"Oxide nanointerface engineering and study for electronic quantum device"

Processes near interfaces of conventional material like semiconductors and normal metal are at the basis of present electronics. The atomic scale control of interface structure and properties led to the realization of several milestone devices starting form p-n junctions to FET and more recently to quantum devices. These results triggered intensive researches in the field of material preparation and characterization as well as of lithographic techniques allowing the high-level integration of such devices of the present days. Silicon technology is approaching now its physical limit for scale reduction, because of two main aspects: (a) the limited dielectric properties of the amorphous SiO₂ barrier that also prevents the possibility of vertically stacking multiple devices (b) “large” characteristic lengths (diffusion length, screening length) that limit the size to tens of nanometers. Due to shrunk size and/or thickness of elements of the modern microelectronics their parameters are impacted by electronic configurations at the metal/semiconductor, semiconductor/oxide interfaces. Use of new materials with desirable functionalities in combination with silicon gives real challenge of the advanced nanoelectronics.

Large scientific and technological interest aroused recently on oxide materials, and especially on transition metal oxides, for their rich spectrum of physical properties, which encompasses ferromagnetism, ferroelectricity, superconductivity, semiconducting and metallic behavior. Almost all this phenomenology results from strongly correlated electronic behavior and turned out to be very sensitive to external parameters such as electric and magnetic fields, internal or external pressure and so on [1,2].

Nowadays, oxides are under investigation in view of device applications both by integrating them in the silicon technology (for instance growing crystalline high-K oxides on Si substrates) or in the ambitious field of oxide electronics, which aims to develop new electronic devices based on oxides. Advantages of this new electronics lie in: a) the exploitation of new functionalities exhibited by oxides, which are completely absent in conventional semiconductors, b) their isostructure, which allows the vertical integration of multiple devices through epitaxial heterostructures, c) striking possibility of size reduction due to the nanometric characteristic lengths.

A completely new class of nanodevices can be envisaged and engineered exploiting the functional properties of oxides and, as in the case of conventional semiconductors; a real breakthrough in this field could be obtained by controlling and tailoring the physical properties at the interfaces between different oxide materials. Indeed, interfaces and surfaces in such highly correlated systems are much more complex and offer far more application possibilities than interfaces involving only conventional metals and semiconductors [2]. Interfaces alter the bulk electronic system sometimes with dramatic consequences; interfaces break the translational and the rotational symmetry, induce stresses, consequently altering the distances and bonds between the ions, giving rise to shift and distortion of the electronic states and energy levels and modifying the bands. Outstanding oxide interface properties can be exploited for applications in devices for electronics and optoelectronics. Specific oxide devices relying on interface properties have already been realized and employed, such as Josephson junctions and SQUIDs, magnetic tunnel junctions based on manganites and FeRAMS.

The strategic importance of this emerging field of research is evidenced by the large amount of efforts and investments that Japan and USA put in the development of oxide-based devices and related technologies. The European Science Foundation contributes for increasing the strategic weight of oxide electronics and more close collaboration among European groups in this field, promoting the THIOX programme (Thin Film for novel Oxide Devices). This network, lead by several of the project partners, is the first step to achieve a real progress, but it seems mandatory that teams working in this field strengthen their collaborations integrating their efforts in a common project. Within this framework, seven groups among the leaders in Europe in oxide properties investigation and device realization and a SME involved in thin film technology conceived the project NANOXIDE, to structure the research efforts in this area.

The project integrates knowledge of two scientific groups doing research in the fields of electronics and material-science. Both groups posses a deep knowledge of the features of nucleation and heteroepitaxial growth of thin films [3-5]. However, specific peculiarities of the nucleation of the multicomponent perovskite-type oxides are poor documented in scientific literature. Interplay of mechanical stresses and anisotropy of the surface free energy, as well as possessing conditions determine to a large extent microstructure and stoichiometry of the nucleuses, which in turn affect considerably electronic configuration at interfaces in the multilayer heterostructures. Detailed knowledge of the mechanisms affecting electronic parameters of the interfaces between different types of the perovskite-type oxides is one of the high priority goal of the project. We plan to use the knowledge (determined mechanisms) for tuning parameters of the interfaces in the multilayer heterostructures which will be developed in favor of possible applications [6].

Basically, the project relates to a category of applied research, aiming at: a) proper choice of materials and b) development of scientifically justified designs and technologies, which in aggregate should provide an opportunity for creation of novel unique functional electronic devices on the basis of interfaces in structured at atomic level oxide materials. At the same time, project will contribute to fundamental knowledge of physics and processes, including electronic, structural and microwave characteristics of interfaces in epitaxially grown oxide multiplayer devices. The main objectives are as follows:

The important advantage of novel devices on the basis of interfaces in oxide materials (in comparison with known ones, made from semiconductors and metals) is the opportunity to create mobile and even portable novel functional devices for detecting weak signals of submm electromagnetic radiation, studying and control of atmosphere pollution, measurements of weak magnetic fields for needs of magnetocardiography and magnetoencephalography, for nondestructive evaluation of integrity of metal constructions, amplification with enhanced quality of filtration of microwave signals in modern stations of cellular communication. The list is not limited. An interesting application of novel heterostructures is the development of phase qubit – the base logic cell of quantum computer.

Oxide electronic devices have also a great potential in humane aspects of life improvement. For instance, magnetocardiography provides much higher sensitivity compared to ordinary electrocardiography and allows revealing myocardium deceases at very early stage. Increase of sensitivity and decrease of intermodulation are important problems of modern mobile telecommunications. In this connection, super low noise microwave amplifiers made from superconducting quantum interference filters, manufactured on the same chip with passive band-pass filters (also made from cuprate superconducting films) give opportunity to solve the both problems.