Perovskite Dielectrics Research Articles

(Invited) ALD and AVD Grown Perovskite-type Dielectrics for Metal-Insulator-Metal Applications

Atomic Vapor Deposition (AVD) and Atomic Layer Deposition (ALD) techniques were successfully applied for the depositions of perovskite type dielectrics, namely, Sr-Ta-O, Ti-Ta-O, Sr-Ti-O, Ba-Hf-O, Nb-Ta-O and Ce-Al-O. Thin films were investigated as alternative dielectrics for Metal-Insulator-Insulator (MIM) capacitors. Structural and electrical properties are investigated after depositing the metal oxides on 200 mm TiN/Si (100) substrates within the temperature range of 225-400 ºC. Electrical properties, investigated after sputtering Au top electrodes, revealed that the main characteristics are different for each dielectric. The highest dielectric constants were achieved for crystalline SrTiO3 (k=95), crystalline CeAlO3 (k = 60) and amorphous Ti-Ta-O (k = 50) films. However, Sr-Ta-O based MIM capacitors showed the lowest leakage current densities as well as the smallest capacitancevoltage linearity coefficients.

Integration of perovskite oxide dielectrics into complementary metal–oxide–semiconductor capacitor structures using amorphous TaSiN as oxygen diffusion barrier

The high permittivity perovskite oxides have been intensively investigated for their possible application as dielectric materials for stacked capacitors in dynamic random access memory circuits. For the integration of such oxide materials into the CMOS world, a conductive diffusion barrier is indispensable. An optimized stack p++-Si/Pt/Ta21Si57N21/Ir was developed and used as the bottom electrode for the oxide dielectric. The amorphous TaSiN film as oxygen diffusion barrier showed excellent conductive properties and a good thermal stability up to 700 °C in oxygen ambient. The additional protective iridium layer improved the surface roughness after annealing. A 100-nm-thick (Ba,Sr)TiO3 film was deposited using pulsed laser deposition at 550 °C, showing very promising properties for application; the maximum relative dielectric constant at zero field is κ ≈ 470, and the leakage current density is below 10–6 A/cm2 for fields lower then ± 200 kV/cm, corresponding to an applied voltage of ± 2 V.

Ultrafast dynamics of soft phonon modes in perovskite dielectrics observed by coherent phonon spectroscopy

Coherent phonons were observed in perovskite dielectrics LaAlO${}_{3}$ and KMnF${}_{3}$ using ultrafast polarization spectroscopy. Ultrafast dynamics and softening of phonon modes, which contribute to structural phase transitions, were studied. The temperature dependences of the phonon frequency and relaxation rate were obtained from the observed damped oscillation of coherent phonons. In LaAlO${}_{3}$, typical softening of the phonon frequency was observed toward the phase-transition temperature ${T}_{c}$ ($~750$ K). The observed relaxation for the soft-mode phonons is explained well by a population decay due to anharmonic phonon-phonon coupling. In KMnF${}_{3}$, two structural phase-transition temperatures, ${T}_{c1}$ $(=187$ K) and ${T}_{\mathit{cm}}$ ($~81$ K), were found through birefringence measurement. At temperatures below ${T}_{\mathit{cm}}$, three modes of coherent phonons were observed and the phonon modes were found to undergo softening toward ${T}_{\mathit{cm}}$. The observed dependences of the coherent phonon signal on the pumping polarization and the temperature suggest an orthorhombic structure, instead of a monoclinic structure, at temperatures below ${T}_{\mathit{cm}}$. The sudden decrease in the phonon frequency at ${T}_{\mathit{cm}}$ indicates the first-order character of the phase transition at ${T}_{\mathit{cm}}$. Notable changes in birefringence and phonon frequency were observed at around 90 K, but no distinct anomaly is found, which suggests that the structure undergoes continuous change. Above 100 K, the ${E}_{g}$ mode in the tetragonal symmetry is found to undergo softening toward ${T}_{c1}$. A coherent phonon signal of relaxation type appears at around ${T}_{c1}$ and survives even at near room temperature, which suggests the existence of structural disorder even in the cubic symmetry above ${T}_{c1}$.

