Intrinsic Charge Trapping in Amorphous Oxide Films: Status and Challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states caused by the disorder of amorphous metal oxide films. We start from presenting the results for amorphous (a) HfO<sub>2</sub>, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy (EPDS) measurements and theoretical calculations using density functional theory (DFT) shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO<sub>2</sub>. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modelling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO<sub>2</sub> and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO<sub>2</sub>, a-Al<sub>2</sub>O<sub>3</sub>, a-TiO<sub>2</sub>. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum (CBM) can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO<sub>2</sub> and a-SiO<sub>2</sub> weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O<sup>2-</sup> ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions

Intrinsic electron trapping in amorphous oxide

We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films. The energy distributions of trapped electron states in ultra-pure prototype amorphous (a)-HfO2 insulator obtained from exhaustive photo-depopulation experiments demonstrate electron states in the energy range of 2–3 eV below the oxide conduction band. These energy distributions are compared to the results of density functional calculations of a-HfO2 models of realistic density. The experimental results can be explained by the presence of intrinsic charge trapping sites formed by under-coordinated Hf cations and elongated Hf–O bonds in a-HfO2. These charge trapping states can capture up to two electrons, forming polarons and bi-polarons. The corresponding trapping sites are different from the dangling-bond type defects responsible for trapping in glass-forming oxides, such as SiO2, in that the traps are formed without bonds being broken. Furthermore, introduction of hydrogen causes formation of somewhat energetically deeper electron traps when a proton is immobilized next to the trapped electron bi-polaron. The proposed novel mechanism of intrinsic charge trapping in a-HfO2 represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators

OXIDE-BASED MEMRISTIVE DEVICES BY BLOCK COPOLYMER SELF-ASSEMBLY

Oxide-based memristive systems represent today an emerging class of devices with a significant potential in memory, logic, and neuromorphic circuit applications. These devices have a simple capacitor structure and promise superior scalability together with favorable memory performances. This thesis presents a study of resistive switching phenomena in HfOx-based nanoscale memristive devices, with focus on material properties and development of bottom-up approaches for the fabrication of structures with dimension down to the nanoscale. One of the main issues for practical applications regarding device variability is first assessed by doping hafnium oxide films with different concentrations of aluminum atoms. Testing devices are analyzed by physico-chemical and electrical techniques in order to define the effect of oxide doping on the device properties. In the following part of the thesis, the scalability limit is explored in very high density arrays of nanodevices produced exploiting a lithographic approach based on the bottom-up self-assembly of block copolymer templates. This technique allows a tight control over the size and density of the defined features, and the possibilities offered by block copolymer patterning are here discussed. Electrical measurements of the nanodevices are performed through conductive atomic force microscopy. The device variability is examined and related to the inherent oxide non-homogeneity at the nanoscale, while a non-volatile switching of the resistance of the nanodevices is demonstrated. Further, this analysis draws the attention to a crosstalk phenomenon occurring at the nanoscale in a continuous thin film geometry. This result suggests to select different system configurations. A promising technique based on selective reactions with one copolymer block is finally discussed which allows the direct production of oxide patterns from block copolymer templates avoiding a pattern transfer process. In conclusion, the results reported in this thesis highlight the high scalability potential of oxide-based memristive devices, providing a missing piece of information for the understanding and practical development of very high density arrays

Optoelectronic devices based on atomically thin semiconductors and photo-oxidised HfOx

The direction of research in solid state physics and technology has changed since the discovery of graphene. Now, a plethora of two-dimensional materials are being thoroughly investigated for their unique properties as well as for their implementation in next-generation optoelectronic devices. Of course, much effort is needed in order to reach the current level of modern electronics which is based on decades of research and development. For example, the level of miniaturisation modern technology requires can be achieved with atomically thin materials, driving Moore's Law forward. Conventional dielectrics exhibit high leakage currents when their dimensions are reduced to the nano-scale and the need for alternative materials compatible with two-dimensional electronics arises. However, the techniques that are being used for the growth and processing of conventional semiconducting materials are not always suitable with two-dimensional materials, which need special handling. These are some of the points that will be addressed in this PhD dissertation. Here, a new method for generating a fundamentally two-dimensional high-k dielectric which can be automatically incorporated in atomically thin optoelectronics devices is presented. The photo-oxidation of hafnium disulfide, HfS2, is a straight-forward, non-invasive process that can be used to oxidise pristine few-layered HfS₂, opening new paths for applications ranging from optoelectronics to photonics. The resulting dielectric, Hafnium dioxide, HfO₂, exhibits outstanding properties that exceed those of silicon dioxide, SiO₂ and its atomically thin nature makes it an ideal insulating layer for next-generation nano-electronics. Finally, the last part of this thesis is dedicated to a novel, CVD-grown, n-type monolayer of tungsten diselenide, WSe2. This is the first time negatively doped CVD-grown WSe₂ is reported, which opens the possibility of choosing the doping of the two-dimensional semiconductor before fabrication. For investigating and characterising this novel material, field-effect transistors are fabricated and characterised optoelectronically, shining light on the carriers' behaviour and the ability of the material in light-detection applications. Vacuum and ambient annealing of the WSe2 based devices highlights a possible way to control the doping level of the material, and thus the electrical behaviour of the devices.Engineering and Physical Sciences Research Council (EPSRC

