A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC(000-1) clean surfaces: Significance of the surface Stark effect

Electric field, uniform within the slab, emerging due to Fermi level pinning at its both sides is analyzed using DFT simulations of the SiC surface slabs of different thickness. It is shown that for thicker slab the field is nonuniform and this fact is related to the surface state charge. Using the electron density and potential profiles it is proved that for high precision simulations it is necessary to take into account enough number of the Si-C layers. We show that using 12 diatomic layers leads to satisfactory results. It is also demonstrated that the change of the opposite side slab termination, both by different type of atoms or by their location, can be used to adjust electric field within the slab, creating a tool for simulation of surface properties, depending on the doping in the bulk of semiconductor. Using these simulations it was found that, depending on the electric field, the energy of the surface states changes in a different way than energy of the bulk states. This criterion can be used to distinguish Shockley and Tamm surface states. The electronic properties, i.e. energy and type of surface states of the three clean surfaces: 2H-, 4H-, 6H-SiC(0001), and SiC($000 \bar{1}$) are analyzed and compared using field dependent DFT simulations.Comment: 18 pages, 10 figures, 4 table

Optimization of epitaxial graphene growth for quantum metrology

The electrical quantum standards have played a decisive role in modern metrology, particularly since the introduction of the revised International System of Units (SI) in May 2019. By adapting the basic units to exactly defined natural constants, the quantized Hall resistance (QHR) standards are also given precisely. The Von Klitzing constant RK = h/e2 (h Planck's constant and e elementary charge) can be measured precisely using the quantum Hall effect (QHE) and is thus the primary representation of the ohm. Currently, the QHR standard based on GaAs/AlGaAs heterostructure has succeeded in yielding robust resistance measurements with high accuracy <10−9. In recent years, graphene has been vastly investigated due to its potential in QHR metrology. This single-layer hexagonal carbon crystal forms a two-dimensional electron gas system and exhibits the QHE, due to its properties, even at higher temperatures. Thereby, in the future the QHR standards could be realized in more simplified experimental conditions that can be used at higher temperatures and currents as well as smaller magnetic fields than is feasible in conventional GaAs/AlGaAs QHR. The quality of the graphene is of significant importance to the QHR standards application. The epitaxial graphene growth on silicon carbide (SiC) offers decisive advantages among the known fabrication methods. It enables the production of large-area graphene layers that are already electron-doped and do not have to be transferred to another substrate. However, there are fundamental challenges in epitaxial graphene growth. During the high-temperature growth process, the steps on the SiC surface bunch together and form terraces with high steps. This so-called step-bunching gives rise to the graphene thickness inhomogeneity (e.g., the bilayer formation) and extrinsic resistance anisotropy, which both deteriorate the performance of electronic devices made from it. In this thesis, the process conditions of the epitaxial graphene growth through a so-called polymer-assisted sublimation growth method are minutely investigated. Atomic force microscopy (AFM) is used to show that the previously neglected flow-rate of the argon process gas has a significant influence on the morphology of the SiC substrate and atop carbon layers. The results can be well explained using a simple model for the thermodynamic conditions at the layer adjacent to the surface. The resulting control option of step-bunching on the sub-nanometer scales is used to produce the ultra-flat, monolayer graphene layers without the bilayer inclusions that exhibit the vanishing of the resistance anisotropy. The comparison of four-point and scanning tunneling potentiometry measurements shows that the remaining small anisotropy represents the ultimate limit, which is given solely by the remaining resistances at the SiC terrace steps. Thanks to the advanced growth control, also large-area homogenous quasi-freestanding monolayer and bilayer graphene sheets are fabricated. The Raman spectroscopy and scanning tunneling microscopy reveal very low defect densities of the layers. In addition, the excellent quality of the produced freestanding layers is further evidenced by the four-point measurement showing low extrinsic resistance anisotropy in both micro- and millimeter-scales. The precise control of step-bunching using the Ar flow also enables the preparation of periodic non-identical SiC surfaces under the graphene layer. Based on the work function measurements by Kelvin-Probe force microscopy and X-ray photoemission electron microscopy, it is shown for the first time that there is a doping variation in graphene, induced by a proximity effect of the different near-surface SiC stacks. The comparison of the AFM and low-energy electron microscopy measurements have enabled the exact assignment of the SiC stacks, and the examinations have led to an improved understanding of the surface restructuring in the framework of a step-flow model. The knowledge gained can be further utilized to