Research interests

My current research interests lie in the field of multifunctional oxide and semiconductor materials and nanostructures for electronic and optoelectronic applications. Metal oxides are a vast class of materials that have a wide variety of properties. They can be dielectrics, semiconductors, superconductors; possess piezoelectricity, ferroelectricity, ferromagnetism, nonlinear-optical and electro-optical properties, colossal magnetoresistance, etc. This opens huge opportunities for novel device applications of these materials. Artificially engineered nanostructures, e.g. strained ultrathin films and superlattices made of oxide materials, open new ways to manipulate their properties. I apply optical spectroscopic techniques (Raman, photoluminescence) to study the fundamental physical properties of these materials.

Ferroelectric thin films have been the primary subject of my research for the last few years. Ferroelectric materials exhibit a wide variety of interesting properties, such as high dielectric permittivity, dielectric nonlinearity, polarizability, piezoelectricity, pyroelectricity, electro-optic activity. This makes them extremely attractive for applications in various electronic and optoelectronic devices (DRAM capacitors, gate oxides for MOSFET, tunable microwave devices, non-volatile ferroelectric memories, actuators, ultrasonic transducers and sensors, infrared sensors, thermal infrared switches etc.). For device applications high-quality thin films are required, and the differences between thin film properties and those of the bulk materials has been a major issue. My research focuses on the investigation of the fundamental lattice-dynamical and optical properties of ferroelectric thin films and the influence of strain, interface and structural defects on them. In particular, I have been studying the lattice dynamics and phase transitions in barium strontium titanate (BaSrTiO₃, BST) thin films and single crystals. Knowledge of lattice-dynamics, in particular, the soft phonon mode behavior, is essential for understanding the fundamental properties ferroelectrics.

To study the vibrational properties of thin films I apply Raman spectroscopy, which is one of the most powerful and versatile tools for lattice dynamical studies. It can be applied for studies of most elementary excitations in materials and provides important information about the structure of thin films, symmetry and crystal ordering, composition, strain, defects, size and interface effects, phase transition behavior. Raman spectroscopy, as well as other optical spectroscopies (photoluminescence, infrared spectroscopy), is a contactless and non-destructive technique. It allows studying samples in a variety of forms (single crystal, ceramic, thin film, particles, solutions etc.), and the sample area can be less than 1 µm². Raman and photoluminescence spectroscopy are easy to apply for studies of temperature and pressure effects, electric and magnetic field effects, even possible to use in situ characterization of material processing.

Recently, new artificial nanostructures, such as ferroelectric and dielectric superlattices have attracted a broad interest, stimulated by their principally new properties compared to bulk materials, and technological promise. Ferroelectric nanostructures are now at the forefront of the fundamental and applied research in the field of ferroelectricity. Experimental investigation of the lattice dynamics in nanoscale ferroelectrics is a very difficult task, primarily due to the transparency and small thickness of the samples. Conventional Raman as well as other optical measurements of ultrathin ferroelectric films and nanostructures in the visible and infrared range are often difficult or practically impossible because of the film transparence and small thickness, which leads to extremely low Raman signals from nanometer-thick films and the dominance of a substrate signal in the spectra. Our recent pioneering work on ultraviolet (UV) Raman scattering in nanoscale ferroelectric BaTiO₃/SrTiO₃ superlattices demonstrated that UV Raman spectroscopy is an effective technique to study the phase transitions in ferroelectric nanostructures. We showed the existence of ferroelectricity and determined the phase transition temperature T_c in one-unit-cell thick BaTiO₃ layers embedded in much thicker non-ferroelectric SrTiO₃. We demonstrated the tuning of T_c by ~ 500 K by varying superlattice layer thicknesses, which is an example of dramatic modification of material properties in artificially engineered nanostructures:

Another interesting and rare type of materials is multiferroics, which display both ferroelectric and magnetic ordering. A material with a strong enough coupling between the ferroelectric and magnetic ordering would have a potential for new applications (e.g. memory device that can be written electrically and read magnetically).