Epitaxial Ferrimagnetic Mn4N Thin Films on GaN by Molecular Beam Epitaxy

Abstract

The III-nitride family of wide bandgap semiconductors GaN, AlN, and their alloys are important for diverse applications ranging from solid-state lighting to RF and power electronics [1]. The long spin lifetimes in nitride semiconductor platform [2] makes it attractive for exploiting the spin degree of freedom of conducting electrons. The ferromagnet/semiconductor heterostructure is crucial for fundamental spin-related device building blocks such as spin injection, spin transport, spin detection [3] and spin to charge conversion [4].Memory devices utilizing spin-orbit torques require smooth interfaces between ferromagnets and the heavy metal or topological insulator layers on top for efficient spin transmission [5]. Epitaxial growth of magnetic layers with smooth surfaces on GaN hosting desirable properties for spintronic applications will provide a path towards spintronic devices for energy-efficient memory applications, and its integration with GaN-based RF, photonic, and wide-bandgap CMOS platforms [6].Mn4N, a metallic nitride ferrimagnet, is an attractive candidate for direct epitaxial integration with GaN and AlN for all-nitride ferromagnet/semiconductor heterostructures. MBE grown Mn4N thin films on cubic substrates such as MgO and SrTiO3 (STO) exhibit desirable properties for spintronic applications such as a high critical temperature (TN ∼ 740 K), large spin polarization (P ∼ 70 %), strong perpendicular magnetic anisotropy (Ku = 1.1×105 J/m3), low saturation magnetization (Ms = 7.1×104 A/m on STO), large domains (∼ millimeter size on STO) and high domain wall velocities (up to 900 m/s) [7, 8].Through exploration of nucleation and growth conditions, we uncover plasma-assisted MBE growth conditions needed for significantly improved epitaxial growth of c-axis aligned Mn4N on GaN with smooth surface morphologies. A series of four samples were grown in which the growth temperature of the 80 nm Mn4N layer was varied from Ts = 150 C to Ts = 375 C, after the deposition of 100 nm homoepitaxial undoped GaN buffer layers at Ts = 670 C, as shown in Fig. 1. Instead of out-of-plane [111] orientation of Mn4N on GaN as found in reactive MBE [9], Mn4N layers grown using plasma-assisted MBE in this work are dominated by [001] orientation, and exhibit 12-fold in-plane symmetry in the diffraction pattern. Smooth Mn4N layers are obtained at low growth temperatures of Ts <= 300 C, though deep pits with depth ~ 10 nm measured by atomic force microscopy are present in the film grown at Ts = 300 C. The magnetic properties of Mn4N grown on hexagonal GaN are comparable to those in earlier reports on cubic substrates such as MgO, and can be tailored by varying the growth temperature. For example, with the increase of growth temperature, the anomalous Hall resistance hysteresis loop (Fig. 2) not only becomes squarer but also exhibits an interesting sign-flip from n-type to p-type between Ts = 225 C and Ts = 300 C. Details about the structural and magnetic properties of Mn4N on GaN will be presented. **