improve the performance of epitaxial graphene quantum resistance standard, and overall, the graphene-based electronic devices. Finally, the QHR measurements have been shown on the optimized graphene monolayers. In order to operate the graphene-based QHR at desirably low magnetic field ranges (B < 5 T), two known charge tuning techniques are applied, and the results are discussed with a view to their further implementation in the QHR metrology. Keywords: Quantum resistance metrology, epitaxial graphene growth, silicon carbide, resistance anisotropy, argon flow-rate, homogenous quasi-freestanding grapheneElektrische Quantennormale spielen eine wichtige Rolle in der modernen Metrologie, besonders seit der Einführung des revidierten Einheitensystems (SI) im Mai 2019. Durch die Zurückführung der Basiseinheiten auf exakt definierte Naturkonstanten sind auch die quantisierten Werte von Widerstandsnormalen (QHR) exakt gegeben. Die Von-Klitzing-Konstante RK = h/e2 (h Planck-Konstante und e Elementarladung) lässt sich mittels des Quanten-Hall-Effekts (QHE) präzise messen und ist somit die primäre Darstellung des Ohm. Die Quanten-Widerstandsnormale bestehen aktuell aus robusten GaAs/AlGaAs-Heterostrukturen, die eine Genauigkeit <10−9 für die Widerstands-Messung erlauben. In den letzten Jahren wird verstärkt Graphen auf sein Potenzial für die Widerstandmetrologie untersucht. Der einlagige hexagonale Kohlenstoffkristall bildet ebenfalls ein zweidimensionales Elektrongas aus, das den Quanten-Hall-Effekt zeigt – und dies auf Grund seiner Eigenschaften schon bei höheren Temperaturen. Damit könnten in Zukunft Widerstandsnormale für vereinfachte experimentelle Bedingungen realisiert werden, die bei höheren Temperaturen und Strömen oder kleineren Magnetfeldern eingesetzt werden können, als es mit konventionellen GaAs/AlGaAs- QHR möglich ist. Für den Einsatz als Widerstandsnormal ist die Qualität des Graphens von entscheidender Bedeutung. Unter den bekannten Herstellungsmethoden bietet das epitaktische Wachstum von Graphen auf Siliciumcarbid (SiC) entscheidende Vorteile. Es lassen sich damit großflächige Graphenschichten herstellen, die nicht auf ein anderes Substrat übertragen werden müssen. Allerdings gibt es grundlegende Herausforderungen beim epitaktischen Wachstum. So tritt bei hohen Prozesstemperaturen eine Bündelung der Kristallstufen auf der SiC-Substratoberfläche auf (Step-bunching), was zu einer bekannten extrinsischen Widerstandsanisotropie führt und darüber hinaus die Bildung von Bilagen-Graphen begünstigt. Beides verschlechtert die Eigenschaften der daraus hergestellten Widerstandsnormale. In dieser Dissertation werden zunächst die Prozessbedingungen des mittels der sogenannten Polymer-Assisted-Sublimations-Growth-Methode hergestellten epitaktischen Graphens auf SiC genauer untersucht. Mithilfe der Rasterkraft-Mikroskopie (Atomic-Force-Microscopy, AFM) wird gezeigt, dass es einen erheblichen Einfluss der bisher wenig beachteten Flussrate des Prozessgases Argon auf die Morphologie des SiC-Substrates und der oberen Kohlenstoffschichten gibt. Anhand eines einfachen Modells für die thermodynamischen Verhältnisse in einer oberflächennahen Schicht lassen sich die Ergebnisse hervorragend erklären. Die sich daraus ergebende Kontrollmöglichkeit des Step-bunching auf Sub-Nanometer-Skalen wird genutzt, um ultraflache, monolagige Graphenschichten ohne Bilageneinschlüsse herzustellen, die eine verschwindende Widerstandsanisotropie aufweisen. Der Vergleich von Vierpunkt-Messungen und Scanning-Tunneling-Potentiometery-Messungen zeigt, dass die verbleibende geringe Anisotropie das ultimative Limit darstellt, die allein durch die verbleibenden Widerstände an den SiC-Terrassenstufen gegeben ist. Dank der fortschrittlichen Wachstumskontrolle werden auch großflächige, homogene quasi-freistehende Monolage- und Bilage-Graphenschichten hergestellt. Die Raman-Spektroskopie und die Rastertunnel-Mikroskopie zeigen sehr geringe Defektdichten der Schichten. Darüber hinaus wird die hervorragende Qualität der hergestellten quasi-freistehenden Schichten durch die Vierpunkt-Messung unter Beweis gestellt, die eine geringe extrinsische Widerstandsanisotropie zeigt. Die präzise Kontrolle des Step-bunching mittels Ar-Fluss ermöglicht auch die gezielte Präparation von periodischen, nicht-identischen SiC-Oberflächen unter der Graphenlage. Anhand von Messungen der Austrittsarbeit mit Kelvin-Probe-Force-Microscopy und X-ray Photoemission-Electron-Microscopy konnte erstmals gezeigt werden, dass es eine Variation der Graphendotierung, induziert durch einen Proximity Effekt der unterschiedlichen oberflächennahen SiC-Stapel, gibt. Der Vergleich von AFM und Low-Energy-Electron-Microscopy-Messungen ermöglicht die genaue Zuordnung der SiC-Stapel und die Untersuchungen führen insgesamt zu einem verbesserten Verständnis der Oberflächen-Umstrukturierung im Rahmen eines adäquaten Step-Flow-Modells. Die gesammelten Erkenntnisse können zur Verbesserung der Eigenschaften von Graphen-Quantennormalen und auch allgemein von graphenbasierten Bauteilen genutzt werden. Abschließend werden QH-Widerstandsmessungen an optimierten Graphen-Monolagen gezeigt. Um den Magnetfeldbereich (B < 5 T) einzuschränken, werden zwei bekannte extrinsische Dotiertechniken verwendet und die Ergebnisse werden im Hinblick auf den weiteren Einsatz in der QH-Metrologie diskutiert. Schlüsselwörter: Wachstum des epitaktischen Graphens, Siliciumcarbid, Argon-Flussrate, Widerstandsanisotropie, homogenes quasi-freistehendes Graphe

