Structural and Optical Characterization of ZnS Ultrathin Films Prepared by Low-Temperature ALD from Diethylzinc and 1.5-Pentanedithiol after Various Annealing Treatments

The structural and optical evolution of the ZnS thin films prepared by atomic layer deposition (ALD) from the diethylzinc (DEZ) and 1,5-pentanedithiol (PDT) as zinc and sulfur precursors was studied. A deposited ZnS layer (of about 60 nm) is amorphous, with a significant S excess. After annealing, the stoichiometry improved for annealing temperatures ≥400 °C and annealing time ≥2 h, and 1:1 stoichiometry was obtained when annealed at 500 °C for 4 h. ZnS crystallized into small crystallites (1–7 nm) with cubic sphalerite structure, which remained stable under the applied annealing conditions. The size of the crystallites (D) tended to decrease with annealing temperature, in agreement with the EDS data (decreased content of both S and Zn with annealing temperature); the D for samples annealed at 600 °C (for the time ≤2 h) was always the smallest. Both reflectivity and ellipsometric spectra showed characteristics typical for quantum confinement (distinct dips/peaks in UV spectral region). It can thus be concluded that the amorphous ZnS layer obtained at a relatively low temperature (150 °C) from organic S precursor transformed into the layers built of small ZnS nanocrystals of cubic structure after annealing at a temperature range of 300–600 °C under Ar atmosphere.