Volume 100, Issue 5, 01 September 2006
- special topic: science of ferroelectric thin films and application to devices
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- SPECIAL TOPIC: SCIENCE OF FERROELECTRIC THIN FILMS AND APPLICATION TO DEVICES
100(2006); http://dx.doi.org/10.1063/1.2336995View Description Hide Description
100(2006); http://dx.doi.org/10.1063/1.2337360View Description Hide Description
A thermodynamic theory is developed for equilibrium 180° stripe domains in ferroelectric thin films on insulating substrates. Such stripe domains form to minimize the energy of the depolarizing field, and lead to a suppression of in thin films. Expressions including depolarizing field and domain wall energy are developed and applied to coherently strained films on substrates, with an upper boundary condition of either a dielectric, a conductor, or vacuum. An elastic solution appropriate for epitaxially strained stripe domains and 180° domain walls is presented. We minimize the full nonlinear free energy using a numerical technique to obtain equilibrium polarization and field distributions, and determine the equilibrium stripe period as a function of temperature and film thickness for each upper boundary condition. While the stripe periods found agree reasonably well with the existing analytical solution using a linearized free energy, the suppression of as film thickness decreases is as much as a factor of 10 smaller than that given by the linear solution.
Controlled crystal growth of layered-perovskite thin films as an approach to study their basic properties100(2006); http://dx.doi.org/10.1063/1.2337357View Description Hide Description
This article describes the current progress in thin bismuth layer-structured ferroelectricfilms (BLSFs) including and , particularly those developed in the last ten years. BLSF thin films can be applied to ferroelectric random access memories because of their durable fatigue-free properties and lead-free composition. We will briefly introduce epitaxialthin filmsgrown on a variety of substrates. Because of the difficulty in growingsingle crystals of sufficient size to characterize the ferroelectric behavior in specific crystal growth directions, we will characterize epitaxially grownthin films to obtain basic information about the anisotropic switching behavior, which is important for evaluating the performance of emerging materials. We will then discuss the fiber-textured growth on the (111)Pt-covered Si substrates of and thin films. Because we expect that the spread crystal orientation will affect the bit-to-bit errors, we believe that the fiber-textured growth and the characterization technique for the deposited film orientation are interesting from a practical standpoint. Another specific challenge of thin film growth is the growth of -axis-(polar axis)-oriented films.-axis-oriented films are characterized both crystallographically and by electric hysteresis loop. The hysteresis performance was in accordance with the volume fraction of the domains; however, no evidence for 90° switching of the domain by an external electric field was obtained. The control of film orientation also allows systematic studies on the effects of a structural modification and relation between spontaneous polarization and Curie temperature, examples of which are given in this paper. After a short description of the piezoelectric properties, we will conclude with a summary and the future prospects of BLSF thin films for research and applications.
100(2006); http://dx.doi.org/10.1063/1.2337359View Description Hide Description
Ferroelectric random access memory (FeRAM) is an attractive candidate technology for embedded nonvolatile memory, especially in applications where low power and high program speed are important. Market introduction of high-density FeRAM is, however, lagging behind standard complementary metal-oxide semiconductor (CMOS) because of the difficult integration technology. This paper discusses the major integration issues for high-density FeRAM, based on (strontium bismuth tantalate or SBT), in relation to the fabrication of our stacked cell structure. We have worked in the previous years on the development of SBT-FeRAM integration technology, based on a so-called pseudo-three-dimensional (3D) cell, with a capacitor that can be scaled from quasi two-dimensional towards a true three-dimensional capacitor where the sidewalls will importantly contribute to the signal. In the first phase of our integration development, we integrated our FeRAM cell in a CMOS technology. In a second phase, then, possibility of scaling of our cell is demonstrated in technology. The excellent electrical and reliability properties of the small integrated ferroelectriccapacitors prove the feasibility of the technology, while the verification of the potential 3D effect confirms the basic scaling potential of our concept beyond that of the single-mask capacitor. The paper outlines the different material and technological challenges, and working solutions are demonstrated. While some issues are specific to our own cell, many are applicable to different stacked FeRAM cell concepts, or will become more general concerns when more developments are moving into 3D structures.