Diamond Schottky barrier diodes

Research on wide band gap semiconductors suitable for power electronic devices has spread rapidly in the last decade. The remarkable results exhibited by silicon carbide (SiC) Schottky batTier diodes (SBDs), commercially available since 2001, showed the potential of wide band gap semiconductors for replacing silicon (Si) in the range of medium to high voltage applications, where high frequency operation is required. With superior physical and electrical properties, diamond became a potential competitor to SiC soon after Element Six reported in 2002 the successful synthesis of single crystal plasma deposited diamond with high catTier mobility. This thesis discusses the remarkable properties of diamond and introduces several device structures suitable for power electronics. The calculation of several figures of merit emphasize the advantages of diamond with respect to silicon and other wide band gap semiconductors and clearly identifies the areas where its impact would be most significant. Information regarding the first synthesis of diamond, which took place back in 1954, together with data regarding the modern technological process which leads nowadays to high-quality diamond crystals suitable for electronic devices, are reviewed. Models regarding the incomplete ionization of atomic dopants and the variation of catTier mobility with doping level and temperature have been elaborated and included in numerical simulators. The study introduces the novel diamond M-i-P Schottky diode, a version of power Schottky diode which takes advantage of the extremely high intrinsic hole mobility. The structure overcomes the drawback induced by the high activation energies of acceptor dopants in diamond which yield poor hole concentration at room temperature. The complex shape of the on-state characteristic exhibited by diamond M-i-P Schottky structures is thoroughly investigated by means of experimental results, numerical simulations and theoretical considerations. The fabrication of a ramp oxide termination on a diamond device is for the first time reported in this thesis. Both experimental and simulated results show the potential of this termination structure, previously built on Si and SiC power devices. A comprehensive comparison between the ramp oxide and two other versions of the field plate termination concept, the single step and the three-step structures, has been performed, considering aspects such as electrical performance, occupied area, complexity of technological process and cost. Based on experimental results presented in this study, together with predictions made via simulations and theoretical models, it is concluded that diamond M-i-P Schottky diodes have the ability to deliver significantly higher performance compared to that of SiC SBDs if issues such as material defects, Schottky contact formation and measurement of reliable ionization coefficients are carefully addressed in the near future

