1-xGex)S4, allowed the observation of the second- and the third-order scattering processes. Along with overtones and combinations of LO-lines that are typical for resonant Raman spectra of polar semiconductors, we also observed overtones of non-polar fully symmetric A-lines, as well as the combinations nA + mLO. The effects of structural (positional) disorder of mixed crystals were analyzed based on Raman scattering as well as the photoluminescence results. For film samples CZTSe the dependence of the change of stoichiometry and the quality of a crystal lattice with a change in film thickness is demonstrated. The peculiarities of the behavior of phonon spectra in the transition from bulk materials to nanocrystals (NCs) of the compound Cu2ZnSnS4, as well as all possible secondary phases, are studied. Size-selected CZTS NCs reveal phonon confinement that manifests itself in an upward shift of the main phonon peak by about 3–4 cm-1 by varying the NC diameter from 3 to 2 nm. A non-monotonous shift and narrowing of the main peak are attributed to the special shape of the phonon dispersion in this material. The possibility of secondary phases formation in the same synthesize conditions was shown. The Raman spectra of most of them are qualitatively similar to the spectra of bulk materials. One The Raman spectra for Zn-Sn-S was not studied before. The strongly different from convention picture was Raman spectra for ZnS NCs. The absence of LO mode, but the presence of a lot of two-phonon modes we explain by very strong electron-two-phonon interaction due to ultrasmall (≈ 2 nm) size of ZnS NCs. The influence of the conditions of the formation of nanoparticle films and their processing by flash annealing method on the quality of the final material is shown. The method of sample preparation, the nature of the supporting substrate and the photoexcitation regime are found to crucially influence the Raman spectra of the CZTS samples. Particularly, the possible oxidation and hydrolysis of CZTS NCs with the concomitant formation of a Cu–S phase are systematically investigated. The flash-lamp annealing with low power density found to be very useful to improving quality of CZTS nanocrystals. The improving of quality of NCs also was observed and for high annealing power density, in the same time, the decay of CZTS NCs to secondary phases in intermediate power density was observed. The peculiarities of the rearrangement of the phonon spectrum with the change in the component composition in AgxGaxGe1-xSe2 compounds promising for the nonlinear optics of the medium IR range have been established, and the possible presence of secondary defective phases GeS and Se is shown as microinclusions in the bulk material.