Integration of lead zirconium titanate thin films for high density ferroelectric random access memory100(2006); http://dx.doi.org/10.1063/1.2337361View Description Hide Description
Interests are being focused on types of nonvolatile memories such as ferroelectric random access memory (FRAM), phase change random access memory, or magnetoresistance random access memory due to their distinct memory properties such as excellent write performance which conventional nonvolatile memories do not possess. Among these types of nonvolatile memories, FRAM whose cell structure and operation are almost identical to dynamic random access memory (DRAM) can ideally realize cell size and speed of DRAM. Thus FRAM is the most appropriate candidate for future universal memory where all memory functions are performed with a single chip solution. Due to the poor ferroelectric properties of downscaled ultrathin lead zirconium titanate (PZT)capacitors as well as technical issues such as hydrogen and plasma related degradation arising from embedding ferroelectric metal-insulator-metal capacitors into conventional complementary metal oxide semiconductor processes, current FRAM still falls far below its ideally attainable cell size and performance. In this paper, based upon PZTcapacitor, current mass-productive one pass transistor and one storage capacitor (1T1C), capacitor over bit line (COB) cell technologies are introduced upon which cell size of at minimum feature size technology node has been realized. And then, most recent 1T1C, COB cell technologies are discussed from which cell size of at minimum feature size technology node has been realized, and finally future three dimensional capacitor technologies for the FRAM with cell size of less than beyond minimum feature size technology node are suggested.
100(2006); http://dx.doi.org/10.1063/1.2337362View Description Hide Description
An overview is given on nucleation phenomena of (PZT)thin films on Pt(111)-based substrates. Emphasis is given on in situgrowth methods, particularly in situ reactive sputtering from three metallic targets. Growth of PZTthin films is discussed from the point of view of the phase diagram, PbO vapor pressure, and classical nucleation theory. The role of thin affinity layers and spots is explained in the frame of this theory. Activation energies for desorption and chemisorption are adapted to comply with the fact that nucleation rates on are much larger than the ones on bare Pt(111). The model reproduces well the PbO surface flux from bare Pt(111) to the affinity spots in the case of nucleation and the reversed tendency in the case of PZTnucleation, explaining experimental observations. The critical size of nuclei was calculated to contain cells for nucleation and 14–17 for PZT/Pt nucleation.
100(2006); http://dx.doi.org/10.1063/1.2336999View Description Hide Description
An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems’ applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials,structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectricfilms which is relevant to microsystems’ applications, and permittivity and loss in ferroelectric films—important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures.
100(2006); http://dx.doi.org/10.1063/1.2337009View Description Hide Description
This article reviews the existing theoretical models describing the interface-induced phenomena which affect the switching characteristics and dielectric properties of ferroelectric thin films. Three groups of interface-induced effects are addressed—namely, “passive-layer-type” effects, ferroelectric-electrode contact potential effects, and the poling effect of the ferroelectric-electrode interface. The existing experimental data on dielectric and switching characteristics of ferroelectric thin filmcapacitors are discussed in the context of the reviewed theories. Special attention is paid to the case of internal bias field effects.
100(2006); http://dx.doi.org/10.1063/1.2337356View Description Hide Description
Atomic force microscopy was used to investigate ferroelectric switching and nanoscale domain dynamics in epitaxialthin films. Measurements of the writing time dependence of domain size reveal a two-step process in which nucleation is followed by radial domain growth. During this growth, the domain wall velocity exhibits a dependence on the electric field, characteristic of a creep process. The domain wall motion was analyzed both in the context of stochastic nucleation in a periodic potential as well as the canonical creep motion of an elastic manifold in a disorder potential. The dimensionality of the films suggests that disorder is at the origin of the observed domain wallcreep. To investigate the effects of changing the disorder in the films,defects were introduced during crystal growth (-axis inclusions) or by heavy ion irradiation, producing films with planar or columnar defects, respectively. The presence of these defects was found to significantly decrease the creep exponent , from 0.62–0.69 to 0.38–0.5 in the irradiated films and 0.19–0.31 in the films containing -axis inclusions.
Scaling of structure and electrical properties in ultrathin epitaxial ferroelectric heterostructures100(2006); http://dx.doi.org/10.1063/1.2337363View Description Hide Description
Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality is maintained down to thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization using a pulsed probe setup and the out-of-plane piezoelectric response revealed a systematic drop from and for a thick film to and for a thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180° polydomain state for the thinnest films. Existence of a polydomain state is demonstrated by piezoresponse force microscopy images of the ultrathin films. These results suggest that the limit for a ferroelectric memory device may be much larger than the fundamental limit for ferroelectricity.
