Heteroepitaxial thin films are widely used in a broad range of modern electronic devices, for example as the high-dielectric-constant (high-k ) gates in field effect transistors. In order to improve the crystal quality of epitaxial layers, researchers develop intricate and costly deposition processes. An alternative paradigm should involve deposition of the complex compounds that could exhibit unusual behavior and yield the high quality films by simple and industry-friendly processes. Successful search for the new complex compounds must employ combinatorial methods. In this publication, we demonstrate fabrication of composition spread materials libraries by Combi-PLD and their high-throughput characterization by two different XRD techniques. Generally, the composition spreads have polycrystalline, textured, and epitaxial areas of varied quality. We use statistical analysis to establish general guidelines for the epitaxial growth. Using the NiO-Co3O4-Mn2O3 system on SrTiO3 as an example, we demonstrate that for the chemically inert interfaces lattice mismatch is the most important factor of the commensurate growth. For example, the rocksalt-perovskite interface requires the optimal lattice mismatch of ∼6.5%. Examination of the Y2O3-HfO2-Al2O3 system on Si(100) revealed a narrow epitaxial area comprised of a new metastable crystal phase. Thus, stable epitaxial interface is attainable even for the chemically reactive boundaries. In addition to lattice mismatch, the high-quality heteroepitaxial film requires balancing the reaction with the outdiffusion of the reaction products away from the interface.
Electric field induced resistance switching and memory effect has been investigated for perovskite-oxide heterojunctions consisting of various metal electrodes and p-type or n-type semiconducting perovskite oxides such as Pr0.7Ca0.3MnO3 and Nb-doped SrTiO3(Nb : STO). The metal/perovskite-oxide heterojunction devices show either ohmic or rectifying current-voltage (I-V ) characteristics similar to those of a Schottky junction, depending on the work function of the metals. In addition, the rectifying I-V characteristics have large hysteresis that can host the resistance switching effect. The dependence of the I-V characteristics on the Nb-doping level in the heteroepitaxial SrRuO3/Nb : STO junctions suggests that the threshold of resistance switching is determined by the potential profile of the Schottky barrier under bias voltage. From the experimental results, we propose the possible origin of the resistance switching effect, that is a charging effect at the Schottky-like metal/perovskite-oxide interfaces.
Magnetic tunnel junctions (MTJs) consisting of two ferromagnetic electrode layers separated by a crystalline MgO(001) tunneling barrier exhibit a giant tunneling magnetoresistance (TMR) effect at room temperature. The effect is a key for the development of next-generation spintronic devices such as magnetoresistive random access memory (MRAM) and ultrahigh-density hard disk drives. First-principle theories predicted that no oxidation of magnetic atoms at the barrier/electrode interfaces is essential for the giant TMR effect. Wehave fabricated fully epitaxial MTJs such as Fe(001)/MgO(001)/Fe(001) using MBE technique and achieved huge magnetoresistance ratios above 400% at room temperature, the highest value reported so far. We investigated the interface structure using x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD), and confirmed that the interface Fe atoms are not oxidized and have an enhanced magnetic moment although they are neighboring the O atoms of MgO.
For the development of advanced LSIs, ultra-thin high-k gate dielectrics with a direct interface between a high-k film and a Si substrate will be necessary to achieve equivalent oxide thickness (EOT) of 0.5 nm or less. LaAlO3 that has both a high dielectric constant of 25 and good interfacial thermal stability with Si, has been considered to have a high potential for realizing such a small EOT value. In this study, a LaAlO3 gate dielectric with both ultra-thin EOT and low leakage current was successfully fabricated by optimizing the deposition process of the dielectric. We found that the deposition of LaAlO3 at high temperature is quite important for the excellent characteristics of LaAlO3 gate dielectric.
Recent two topics on Schottky barrier at metal/Hf-based dielectric interfaces and atom intermixing at metal/semiconductor interfaces are reviewed. We have constructed a universal theory of Schottky barriers based on a concept of generalized charge neutrality levels. This theory systematically explains barriers of various gate materials, in particular the unusual behavior of p-metals, and naturally reproduces band offsets at various semiconductor/semiconductor interfaces. On the other hand, we have clarified the mechanisms of atom intermixing at various metal/semiconductor interfaces by analyzing the thermal diffusion. It is shown that atom rebonding, screening of bonds, and strain relaxation are important factors to realize the intermixing.
Since the discovery of C60 nanowhiskers in 2001, fullerene nanowhiskers with various morphologies and compositions have been synthesized, and mechanical, thermal, electrical and magnetic properties of C60 nanowhiskers have been investigated. This paper reviews the recent research progress in the synthesis of fullerene nanowhiskers and characterization of their properties.
It is important to understand the interfacial properties of the nanotube/metal contacts because, in the case of carbon nanotube field-effect transistors (NT-FETs), the FET action is dominated by the Schottky barrier formed at the interface of the source contact. In this study, we have studied the interfacial properties from the electrical characterization of NT-FETs. First, the conduction pass in the vicinity of the contact electrodes has been investigated using a multi-terminal device. Then, the relation between conduction property of NT-FETs and work function of the contact metals has been investigated. The interfacial band structure has been discussed in terms of the temperature dependence of the drain current.