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As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices1–9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries12,13, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses., The vibrational states emerging at the interface in oxide superlattices are characterized theoretically and at atomic resolution, showing the impact of material length scales on structure and vibrational response.

Ferroelectric hafnium zirconium oxide holds great promise for a broad spectrum of complementary metal-oxide-semiconductor (CMOS) compatible and scaled microelectronic applications, including memory, low-voltage transistors, and infrared sensors, among others. An outstanding challenge hindering the implementation of this material is polarization instability during field cycling. In this study, the nanoscale phenomena contributing to both polarization fatigue and wake-up are reported. Using synchrotron X-ray diffraction, the conversion of non-polar tetragonal and polar orthorhombic phases to a non-polar monoclinic phase while field cycling devices comprising noble metal contacts is observed. This phase exchange accompanies a diminishing ferroelectric remanent polarization and provides device-scale crystallographic evidence of phase exchange leading to ferroelectric fatigue in these structures. A reduction in the full width at half-maximum of the superimposed tetragonal (101) and orthorhombic (111) diffraction reflections is observed to accompany wake-up in structures comprising tantalum nitride and tungsten electrodes. Combined with polarization and relative permittivity measurements, the observed peak narrowing and a shift in position to lower angles is attributed, in part, to a phase exchange of the non-polar tetragonal to the polar orthorhombic phase during wake-up. These results provide insight into the role of electrodes in the performance of hafnium oxide-based ferroelectrics and mechanisms driving wake-up and fatigue, and demonstrate a non-destructive means to characterize the phase changes accompanying polarization instabilities.

Electronic conduction pathways in dielectric thin films are explored using automated experiments in scanning probe microscopy (SPM). Here, we use large field of view scanning to identify the position of localized conductive spots and develop a SPM workflow to probe their dynamic behavior at higher spatial resolution as a function of time, voltage, and scanning process in an automated fashion. Using this approach, we observe the variable behaviors of the conductive spots in a 20 nm thick ferroelectric Hf0.54Zr0.48O2 film, where conductive spots disappear and reappear during continuous scanning. There are also new conductive spots that appear during scanning. The automated workflow is universal and can be integrated into a wide range of microscopy techniques, including SPM, electron microscopy, optical microscopy, and chemical imaging., 12 pages, 6 figures

Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO

We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca0.5Sr0.5TiO3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from ∼1 to ∼10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin "vibrational bridge" layer between two solids.

Ferroelectric oxides interfaced with transition metal dichalcogenides (TMDCs) offer a promising route toward ferroelectric-based devices due to lack of dangling bonds on the TMDC surface leading to a high-quality and abrupt ferroelectric/TMDC interface. In this work, the thermal stability of this interface is explored by first depositing hafnium zirconium oxide (HZO) directly on geological MoS2 and as-grown WSe2, followed by sequential annealing in ultra-high vacuum (UHV) over a range of temperatures (400-800 {\deg}C), and examining the interface through X-ray photoelectron spectroscopy (XPS). We show that the nucleation and stability of HZO grown through atomic layer deposition (ALD) varied depending on functionalization of the TMDC, and the deposition conditions can cause tungsten oxidation in WSe2. It was observed that HZO deposited on non-functionalized MoS2 was unstable and volatile upon annealing, while HZO on functionalized MoS2 was stable in the 400-800 {\deg}C range. The HZO/WSe2 interface was stable until 700 {\deg}C, after which Se began to evolve from the WSe2. In addition, there is evidence of oxygen vacancies in the HZO film being passivated at high temperatures. Lastly, X-ray diffraction (XRD) was used to confirm crystallization of the HZO within the temperature range studied., Comment: 33 pages, 11 figures