100(2006); http://dx.doi.org/10.1063/1.2336996View Description Hide Description
This article gives an overview of recent developments in the search for the next-generation dielectric for the complementary metal-oxide semiconductor gate stack. After introducing the main quantities of interest, the paper concentrates on a figure of merit that connects two main properties of the gate stack, namely, the leakage current and the capacitance. This is done for single layers as well as for bilayers consisting of interfacial and a high-dielectric. In the case of the bilayers, the impact of the interfacial layer is enormous, reducing the leakage current by an order of magnitude per monolayer. This extreme dependance makes a good correlation between the leakage and the structural parameters nearly impossible. This is illustrated using numerical examples designed to help the reader evaluate the orders of magnitude involved. The origin of the interfacial layer is traced back by means of thermodynamic considerations. As the estimates put forward in the literature do not correspond to the results observed, a detailed review is made, and additional mechanisms are suggested. By using reasonable values for the Gibbs free energy of an interfacial solid silicon oxide phase it is demonstrated how the reaction equilibria shift. Such an interface phase may fundamentally change the stability criteria of oxides on Si. Furthermore, it can also provide a major source of electronic defects that will affect the device performance. Finally, a second figure of merit is introduced that connects the capacitance with a strongly reduced carrier mobility, which might also be related to the same electronic defects.
100(2006); http://dx.doi.org/10.1063/1.2337078View Description Hide Description
Hysteretic resistance effects based on a correlation between ferroelectricpolarization and conductivity might become of particular interest for nonvolatile memory applications, because they are not subject to the scaling restrictions of charge based memories such as the ferroelectric random access memory. Two basic concepts, a metal-ferroelectric-metal structure and a metal-ferroelectric-semiconductor structure are discussed in the literature. This contribution discusses the principle of operation of those concepts in terms of the band model. A generalized model is proposed, which is based on a conductive metal-ferroelectric-semiconductor-metal structure. Here, the existence of a low and a high conductive state originates from a switch of the polarization in the ferroelectric layer and a resulting positive or negative polarizationcharge at the ferroelectric-semiconductor interface. Charge carriers in the film are attracted by or depleted at the interface giving rise to different local conductivities. By simulation, the effect of internal screening caused by mobile charge carriers on the hysteretic current-voltage behavior and the depolarizing field in the ferroelectric are estimated. The simulation discloses a switching ratio up to several orders of magnitude and a conductivity window, which scales with the donor concentration. It may also explain resistive switching in systems consisting only of one ferroelectric layer by assuming the presence of nonferroelectric interface layers.
Homogeneity of thin films by chemical solution deposition: Extended x-ray absorption fine structure spectroscopy study of zirconium local environment100(2006); http://dx.doi.org/10.1063/1.2337079View Description Hide Description
Sols for (PZT)thin films were prepared by 2-methoxyethanol route from lead acetate, titanium -propoxide, and zirconium-propoxide, the latter either unmodified or modified with acetylacetone or acetic acid in a molar ratio and deposited on sapphire (0001). By Zr -edge extended x-rayabsorption fine structure(EXAFS)spectroscopy, the structural changes in the Zr local environment, induced by the addition of the two modifiers, were followed from the synthesis of the PZT sol to the transition to the amorphous film. In the unmodified PZT sol segregation of Zr species occurs from the original dimers present in the Zr propoxide solution in 2-methoxyethanol. The immediate neighborhood of Zr atoms changes markedly at the transition from the sol to the amorphous film: the local structure around Zr atoms is similar to the one found in tetragonal zirconia particles. The modification of Zr propoxide with acetylacetone in 2-methoxyethanol results in Zr monomers. In PZT sol, clustering of Zr species is observed continuing into the amorphous film. By modification with acetic acid the original dimeric structure of the Zr precursor is retained in the PZT sol and further in the amorphous film. Selective modification of Zr propoxide with acetic acid therefore results in a more homogeneous distribution of Zr atoms in the PZT sol and amorphous film than in both as-received and acetylacetone-modified Zr propoxide.