Development of 4H-SiC power MOSFETs for high voltage applications

Silicon carbide is a promising wide bandgap semiconductor for high-power, high-temperature and high frequency devices, owing to its high breakdown electric field strength, high thermal conductivity and ability to grow high quality SiO2 layers by thermal oxidation. Although the SiC power MOSFET (metal-oxide-semiconductor field effect transistor) is preferred as a power switch, it has suffered from low channel mobility with only single digit field effect mobility achieved using standard oxidation process (1200◦C thermal oxidation). As such, this thesis is focussed on the development of 4H-SiC MOSFETs (both lateral and vertical MOSFETs) to improve the channel mobility and breakdown characteristics of these devices. In this work, high temperature nitridation using N2O has been investigated on MOS capacitors and MOSFETs, both with gate oxides grown directly in N2O environment or in a O2 ambient followed by a N2O post-oxidation annealing process. Results have demonstrated that at high temperature (>1200◦C) there is a significant improvement in the interface trap density to as low as (1.5x10^11cm-2eV-1) and field effect channel mobility (19cm2/V.s) of 4H-SiC MOSFET compare with a lower temperature (between 800 and 1200◦C) oxidation (1x10^12cm-2eV-1 and 4cm2/V.s). Nitridation temperatures of 1300◦C was found to be the most effective method for increasing the field effect channel mobility and reducing threshold voltage. The number of working devices per sample also increased after N2O nitridation at 1300◦C as observed for both lateral and vertical MOSFETs. Other post oxidation techniques have also been investigated such as phosphorous passivation using solid SiP2O7 planar diffusion source (PDS). The peak value of the field effect mobility for 4H-SiC MOSFET after phosphorus passivation is approximately 80cm2/V.s, which is four times more than the valued obtained using high temperature N2O annealing. Different JTE structures have been designed and simulated including single-zone JTE, space modulated JTE (SMJTE) and the novel two-step mesa JTE structures. It was found that for the same doping concentration the SM two-zone JTE and SMJTE have higher breakdown voltage than the single zone JTE. With SMJTE, the device could achieve more than 90% of the ideal parallel plane voltage from simulations and 86% from the breakdown test of the fabricated devices

Evaluation of 4h-Sic Photoconductive Switches for Pulsed Power Applications Based on Numerical Simulations

Since the early studies by Auston, photoconductive semiconductor switches (PCSSs) have been investigated intensively for many applications owing to their unique advantages over conventional gas and mechanical switches. These advantages include high speeds, fast rise times, optical isolation, compact geometry, and negligible jitter. Another important requirement is the ability to operate at high repetition rates with long device lifetimes (i.e., good reliability without degradation). Photoconductive semiconductor switches (PCSSs) are low-jitter compact alternatives to traditional gas switches in pulsed power systems. The physical properties of Silicon Carbide (SiC), such as a large bandgap (3.1-3.35 eV), high avalanche breakdown field (~3 MV/cm), and large thermal conductivity (4-5 W/cm-K) with superior radiation hardness and resistance to chemical attack, make SiC an attractive candidate for high voltage, high temperature, and high power device applications. A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC was carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, were comprehensively included. Results of our model matched the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters were also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples could be discounted up to applied fields as high as 275 kV/cm. In addition, calculations of electric field distributions in a SiC photoconductive semiconductor switch structure with metal contacts employing contact extensions on a high-k HfO2 dielectric were carried out, with the goal of assessing reductions in the peak electric fields. For completeness, analysis of thermal heating in a lateral PCSS structure with such modified geometries after photoexcitation was also included. The simulation results of the electric field distribution show that peak electric fields, and hence the potential for device failure, can be mitigated by these strategies. A combination of the two approaches was shown to produce up to a ~67% reduction in peak fields. The reduced values were well below the threshold for breakdown in SiC material using biasing close to experimental reports. The field mitigation was shown to depend on the length of the metal overhang. Further, the calculations show that, upon field mitigation, the internal temperature rise would also be controlled. A maximum value of 980 K was obtained here for an 8 ns electrical pulse at a 20 kV external bias, which is well below the limits for generating local stress or cracks or defects