We present here a multi-length scale integration of compositionally tailored NaSICON-based Na+ conductors to create a high Na+ conductivity system resistant to chemical attack in strongly alkaline aqueous environments. Using the Pourbaix Atlas as a generalized guide to chemical stability, we identify NaHf2P3O12 (NHP) as a candidate NaSICON material for enhanced chemical stability at pH > 12, and demonstrate the stability of NHP powders under accelerated aging conditions of 80 °C and pH = 13–15 for a variety of alkali metal cations. To compensate for the relatively low ionic conductivity of NHP, we develop a new low temperature (775 °C) alkoxide-based solution deposition chemistry to apply dense NHP thin films onto both platinized silicon wafers and bulk, high Na+ conductivity Na3Zr2Si2PO12 (NZSP) pellets. These NHP films display Na+ conductivities of 1.35 × 10−5 S cm−1 at 200 °C and an activation energy of 0.53 eV, similar to literature reports for bulk NHP pellets. Under aggressive conditions of 10 M KOH at 80 °C, NHP thin films successfully served as an alkaline-resistant barrier, extending the lifetime of NZSP pellets from 4.26 to 36.0 h. This integration of compositionally distinct Na+ conductors across disparate length scales (nm, mm) and processing techniques (chemically-derived, traditional powder) represents a promising new avenue by which Na+ conducting systems may be utilized in alkaline environments previously thought incompatible with ceramic Na+ conductors.

Epsilon-near-zero materials provide a new path for tailoring light-matter interactions at the nanoscale. In this paper, we analyze a compact electroabsorption modulator based on epsilon-near-zero confinement in transparent conducting oxide films. The nonresonant modulator operates through field-effect carrier density tuning. We compare the performance of modulators composed of two different conducting oxides, namely, indium oxide (In2O3 ) and cadmium oxide (CdO), and show that better modulation performance is achieved when using high-mobility (i.e., low loss) epsilon-near-zero materials such as CdO. In particular, we show that nonresonant electroabsorption modulators with submicron lengths and greater than 5 dB extinction ratios may be achieved through the proper selection of high-mobility transparent conducting oxides, opening a path for device miniaturization and increased modulation depth.

A new statistical approach for modeling diffraction profiles is introduced, using Bayesian inference and a Markov chain Monte Carlo (MCMC) algorithm. This method is demonstrated by modeling the degenerate reflections during application of an electric field to two different ferroelectric materials: thin-film lead zirconate titanate (PZT) of composition PbZr0.3Ti0.7O3 and a bulk commercial PZT polycrystalline ferroelectric. The new method offers a unique uncertainty quantification of the model parameters that can be readily propagated into new calculated parameters.

Ferroelastic domain walls provide opportunities for deterministically controlling mechanical, optical, electrical, and thermal energy. Domain wall characterization in micro- and nanoscale systems, where their spacing may be of the order of 100 nm or less is presently limited to only a few techniques, such as piezoresponse force microscopy and transmission electron microscopy. These respective techniques cannot, however, independently characterize domain polarization orientation and domain wall motion in technologically relevant capacitor structures or in a non-destructive manner, thus presenting a limitation of their utility. In this work, we show how backscatter scanning electron microscopy utilizing channeling contrast yield can image the ferroelastic domain structure of ferroelectric films with domain wall spacing as narrow as 10 nm. Combined with electron backscatter diffraction to identify grain orientations, this technique provides information on domain orientation and domain wall type that cannot be readily measured using conventional non-destructive methods. In addition to grain orientation identification, this technique enables dynamic domain structure changes to be observed in functioning capacitors utilizing electrodes that are transparent to the high-energy backscattered electrons. This non-destructive, high-resolution domain imaging technique is applicable to a wide variety of ferroelectric thin films and a multitude of material systems where nanometer-scale crystallographic twin characterization is required.