Atomistic determination of flexoelectric properties of crystalline dielectrics

Upon application of a uniform strain, internal sublattice shifts within the unit cell of a noncentrosymmetric dielectric crystal result in the appearance of a net dipole moment: a phenomenon well known as piezoelectricity. A macroscopic strain gradient on the other hand can induce polarization in dielectrics of any crystal structure, even those which possess a centrosymmetric lattice. This phenomenon, called flexoelectricity, has both bulk and surface contributions: the strength of the bulk contribution can be characterized by means of a material property tensor called the bulk flexoelectric tensor. Several recent studies suggest that strain-gradient induced polarization may be responsible for a variety of interesting and anomalous electromechanical phenomena in materials including electromechanical coupling effects in nonuniformly strained nanostructures, ``dead layer'' effects in nanocapacitor systems, and ``giant'' piezoelectricity in perovskite nanostructures among others. In this work, adopting a lattice dynamics based microscopic approach we provide estimates of the flexoelectric tensor for certain cubic crystalline ionic salts, perovskite dielectrics, $III\text{\ensuremath{-}}V$ and $II\text{\ensuremath{-}}VI$ semiconductors. We compare our estimates with experimental/theoretical values wherever available and also revisit the validity of an existing empirical scaling relationship for the magnitude of flexoelectric coefficients in terms of material parameters. It is interesting to note that two independent groups report values of flexoelectric properties for perovskite dielectrics that are orders of magnitude apart: Cross and co-workers from Penn State have carried out experimental studies on a variety of materials including barium titanate while Catalan and co-workers from Cambridge used theoretical ab initio techniques as well as experimental techniques to study paraelectric strontium titanate as well as ferroelectric barium titanate and lead titanate. We find that, in the case of perovskite dielectrics, our estimates agree to an order of magnitude with the experimental and theoretical estimates for strontium titanate. For barium titanate however, while our estimates agree to an order of magnitude with existing ab initio calculations, there exists a large discrepancy with experimental estimates. The possible reasons for the observed deviations are discussed.

Hf- and Zr-based alkaline earth perovskite dielectrics for memory applications

We investigate a group of Hf- and Zr-based dielectrics crystallizing in the cubic perovskite structure for memory applications. The dielectrics are deposited in the form of thin layers (<30 nm) onto TiN substrates using molecular beam deposition. Prepared layers are characterized with respect to their dielectric constant, crystallization temperature as well as some aspects of leakage current flowing across group-II hafnates and zirconates. We show that dielectric constants of about 40 can be achieved for BaHfO 3, BaZrO 3, and SrHfO 3 on TiN substrates. On an example of BaHf 1− x Ti x O 3, we demonstrate that a significant gain in the dielectric constant and capacitance density can be achieved by partial substitution of Hf atoms by Ti in BaHfO 3. This process leads also to a reduction of the crystallization temperature of the layers.

異常に広い固溶域を持つ(Ba1-xCax)(Sc1/2Nb1/2)O3系ペロブスカイトマイクロ波誘電体の奇妙な挙動

異常に広い固溶域を持つ(Ba1-xCax)(Sc1/2Nb1/2)O3系ペロブスカイトマイクロ波誘電体の奇妙な挙動

Superconducting Interfaces Between Insulating Oxides

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of congruent with 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of congruent with 10 nanometers.

Interface Controlled Electronic Charge Inhomogeneities in Correlated Heterostructures

For heterostructures of ultrathin, strongly correlated copper-oxide films and dielectric perovskite layers, we predict inhomogeneous electronic interface states. Our study is based on an extended Hubbard model for the cuprate film. The interface is implemented by a coupling to the electron and phonon degrees of freedom of the dielectric oxide layer. We find that electronic ordering in the film is associated with a strongly inhomogeneous polaron effect. We propose to consider the interfacial tuning as a powerful mechanism to control the charge ordering in correlated electronic systems.

