Experimental evidence of polarons in silver doped ZnO films

Pure and silver (0.27 at.%)-doped zinc oxide (Ag:ZnO) films were deposited on sapphire substrates by e-beam evaporation technique at 290 °C and post-annealed at 350 °C for 1 h under ambient air. The XRD analysis confirms the formation of wurtzite hexagonal structure. From the UV–Vis absorption spectra, the incorporation of Ag into the ZnO involves a slight decrease in the optical band gap as compared to that of undoped ZnO film. Infrared reflection spectral data of the Ag:ZnO films indicate hole polaron states placed in the range of 0.136–0.439 eV above the valence band maximum. According to the calculated electron–phonon coupling strength (1.04) and binding energy (− 76 meV), the polaron of large radius is defined. The polaron optical conductivity and carrier relaxation time are determined from the experimental spectra using Kramers–Kronig relations.