engleski

Structural investigation of nanocrystalline ZnO:Al thin films deposited by PLD in RF excited oxygen atmosphere

Nanocrystalline Al-doped ZnO thin films were deposited in vacuum and in oxygen atmosphere with and without RF excitation by pulsed laser deposition (PLD). The gas pressures during deposition were varied between 10 and 70 Pa in order to investigate the influence on structural properties and point defect dynamics. The surface structures were monitored by atomic force microscopy (AFM) and field-emission-gun scanning electron microscope (FEG-SEM), while the obtained nanostructural changes were analyzed by x-ray scattering at grazing incidence angles. The imaged surfaces, obtained from AFM, are not homogenous and flat, but show a granular structure. The surface roughness mostly follows the trend, where the roughness increases with the oxygen pressure, and also by applying RF excitation. Grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXRD) showed also the trend, that the roughness of film surfaces increases with increasing gas pressure, and the density decreases due to the formation of nanovoids. The nanocrystals sizes estimated from GIXRD were around 20 nm, while the sizes of the nanovoids increased from 1 to 2 nm with the oxygen pressure. Furthermore, as seen from the SEM micrographs, various larger particles are present on the surface, possibly as a consequence of droplet formation in the laser plume. The samples deposited under vacuum condition and at lower pressures, contain on the surface nanostructures with cylindrical shape, hollow at lower pressures, while solid at higher pressures. At 70Pa, the cylindrical shapes turned to spherical. The addition of excited oxygen particles from a RF plasma improves the structural ordering by lowering the defect level, which is evident from GISAXS and photoluminescence (PL) measurements. The PL consisted of 3 well defined peaks, UV emission that corresponds to a band-to-band transition, blue emission that appeared due to Zn vacancies, VZn, and red emission that is probably due to oxygen interstitials, Oi. For all pressures the RF excitation lowered the defect level related to blue emission and resulted in a narrower UV luminescence peak, indicating better structural ordering. The red emission peak is only seen using a RF excited oxygen atmosphere at 70Pa. The observed influence of the pressure and RF excitation on the films properties is a consequence of two main effects: the variation of the energy transfer from the laser plume to the growing film due to a different collision rate in the gaseous phase and changes of the growth chemistry due to various concentrations of active oxygen species.

ZnO:Al ; PLD ; FEG-SEM ; AFM ; GIXRD

nije evidentirano