Solution-Processed Oxide and Sulfide Semiconductors for Thin-Film Electronics

Department of PhysicsSolution-processing of metal oxides and sulfides is considered as one of the promising electronics technologies due to its simple, cheap, and large-area processability. This thesis covers developing solution-processing techniques for metal oxides and sulfides, and their device applications. Firstly, newly developed sol-gel method for amorphous HfO2, ZrO2, and Ta2O5 is introduced. The processed oxides exhibited high visible transparency and dielectric constant (??), and excellent electrical insulation. Then, they were applied to thin film transistors (TFTs) as high-?? gate dielectrics. The incorporation of solution-processed metal oxides led to the devices exhibiting comparable field-effect mobility and significantly reduced operating voltage (~ 95% reduction) than the devices with a conventional SiO2 gate dielectric. Next chapter is devoted to the introduction of efficient and universal metal sulfide precursors. Simple mixing of alkanethiolates with metal acetates (or isopropoxides) allowed the formation of soluble metal thiolates, which are the resultant precursors for metal sulfides. Decomposition of metal thiolates occurred via SN1 reaction, giving carbon-free metal sulfide films with volatile dialkylsulfide byproducts. It is further found that mixing two (or three) precursors results in ternary (or quaternary) sulfide compounds. Based on this, CdS TFTs and CuInS2 thin film solar cells are fabricated and characterized. Following chapters deal with another solution-processing approach, synthesizing colloidal nanoparticles. To prepare efficient electron transporting layer for polymer solar cells, colloidal ZnO and TiO2 nanoparticles are prepared. By using the nanoparticle solutions, ZnO and TiO2 thin films were successfully deposited on ITO substrates without any possible damages to pre-coated Ag quantum dots on the substrates. It is observed that surface plasmon resonance peaks are strongly affected by the refractive index of surrounding oxide medium. Computer simulation further unveils detailed behavior of incident photons interacting with Ag quantum dots surrounded by solution-processed metal oxide media. Finally, simple and efficient strategy to boost colloidal stability of ZnO nanoparticles are discussed. The addition of a coordination complex, titanium diisopropoxide bis(acetylacetonate) significantly improved colloidal stability of ZnO nanoparticles in methanol, isopropanol, and chlorobenzene. Acetylacetonates on the surface of the nanoparticles effectively reduced the aggregation between the nanoparticles and the electron donation from Ti filled up deep level traps, which results in the reduced trap-assisted radiative recombination. In the device applications, it is confirmed that the colloidal stability of functionalized ZnO nanoparticles is prolonged at least 2 months. Not only comparable material properties of solution-processed oxide and sulfide thin films with those of the films prepared via conventional processing, but additional positive contributions to the device performance show their great potential for thin film electronics technology.clos

Nanoscale Ferroic Materials—Ferroelectric, Piezoelectric, Magnetic, and Multiferroic Materials

Ferroic materials, including ferroelectric, piezoelectric, magnetic, and multiferroic materials, are receiving great scientific attention due to their rich physical properties. They have shown their great advantages in diverse fields of application, such as information storage, sensor/actuator/transducers, energy harvesters/storage, and even environmental pollution control. At present, ferroic nanostructures have been widely acknowledged to advance and improve currently existing electronic devices as well as to develop future ones. This Special Issue covers the characterization of crystal and microstructure, the design and tailoring of ferro/piezo/dielectric, magnetic, and multiferroic properties, and the presentation of related applications. These papers present various kinds of nanomaterials, such as ferroelectric/piezoelectric thin films, dielectric storage thin film, dielectric gate layer, and magnonic metamaterials. These nanomaterials are expected to have applications in ferroelectric non-volatile memory, ferroelectric tunneling junction memory, energy-storage pulsed-power capacitors, metal oxide semiconductor field-effect-transistor devices, humidity sensors, environmental pollutant remediation, and spin-wave devices. The purpose of this Special Issue is to communicate the recent developments in research on nanoscale ferroic materials

