Electrical and material properties of thin film perovskites

PhD ThesisThis thesis presents a study of negative capacitance in the robust perovskite BaTiO3. Negative capacitance is an unstable state in ferroelectrics, which explains why there is a lack of experimental evidence in the literature. A positive capacitance in a series capacitor configuration allows stabilisation of negative capacitance. The key finding is the stabilisation of negative capacitance at room temperature in BaTiO3. Temperature constraints in back-end-of-line processing should be at 500 °C or below in order to avoid diffusion of dopants and to inhibit high resistivity silicide phases. Three deposition techniques, pulsed laser deposition, atomic layer deposition and sputter deposition are used to investigate the material and electrical properties of perovskites for back-end integration within this temperature constraint. SrTiO3, Ba0.8Sr0.2TiO3 and BaTiO3 are all explored as possible solutions for tunable capacitance under low temperature processing. Evidence is shown for SrTiO3 displaying fully crystallised structures through pulsed laser deposition at 500 °C growth temperature. A refined model of effective oxide thickness is used to calculate interfacial layers that impact metal-oxide-semiconductor capacitors. The model is applied to SrTiO3 metal-insulator-metal capacitors in terms of a dead layer. Calculation of the dead layer thickness, which has been previously unattainable using solely the series capacitance model, is carried out using the effective oxide thickness model. However, transmission electron microscopy images suggest that a physical layer of ‘dead’ material is abesnt in the capacitors. The results support the hypothesis of an intrinsic explanation to the dead layer phenomenon. ii Finally, pulsed laser deposited BaTiO3 is explored in terms of ferroelectricity when integrated with Si using Pt/Ti/SiO2/Si substrates. Here, a mixed phase relationship is shown in the films of BaTiO3 in which the cubic phase, responsible for paraelectricity, dominates at room temperature. Increasing film thickness also correlates with higher remnant polarization in the films. The result confirms a size driven phase transition in thin film BaTiO3 which has preveously been studied on perovskite free-standing films or nanoparticles

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Leakage current and resistive switching mechanisms in SrTiO3

PhD ThesisResistive switching random access memory devices have attracted considerable attention due to exhibiting fast programming, non-destructive readout, low power-consumption, high-density integration, and low fabrication-cost. Resistive switching has been observed in a wide range of materials but the underpinning mechanisms still have not been understood completely. This thesis presents a study of the leakage current and resistive switching mechanisms of SrTiO3 metal-insulator-metal devices fabricated using atomic layer deposition and pulse laser deposition techniques. First, the conduction mechanisms in SrTiO3 are investigated. The leakage current characteristics are highly sensitive to the polarity and magnitude of applied voltage bias, punctuated by sharp increases at high field. The characteristics are also asymmetric with bias and the negative to positive current crossover point always occurs at a negative voltage bias. A model comprising thermionic field emission and tunnelling phenomena is proposed to explain ii the dependence of leakage current upon the device parameters quantitatively. SrTiO3 also demonstrates bipolar switching behaviour where the current-density versus voltage (J-V) characteristics show asymmetry at all temperatures examined, with resistive switching behaviour observed at elevated temperatures. The asymmetry is explained by the relative lack of electron traps at one electrode, which is determined from the symmetric J-V curve obtained at room temperature due to the redistribution of the dominant electrical defects in the film. Evidence is presented for a model of resistive switching that originates from defect diffusion (possibly oxygen vacancies) at high temperatures. Finally, a peculiar resistive switching behaviour was observed in pulse laser deposited SrTiO3. This switching depends on both the amplitude and polarity of the applied voltage, and cannot be described as either bipolar or unipolar resistive switching. This behaviour is termed antipolar due to the opposite polarity of the set voltage relative to the previous reset voltage. The proposed model based on electron injection by tunnelling at interfaces and a Poole-Frenkel mechanism through the bulk is extended to explain the antipolar resistive switching behaviour. This model is quantified by use of a simple mathematical equation to simulate the experimental results

Modeling and Simulations of Electrical Breakdown and Thermal Failure in Zinc Oxide and Titanium Dioxide for High-Voltage Dielectric Applications