Phonon scattering in crystalline systems can be strongly dictated by a wide array of defects, many of which can be difficult to observe via standard microscopy techniques. We experimentally demonstrate that the phonon thermal conductivity of MgO thin films is proportional to the crystal’s coherence length, a property of a solid that quantifies the length scale associated with crystalline imperfections. Sputter-deposited films were prepared on (100)-oriented silicon and then annealed to vary the crystalline coherence, as characterized using x-ray diffraction line broadening. We find that the measured thermal conductivity of the MgO films varies proportionally with crystalline coherence length, which is ultimately limited by the grain size. The microstructural length scales associated with crystalline defects, such as small-angle tilt boundaries, dictate this crystalline coherence length, and our results demonstrate the role that this length scale has on the phonon thermal conductivity of thin films. Our results suggest that this crystalline coherence length scale provides a measure of the limiting phonon mean free path in crystalline solids, a quantity that is often difficult to measure and observe with more traditional imagining techniques.

Ferroelectric materials are well-suited for a variety of applications because they can offer a combination of high performance and scaled integration. Examples of note include piezoelectrics to transform between electrical and mechanical energies, capacitors used to store charge, electro-optic devices, and nonvolatile memory storage. Accordingly, they are widely used as sensors, actuators, energy storage, and memory components, ultrasonic devices, and in consumer electronics products. Because these functional properties arise from a noncentrosymmetric crystal structure with spontaneous strain and a permanent electric dipole, the properties depend upon physical and electrical boundary conditions, and consequently, physical dimension. The change in properties with decreasing physical dimension is commonly referred to as a size effect. In thin films, size effects are widely observed, whereas in bulk ceramics, changes in properties from the values of large-grained specimens is most notable in samples with grain sizes below several micrometers. It is important to note that ferroelectricity typically persists to length scales of about 10 nm, but below this point is often absent. Despite the stability of ferroelectricity for dimensions greater than ~10 nm, the dielectric and piezoelectric coefficients of scaled ferroelectrics are suppressed relative to their bulk counterparts, in some cases by changes up to 80%. The loss of extrinsic contributions (domain and phase boundary motion) to the electromechanical response accounts for much of this suppression. In this article, the current understanding of the underlying mechanisms for this behavior in perovskite ferroelectrics is reviewed. We focus on the intrinsic limits of ferroelectric response, the roles of electrical and mechanical boundary conditions, grain size and thickness effects, and extraneous effects related to processing. In many cases, multiple mechanisms combine to produce the observed scaling effects.

Effective thermal conductivity as a function of domain structure is studied by solving the heat conduction equation using a spectral iterative perturbation algorithm in materials with inhomogeneous thermal conductivity distribution. Using this proposed algorithm, the experimentally measured effective thermal conductivities of domain-engineered {001}p-BiFeO3 thin films are quantitatively reproduced. In conjunction with two other testing examples, this proposed algorithm is proven to be an efficient tool for interpreting the relationship between the effective thermal conductivity and micro-/domain-structures. By combining this algorithm with the phase-field model of ferroelectric thin films, the effective thermal conductivity for PbZr1-xTixO3 films under different composition, thickness, strain, and working conditions is predicted. It is shown that the chemical composition, misfit strain, film thickness, film orientation, and a Piezoresponse Force Microscopy tip can be used to engineer the domain structures and tune the effective thermal conductivity. Therefore, we expect our findings will stimulate future theoretical, experimental and engineering efforts on developing devices based on the tunable effective thermal conductivity in ferroelectric nanostructures.

Preparation of sodium zirconium silicate phosphate (NaSICon), Na1+xZr2SixP3–xO12 (0.25 ≤ x ≤ 1.0), thin films has been investigated via a chemical solution approach on platinized silicon substrates. Increasing the silicon content resulted in a reduction in the crystallite size and a reduction in the measured ionic conductivity. Processing temperature was also found to affect microstructure and ionic conductivity with higher processing temperatures resulting in larger crystallite sizes and higher ionic conductivities. The highest room temperature sodium ion conductivity was measured for an x = 0.25 composition at 2.3 × 10–5 S/cm. In conclusion, the decreasing ionic conductivity trends with increasing silicon content and decreasing processing temperature are consistent with grain boundary and defect scattering of conducting ions.

