A new method for estimating the conductive filament temperature in OxRAM devices based on escape rate theory

Because of the atomic nature of the system under study, an estimation of the temperature of the conductive filament (CF) in OxRAM devices as a function of the applied bias can only be obtained by means of indirect methods, usually electrothermal simulations. In this paper, a heuristic approach that combines time-dependent dielectric breakdown (TDDB) statistics for the electroformed device with field and temperature-assisted ionic transport within the framework of escape rate theory is presented. Extended expressions for the time-to-failure acceleration law (E-model) and for the Kramers' rate compatible both with the standard models at moderate/high biases and with the principle of detailed balance at equilibrium are proposed. An approximate expression for the CF temperature is reported. For the investigated stress voltage range (0.30 V-0.65 V), the estimated CF temperature at the SET condition is found to be in the range 350 K-600 K

Substoichiometric Phases of Hafnium Oxide with Semiconducting Properties

Since the dawn of the information age, all developments that provided a significant improvement in information processing and data transmission have been considered as key technologies. The impact of ever new data processing innovations on the economy and almost all areas of our daily lives is unprecedented and a departure from this trend is unimaginable in the near future. Even though the end of Moore's Law has been predicted all too often, the steady exponential growth of computing capacity remains unaffected to this day, due to tremendous commercial pressure. While the minimum physical size of the transistor architecture is a serious constraint, the steady evolution of computing effectiveness is not limited in the predictable future. However, the focus of development will have to expand more strongly to other technological aspects of information processing. For example, the development of new computer paradigms which mark a departure from the digitally dominated van Neumann architecture will play an increasingly significant role. The category of so-called next-generation non-volatile memory technologies, based on various physical principles such as phase transformation, magnetic or ferroelectric properties or ion diffusion, could play a central role here. These memory technologies promise in part strongly pronounced multi-bit properties up to quasi-analog switching behavior. These attributes are of fundamental importance especially for new promising concepts of information processing like in-memory computing and neuromorphic processing. In addition, many next-generation non-volatile memory technologies already show advantages over conventional media such as Flash memory. For example, their application promises significantly reduced energy consumption and their write and especially read speeds are in some cases far superior to conventional technology and could therefore already contribute significant technological improvements to the existing memory hierarchy. However, these alternative concepts are currently still limited in terms of their statistical reliability, among other things. Even though phase change memory in the form of the 3D XPoint, for example, has already been commercialized, the developments have not yet been able to compete due to the enormous commercial pressure in Flash memory research. Nevertheless, the further development of alternative concepts for the next and beyond memory generations is essential and the in-depth research on next-generation non-volatile memory technologies is therefore a hot and extremely important scientific topic. This work focuses on hafnium oxide, a key material in next-generation non-volatile memory research. Hafnium oxide is very well known in the semiconductor industry, as it generated a lot of attention in the course of high-k research due to its excellent dielectric properties and established CMOS compatibility. However, since the growing interest in so-called memristive memory, research efforts have primarily focused on the value of hafnium oxide in the form of resistive random-access memory (RRAM) and, with the discovery of ferroelectricity in HfO₂, ferroelectric resistive random-access memory (FeRAM). RRAM is a next-generation non-volatile memory technology that features a simple metal-insulator-metal (MIM) structure, excellent scalability, and potential 3D integration. In particular, the aforementioned gradual to quasi-continuous switching behavior has been demonstrated on a variety of RRAM systems. A significant change of the switching properties is achievable, for example, by the choice of top and bottom electrodes, the introduction of doping elements, or by designated oxygen deficiency. In particular, the last point is based on the basic physical principle of the hafnium oxide-based RRAM mechanism, in which local oxygen ions are stimulated to diffuse by applying an electrical potential, and a so-called conducting filament is formed by the remaining vacancies, which electrically connects the two electrode sides. The process is characterized by the reversibility of the conducting filament which can be dissolved by a suitable I-V programming (e.g., reversal of the voltage direction). In the literature there are some predictions of sub-stoichiometric hafnium oxide phases, such as Hf₂O₃, HfO or Hf₆O, which could be considered as conducting filament phases, but there is a lack of conclusive experimental results. While there are studies that assign supposed