All Epitaxial Heterostructure for Spin Injection from a Half Metal into Silicon

Using reactive molecular-beam epitaxy, epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ / SrTiO$_{3}$ / Si heterostructures have been grown. The SrTiO$_{3}$ layer, just a few unit cells thick, serves simultaneously as a tunnel barrier and as a means to reduce reaction between the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and the underlying Si. The growth of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ at MBE-compatible pressures requires ozone, which readily oxidizes bare Si and would destroy the chances for epitaxial growth. In contrast, epitaxial SrTiO$_{3}$ can be grown on (001) Si using molecular oxygen via a complex, but established process. Once the SrTiO$_{3}$ film is complete, ozone is turned on for the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ growth. The thin SrTiO$_{3}$ layer acts as a diffusion barrier for oxygen limiting the formation of SiO$_{2}$ at the SrTiO$_{3}$/Si interface. X-ray diffraction measurements show that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer has good crystalline quality with rocking curve full width at half maximum values of the 200 peak of less than 0.5\r{ }. Furthermore, electrical transport measurements indicate that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer is ferromagnetic and metallic below $\sim $370 K with a resistivity $<$100 $\mu \Omega $--cm at 4.2 K. Possible devices for tunneling spins into Si and for detecting spin carrier density inside a Si channel will be discussed.