Electronic and vibrational spectra of III–VI layered semiconductors

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 (quasi-two-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 $p_z$, 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.