High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layer

High electron mobility is measured down to low temperature in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Electron mobility is evaluated by both magnetoresistance and resistivity-Hall effect data. Values up to 400cm2∕Vs are found below 50 K in epitaxial thin films grown by a two-step method: first a 100-nm-thick ZnO relaxing layer is deposited on the SrTiO3 (110) substrate at relatively low temperature (550–600 °C) and then the deposition temperature is raised up to 800 °C for the growth of a second ZnO layer. Reflection high energy electron diffraction analysis during the deposition, ex situ x-ray diffraction and AFM morphology studies performed separately on each layer reveal that the first layer grows in a quasi-two-dimensional mode while the increased temperature in the second step improves the crystalline quality of the film. The integration of ZnO transparent semiconductor with high-k dielectric perovskite substrates may lead to a wide variety of new electronic∕optoelectronic devices.

Permittivity and temperature response of complex niobate perovskite dielectrics

Dielectric materials in the barium strontium zinc cobalt niobate system have been prepared to explore the consequences of independent variation of the population of the A and B sites in the perovskite structure. Results for the temperature coefficient of the resonant frequency follow the empirical relationship with the “ t” factor established by Reaney. Permittivity results show deviation from linear behaviour with composition as previously observed by Kim. By comparing ionic polarisibilities calculated from the measured permittivities using the Clausius–Mossotti equation and from the ion additivity rule we show that the anomalous results are those for the high barium compositions. We demonstrate that these can be explained by compression of the A site ions and propose a simple calculation to estimate the extent of this effect which agrees well with the experimental results.

Ordered domains and boundary structure in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics

Ordered domains and boundaries have been observed in Ba(Cd1/3Ta2/3)O3 perovskite dielectrics by electron microscopy. Individual nanoscale domains were found to be well ordered with hexagonal symmetry. Electron diffraction revealed a twinned crystallographic relationship between domains. High-resolution electron microscopy showed further details of the boundary structure. Conservative twin boundaries along the (001) and (110) planes and nonconservative antiphase boundaries with a projected displacement vector of the type [001] were observed. The relationship between the energy and structure of the domain boundaries is briefly discussed.

Temperature stable Ba 1− xCd xTiO 3 dielectrics

A new system, Ba 1− x Cd x TiO 3, was found as a perovskite dielectrics with some advantageous properties. Doping Nb 2O 5 and rare earth elements in Ba 1− x Cd x TiO 3 resulted in the dielectric ceramics with high permittivity and satisfying EIA X7R specifications, which sintered as low as 1150 °C.

Study of poly-Si/TaSiN/Pt structure for stacked capacitors

ABSTRACTDue to its high oxidation resistance, TaSiN is a promising candidate as an electrically conductive barrier layer for integration of high permittivity oxides in advanced memory devices. In this work, we report on the properties of TaSiN thin films deposited by reactive magnetron sputtering of a TaSi2 target. We have mainly studied the influence of deposition pressure and power density on film properties (composition, density, resistivity). The oxidation resistance of TaSiN films has been investigated at typical conditions for crystallization of perovskite dielectrics. The as-deposited and annealed samples were characterized using Rutherford backscattering spectroscopy and nuclear reaction analysis for atomic composition and XPS for chemical bonding. To study oxidation resistance, films have been processed in 18O2. The concentration depth profiles of 18O was measured after thermal treatments via the narrow resonances of 18O(p,α)15N at 151 keV (fwhm=100eV). The different results suggest that a pressure of 0.5 Pa, a power density of 2.63W/cm2 and a gas flow ratio N2/Ar of 5% allow to perform TaSiN films with high density, low resistivity and good oxidation resistance.