Defect Engineering in HfO2/TiN-based Resistive Random Access Memory (RRAM) Devices by Reactive Molecular Beam Epitaxy

Recently, there has been huge interest in emerging memory technologies, spurred by the ever increasing demand for storage capacities in various applications like Internet of Things (IoT), Big Data, etc. CMOS based flash memory, the current mainstay of the memory technology, has been able to increase its density by scaling down to a 16 nm node and further implementation of 3D architectures. However, flash memory is expected to soon run into disadvantage due to challenges in further scaling. Therefore, extensive efforts are being made towards developing new devices for the next generation of non-volatile memories with the combined advantages of flash memory like non-volatility, high density, low cost and low power consumption as well as high speed performance of DRAM. Among the many competitors, resistive random access memories (RRAM) based on resistive switching in oxides are promising due to its simple metal-insulator-metal (MIM) structure, fast switching speeds (<10 ns), excellent scalability (<10 nm) and potential for multi-level switching. RRAM devices based on the popular dielectric-metal gate combination of hafnium oxide (HfO2) and titanium nitride (TiN), which is the subject of research in this work, are particularly interesting due to its compatibility with existing CMOS technology in addition to the aforementioned advantages. Though prototype RRAM chips have already been demonstrated, key problems for commercial realization of RRAM include large variability and insufficient understanding of the complex switching physics. Resistive switching mechanism in oxides is generally understood to be mediated via the transport of oxygen ions leading to the formation of a conductive filament composed of oxygen vacancy defects. Appropriate defect engineering approaches offer potential towards tailoring the switching behavior as well as improving the performance and yield of HfO2-RRAM. In this thesis, the impact of pre-induced defects on the resistive switching behavior of HfO2-RRAM is investigated in detail and our results are presented. Defect engineered oxide thin films were deposited using reactive molecular beam epitaxy (RMBE) to fabricate metal oxide/TiN based devices. RMBE technique offers the unique possibility to precisely and reproducibly control the oxygen stoichiometry of the thin films in a wide range. Using RMBE, defects were introduced in polycrystalline HfOx thin films intrinsically by oxygen stoichiometry engineering and extrinsically via impurity doping (trivalent lanthanum and pentavalent tantalum). Both the studies were performed at at CMOS compatible deposition temperatures (< 450 °C) with an eye on practical applications. Prior to tantalum doping in HfO2, oxygen stoichiometry engineering studies were also performed in amorphous tantalum oxide (TaOx) thin films to identify the oxidation conditions of tantalum metal. The density of oxygen stoichiometry engineered thin films of HfOx and TaOx could be tuned in a wide range from that of the bulk oxide density to close to metallic density. High degree of oxygen deficiency in oxides led to the formation of defect states near the Fermi level as well as multiple oxidation states of the metal, as observed by X-ray photoelectron spectroscopy (XPS). The pure stoichiometric hafnium oxide films crystallize as expected in a stable monoclinic structure (m-HfO2) whereas, oxygen deficient HfOx thin films were found to crystallize in vacancy stabilized tetragonal like structure (t-HfO2-x). Impurity doping also led to the stabilization of higher symmetry tetragonal (t-Ta:HfOx) or cubic structures (c-La:HfOx) depending on the ionic radii of the dopant. The growth of TiN thin films was also investigated using RMBE. The devices used for electrical studies in this work mostly involved deposition of oxides by RMBE on polycrystalline TiN/Si electrodes after ex-situ transfer for further deposition. Therefore, RMBE grown TiN thin film electrodes with similar or better quality would allow in-situ uninterrupted deposition of subsequent oxide layers in future to form cleaner interfaces. Optimized conditions for growth of epitaxial TiN films on the commercially relevant (001) oriented silicon and c-cut sapphire substrates were established, with focus on achieving smooth surfaces and low resistivity. High quality epitaxial TiN(111)||Al2O3(0001) and TiN(001)||Si(001) films with a low resistivity (20-200 uOhm.cm) were achieved, in spite of the large lattice mismatch. Very low surface roughness, characterized by a streaky reflection high energy electron diffraction (RHEED) pattern during TiN film growth was additionally obtained, by tuning the Ti/N flux ratios. Oxygen engineered HfOx/TiN devices were further electrically characterized to obtain I-V characteristics during quasi-static DC switching. Usually, an initial electroforming step (high voltages) is required to obtain further reproducible switching operation (at lower voltages). High device to device variability in RRAM is typically associated with the stochastic nature of electroforming process which increases at higher forming voltages. Using highly oxygen deficient HfOx and TaOx films, the forming voltages were found to be reduced to levels close to operating voltages, paving the way for forming-free devices. However, the use of high defect concentration adds to increasing the complexity of the switching mechanism. This is reflected in the rather complex and dissimilar switching behaviors observed in the myriad of similar RRAM devices reported in the rapidly growing literature. Using model Pt/HfOx/TiN-based device stacks; it is shown that a well-controlled oxygen stoichiometry governs the filament formation and the (partial) occurrence of multiple resistive switching modes (bipolar, unipolar, threshold, complementary). These findings fuel a better fundamental understanding of the underlying phenomena for future theoretical considerations. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. While a bipolar switching occurs in all the devices irrespective of defect concentration, switching modes like unipolar and threshold switching is favored only at higher oxygen stoichiometry. This suggests the suppression of thermal effects via higher heat dissipation and lowered concentration gradient of oxygen vacancies in oxygen deficient devices. A qualitative switching model based on the drift, diffusion and thermophoresis of oxygen ions is suggested to account for the partial occurrence of various switching modes depending on the oxygen stoichiometry. Further, the evolution or drift of high resistance states during endurance test of the common bipolar operation is compared for HfO2 and HfO1.5 based devices and interpreted using the quantum point contact (QPC) model. Similar observations regarding switching modes were also obtained in oxygen engineered Pt/TaOx/TiN devices, therefore allowing the findings to be generalized to other filamentary resistive switching oxides and contributing towards developing a unified switching model. Besides finding application as non-volatile memory, RRAM devices are also promising for hardware implementation of neuromorphic computing. This is motivated by the possibility of multi-level switching or gradual (analog) modulation of resistance in an RRAM device which can emulate biological synapses. Defect engineering approaches have thus been investigated in Pt/hafnium oxide/TiN devices for tuning the DC I-V switching dynamics to achieve multi-level or gradual switching electronic synapses. Higher contribution of thermal effects in pure stoichiometric HfO2 typically results in a single sharp set process and abrupt sharp current jumps during the reset process during a conventional bipolar operation. By using ~18% La-doped HfOx based device, a completely gradual reset behavior with a higher ON/OFF ratio could be achieved during the bipolar reset operation. This is likely related to filament stabilization around the dopant sites allowing a uniform rupture during reset. More interestingly, in oxygen deficient HfO1.5 based devices, intermediate conductance states corresponding to integer or half-integer multiples of quantum conductance (G0) was observed during both the set and reset operations at room temperature. These are related to the better stabilization of intermediate atomic size filament constrictions during the switching process. Occurrence of these intermediate quantum conductance states, especially during the typically abrupt set process, is likely aided by a weaker filament and better thermal dissipation in the highly oxygen deficient devices. These results suggest that a combination of doping and high oxygen vacancy concentration may lead to improved synaptic functionality with concurrent gradual set and reset behaviors

