Interfaces and Interdiffusion in Heusler-alloy/GaAs(001) Hybrid Structures

Integration of ferromagnetic properties into semiconductor devices has become a focus of worldwide research. It promises access and control of the spin information of the electron, in addition to its charge currently used in the transistor, and accordingly advanced applications in the fields of a future beyond-silicon electronics. Of particular interest are ferromagnetic layers with a high spin polarization that may serve as efficient spin injectors into the semiconductor for spintronics applications or as high- performance tunneling magnetoresistive (TMR) elements for magnetoelectronic devices. Selected compounds of the Heusler alloys are so-called half-metallic ferromagnets that exhibit a band structure being metallic for one sort of spin and semiconducting for the other. Since half-metallic ferromagnets exhibit conduction electrons with 100% spin polarization they are perfect spin-injectors and promise magnetic tunneling junctions (MTJs) with ideally an infinite TMR ratio. According to recent theory, for attaining high spin-polarization perfect crystallographic order is essential. Furthermore, for efficient spin injection the interface has to be abrupt and even more annoying, it seems to depend dramatically on the type of alloy atoms that participate in the contact layer to the substrate. Consequently interface disorder due to interdiffusion or interface reactions between film and substrate is detrimental for technological application. Our recent investigations of Fe1-x Si x /GaAs(001) evidence strong interdiffusion at the interface region. Real-time-stress measure- ments combined with spatially-resolved determination of the chemical deposition reveal considerable diffusion of both, Fe and Si, into the GaAs substrate as well as of As and Ga into the Fe1-x Si x films, creating intermixed layers of 23 nm thickness in both, film and substrate. Interdiffusion is already dominant at the moderate growth-temperatures typically required for crystallographic ordering and lithographic processing, which explains the obtained low spin-injection despite of excellent abrupt interfaces indicated by transmission electron microscopy and x-ray diffraction.. In view of our alarming results, the development of appropriate diffusion-barrier layers is essential for ferromagnet/ semiconductor hybrid based devices. The applied project focuses on the development of appropriate diffusion barriers for the growth of half-metallic Heusler compounds on GaAs(001) that may also function as tunnel barriers in order to circumvent the impedance- matching problem in spin-injection. In detail, this concerns the formation of four relevant interfaces: (i) barrier- A/GaAs(001), (ii) Heusler/barrier-A, (iii) barrier-B/Heusler, and (iv) Heusler/ barrier-B. Of particular interest are atomistic details of interface formation and possible interdiffusion during the the nucleation and growth regimes of each individual step. The results of the project will form the basis for the fabrication of reliable spin injectors [steps (i) and (ii)] as well as of high-performance Heusler MTJs [steps (i) to (iv)]. The key experimental techniques of this study are: (i) in-situ stress measurements to investigate nucleation, growth, strain-relaxation, and intermixing in real-time, and (ii) a scanning tunneling microscopy (STM) attached recently to our III/V-semi- conductor/ ferromagnet MBE system for in-situ investigations of interface structure and composition on a local scale with atomic resolution.

Integration of ferromagnetic properties into semiconductor devices has become a focus of worldwide research. It promises access and control of the spin information of the electron, in addition to its charge currently used in the transistor, and accordingly advanced applications in the fields of a future beyond-silicon electronics. Of particular interest are ferromagnetic layers with a high spin polarization that may serve as efficient spin injectors into the semiconductor for spintronics applications or as high-performance tunneling magnetoresistive (TMR) elements for magneto- electronic devices. Selected compounds of the Heusler alloys are so-called half-metallic ferromagnets that exhibit a band structure being metallic for one sort of spin and semiconducting for the other. Since half-metallic ferromagnets exhibit conduction electrons with 100% spin polarization they are perfect spin-injectors and promise magnetoresistive elements with ideally an infinite TMR ratio.For attaining high spin-polarization perfect crystallographic order is essential. Furthermore, for efficient spin injection the interface has to be abrupt and even more annoying, it seems to depend dramatically on the type of alloy atoms that participate in the contact layer to the substrate. Consequently interface disorder due to interdiffusion or interface reactions between film and substrate is detrimental for technological application. For the ternary Heusler compound Co2FeSi on the technologically important semiconductor gallium arsenide (GaAs) it was found that Co, Fe, and Si diffuse about 50 nm deep into the substrate. Our own studies of Fe/GaAs(001) and Fe1-xSix/GaAs(001) revealed considerable diffusion of both, Fe and Si, into the GaAs substrate as well as of As and Ga into the Fe1-xSix films, creating intermixed layers of 2?3 nm thickness in both, film and substrate already at moderate growth temperatures (> 50C). In view of our alarming results, the development of appropriate diffusion-barrier layers is essential for ferromagnet/semiconductor hybrid based devices.In Project P 24335 the growth of two promising dielectrics, magnesium oxide (MgO) and molybdenum trioxide (MoO3), on GaAs(001) has been investigated, both of the capable to act as diffusion and/or tunneling barriers. The focus was laid on the exploration of the atomistic details of structure and composition of the interfaces between subsequent layers of a heterostructure (e.g., MgO/GaAs or Fe/MoO3). The heterostructures were prepared in a multi-chamber molecular-beam-epitaxy system and characterized by various structural and spectroscopic techniques. A central result is that an already ultrathin layer of MgO, consisting of only 14 (!) single crystalline atomic layers, are sufficient to block diffusion of, e.g., Ga and As into an adjacent Fe layer or of Fe into the GaAs substrate. We found that such diffusion barriers actually are too thin for suppressing magnetic coupling between two ferromagnetic layers and, consequently, for guaranteeing independent switching of the magneti- zation of each them. MgO spacer layers therefore are not applicable as diffusion barriers in future magnetologic devices. However, MgO diffusion barriers are thin enough for enabling the flow of a still measurable quantum mechanical tunneling current, which is utilized for the electrical detection of the magnetization direction of a single ferromagnetic layer in spintronics devices (e.g., in MRAM).With the growth of thin MoO3-films on GaAs(001) new scientific ground is broken. Owing to the almost perfect match of the lattice dimensions of the ?-MoO3(010) plane and GaAs(001), nearly strain-free growth of MoO3 on GaAs(001) was expected and realized. Moreover, the electrical resistance of MoO3 films can be tuned via its oxygen concentration over a wide range ? from insulating to (half-) metallic. At the studied growth conditions polycrystalline MoO3 films were obtained characterized by nanoscale single crystal domains. Surprisingly, upon deposition of Fe onto an ultrathin (?3 nm) MoO3 layer, Fe diffuses into and underneath the MoO3 layer. For application as diffusion barriers single crystalline layers of the more dense and compact ?-MoO3 need to be developed.