A STUDY ON ATOMICALLY THIN ULTRA SHORT CONDUCTING CHANNELS, BREAKDOWN, AND ENVIRONMENTAL EFFECTS

We have developed a novel method of producing ultra-short channel graphene field effect devices on SiO2 substrates and have studied their electrical transport properties. A nonlinear current behavior is observed coupled with a quasi-saturation effect. An analytical model is developed to explain this behavior using ballistic transport, where the charge carriers experience minimal scattering. We also observe multilevel resistive switching after the device is electrically stressed. In addition, we have studied the evolution of the electrical transport properties of few-layer graphene during electrical breakdown. We are able to significantly increase the time scale of break junction formation, and we are able to observe changes occurring close to breakdown regime. A decrease in conductivity along with p−type doping of the graphene channel is observed as the device is broken. The addition of structural defects generated by thermal stress caused by high current densities is attributed to the observed evolution of electrical properties during the process of breakdown. We have also studied the effects of the local environment on graphene devices. We encapsulate graphene with poly(methyl methacrylate) (PMMA) polymer and study the electrical transport through in situ measurements. We have observed an overall decrease in doping level after low-temperature annealing in dry-nitrogen, indicating that the solvent in the polymer plays an important role in doping. For few-layer encapsulated graphene devices, we observe stable n−doping. Applying the solvent onto encapsulated devices demonstrates enhanced hysteretic switching between p and n−doped states

Modelling of field-effect transistors based on 2D materials targeting high-frequency applications

New technologies are necessary for the unprecedented expansion of connectivity and communications in the modern technological society. The specific needs of wireless communication systems in 5G and beyond, as well as devices for the future deployment of Internet of Things has caused that the International Technology Roadmap for Semiconductors, which is the strategic planning document of the semiconductor industry, considered since 2011, graphene and related materials (GRMs) as promising candidates for the future of electronics. Graphene, a one-atom-thick of carbon, is a promising material for high-frequency applications due to its intrinsic superior carrier mobility and very high saturation velocity. These exceptional carrier transport properties suggest that GRM-based field-effect transistors could potentially outperform other technologies. This thesis presents a body of work on the modelling, performance prediction and simulation of GRM-based field-effect transistors and circuits. The main goal of this work is to provide models and tools to ease the following issues: (i) gaining technological control of single layer and bilayer graphene devices and, more generally, devices based on 2D materials, (ii) assessment of radio-frequency (RF) performance and microwave stability, (iii) benchmarking against other existing technologies, (iv) providing guidance for device and circuit design, (v) simulation of circuits formed by GRM-based transistors.Comment: Thesis, 164 pages, http://hdl.handle.net/10803/40531

Two-Dimensional Electronics - Prospects and Challenges

During the past 10 years, two-dimensional materials have found incredible attention in the scientific community. The first two-dimensional material studied in detail was graphene, and many groups explored its potential for electronic applications. Meanwhile, researchers have extended their work to two-dimensional materials beyond graphene. At present, several hundred of these materials are known and part of them is considered to be useful for electronic applications. Rapid progress has been made in research concerning two-dimensional electronics, and a variety of transistors of different two-dimensional materials, including graphene, transition metal dichalcogenides, e.g., MoS2 and WS2, and phosphorene, have been reported. Other areas where two-dimensional materials are considered promising are sensors, transparent electrodes, or displays, to name just a few. This Special Issue of Electronics is devoted to all aspects of two-dimensional materials for electronic applications, including material preparation and analysis, device fabrication and characterization, device physics, modeling and simulation, and circuits. The devices of interest include, but are not limited to transistors (both field-effect transistors and alternative transistor concepts), sensors, optoelectronics devices, MEMS and NEMS, and displays

Chemical and biological sensors based on van der Waals heterostructures of graphene and carbon nanomembrane

In this thesis, single-layer graphene (SLG)-based field-effect devices were built to realize two different sensors: a pH sensor and a sensor measuring the concentration of the chemokine CXCL8 in clinical samples. CXCL8 is a signal protein that has various promising new applications in the field of disease diagnostics.20 For building these devices, a recently proposed new functionalization approach for graphene was employed.78 The approach is based on the assembly of an all-carbon van der Waals (vdW) heterostructure of carbon nanomembrane (CNM) and SLG. The effect of different substrates including SiO2, poly(ethylene 2,6-naphthalate) (PEN) and SiC and different types of SLG including graphene grown by chemical vapor deposition (CVD), epitaxial graphene, and nanoporous graphene (NPSLG) on the sensor response was investigated. Devices of increasing complexity were designed and investigated. At first, devices for measurements in vacuum. As a next step, devices for measurements of the pH value, and as final step the devices for biosensing. The fabrication of devices included their successive optimization based on transport measurements, electrical impedance spectroscopy (EIS) measurements, and surface sensitive characterization techniques. The device concept was the solution-gated field-effect transistor (SGFET),33 which has promising applications for point-of-care devices247 and lab-onchip technology