structures in oxygen-deficient hafnium oxide thin films, these assignments are mostly based on references from various stoichiometric hafnium oxide high-temperature phases such as tetragonal t-HfO₂ (P4₂/nmc) or cubic c-HfO₂ (Fm-3m), or high-pressure phases such as orthorhombic o-HfO₂ (Pbca). Furthermore, the structural identification of such thin films proves to be difficult, as they are susceptible to arbitrary texturing and reflection broadening in X-ray diffraction. In addition, such thin films are usually synthesized as phase mixtures with monoclinic hafnium oxide. A further challenge in property determination is given by their usual arrangement in MIM configuration, which is determined by the quality of top and bottom electrodes and their interfaces to the active material. It is therefore a non-trivial task to draw conclusions on individual material properties such as electrical conductivity in such (e.g., oxygen-deficient) RRAM devices. To answer these open questions, this work is primarily devoted to material properties of oxygen-deficient hafnium oxide phases. Therefore, in the first comprehensive study of this work, Molecular-Beam Epitaxy (MBE) was used to synthesize hafnium oxide phases over a wide oxidation range from monoclinic to hexagonal hafnium oxide. The hafnium oxide films were deposited on c-cut sapphire to achieve effective phase selection and identification by epitaxial growth, taking into account the position of relative lattice planes. In addition, the choice of a substrate with a high band gap and optical transparency enabled the direct investigation of both optical and electrical properties by means of UV/Vis transmission spectroscopy and Hall effect measurements. With additional measurements via X-ray diffraction (XRD), X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), the oxygen content-dependent changes in crystal as well as band structure could be correlated with electrical properties. Based on these results, a comprehensive band structure model over the entire oxidation range from insulating HfO₂ to metallic Hf was established, highlighting the discovered intermediate key structures of rhombohedral r-HfO₁.₇ and hexagonal hcp-HfO₀.₇. In the second topic of this work, the phase transition from stoichiometric monoclinic to oxygen-deficient rhombohedral hafnium oxide was complemented by DFT calculations in collaboration with the theory group of Prof. Valentí (Frankfurt am Main). A detailed comparison between experimental results and DFT calculations confirms previously assumed mechanisms for phase stabilization. In addition, the comparison shows a remarkable agreement between experimental and theoretical results on the crystal- and band stucture. The calculations allowed to predict the positions of oxygen ions in oxygen-deficient hafnium oxide as well as the associated space group. Also, the investigations provide information on the thermodynamic stability of the corresponding phases. Finally, the orbital-resolved hybridization of valence states influenced by oxygen vacancies is discussed. Another experimental study deals with the reproduction and investigation, of the aforementioned substoichiometric hafnium oxide phases in MIM configuration which is typical for RRAM devices. Special attention was given to the influence of surface oxidation effects. Here, it was found that the oxygen-deficient phases r-HfO₁.₇ and hcp-HfO₀.₇ exhibit high ohmic conductivity as expected, but stable bipolar switching behavior as a result of oxidation in air. Here, the mechanism of this behavior was discussed and the role of the r-HfO₁.₇ and hcp-HfO₀.₇ phases as novel electrode materials in hafnium oxide-based RRAM in particular. In collaboration with the electron microscopy group of Prof. Molina Luna, the studied phases, which have been characterized by rather macroscopic techniques so far, have been analyzed by wide-ranging TEM methodology. The strong oxygen deficiency in combination with the verified electrical conductivity of r-HfO₁.₇ and hcp-HfO₀.₇ shows the importance of the identification of these phases on the nanoscale. Such abilities are essential for the planned characterization of the "conducting-filament" mechanism. Here, the ability to distinguish m-HfO₂, r-HfO₁.₇, and hcp-HfO₀.₇ using high-resolution transmission electron microscopy (HRTEM), Automated Crystal Orientation and Phase Mapping (ACOM), and Electron Energy Loss Spectroscopy (EELS), is demonstrated and the necessity of combined measurements for reliable phase identification was discussed. Finally, a series of monoclinic to rhombohedral hafnium oxide was investigated in a cooperative study with FZ Jülich using scanning probe microscopy. Since recent studies in particular highlight the significance of the microstructure in stoichiometric hafnium oxide-based RRAM, the topological microstructure in the region of the phase transition to strongly oxygen deficient rhombohedral hafnium oxide was investigated. Special attention was given to the correlation of microstructure and conductivity. In particular, the influences of grain boundaries on electrical properties were discussed. In summary, this work provides comprehensive insights into the nature and properties of sub-stoichiometric hafnium oxide phases and their implications on the research of hafnium oxide-based RRAM technology. Taking into account a wide range of scientific perspectives, both, the validity of obtained results and the wide range of their application is demonstrated. Thus, this dissertation provides a detailed scientific base to the understanding of hafnium oxide-based electronics