100(2006); http://dx.doi.org/10.1063/1.2337364View Description Hide Description
Dielectric behavior on artificial lattices has been investigated along with quantum mechanical simulation (first principles calculation). From the oxide artificial lattice approach, strain manipulation was performed to obtain a wide range of lattice deformation in the consisting and layers, which leads to two important consequences. First, we obtained enhanced dielectric constant and extremely large nonlinearity in the artificial lattices with very short stacking periods. Second, it is found that there exists a maximum dielectric constant in each lattice and lattice at a certain degree of lattice deformation. The first principles study successfully explains the dielectric behavior of strained and lattices, the existence of the maximum dielectric constant. The first principles study on artificial lattices with very short stacking periods also reveals that the artificial lattice undergoes phase transition between the tetragonal and monoclinic phases with a misfit lattice strain and exhibits an anomalous dielectric behavior at the phase boundary. Optical phonon behavior of the artificial lattice resembles that of strained lattice and optical phonon softening primarily derives the anomaly of the dielectric tensor at the phase boundary. The lattice deformation is a primary influencing factor to phonon and dielectric behaviors rather than interface layer effect in artificial lattice with very short stacking periods.
Science and technology of thin films and interfacial layers in ferroelectric and high-dielectric constant heterostructures and application to devices100(2006); http://dx.doi.org/10.1063/1.2337005View Description Hide Description
The fabrication of the next generation of complex oxide thin film-based micro and nanoscale devices, such as, for example, low and high density nonvolatile ferroelectric random access memories (FeRAMS), high-dielectric constant (K) high-frequency devices, and the next generation of complimentary metal oxide semiconductor (CMOS) nanoscale devices based on high-K dielectrics, require understanding and control of film growth and interface processes as well as development of materials integration strategies with atomic scale control. In recent years, we developed and applied a unique combination of integrated film synthesis / in situ characterization and ex situ analytical techniques capable of providing information about thin filmsurface and interface processes at the atomic scale as required for the development of the devices mentioned above. These techniques are also useful for establishing composition-microstructure-property relationships critical for the integration of oxide thin films with semiconductor device platforms for the development of a whole new generation of micro and nanodevices based on film technologies beyond semiconductors and specifically silicon. Our recent work has been focused on developing diffusion barrier layers and heterostructured bottom electrodes that play a critical role in high-density FeRAM integration. We demonstrated that layers can be used as a material with a double diffusion barrier/bottom electrode functionality for integration of ferroelectriccapacitors CMOS devices for fabrication of FeRAMs. We also demonstrated that control of interfaces is critical to the integration of high-K dielectric films with appropriate substrates for the fabrication of high-performance high-frequency devices, and here again a diffusion barrier such as the layer developed by our group is critical for such integration. These studies revealed that when properly oxidized, nanoscale thick amorphous layers exhibit properties that make them strong candidates for application as gate dielectric in the next generation of nanoscale CMOS devices. We discuss here results from systematic studies designed to understand film growth and interface processes and their effect on materials integrations and composition-microstructure-property relationships and oxidation processes using sputter-deposition in conjunction with complementary in situ atomic layer-resolution mass spectroscopy of recoil ion (MSRI) and surface sensitive x-ray photoelectron spectroscopy(XPS) and ex situ transmission electron microscopy and electrical characterization. The unique combination of films synthesis and in situ/ex situ analytical techniques provides a powerful platform for the fundamental and applied materials science needed for the development of the next generation of multifunctional micro and nanoscale devices. A common theme in this article is the science and technology a layer that exhibit multifunctional characteristics as diffusion barrier and bottom electrode for integration of ferroelectric and high-dielectric constant (K) thing films with appropriate platform substrates for FeRAMs and high-frequency devices, and as a promising high-K dielectric layer for the next generation of nanoscale CMOS gates, flash memories, and other micro and nanodevices that require high-K layers in the device architecture.
100(2006); http://dx.doi.org/10.1063/1.2337358View Description Hide Description
Lateral size effects of ferroelastic domain structures in epitaxialthin films were investigated systematically with a viewpoint of misfit strain relaxation mechanism. The epitaxialthin films were patterned into discrete islands and the effects of lateral dimension were analyzed by reciprocal space mapping using synchrotron x-ray diffraction as well as finite element simulation. As the lateral two-dimensional planar size decreases in the patterns on MgO(001), some of the domains turned into domains due to the relaxed tensile strain. In the patterns on , on the other hand, the formation of 90° domains is enhanced by the reduction in compressive misfit strain. As the pattern size decreases further to , the untilted domains arise due to the almost completely relaxed misfit strains. Equilibrium domain structures in the epitaxialthin films and discrete islands are also analyzed by the finite element simulation and found to be consistent with the experimental observation. These results manifest that the domain structure and evolution of the epitaxialthin films could be engineered by the control of misfit strain and its relaxation.