Atomic-scale phenomena fundamentally influence materials form and function that makes the ability to locally probe and study these processes critical to advancing our understanding and development of materials. Atomic-scale chemical imaging by scanning transmission electron microscopy (STEM) using energy-dispersive X-ray spectroscopy (EDS) is a powerful approach to investigate solid crystal structures. Inefficient X-ray emission and collection, however, require long acquisition times (typically hundreds of seconds), making the technique incompatible with electron-beam sensitive materials and study of dynamic material phenomena. Here we describe an atomic-scale STEM-EDS chemical imaging technique that decreases the acquisition time to as little as one second, a reduction of more than 100 times. We demonstrate this new approach using LaAlO3 single crystal and study dynamic phase transformation in beam-sensitive Li[Li0.2Ni0.2Mn0.6]O2 (LNMO) lithium ion battery cathode material. By capturing a series of time-lapsed chemical maps, we show for the first time clear atomic-scale evidence of preferred Ni-mobility in LNMO transformation, revealing new kinetic mechanisms. These examples highlight the potential of this approach toward temporal, atomic-scale mapping of crystal structure and chemistry for investigating dynamic material phenomena.

Composition dependence of second harmonic generation, refractive index, extinction coefficient, and optical bandgap in 20 nm thick crystalline Hf1−xZrxO2 (0 ≤ x ≤ 1) thin films is reported. The refractive index exhibits a general increase with increasing ZrO2 content with all values within the range of 1.98–2.14 from 880 nm to 400 nm wavelengths. A composition dependence of the indirect optical bandgap is observed, decreasing from 5.81 eV for HfO2 to 5.17 eV for Hf0.4Zr0.6O2. The bandgap increases for compositions with x > 0.6, reaching 5.31 eV for Hf0.1Zr0.9O2. Second harmonic signals are measured for 880 nm incident light. The magnitude of the second harmonic signal scales with the magnitude of the remanant polarization in the composition series. Film compositions that display near zero remanent polarizations exhibit minimal second harmonic generation while those with maximum remanent polarization also display the largest second harmonic signal. The results are discussed in the context of ferroelectric phase assemblage in the hafnium zirconium oxide films and demonstrate a path toward a silicon-compatible integrated nonlinear optical material.

Ferroelastic domain walls in ferroelectric materials possess two properties that are known to affect phonon transport: a change in crystallographic orientation and a lattice strain. Changing populations and spacing of nanoscale-spaced ferroelastic domain walls lead to the manipulation of phonon-scattering rates, enabling the control of thermal conduction at ambient temperatures. In the present work, lead zirconate titanate (PZT) thin-film membrane structures were fabricated to reduce mechanical clamping to the substrate and enable a subsequent increase in the ferroelastic domain wall mobility. Under application of an electric field, the thermal conductivity of PZT increases abruptly at ∼100 kV/cm by ∼13% owing to a reduction in the number of phonon-scattering domain walls in the thermal conduction path. The thermal conductivity modulation is rapid, repeatable, and discrete, resulting in a bistable state or a "digital" modulation scheme. The modulation of thermal conductivity due to changes in domain wall configuration is supported by polarization-field, mechanical stiffness, and in situ microdiffraction experiments. This work opens a path toward a new means to control phonons and phonon-mediated energy in a digital manner at room temperature using only an electric field.

The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO2) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO2-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O2 sensor and the transient behavior of the resistance as a proxy. A pO2 range of 10−25–100 atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt–Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO2 on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. An approximately 11 % increase in power factor over a pO2 range of 10−19–10−8 atm at 973 K for the donor-doped single crystals is observed.

