Short- and medium-range order of atomic structure and electronic properties of arsenic salinide and sulphide amorphous films

Full text: The relationship between short- and medium-range order of atomic structure and electronic properties is the object of invariable attention of disordered matter physics. This problem is the most important for non-crystalline semiconductors, and specifically for chalcogenide glassy semiconductors (ChGS) films due to low co-ordination of atoms, which leads to lability of their structure. In this report we present results of atomic structure, electric, optical properties and carrier drift investigation in amorphous films of As2Se3 and As2S3 prepared by thermal evaporation in a vacuum (TE films) and by RF ion-plasma sputtering (RF films). These techniques strongly differing in the conditions of substance vaporization and condensation atoms on a substrate. The short- and medium-range order of the films atomic structure has been studied by X-ray diffraction analysis using the CuKα radiation (λ=1.5418 Angstrom) and by Raman spectroscopy. Raman spectra were measured at room temperature on Perkin-Elmer Spectrum GX Raman FT-IR Spectrometer (180 deg. backscattering). Vibrational modes were excited vertically polarized light of DPY Nd:YAG laser (1.064 μm). The radii of the first and second coordination spheres, number of the nearest neighbours of As and Se (S) atoms in the first coordination sphere, dimension of the medium-range order domain and 'quasi-period' in that region have been determined for the studied samples. The identification of the structural units in matrix of the film has been carried out, too. It is established that the films prepared by different methods have differences in the parameters of short- and medium range orders. It follows from the comparative analysis of Raman spectra that spectrum of RF films is significantly more complex than that of glass and TE films. The matrix of RF films contains, along with the structural units AsSe3/2 (AsS3/2) inherent in TE films and glass, other structural units with As and Se (S) excess. It may be molecule As4Se4 (As4S4) or some new structural unit. Structural investigations show that TE and RF films have significant differences in their local structure. The conductivity, activation energy of conductivity, optical gap, shift of the Fermi level with respect to midgap and carrier drift mobility were determined from investigations of electrical and optical properties as well as from investigations of electrons and holes transient photocurrents in TE and RF As2Se3 and As2S3 films. It is established that electrons parameters of TE and RF films have differences, and the drift mobility of electrons differs the most significantly. It is well known that in TE As2Se3 and As2S3 films only holes are mobile carrier. On the contrary, in RF films of those compounds the drift mobility of electrons and holes are practically the same, and these coincide with hole drift mobility in TE films equal measurement conditions. Note that the main features differences of TE and RF films electronic parameters, as well as those parameters of atomic structures, are the same for films of investigated compounds As2Se3 and As2S3. The analysis of obtained results point to a difference in the spectra of localized and extended electronic states in TE and RF films, i.e. in their electronic structure, and mainly in the spectra of localized states, which control electron transport. Consequently, the amorphous films of As2Se3 and As2S3 compounds prepared by thermal evaporation in a vacuum and by RF ion-plasma sputtering have differences both in local atomic structure and in electronic structure. It is observed the relationship between electronic structure and local atomic structure of As2Se3 and As2S3 films