Design and fabrication of high voltage 4H-SiC Schottky barrier diodes

A novel design of mesa-etch termination and Superjunction JBS diode structure has been proposed and optimized. The new mesa-etch termination can achieve over 90% of ideal maximal breakdown voltage within a wide sidewall implant dose window (~9e16 cm⁻³). Besides the high tolerance on implant dose, the proposed design also exhibits high tolerance on the etch sidewall angle: minimal maximum breakdown voltage was observed with etch sidewall angle variations. The Superjunction JBS diode can obtain both 96.4% maximum super junction breakdown voltage and 76.6% JBS Schottky surface electric field reduction. The super junction maximal breakdown voltage is 1.5 times large as the conventional Schottky diode breakdown voltage and the leakage current is logarithmically related to the surface electric field. The superior breakdown voltage represents a large improvement on the power rectifier performance. Based on these structure improvements, vertical 4H-SiC Schottky Diodes have been fabricated and tested. Vertical 4H-SiC Schottky Diode without any edge termination has a breakdown voltage as large as 692 V and exhibits an on-state specific resistance as small as 7.9 mΩ*cm². Such breakdown voltage is much higher than simulation results. In the meantime, on-state resistance is also much larger than the simulation results. The mechanism for these improved power rectifier performances will be furthered investigated in future studies.Electrical and Computer Engineerin

Diamond power devices: State of the art, modelling and figures of merit

With its remarkable electro-thermal properties such as the highest known thermal conductivity (~22W/cmbold dotK at room temperature) of any material, high hole mobility (> 2000cm2/Vbold dots), high critical electric field (>10MV/cm), and large bandgap (5.47eV), diamond has overwhelming advantages over silicon and other wide bandgap semiconductors (WBG) for ultra-high- voltage and high temperature applications (>3kV and >450 K, respectively). However, despite their tremendous potential, fabricated devices based on this material have not delivered yet the expected high-performance. The main reason behind this is the absence of shallow donor and acceptor species. The second reason is the lack of consistent physical models and design approaches specific to diamond-based devices that could significantly accelerate their development. The third reason is that the best performances of diamond devices are expected only when the highest electric field in reverse bias can be achieved, something that has not been widely obtained yet. In this context, high temperature operation and unique device structures based on the 2DHG formation represent two alternatives which could alleviate the issue of the incomplete ionization of dopant species. Nevertheless, ultra-high temperature operations and device parallelization could result in severe thermal management issues and affect the overall stability and long-term reliability. Additionally, problems connected to the reproducibility and the long-term stability of 2DHG based-devices still need to be resolved. This review paper aims at addressing these issues by providing the power device research community with a detailed set of physical models, device designs and challenges associated to all the aspects of the diamond power device value chain, from the definition of figures of merits, the material growth and processing conditions, to packaging solutions and targeted applications. Finally, the paper will conclude with suggestions on how to design power converters with diamond devices and will provide the roadmap of diamond devices development for power electronics.This work was supported by the U.K. Engineering and Physical Sciences Research Council for the University of Cambridge Centre for Doctoral Training under Grant EP/M506485/1 and by the French ANR Research Agency under grant ANR-16-CE05-0023 #Diamond-HVDC. The research leading to these results has been performed within the GREENDIAMOND project and received funding from the European Community's Horizon 2020 Programme (H2020/2014–2020) under grant agreement no. 640947

Diamond power devices: state of the art, modelling, figures of merit and future perspective

Abstract: With its remarkable electro-thermal properties such as the highest known thermal conductivity (~22 W cm−1∙K−1 at RT of any material, high hole mobility (>2000 cm2 V−1 s−1), high critical electric field (>10 MV cm−1), and large band gap (5.47 eV), diamond has overwhelming advantages over silicon and other wide bandgap semiconductors (WBGs) for ultra-high-voltage and high-temperature (HT) applications (>3 kV and >450 K, respectively). However, despite their tremendous potential, fabricated devices based on this material have not yet delivered the expected high performance. The main reason behind this is the absence of shallow donor and acceptor species. The second reason is the lack of consistent physical models and design approaches specific to diamond-based devices that could significantly accelerate their development. The third reason is that the best performances of diamond devices are expected only when the highest electric field in reverse bias can be achieved, something that has not been widely obtained yet. In this context, HT operation and unique device structures based on the two-dimensional hole gas (2DHG) formation represent two alternatives that could alleviate the issue of the incomplete ionization of dopant species. Nevertheless, ultra-HT operations and device parallelization could result in severe thermal management issues and affect the overall stability and long-term reliability. In addition, problems connected to the reproducibility and long-term stability of 2DHG-based devices still need to be resolved. This review paper aims at addressing these issues by providing the power device research community with a detailed set of physical models, device designs and challenges associated with all the aspects of the diamond power device value chain, from the definition of figures of merit, the material growth and processing conditions, to packaging solutions and targeted applications. Finally, the paper will conclude with suggestions on how to design power converters with diamond devices and will provide the roadmap of diamond device development for power electronics