PNNL

Abstract

We report on the preparation and detailed characterization of ferromagnetic Zn1-xCrxO thin films deposited on Si substrates using reactive co-sputtering of Cr and Zn in controlled oxygen atmospheres. X-ray diffraction data showed wurtzite ZnO peaks in the ferromagnetic films prepared with lower Cr powers, whose position and intensities are influenced by Cr doping. However, samples prepared with higher Cr powers did not show ferromagnetism but displayed evidence of Cr2O3 and ZnCr2O4 phases with no ZnO phase. The magnetization is higher (saturation magnetization Ms = 18 emu/cm3) for lower Cr concentrations and decreases for higher Cr doping. The samples were investigated extensively to understand the film composition, dopant distribution, homogeneity and potential origin of the observed ferromagnetism. Particle induced x-ray emission studies were employed to determine the chemical composition as well as the Cr/Zn ratio in the films. Film uniformity and homogeneity, investigated using Rutherford backscattering spectrometry, showed a relatively uniform ZnO layer in the as-prepared samples but, in a sample annealed at 800 oC, showed some diffusion of Si from the substrate. X-ray photoelectron spectroscopy studies indicated that Cr ions are in the oxidized state, but showed changes in the binding energy and Cr concentration when measured after removing 10 nm from the surface using Ar ion sputtering. Possible origins of the observed ferromagnetic behavior are discussed based on the comprehensive characterization results.