Ion irradiation effects during growth of Mo/V(001) superlattices by dual-target magnetron sputtering

Abstract

Epitaxial (001)-oriented Mo/V superlattice films with wavelengths of ≈ 5 nm have been grown on MgO(001) substrates, kept at 700°C, by dual-target unbalanced magnetron sputter deposition in Ar discharges. Low-energy (15–250 eV) Ar ion irradiation with incident ion-to-metal flux ratio of ≈ 1 during film growth was obtained through the application of a negative potential V s to the substrate. The effects of ion bombardment on interface roughness and mixing, resputtering rates, and defect structure were investigated using a combination of cross-sectional transmission electron microscopy (XTEM), X-ray diffraction (XRD), and simulation of XRD patterns. High-resolution XTEM images showed that the interfaces were relatively sharp for V s ⩽ 100 V while higher V s values resulted in more diffuse interfaces indicating ion-induced intermixing. By using a kinematical model of diffraction, and comparing with experimental XRD results, it could be concluded that the intermixing increased from ≈ 0.3 nm (2 monolayers) at V s = 15 V to & 0.9 nm (6 monolayers) at V s = 250 V. The inhomogeneous strain showed a large increase for V s & 50 V . This is explained by an incorporation of point defects. Coherency strain relaxation between layers is suggested to take place through the formation of edge dislocations with Burgers vector 〈110〉 by climb processes. Finally, increasing V s also resulted in resputtering, preferentially from the V layers.