An analysis is made of the many theoretical and experimental studies of layered crystals. The results of this analysis for the particular case of group III-VI semiconductors are used to investigate the features in the electronic and phonon spectra which are due to the manifest structural anisotropy of these compounds. It is shown that in both the phonon and electronic spectra there are branches (bands) in which there is practically no dispersion in the direction of the C axis of the crystal. These vibrational branches (the so-called "intralayer vibrations") and energy bands (made up, for example, of the metal orbitals) are extremely anisotropic (quasitwo-dimensional). The intralayer vibrations contribute substantially to the phonon subsystem of layered crystals, and, therefore, all the properties which are determined to an appreciable degree by the phonon subsystem are highly anisotropic. The basic electronic properties of the semiconductors, being determined by the structure of only two bands (the conduction and valence bands), are almost isotropic. The reason for this is that these bands are made up of the p, orbitals of the metalloid, which, according to theoretical calculations of the electron density distribution, overlap with the corresponding states of the neighboring layers. This conclusion is supported by numerous experimental studies of the exciton spectra and photoemission of III-VI layered semiconductors.
Strong evidences for existence of indirect excitons in Gas are obtained a s a result of an analysis of differential absorption and photoluminescence spectra registered in the 4.2 to 100 K temperature range in polarized light. The location of the exciton band in gallium sulphide E,,, = = (2594 & 2) meV and the energy of phonons which take part in indirect transitions are determined. Lines which are interpreted as a result of non-phonon transitions are observed in the absorption and emission spectra. Lines which arise from the decomposition of bound excitons are found in emission spectra as well. It is shown that polarization pecularities of Gas absorption and luminescence spectra are explained in terms of the band scheme of gallium selenide which is structurally analogous to Gas. The possible nature of the anisotropy of excitons in Gas is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.