Y. L. Li

Bio: Y. L. Li is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Ferroelectricity & Dielectric. The author has an hindex of 15, co-authored 15 publications receiving 2929 citations. Previous affiliations of Y. L. Li include Los Alamos National Laboratory.

Room-temperature ferroelectricity in strained SrTiO3.

Abstract: Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

A Phenomenological Thermodynamic Potential for BaTiO3 Single Crystals

Abstract: A phenomenological thermodynamic potential was constructed based on the properties of bulk BaTiO3 single crystals. An eighth-order polynomial of Landau-Devonshire expansion was employed. It reproduces bulk properties including the three possible ferroelectric transition temperatures and their dependence on electric fields, as well as the dielectric and piezoelectric constants. Different from the existing thermodynamic potential, it is applicable to predicting the ferroelectric phase transitions and properties of BaTiO3 thin films under large compressive biaxial strains.

Temperature-strain phase diagram for BaTiO3 thin films

Abstract: The shifts of ferroelectric phase transition temperatures and domain stabilities of BaTiO3 thin films as a function of strain and temperature were studied using phase-field simulations. A new Landau-Devonshire thermodynamic potential based on an eighth-order polynomial was employed for describing the bulk free energy of BaTiO3 single crystals, which allows the exploration of domain stability in the full range of experimentally accessible compressive and tensile strains. Based on the simulation results, a phase diagram was constructed, which displays the stability of various ferroelectric phases and domain structures as a function of temperature and strain.

c-axis oriented epitaxial BaTiO3 films on (001) Si

Abstract: c-axis oriented epitaxial films of the ferroelectric BaTiO3 have been grown on (001) Si by reactive molecular-beam epitaxy The orientation relationship between the film and substrate is (001) BaTiO3‖(001) Si and [100] BaTiO3‖[110] Si The uniqueness of this integration is that the entire epitaxial BaTiO3 film on (001) Si is c-axis oriented, unlike any reported so far in the literature The thermal expansion incompatibility between BaTiO3 and silicon is overcome by introducing a relaxed buffer layer of BaxSr1−xTiO3 between the BaTiO3 film and silicon substrate The rocking curve widths of the BaTiO3 films are as narrow as 04° X-ray diffraction and second harmonic generation experiments reveal the out-of-plane c-axis orientation of the epitaxial BaTiO3 film Piezoresponse atomic force microscopy is used to write ferroelectric domains with a spatial resolution of ∼100nm, corroborating the orientation of the ferroelectric film

Effect of substrate-induced strains on the spontaneous polarization of epitaxial BiFeO3 thin films

Abstract: A single-domain thermodynamic theory is employed to predict the spontaneous polarizations of (001)c, (101)c, and (111)c oriented epitaxial BiFeO3 thin films grown on dissimilar substrates. The effects of various substrate-induced strains on the spontaneous polarization were studied. The dependences of the spontaneous polarization on film orientations and the types of substrate-induced strains were analyzed.

Physics and Applications of Bismuth Ferrite

Abstract: BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Multiferroics: progress and prospects in thin films.

Abstract: Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Physics of thin-film ferroelectric oxides

Abstract: This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices

Abstract: It is well-known that conventional field effect transistors (FETs) require a change in the channel potential of at least 60 mV at 300 K to effect a change in the current by a factor of 10, and this minimum subthreshold slope S puts a fundamental lower limit on the operating voltage and hence the power dissipation in standard FET-based switches. Here, we suggest that by replacing the standard insulator with a ferroelectric insulator of the right thickness it should be possible to implement a step-up voltage transformer that will amplify the gate voltage thus leading to values of S lower than 60 mV/decade and enabling low voltage/low power operation. The voltage transformer action can be understood intuitively as the result of an effective negative capacitance provided by the ferroelectric capacitor that arises from an internal positive feedback that in principle could be obtained from other microscopic mechanisms as well. Unlike other proposals to reduce S, this involves no change in the basic physics of the FET and thus does not affect its current drive or impose other restrictions.

Enhancement of ferroelectricity in strained BaTiO3 thin films.

Abstract: Biaxial compressive strain has been used to markedly enhance the ferroelectric properties of BaTiO 3 thin films. This strain, imposed by coherent epitaxy, can result in a ferroelectric transition temperature nearly 500°C higher and a remanent polarization at least 250% higher than bulk BaTiO 3 single crystals. This work demonstrates a route to a lead-free ferroelectric for nonvolatile memories and electro-optic devices.