The development of transmission lines with higher energy storage capabilities is an important goal for compact pulsed power systems. In this context, ceramic dielectrics are promising candidates from the standpoint of high dielectric constants and breakdown strength. Though such materials look promising, their breakdown response characteristics have not been well studied, nor adequately understood. The electrical response of dielectrics also seems to depend on the internal structure and its granularity. For example, the breakdown strength of nano-crystalline insulators such as titania and zinc oxide have been observed to depend on the internal grain size. In general, the hold-off voltage increases monotonically with decreases in grain sizes. For example, nano-crystalline TiO2 exhibits higher breakdown strength as compared to micron size TiO2. In this dissertation, time-dependent, two-dimensional simulations based on random Voronoi networks have been developed to study the electrical breakdown and thermal failure in ceramic dielectrics in ZnO varistors in response to high-voltage pulsing. Our simulations allow for dynamic predictions of internal failures and to track the progression of hot-spots and thermal stresses in samples. The focus is on internal grain-size variations and relative disorder. Our results predict that parameters such as the device hold-off voltage, the average internal temperature, and average dissipated energy density, and applied pulse-durations would be higher with more uniform grains. Furthermore, scaling down the average grain size offers similar advantages. Finally, it is shown that for the situations studied, the principle failure mechanism arises from internal localized melting, while thermal stresses are well below the thresholds for cracking. In addition, the somewhat surprising observation of lower breakdown fields for TiO2 under pulsed conditions as compared to quasi-DC biasing, was studied. Our simulation results indicated that electrical breakdown of TiO2 under multiple pulsed conditions can occur at lower voltages as compared to quasi-DC biasing. We hypothesize that the lower breakdown voltages observed in TiO2 under pulsed conditions, is a direct rise-time effect, coupled with cumulative detrapping. Finally, the role of granular dielectrics having non-linear, voltage-dependent capacitances on pulse rise-time sharpening was also probed and has been discussed

Development of Chemical Solution Deposition Derived (001)-Oriented Epitaxial Bismuth Ferrite Thin-Films with Robust Ferroelectric Properties

Bismuth ferrite (BiFeO3, BFO) has attracted recent attention due to its multi-functional properties, including multiferroism, resistive switching and photovoltaic effects. In particular, epitaxial BFO has been shown to demonstrate giant polarization, polarization-mediated resistive switching and unique magnetic properties. Until now, the most popular methods to obtain epitaxial BFO films with robust properties have been pulsed laser deposition (PLD) and radio frequency (RF) sputtering. Films made using these methods have been reported to have a high spontaneous polarization of up to 130 µC/cm2 and a switchable diode effect. Chemical deposition techniques, such as chemical solution deposition (CSD), have attracted recent interest for the preparation of BFO films and owing to them offering a cost-effective and more convenient manufacturing method compared with PLD and RF sputtering, an aspect of particular importance in an industrial context. However, the large scale adoption of CSD-derived BFO thin films for a variety of applications has been stymied by a number of significant limitations and challenges including: (1) the imprecision of the starting chemical composition and the subsequent volatilisation of Bi during the annealing step leading to the formation of secondary phases and/or highly conductive films with very poor leakage resistance; (2) variable sintering and densification behaviour leading to films having porosity and poor microstructures; and, (3) limited epitaxy between the film and substrate. Collectively, these dramatically impair the structural and electromechanical properties of the BFO films rendering them unsuitable for practical application. Thus, there is the important need to optimize the CSD preparation process for obtaining pure-phase epitaxial BFO. In this thesis, a non-aqueous CSD route was developed and studied with the aim to optimise it for the preparation of epitaxial (001) BFO thin films with robust (square) polarization hysteresis loops, high dielectric constant, strong piezoelectric response and distinct diode behavior. Molecular changes in the organic precursors on heating (determined by NMR and FTIR) and the effects of gelation temperature–time and thickness on film morphology were studied to develop an optimal deposition–gelation process for the synthesis of homogenous, defect-free gel films suitable for subsequent crystallization. The key to obtaining a homogenous gel was control of the delicate balance between gelation and salt (metal nitrate) precipitation through solvent evaporation. The optimized synthesis route consists of spin-coating 0.25 M precursor on 70°C preheated substrate at 3000 rpm for 30 seconds then gelating at 90°C then drying at 270°C. The crystallization of optimized gel films was studied as a function of Bi/Fe concentration and stoichiometry in the precursor solution, film thickness and single versus multiple depositions, crystallization temperature and atmosphere. Oxygen atmosphere was found to be essential for suppression of Bi volatilization and promotion of film epitaxial orientation. Pure-phase, epitaxial BFO thin film on (001)-strontium titanate (STO) substrate was obtained by rapidly heating the thin film to 650°C in an oxygen atmosphere and holding at the temperature for 30 minutes. A multi-layer deposition process for fabrication of films of various thicknesses was optimised by study of the deposition-heating sequence. The ferroelectric properties of pure-phase, epitaxial BFO thin films on lanthanum strontium manganite buffered (001)-STO substrates were studied as a function of thickness (40, 70, and 150 nm). The 70 and 150 nm films exhibited exhibited square hysteresis loops at room temperature with high remanent polarization (2Pr up to 100 μC/cm2), low coercive field (2Ec down to 193 kV/cm), and high relative dielectric constant (up to 613). High-cycle fatigue tests showed that these films are resistant to polarization fatigue (up to 108 cycles). All thicknesses showed resistive switching behaviour and a polarization-mediated diode effect both of which became more pronounced with decreasing thickness. The CSD technique developed in this work yielded high-quality BFO thin films and offers a viable low-cost alternative to current BFO deposition techniques