- LASERS, OPTICS, AND OPTOELECTRONICS
100(2006); http://dx.doi.org/10.1063/1.2337101View Description Hide Description
Influence of lithium tetraborate (, TBL) glass matrix on the luminescent properties of the ions emission was investigated. It was demonstrated that the decrease of matrix long-range ordering leads to substantial widening of corresponding peaks in the emission spectra in comparison with crystalline matrices. During the decrease of temperature from 292 down to a distinct low-energy spectral shift of the principal red luminescent band from is observed, which is a consequence of a coexistence of several structural borate fragments. Simulations of incorporation the ions into the TBL glasslike matrix were carried out using the Langevin molecular dynamics simulations and quantum chemical simulations. Possibility of partial substitution of boronions by ions is demonstrated. The contribution of the electron-phonon subsystems to the spectral broadening of the corresponding emission red lines was evaluated. It was shown that the main contribution to the emission bands gives harmonic electron-phonon interactions contrary to the generally adopted model assuming prevailing role of anharmonic electron-phonon interactions.
100(2006); http://dx.doi.org/10.1063/1.2335772View Description Hide Description
The gain coefficient of several quantum-cascade lasers (QCLs) with a nominal Al content of has been calculated as a function of transition energy and electric field strength. We solve the Schrödinger and Poisson equations self-consistently in the framework of a linear scattering-rate model with periodic boundary conditions. The actual layer thicknesses as well as the Al content of the barriers have been obtained from x-ray diffraction. The calculated gain characteristics exhibit a large range of transition energies and a corresponding range of possible operating field strengths. These results may provide an explanation for the observation of a rather large range of the lasing energies for QCLs with nominally identical cascade structures. The comparison with the experimental values of the lasing energy reveals that most of the lasers operate below the designed field strength. The analysis of the operating field strengths and the threshold current densities indicates larger losses for higher field strengths probably due to leakage currents.
Spatially resolved distribution of dislocations and crystallographic tilts in GaN layers grown on Si(111) substrates by maskless cantilever epitaxy100(2006); http://dx.doi.org/10.1063/1.2234807View Description Hide Description
The spatial distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) substrates by maskless cantilever epitaxy was studied by polychromatic x-ray microdiffraction, high resolution monochromatic x-ray diffraction, and scanning electron microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A higher lateral to vertical growth ratio suppressed sidewall deposition and was found to produce larger lattice tilts in the GaNfilms. Two kinds of crystallographic tilts are observed in the films. The measurements revealed that the free-hanging wings are tilted upward at room temperature in the direction perpendicular to the stripes. Finite element simulations of the thermally induced part of the wing tilt are presented. Moreover, a misorientation between the GaN(0001) and the Si(111) planes is observed in the parallel to the stripe direction. Its origin is discussed with respect to the strain of the epitaxialGaN on a miscut Si(111) surface and misfit dislocations formed at the interface.
100(2006); http://dx.doi.org/10.1063/1.2220646View Description Hide Description
A study was made of the cooling of the laser induced vapor plume in background air. The temperature and size variations of the vapor plume were determined from spectroscopic measurements during the first few tens of microseconds after the laser pulse. Experiments were carried out over a range of laser spot sizes and energies. The energy transport by thermal radiation from the vapor plume to the background air and to the test sample was formulated. Spectral line by line calculations were made by (a) calculating the detailed line emission profiles (valid for all optical depths), as well as by (b) dividing the lines into being either optically thin or optically thick. The calculations agreed with one another and with the experimental results for the decreasing vapor plume temperature. It was also shown that for optically thin conditions, which are often valid for small vapor plumes, the variation of the surface reflectivity of the test sample had very little effect on the cooling process. For optically thin conditions, the temperature decrease of the vapor plume was independent of the plume size, shape, and position. For larger optical thicknesses of the vapor plume, the calculations showed that the reflectivity of the sample surface and the size of the vapor plume would dramatically affect cooling of the vapor plume.