Epitaxial Growth and Surface Roughness Control of Ferromagnetic Thin Films on Si by Sputter Deposition

Abstract

Epitaxial growth of ferromagnetic (FM) thin-film electrodes on Si substrates is attracting increasing attention for magnetic tunnel junction (MTJ) studies as it enables both a high tunneling magnetoresistance (TMR) for practical applications and a well-defined junction structure for fundamental research. In this paper, we report epitaxial growth of both conventional FM and emerging Heusler FM electrodes by magnetron sputter deposition on Si substrates via appropriate buffer layers. Our primary focus is not only on the ability to grow epitaxial structures of the FM electrodes but also to achieve the desired smooth surfaces of these electrodes. The crystalline structures of the␣FM electrodes are known to play a critical role on the TMR of an MTJ. On the other hand, surface roughness of the FM electrode significantly degrades the performance of these junctions by interface scattering of spin-polarized electrons. We successfully grew various epitaxial face-centered cubic, hexagonal close-packed and body-centered cubic structures for Ni, Co, Fe, and their alloys in addition to the ordered intermetallic Heusler alloys, Co₂MnSi and Co₂MnAl, with appropriate buffer layers such as Ag between the FM layers and the Si substrates, followed by post-annealing. This annealing process greatly improves the quality of the epitaxy and the surface roughness of the FM layers deposited at room temperature. The crystallographic orientations of the ferromagnetic layers as well as the surface roughness were also found to vary with the orientation of the Si substrate.