Graphene field-effect devices at high frequencies

In this work, the high frequency response of several types of graphene field-effect transistors (GFET) is analyzed. In the first part, insulating substrates such as sapphire and hexagonal boron nitride are used to optimize device performance. In the second part, few-layer graphene is used as gate material to obtain ultra-thin GFET. Using large area CVD-grown graphene, an array of similar GFETs for improved device comparability and reproducibility is presented in the last part

Graphene for Electronics

Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional (2D) honeycomb lattice. Graphene's unique properties of thinness and conductivity have led to global research into its applications as a semiconductor. With the ability to well conduct electricity at room temperature, graphene semiconductors could easily be implemented into the existing semiconductor technologies and, in some cases, successfully compete with the traditional ones, such as silicon. This reprint presents very recent results in the physics of graphene, which can be important for applying the material in electronics

Linear optical characterization of graphene structure

Graphene as a newly developed 2D material has attracted a lot of attention for its promising applications in optoelectronic fields. To pursue a profound understanding of its optical properties, this thesis presents the optical refractive index in response to the infrared incidents, and its modulations under external electric field. We tested the optical reflection response of monolayer graphene on an SiO2/Si substrate at 1550nm laser incident. The derived value of the graphene optical refractive index was: 2.75-1.56i at 1550nm, which made up for the deficiency of graphene optical properties in the infrared region. We also compared the results for the current work with studies in the visible spectrum, and we provide a value range for graphene RI, which can be used to estimate the monolayer graphene optical response to different incidents and substrates. Finally, we checked the graphene optical reflection changes in response to an external electric field using a top-gated graphene samples at 1064nm incident. We found that the tunability of complex refractive index of graphene verified according to gate voltage. Additionally, through comparison with other experimental work, we have found the optical refractive index trends are similar in infrared range

Theory, Modelling and Implementation of Graphene Field-Effect Transistor

PhDTwo-dimensional materials with atomic thickness have attracted a lot of attention from researchers worldwide due to their excellent electronic and optical properties. As the silicon technology is approaching its limit, graphene with ultrahigh carrier mobility and ultralow resistivity shows the potential as channel material for novel high speed transistor beyond silicon. This thesis summarises my Ph.D. work including the theory and modelling of graphene field-effect transistors (GFETs) as well as their potential RF applications. The introduction and review of existing graphene transistors are presented. Multiscale modelling approaches for graphene devices are also introduced. A novel analytical GFET model based on the drift-diffusion transport theory is then developed for RF/microwave circuit analysis. Since the electrons and holes have different mobility variations against the channel potential in graphene, the ambipolar GFET cannot be modelled with constant carrier mobility. A new carrier mobility function, which enables the accurate modelling of the ambipolar property of GFET, is hence developed for this purpose. The new model takes into account the carrier mobility variation against the bias voltage as well as the mobility difference between electrons and holes. It is proved to be more accurate for the DC current calculation. The model has been written in Verilog-A language and can be import into commercial software such as Keysight ADS for circuit simulation. In addition, based on the proposed model two GFET non-Foster circuits (NFCs) are conducted. As a negative impedance element, NFCs find their applications in impedance matching of electrically small antennas and bandwidth improvement of metasurfaces. One of the NFCs studied in this thesis is based on the Linvill's technique in which a pair of identical GFETs is used while the other circuit utilises the negative resistance of a single GFET. The stability analysis of NFCs is also presented. Finally, a high impedance surface loaded with proposed NFCs is also studied, demonstrating significant bandwidth enhancement.Engineering and Physical Sciences Research Council (EPSRC) Grant on `Graphene Flexible Electronics and Optoelectronics' (EP/K01711X/1), the EU Graphene Flagship (FP7-ICT-604391) and Graphene Core 1 (H2020 696656

