Epitaxial growth and characterization of Ni films grown on MgO(001) by biased direct-current sputter deposition

Abstract

Ni films thinner than 180 nm are deposited on MgO(001) substrates at a temperature Ts of 190 or 280 °C by dc sputtering at 2.5 kV in Ar gas. A dc bias voltage Vs between 0 and -140 V is applied to the substrate during the deposition. A study of structural and physical properties of the Ni film is made by the use of reflection high-energy electron diffraction (RHEED), cross-sectional transmission electron microscopy (XTEM), x-ray reflection diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and by measuring (TCR) in the temperature range from 35 to 135 °C. When Ts=190 °C the Ni film retains a polycrystalline structure at any Vs. When Ts=280 °C, as Vs increases from 0 to −140 V the film transforms from the polycrystal to the single crystal with the orientation as Ni(001)∥MgO(001) and Ni〈010〉∥MgO〈010〉, indicating that an optimal value of Vs for the epitaxial growth ranges from −80 to −110 V. Besides, an analysis of RBS spectra in comparison with XTEM images explains that the atomic density of the Ni film is the highest at Vs=−80 V. The optimal condition for the epitaxial growth is also confirmed by the change of TCR as a function of Vs. In conclusion, the epitaxial growth of the Ni film with the lower defect density is dominated at Vs=−80 to −110 V by the bombardment of both energetic ions and fast neutrals of Ar to increase the mobility of Ni adatoms and to resputter impurities during the film formation. This effect is pronounced at Ts≥280 °C.