18O study of the oxidation of reactively sputtered Ti1−xAlxN barrier

The preparation of high-permittivity perovskite materials requires high-temperature (550–750 °C) oxidizing environments, providing stringent limitations on the choice of electrode materials. To minimize interdiffusion and oxidation reactions, an electrically conductive diffusion barrier such as Ti1−xAlxN is needed below the electrode material (Pt, IrO2, RuO2…). Ti1−xAlxN films were deposited by multitarget reactive sputtering in a mixture of Ar and N2. The stability of these films has been investigated under typical conditions for crystallization of perovskite dielectrics. Sample composition was characterized using Rutherford backscattering spectroscopy and nuclear reaction analysis. In particular, the concentration depth profiles of both 18O and 27Al were measured before and after RTA treatments via the narrow resonances of 18O(p,α)15N at 151 keV (FWHM = 100 eV) and 27Al(p,γ)28Si at 992 keV (FWHM = 100 eV). The different 18O excitation curves show that the oxidation resistance increases with Al incorporation. The Al excitation curves indicate a uniform Al content for as-deposited TixAl1−xN and reveal Al diffusion to the surface during the oxidation process which indicates the formation of an Al-rich oxide layer at the TixAl1−xN surface, leaving a layer depleted in Al below it.

Optimization of Ta–Si–N thin films for use as oxidation-resistant diffusion barriers

We have demonstrated that the optimum Ta–Si–N compositions for use as oxygen diffusion barriers in stacked-capacitor dynamic random-access memory structures with perovskite dielectrics are in the range Ta(20–25 at.%)–Si(20–45 at.%)–N(35–60 at.%). Twenty-two different Ta–Si–N compositions were evaluated, starting from six sputter-deposited Ta–Si alloys of which four were reactively deposited in 2–8% nitrogen in an argon plasma. The barriers were evaluated after an aggressive 650 °C/30 min oxygen anneal to determine if they remained electrically conductive, prevented oxygen diffusion and formation of low dielectric constant oxides, and had minimal interaction with the Pt electrode and underlying Si plug. Rutherford backscattering spectroscopy, four-point probe sheet resistance, through-film-resistance, and x-ray diffraction analysis techniques were used in the evaluation.

Dielectric‐State Analysis in Solid‐Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 −x)Pb(Yb1/2Ta1/2)O3–x Pb(Lu1/2Nb1/2)O3. Superlattice reflections for the compositions 0.8 &lt; x &lt; 1.0 were observed by X‐ray diffractometry, and the temperature‐composition dielectric‐state diagram was determined. In the present study, the disordered middle composition, with 0.2 &lt; x &lt; 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤x &lt; 0.2 and 0.8 &lt; x≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization (P) with electric field (E).

Layered TaSiN as an Oxidation Resistant Electrically Conductive Barrier

TaSiN films deposited as layered TaN–SiN structures of various compositions have been examined for their oxidation resistant properties during annealing in oxygen at annealing conditions commonly used to prepare perovskite dielectrics. The films have been characterized by Rutherford backscattering analysis (RBS), x-ray diffraction (XRD), and electrical resistivity measurements. Films with less than 15 at.% Si showed some resistance to oxidation after annealing for 1 min at 650 °C but became fully oxidized after longer anneals. Increasing the Si content up to 28 at.% increasingly improved the oxidation resistance of the alloys to the point where the films resisted complete oxidation for up to 5 min at 700 °C. For alloys with greater than 28 at.% Si, no oxidation could be detected by RBS or electrical measurements for anneals up to 5 min at 700 °C. Furthermore, these high Si content alloys were still conductive with resistivities of near 1000 μΩ cm. It was also found that TaSiN and lead lanthanum titanate (PLT) interact strongly during annealing, and another nonoxidizing barrier metal, such as Pt, is required between the two materials if TaSiN is to be used as an electrode/barrier with lead-based perovskites.