グラフェン電界効果トランジスタに関する研究―溶液法によるゲート絶縁膜形成とUVオゾン法による接触抵抗―

Tohoku University末光眞希課

Epitaxial Graphene and its Electronic Device Applications

Department of Electrical EngineeringGraphene is a two-dimensional material in which carbon atoms are bonded in honeycomb lattice. It has a unique electronic band structure that shows zero band gap energy and linear dispersion relation near the Dirac point. Because of to its outstanding electrical and mechanical properties, graphene has been actively studied in various fields. After successfully separating graphene from highly oriented pyrolytic graphite (HOPG), a variety of methods for obtaining high quality and large area graphene have been studied. Especially, a method of growing epitaxial graphene (EG) on a SiC substrate has attracted much attention as a material for next generation electronic devices. This allows the growth of large area graphene and it is not necessary for transfer process because semi-insulating SiC wafer can be used as a substrate. However, it has disadvantages of requiring high temperature (> 1300 ??C) for high quality EG growth and the single crystalline SiC substrate are too expensive. In order to overcome these problem, we propose two effective methods for growth of EG on SiC. Firstly, the high quality EG is grown on 4H-, 6H SiC substrate by molybdenum plate (Mo-plate) capping during annealing process. Mo-plate capping causes the heat accumulation on SiC surface by preventing loss of thermal radiations from SiC surface, and increase the Si vapor pressure on SiC surface by enclosing the sublimated Si atoms. Therefore, the temperature of the SiC surface becomes higher than surrounding temperature, and the Si sublimation rate is reduced. These factors enable high quality EG growth at relatively low power assumption (chamber temperature). The quality enhancement of the grown EG with Mo-plate capping is demonstrated by Raman spectra, compared to EG without Moplate capping. Secondly, the graphene is formed on SiC thin film surface at relatively low temperature by electron beam (e-beam) irradiation with low acceleration voltage. The e-beam irradiation with low acceleration voltage induces the heat accumulation within several layers of SiC thin film surface due to its shallow penetration depth. The thermalized electrons weaken the bond strength of the Si-C atoms while staying within a few layers of SiC thin film surface, which reduce the heat energy required for sublimating Si atoms. As the electron fluency increase, the crystallinity and uniformity of grown graphene are improved, which is confirmed by Raman spectra and scanning electron microscopy (SEM) images. We propose the cleanly patterning method for graphene using Al thin film as etching mask because general patterning methods such as electron beam lithography and photolithography induce the degradation of graphene quality due to polymer residue. The properties of fabricated graphene device using Al thin film are confirmed by Hall measurement and Raman spectra, compared with graphene sample patterned with conventional photolithography. In particular, the apparent Shubnikov-de Haas (SdH) oscillation measured in graphene device patterned with Al thin film demonstrates better homogeneity and 2DEG system. The carrier density and Hall mobility in Al patterned EG device are measured to be 9.16 ?? 10^12 cm-2 and ~ 2100 cm2/Vs, respectively. The complementary logic inverter having graphene channel is fabricated by using selective doping of graphene. Ti or Al adsorbed graphene is doped n-type, because Ti or Al with lower work function than graphene induces the charge transfer from the Ti or Al to graphene. On the other hand, the SiO2 adsorbed graphene is doped to p-type by the dangling bonds of SiO2 surface. The doping concentration and type of graphene are confirmed by Raman spectra and electrical measurements. We fabricated two kinds of inverter doped with Al-SiO2 and Ti-SiO2 materials. These inverters exhibit a clear voltage inversion as function of Vin at a wide range of VDD from 0.5 V to 20 V, and the highest voltage gains are ~0.93 and ~0.86, respectively. These properties can be improved by using insulating layer of higher dielectric constant and reducing thickness of gate oxide. We propose a new structure of multifunctional capacitive sensor to surmount the limitations the previous single-capacitor sensor. The proposed dual-capacitor sensor composes of two capacitors stacking vertically in a pixel which detects strength information and surface-normal directionality of external stimuli, and clearly classifies the types of stimuli. These properties have been demonstrated by detecting and distinguishing the curvature, pressure, touch and strain stimuli through the capacitances changes of the two capacitors. We successfully fabricated a stable n-type InAs NW FET with a very simple fabrication process using pre-deposition of Al2O3 layer. This oxide layer of 10 nm thickness is uniformly formed on entire surface of NW channel by ALD. It serves not only as a gate oxide but also as a protective layer of the NW channel. The structure of completed device is demonstrated by TEM images and EDX electron mapping. The n-InAs NW FET shows good current saturation and low voltage operation, the peak transconductance (gm) is extracted to be 13.4 mS/mm, the field effect mobility (??FE) is calculated to be ~1039 cm2/Vs at VDS = 0.8 V and current on/off ratio is about ~750.ope