Optoelectronic devices based on van der Waals heterostructures

In this thesis we investigate the use of van der Waals heterostructures in optoelec- tronic devices. An improvement in the optical and electronic performance of specific devices can be made by combining two or more atomically thin materials in layered structures. We demonstrate a heterostructure photodetector formed by combining graphene with tungsten disulphide. These photodetectors were found to be highly sensitive to light due to a gain mechanism that produced over a million electrons per photon. This arises from the favourable electrical properties of graphene and the strong light-matter interaction in WS2 . An analysis of the photodetector per- formance shows that these devices are capable of detecting light under moonlight illuminations levels at video-frame-rate speeds with applications in night vision ima- ging envisaged. We also report a novel method for the direct laser writing of a high-k dielectric embedded inside a van der Waals heterostructure. Such structures were shown to be capable of both light-detection and light-emission within the same de- vice architecture, paving the way for future multifunctional optoelectronic devices. Finally we address a more fundamental problem in the properties of aligned grap- hene/hBN heterostructures. Strain distributions are shown to modify the electronic properties of graphene due to a change in the interlayer interaction. We demon- strates a method to engineer these strain patterns by contact geometry design and thermal annealing strategies.Engineering and Physical Sciences Research Council (EPSRC

Ferroelectric-Domain-Patterning-Controlled Schottky Junction State in Monolayer MoS\u3csub\u3e2\u3c/sub\u3e

We exploit scanning-probe-controlled domain patterning in a ferroelectric top layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS2 between a transistor and a junction state. In the presence of a domain wall, MoS2 exhibits rectified I-V characteristics that are well described by the thermionic emission model. The induced Schottky barrier height ΦeffB varies from 0.38 to 0.57 eV and is tunable by a SiO2 global back gate, while the tuning range of ΦeffB depends sensitively on the conduction-band-tail trapping states. Our work points to a new route to achieving programmable functionalities in van der Waals materials and sheds light on the critical performance limiting factors in these hybrid systems