Vibrational properties and structure of undoped and Al-doped ZnO films deposited by RF magnetron sputtering

Abstract

Highly conductive and transparent in the visible range Al-doped ZnO (ZnO:Al) and undoped ZnO films have been deposited by RF magnetron sputtering. Reflection high-energy electron diffraction observations characterized them as textured. The habitus of the microcrystallites forming the texture depends on the Al doping. The layer texture of undoped ZnO films has texture axis parallel to the substrate. The ZnO:Al films, instead, show a columnar texture with texture axis perpendicular to the substrate. The Raman spectra of the films obtained by non-resonant excitation are completely different from those of the target material which is polycrystalline ZnO. For the interpretation of the different bands in the Raman spectra the existence of a depletion region near the grain boundaries has been assumed. The most intensive band in the Raman spectra at approximately 570 cm −1 has been assigned to electric field-induced Raman scattering on longitudinal optical phonons. The built-in electric field in the depletion region induces the Raman activity of the B 2 modes and a band at 276 cm −1 appears in the spectra. Phonon modes highly localized near the grain boundaries have been detected at 516 cm −1 and 468 cm −1 which are well pronounced in the Raman spectra for the doped samples. Localized modes were observed also in the infrared reflection spectra of the doped films. Surface enhanced Raman scattering has been applied and the band in the range 830–920 cm −1 has been interpreted as due to adsorbates from the ambient air. It has been shown that the non-resonant Raman scattering can be used for qualitative study of some details of the microstructure of the zinc oxide films like the built-in electric field and the adsorbates in the films.