The effects of chemistry, substrate, and processing conditions on through-thickness cation distributions are explored in solution-derived morphotropic composition lead zirconate titanate (PZT) films. Films prepared from chelate-based and conventional sol–gel chemistries were spin cast onto Pt/ZnO/SiO2/Si and Pt/Ti/SiO2/Si substrates and pyrolyzed at 300°C, 350°C, and 400°C prior to crystallization at 700°C either in a preheated furnace or via rapid thermal processing. For films crystallized within a conventional furnace on Pt/ZnO/SiO2/Si substrates no chemical gradients were observed. All films prepared on Pt/Ti/SiO2/Si substrates had increased titanium concentrations near the PZT/Pt interfaces, and the source is shown to be titanium diffusing from the substrate metallization stack. The effect of heating method and rate was explored in films prepared on Pt/ZnO/SiO2/Si substrates with 15°C, 50°C, and 100°C/s heating rates within a rapid thermal annealer. Only one solution chemistry-heating rate combination resulted in the formation of a chemical gradient: a conventional sol–gel chemistry and a 50°C/s heating rate. Infrared spectroscopy of pyrolyzed gel films showed absorption spectra differences in the bonding structure between the two chemistries with the conventional sol–gel-derived films exhibiting a signature more similar to that of a PbTiO3 gel, suggestive of a gel-structure source of gradient formation during crystallization.

Epitaxial perovskite oxide thin films and heterostructures are a highly active materials research topic generating both fundamental and applied interest. This manuscript reports the roles of oxidation kinetics and film deposition rate on the growth of phase-pure complex oxides by molecular-beam epitaxy, one of the premier techniques for these materials, in a regime that is traditionally described as being governed by thermodynamics. It is found that oxidation kinetics on the film surface are surprisingly important for the growth of PbTiO${}_{3}$ and for formulating a simple kinetic theory to interpret experimental observations. The results are also enlightening to predict the conditions for improved growth of BiFeO${}_{3}$ and other complex oxides by MBE, of high interest to the broad community studying oxide films and heterostructures.

We study the role of carrier mobility in transparent conducting oxides integrated into epsilon-near-zero modulators. High-mobility materials including CdO enable sub-micron length electroabsorption modulators through >4dB/µm extinction ratios.

We have measured the cross-plane thermal conductivity (κ) of (010)-textured, undoped, and lanthanum-doped strontium niobate (Sr2-xLaxNb2O7-δ) thin films via time-domain thermoreflectance. Then the thin films were deposited on (001)-oriented SrTiO3 substrates via the highly-scalable technique of chemical solution deposition. We find that both film thickness and lanthanum doping have little effect on κ, suggesting that there is a more dominant phonon scattering mechanism present in the system; namely the weak interlayer-bonding along the b-axis in the Sr2Nb2O7 parent structure. We also compare our experimental results with two variations of the minimum-limit model for κ and discuss the nature of transport in material systems with weakly-bonded layers. The low cross-plane κ of these scalably-fabricated films is comparable to that of similarly layered niobate structures grown epitaxially.

Atom probe tomography (APT) is a powerful materials characterization technique capable of ppm chemical resolution and near atomic scale spatial resolution. However, owing to a number of factors, the technique has not been widely applied to insulating materials and even less to complex oxides. In this study, we outline the methodology necessary to obtain high-quality results on a technologically relevant complex oxide Pb(Zr,Ti)O3 (or PZT) using laser-assisted APT on both bulk and thin film specimens. We show how, with optimized and well-controlled conditions, APT complements conventional techniques such as STEM-EDS. The correlative information can be used to obtain the nanoscale 3-D chemical information and investigate the nanoscale distribution of cations. Using nearest-neighbor cluster analysis routines, 5–10 nm segregation of B-site cations was detected in bulk sintered PZT 53/47 from chemically prepared powders. No statistically significant segregation of B-site cations was observed in thin film specimens. This work opens new avenues toward understanding the process-structure properties in complex materials at length scales heretofore unachievable.

