Laser‐Induced Formation of CdS Crystallites in Cd‐Doped Amorphous Arsenic Sulfide Thin Films

Abstract

As 2 S 3 :Cd films with nominal Cd content x up to 4 at.% were prepared by thermal evaporation. Their amorphous structure is confirmed by Raman spectroscopy. The film surface roughness estimated from atomic force microscopy does not exceed 1 nm. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy show a strong decrease of Cd content with the film depth. Peaks of CdS longitudinal optical phonon and its overtones emerging in the Raman spectra of films with x ! 2 at.% Cd are the evidence for the laser irradiation-induced formation of CdS crystallites in the As 2 S 3 :Cd films. The downward shift of the CdS Raman peak frequencies is mainly explained by tensile strain undergone by the CdS crystallites in the films caused by material transfer from the laser spot due to the photoplastic effect.

Ternary CdS1–xSexnanocrystals formed in Cd‐doped As–Se–S films due to photoenhanced diffusion during micro‐Raman measurement

“…2), three broad peaks near 168, 233, and 341 cm −1 (the most intense one) are revealed in the Raman spectrum which is typical for amorphous arsenic sulfide. [ 19,48–52 ] It is important to note that in our case, the As 2 S 3 films prepared by thermal evaporation definitely exhibit amorphous‐like Raman spectra contrary to earlier data for fresh As 2 S 3 films obtained by vacuum thermal or flash evaporation [ 53–56 ] where the Raman spectra contained more narrow features on the background of a glass‐like spectrum with broad maxima. These features were related to the vibrations of As 4 S 4 , As 4 S 3 , and other pronounced structural units in the glass network and disappeared only after annealing at 160–180°C for 1 h.…”

“…The samples exhibit rather uniform patterns; the surface roughness values R a calculated from the AFM images were in both cases about 0.8 nm. Note that in our recent studies of Cd‐doped As 2 Se 3 [ 18 ] and As 2 S 3 ,[ 19 ] films prepared by a similar technique, we observed surface roughness of the same scale (below 1 nm) practically independent of the cadmium content in the film. In addition, no significant differences in the surface roughness were observed for films prepared on silicon and silicate glass substrates.…”

“…It is known that all mentioned dopants concentrate in the near‐surface film layers. [ 18–20,33 ] Therefore, one can suppose that the decomposition of As 2 S 3 and As 2 Se 3 at the evaporation enables reaction of free sulfur and selenium with metallic dopants leading to the formation of chalcogenides with lower equilibrium vapor pressure values. This results in the corresponding species being evaporated at the end of the process.…”

“…A similar distribution of group II or III dopants in amorphous arsenic chalcogenide films is known for As 2 Se 3 and As 2 S 3 from our earlier studies. [ 19,20,33 ] It is also known that, even though the nominal doping concentration in this case cannot be considered a realistic characteristic of the entire film material, it qualitatively correlates with the dopant content on the film surface and with the average dopant content within the dopant‐containing layer. Hereinafter, we keep on referring to the nominal concentration of Cd in the films for the sake of sample labeling and relative estimation of the doping level.…”

“…In amorphous Cd‐ or Zn‐doped As 2 Se 3 or As 2 S 3 films, this can result in fast aggregation of the dopant (Cd or Zn) and chalcogen (S or Se) atoms and formation of binary (CdSe, CdS, ZnSe, or ZnS) II–VI nanocrystals (NCs) directly during the Raman measurement. [ 18–21 ] In the present paper, we investigate Cd‐doped As–Se–S films using X‐ray diffraction (XRD), atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS), photoluminescence (PL), and micro‐Raman spectroscopy. One could expect that in the As–Se–S:Cd films, illumination by laser light would lead to the formation of ternary CdS 1– x Se x nanocrystals (NCs).…”