Al–Ta Bilayer as an Oxidation Resistant Barrier for Electrode Structures in High Dielectric Constant Capacitors

Aluminum-tantalum bilayers have been investigated for their potential to serve as conductive barriers to oxygen diffusion when annealed at conditions corresponding to crystallization of perovskite dielectrics such as lead lanthanum titanate (PLT). Ta (50 nm)/Al (15 nm) structures have been deposited on Si substrates and annealed in oxygen at 650 and 700 °C for various amounts of time. The as-deposited and annealed structures have been characterized by x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and Auger electron spectroscopy (AES) analysis and by four-point probe electrical measurements. It has been found that the Al–Ta structures can withstand complete oxidation when exposed to oxygen at 650 °C for 30 min or 700 °C for 1 min and the oxide layer formed at the surface of the structure acts as a barrier to further oxygen diffusion. When a PLT film was deposited directly on the Al–Ta structures intermixing took place. It was therefore necessary to insert a Pt layer between the Al–Ta barrier and PLT layer. In such a case the PLT showed electrical properties similar to those obtained when deposited on SiO2/Pt; however, the Al–Ta structure did interact with Pt during the perovskite formation anneal. It has been found that this interaction can be prevented by preannealing the Al–Ta, in oxygen, prior to the deposition of Pt.

Pyrochlore–perovskite phase transformation in highly homogeneous (Pb,La)(Zr,Sn,Ti)O3 powders

Control of undesirable pyrochlore is critical to the processing of lead perovskite dielectrics and ferroelectrics. In this work, a homogeneous and stoichiometric fine powder of the ferroelectric Pb0.97La0.02(Zr0.64Sn0.25Ti0.11)O3 (PLZST) has been prepared by a hydroxide coprecipitation and freeze-drying method. Through systematic variation of processing temperature and time, we have characterized the pyrochlore-to-perovskite crystallization process of the powder. Studies of the crystallization behavior of the precursor as a function of temperature by X-ray powder diffraction and transmission electron microscopy showed that the pyrochlore phase forms from an amorphous precursor, initially at low temperatures around 500–550 °C. Further heat treatment to 750 °C resulted in development of the perovskite phase with no significant pyrochlore crystallite growth. At intermediate temperatures the precursor yielded a fine mixture of pyrochlore and perovskite phases. When the pyrochlore phase was heat-treated in air a slight weight increase was observed in the temperature range of 300–700 °C, which is attributed to oxygen absorption. The weight increase was not observed upon firing in argon atmosphere; instead, a weight loss occurred near 700 °C, which was identified as being mostly due to CO2 gas evolution. This implies that the pyrochlore phase is crystallographically and thermodynamically metastable. An apparent activation energy of 53.9 kcal mol–1 was estimated for the pyrochlore–perovskite phase transformation.

Narrow Resonance Profiling Study of the Oxidation of Ti1-xALxN Barrier Layer

AbstractThe preparation of ferroelectric and high dielectric perovskite materials, which is performed at high temperature (550–750°C) in oxidizing environments, provides strong limitations on the choice of electrode materials which have to be used for integration with semiconductor devices. To minimize interdiffusion and oxidation reactions, a diffusion barrier must be placed between Si substrate and electrode material (Pt, lrO2, RuO2.…). Ti1-xAlxN thin films, deposited by reactive sputtering, are promising materials as electrically conductive layers and robust diffusion barriers. The stability of these films has been investigated at conditions typically used for crystallization of perovskite dielectrics. Films with various Al composition (x=0, 0.1, 0.2 and 0.4) have therefore been annealed by RTA (rapid thermal annealing) in 18O2 at 550–750°C. The concentration depth profiles of both 18O and 27A1 were measured before and after the RTA treatments via the narrow resonances of 18O(p,α)15N at 151 keV (fwhm=100 eV) and 27Al(p,γ)28Si at 992 keV (fwhm=100 eV). Al incorporation in the films reduces oxide growth especially at high annealing temperatures. The Al excitation curves indicate a uniform Al content for as deposited Ti1−xAlxN, and reveal Al diffusion to the surface during oxidation which indicates the formation of an Al rich oxide layer at the Ti1-xAlxN surface.