The crystallization of lead zirconate titanate (PZT) thin films was evaluated on two different platinum-coated Si substrates. One substrate consisted of a Pt coating on a Ti adhesion layer, whereas the other consisted of a Pt coating on a TiO2 adhesion layer. The Pt deposited on TiO2 exhibited a higher degree of preferred orientation than the Pt deposited on Ti (as measured by the Full Width at Half Maximum of the 111 peak about the sample normal). PZT thin films with a nominal Zr/Ti ratio of 52/48 were deposited on the substrates using the inverted mixing order (IMO) route. Phase and texture evolution of the thin films were monitored during crystallization using in situ X-ray diffraction at a synchrotron source. The intensity of the Pt3Pb phase indicated that deposition on a highly oriented Pt/TiO2 substrate resulted in less diffusion of Pb into the substrate relative to films deposited on Pt/Ti. There was also no evidence of the pyrochlore phase influencing texture evolution. The results suggest that PZT nucleates directly on Pt, which explains the observation of a more highly oriented 111 texture of PZT on the Pt/TiO2 substrate than on the Pt/Ti substrate.

Ferroelectricity in doped and alloyed hafnia thin films has been demonstrated using several different electrodes, with TiN and TaN being most prominent. In this work, we demonstrate ferroelectric Hf0.58Zr0.42O2 thin films with superconducting NbN electrodes at cryogenic temperatures. Demonstration of polarization—electric field [P(E)] response at liquid helium cryogenic temperatures, 4 K, suggests that the polarization is switchable over a wide temperature range after an initial 600 °C anneal. Further, room temperature P(E) and capacitance measurements demonstrate an expected polarization response with wake-up required to reach the steady state. Wake-up cycling at 4 K is observed to have no effect upon the ferroelectric phase suggesting an oxygen vacancy mobility freeze out whereas wake-up cycling at 294 K demonstrates close to a 3× increase in remanent polarization. This integration of a ferroelectric Hf0.58Zr0.42O2 thin film with NbN demonstrates the suitability of a highly scalable ferroelectric in applications for cryogenic technologies.

Uniform metal contacts are critical for advanced thermoelectric devices. The uniformity of the contact resistance for gold, tungsten, and SrRuO3 electrodes on polycrystalline ternary Bi2Te3-based alloys for different types of surface cleaning procedures was characterized. The presence of a nanometer-thick native oxide layer on the Bi2Te3 surface leads to large and non-uniform contact resistance. Surface treatments included solvent cleans and chemical and dry etching prior to metallization of the Bi2Te3. Only etching the surface led to a significant improvement in contact resistance uniformity. None of the tested contacts reacted with the underlying Bi2Te3 substrate. Etching resulted in the removal of the native oxide on the Bi2Te3 surface, which was characterized using X-ray photoelectron spectroscopy (XPS). The average thickness, chemistry, and dry etch rate of the native oxide was further characterized using XPS. The non-uniformity in contact resistance suggests that the native oxide grows non-uniformly on polycrystalline bismuth telluride surfaces.

We present an in situ high-temperature confocal laser microscopy study on the thermal stability of 40 nm thick gold thin films grown on 40 nm Ti, TiO x and ZnO adhesion layers on (0 0 1) Si. In situ observation of the dewetting process was performed over a wide range of set temperatures (400–800 °C) and ramp rates (10–50 °C min −1 ) for each gold/adhesion layer combination. We found that significant dewetting and subsequent formation of gold islands occurs only at and above 700 °C for all adhesion layers. The dewetting is driven to equilibrium for gold/ZnO compared to gold/Ti and gold/TiO x as confirmed by ex situ X-ray diffraction and scanning electron microscopy characterization. Quantification of the in situ data through stretched exponential kinetic models reveals an underlying apparent activation energy of the dewetting process. This energy barrier for dewetting is higher for gold/Ti and gold/TiO x compared to gold/ZnO, thus confirming the ex situ observations. We rationalize that these apparent activation energies correspond to the underlying thermal stability of each gold/adhesion layer system.