Carrier transport engineering in wide bandgap semiconductors for photonic and memory device applications

Wide bandgap (WBG) semiconductors play a crucial role in the current solid-state lighting technology. The AlGaN compound semiconductor is widely used for ultraviolet (UV) light-emitting diodes (LEDs), however, the efficiency of these LEDs is largely in a single-digit percentage range due to several factors. Until recently, AlInN alloy has been relatively unexplored, though it holds potential for light-emitters operating in the visible and UV regions. In this dissertation, the first axial AlInN core-shell nanowire UV LEDs operating in the UV-A and UV-B regions with an internal quantum efficiency (IQE) of 52% are demonstrated. Moreover, the light extraction efficiency of this UV LED can be further improved by 63% by utilizing appropriate hexagonal photonic crystal structures. The carrier transport characteristics of the LEDs have been carefully engineered to enhance the carrier distributions and reduce the current leakage, leading to a significantly improved IQE of the LEDs. In this regard, the p-type AlGaN electron blocking layer (EBL) has been utilized to suppress electron leakage. Although the EBL can suppress the electron leakage to an extent, it also affects the hole injection due to the generation of positive polarization sheet charges at the hetero interface of EBL and the last quantum barrier (QB). Moreover, the Mg acceptor activation energy of the Al-rich AlGaN EBL layer is elevated, affecting the Mg doping efficiency. To mitigate this problem, in this dissertation, EBL-free UV LED designs are proposed where the epilayers are carefully band-engineered to notably improve the device performance by lowering the electron overflows. The proposed EBL-free strip-in-a-barrier UV LED records the maximum IQE of -61.5% which is -72% higher, and IQE droop is -12.4%, which is -333% less compared to the conventional AlGaN EBL LED structure at 284.5 nm wavelength. Moreover, it is shown that the EBL-free AlGaN deep UV LED structure with linearly graded polarization-controlled QBs instead of conventional QBs in the active region could drastically reduce the electrostatic field in the quantum well (QW) region due to the decreased lattice mismatch between the QW and the QB. The carrier transport in the EBL-free deep UV LEDs is significantly improved, attributed to the increased radiative recombination, quantum efficiency, and output power compared to the conventional EBL LEDs. Overall, the study of EBL-free UV LEDs offers important insights into designing novel, high-performance deep UV LEDs for practical applications. Further, it is demonstrated that novel WBG materials could be perfectly employed for emerging non-volatile memory (resistive random access memory, RRAM) applications. The resistive switching (RS) capability has been observed in Ga2O3 at low power operation. Importantly, for the first time, the multi-bit storage capability of this types of RRAM devices with a reasonably high Roff/Ron ratio is experimentally demonstrated. In addition, integrating a thin SiNx layer in the conventional SiO2 RRAM device could effectively facilitate the formation of a conducting filament. It is reported that the proposed RRAM device exhibits excellent RS characteristics, such as highly uniform current-voltage characteristics with concentrated SET and RESET voltages, excellent stability, and high Roff/Ron (\u3e 103) even at ultra-low current (10 nA) operation. The multi-bit RS behavior has been observed in these RRAM devices, which pave the way for low-power and high-density data storage applications

Chalcogenide and metal-oxide memristive devices for advanced neuromorphic computing

Energy-intensive artificial intelligence (AI) is prevailing and changing the world, which requires energy-efficient computing technology. However, traditional AI driven by von Neumann computing systems suffers from the penalties of high-energy consumption and time delay due to frequent data shuttling. To tackle the issue, brain-inspired neuromorphic computing that performs data processing in memory is developed, reducing energy consumption and processing time. Particularly, some advanced neuromorphic systems perceive environmental variations and internalize sensory signals for localized in-senor computing. This methodology can further improve data processing efficiency and develop multifunctional AI products. Memristive devices are one of the promising candidates for neuromorphic systems due to their non-volatility, small size, fast speed, low-energy consumption, etc. In this thesis, memristive devices based on chalcogenide and metal-oxide materials are fabricated for neuromorphic computing systems. Firstly, a versatile memristive device (Ag/CuInSe2/Mo) is demonstrated based on filamentary switching. Non-volatile and volatile features are coexistent, which play multiple roles of non-volatile memory, selectors, artificial neurons, and artificial synapses. The conductive filaments’ lifetime was controlled to present both volatile and non-volatile behaviours. Secondly, the sensing functions (temperature and humidity) are explored based on Ag conductive filaments. An intelligent matter (Ag/Cu(In, Ga)Se2/Mo) endowing reconfigurable temperature and humidity sensations is developed for sensory neuromorphic systems. The device reversibly switches between two states with differentiable semiconductive and metallic features, demonstrating different responses to temperature and humidity variations. Integrated devices can be employed for intelligent electronic skin and in-sensor computing. Thirdly, the memristive-based sensing function of light was investigated. An optoelectronic synapse (ITO/ZnO/MoO3/Mo) enabling multi-spectrum sensitivity for machine vision systems is developed. For the first time, this optoelectronic synapse is practical for front-end retinomorphic image sensing, convolution processing, and back-end neuromorphic computing. This thesis will benefit the development of advanced neuromorphic systems pushing forward AI technology

A new method for estimating the conductive filament temperature in OxRAM devices based on escape rate theory

Because of the atomic nature of the system under study, an estimation of the temperature of the conductive filament (CF) in OxRAM devices as a function of the applied bias can only be obtained by means of indirect methods, usually electrothermal simulations. In this paper, a heuristic approach that combines time-dependent dielectric breakdown (TDDB) statistics for the electroformed device with field and temperature-assisted ionic transport within the framework of escape rate theory is presented. Extended expressions for the time-to-failure acceleration law (E-model) and for the Kramers' rate compatible both with the standard models at moderate/high biases and with the principle of detailed balance at equilibrium are proposed. An approximate expression for the CF temperature is reported. For the investigated stress voltage range (0.30 V-0.65 V), the estimated CF temperature at the SET condition is found to be in the range 350 K-600 K