Deposition and characterisation of bismuth layer-structured ferroelectric films

Bismuth layer-structured ferroelectrics have been recognised as promising film materials for ferroelectric random access memory application due to their excellent fatigue resistance and other electrical properties. This work deals with the deposition and characterisation of epitaxial and polycrystalline W-doped SrBi2Ta2O9 (SBT) and lanthanide-doped bismuth titanate (BiT) films. SBT and W-doped SBT films were fabricated by pulsed laser deposition (PLD) on platinised silicon substrates. The effects of fabrication temperature and W-doping level on film properties were studied. The crystallinity of SBTW films improved with increasing fabrication temperatures, resulting in enhanced ferroelectric properties and dielectric properties above the fabrication temperature of 750 ºC. Dense ceramic samples of Nd- and Sm-doped BiT (BNdT and BSmT) were successfully fabricated for PLD targets by solid state processing. Highly epitaxially (001)-, (118)-, and (104)-oriented Nd-doped bismuth titanate (BNdT) films were grown by PLD on (001)-, (011)-, and (111)-oriented SrTiO3 (STO) single crystal substrates, respectively. A three-dimensional orientation relationship between films and substrates was derived as: BNdT(001)//STO(001), BNdT[ 110 ]//STO[100]. Films showed strong dependence of structural and ferroelectric properties on the crystal orientation. PLD-grown BSmT films on platinised silicon substrates were studied as a function of fabrication temperature, effects of Pt bottom layer orientation, Sm doping level, and LaNiO3 buffer layer. An alkoxide-salt chemical solution deposition (CSD) method was adopted to prepare the precursors for BSmT (BNdT) film fabrication. Precursors of Bi-Sm(Nd)-Ti which were stable for at least eight months in air ambient were successfully developed. In-situ FT-IR studies suggest that acetic acid serves as chelating agent to improve the homogeneity of the precursor solution by generating a dense and homogeneous Ti-O-Ti polymeric network. The electrical properties of the films fabricated in this study (dielectric and ferroelectric properties, leakage current characteristics and electrical fatigue properties), are comparable or superior to these previously reported for similar films developed by other techniques or with other doping elements. Low temperature electrical properties of BSmT films suggest that the films are very promising for extremely low temperature nonvolatile memory applications. The results of BNdT films annealed at different oxygen partial pressure (O2, air, N2) showed that oxygen ambience affected structural properties of the films by enhancing the growth of perovskite phase (phase formation), increasing grain size (grain growth), and assisting the growth of (117)-oriented grains (crystallographic orientations). Piezoresponse force microscopy (PFM) was adopted to characterise BSmT films. Domain structures were clearly observed in a PLD-grown BSmT film, which were closely related to the grain structures. Domain manipulation was carried out in a CSD-derived BSmT film, showing that the film can be nearly uniformly polarised, which can be used in nanoscale device fabrication. Clear hysteresis loops were measured by PFM, which was an important proof of ferroelectricity. Large spatial variations of piezoelectric hysteresis loops of a CSD-derived BSmT film were observed across the film surface. Effective electrostriction coefficient (Qeff) of a PLD-grown BSmT film was measured, showing that BSmT films had better piezoelectric properties (higher Qeff, higher dzz) than SBT films, un-doped BiT ceramics and films. It suggests that BSmT films are promising piezoelectric materials for MEMS use