The crystallization behavior of solution-derived lead zirconate titanate (PZT) thin films in different atmospheric environments was studied using in situ X-ray diffraction. The stability of the transient intermetallic Pt3Pb phase and perovskite PZT is dependent on oxygen partial pressure during crystallization. Based on the relationship between oxygen partial pressure and the resultant phase stability of intermediate phases, a new route to produce PZT thin films was developed. The new route involves switching atmospheres during crystallization and is shown to mitigate the formation of the transient intermetallic Pt3Pb phase and to promote the perovskite PZT phase. The route evidences a new and significant variable controlling film synthesis and film microstructure.

Strontium niobate (Sr:Nb = 1:1) thin films were prepared via chemical solution deposition on (001)-oriented SrTiO3, (001)p-oriented LaAlO3, (0001)-oriented sapphire, and polycrystalline alumina substrates. Crystallization in oxygen at 1000°C yielded Sr2Nb2O7 films on all substrates with strong (010) orientation. Films on LaAlO3 and SrTiO3 single-crystal substrates possessed a small amount of preferred in-plane orientation, whereas films prepared on sapphire and polycrystalline alumina substrates were fiber textured. Films crystallized at 900°C in a low oxygen atmosphere (~10−21 atm pO2) formed a randomly oriented polycrystalline perovskite, SrNbO3−δ on all substrates. A similar set of films crystallized at 900°C at a slightly higher oxygen partial pressure (~10−15 atm pO2) was comprised of Sr2Nb2O7 and SrNbO3−δ phases, exposing the dependence of phase formation on oxygen partial pressure. When subjected to a high-temperature anneal in oxygen, the SrNbO3−δ phase is shown to transform into Sr2Nb2O7, however, Sr2Nb2O7 did not significantly reverse transform into SrNbO3−δ after annealing in low oxygen partial pressure atmospheres.

A long-standing challenge to the widespread application of complex oxide thin fi lms is the stable and robust integration of noble metal electrodes, such as platinum, which remains the optimal choice for numerous applications. By considering both work of adhesion and stability against chemical diffusion, it is demonstrated that the use of an improved adhesion layer (namely, ZnO) between the silicon substrate and platinum bottom electrode enables dramatic improvements in the properties of the overlying functional oxide fi lms. Using BaTiO 3 and Pb(Zr,Ti)O 3 fi lms as test cases, it is shown that the use of ZnO as the adhesion layer leads directly to increased process temperature capabilities and dramatic improvements in chemical homogeneity of the fi lms. These result in signifi cant property enhancements (e.g., 300% improvement to bulk-like permittivity for the BaTiO 3 fi lms) of oxide fi lms prepared on Pt/ZnO as compared to the conventional Pt/Ti and Pt/TiO x stacks. A comparison of electrical, structural, and chemical properties that demonstrate the impact of adhesion layer chemistry on the chemical homogeneity of the overlying complex oxide is presented. Collectively, this analysis shows that in addition to the simple need for adhesion, metal-oxide layers between noble metals and silicon can have tremendous chemical impact on the terminal complex oxide layers.

In the field of radiation effects in materials, a detailed and precise description of the radiation environment used to damage samples is often required to make sense of subsequent materials analysis. The types of reactions and extent of damage that occur during irradiation strongly depend on the flux spectrum of the particular facility. Different neutron activation techniques for characterizing neutron flux spectra were performed on the University of Texas at Austin TRIGA research reactor’s in-core facilities. The results were compared in terms of spectral detail and precision. Activation of Au foils with multiple correction factors, and multiple foil activation employing different deconvolution techniques comprise the methods tested.

Capacitor technologies are as varied as the applications that they enable, but one of the common themes in advanced capacitors for consumer electronics is a desire for increased capacitance in smaller areas/volumes. The heroic advances of discrete capacitor manufacturers have kept pace with the increasing demands of miniaturization, but a time is quickly approaching when it appears that powder-based fabrication techniques simply will not be able to achieve desired layer thicknesses and capacitance densities. Here, we review the current state of the art and recent advances in the processing science and technology of high-permittivity thin films with a focus on industrially scalable solution-based fabrication processes of perovskite ferroelectric systems that appear to offer the greatest promise for the fabrication of future nanoscale capacitors.