Photo-effects on Current Transport in Back-gate Graphene Field-effect Transistor

Graphene, which has attracted wide attention because of its two-dimensional structure and high carrier mobility, is a promising candidate for potential application in optics and electronics. In this dissertation, the photonic effects on current transport in back-gate graphene field-effect transistor is investigated. Chemical vapor deposition (CVD) on metal provides a promising way for large area, controllability and high quality graphene film. The transfer and back-gate transistor fabrication processes are proposed in this dissertation. The theoretical analysis of photodetector based on back-gate graphene field-effect transistor has been done. It is shown that the photo-electronic current consists of current contributions from photovoltaic, photo-thermoelectric and photo-bolometric effects. A maximum external responsivity close to 0.0009A/W is achieved at 30μW laser power source and 633nm wavelength. The photodiode based on graphene/silicon Schottky barrier is also. A computed 238.8 W-1 photocurrent to dark current ratio normalized by the power source (633nm wavelength and 10mW laser) is obtained. An equivalent circuit model of the graphene/silicon Schottky barrier diode compatible with SPICE simulation is developed and simulated photo-response characteristics are presented using analog behavior modeling which are in close agreement with the theoretical analysis. Besides the optical applications, graphene based-transistors can also be used in applications related to space electronics. The irradiation effects including oxide trap charge and graphene layer traps charges are investigated. A semi-empirical model of graphene back-gate transistors before and after irradiation is predicted

Electronic, optical, mechanical and thermoelectric properties of graphene

Graphene, a two-dimensional allotrope of graphite with sp2 bonded carbon atoms, is arranged in honeycomb structure. Its quasi one-dimensional form is graphene nanoribbon (GNR). Graphene related materials have been found to display excellent electronic, chemical, mechanical properties along with uniquely high thermal conductivity, electrical conductivity and high optical transparency. With excellent electrical characteristics such as high carrier transport properties, quantum Hall effect at room temperature and unusual magnetic properties, graphene has applications in optoelectronic devices. Electronically, graphene is a zero bandgap semiconductor making it essential to tailor its structure for obtaining specific band structure. Narrow GNRs are known to open up bandgap and found to exhibit variations for different chiralities i.e., armchair and zigzag. Doping graphene, with p- or n- type elements, is shown to exhibit bandgap in contrast to pristine graphene. In this study, optical properties including dielectric functions, absorption coefficient, transmittance, and reflectance, as a function of wavelength and incident energy, are studied. Refractive index and extinction coefficient of pristine graphene are presented. A key optical property in the infrared region, emissivity, is studied as a function of wavelength for various multilayered configurations having graphene as one of the constituent layers. Application of such a structure is in the fabrication of a Hot Electron Bolometer (a sensor that operates on the basis of temperature-dependent electrical resistance). Graphene is found to have very high elastic modulus and intrinsic strength. Nanoindentation of graphene sheet is simulated to study the force versus displacement curves. Effects of variation of diameter of indenter, speed of indentation and number of layers of graphene on the mechanical properties are presented. Shrinking size of electronic devices has led to an acute need for thermal management. This prompted the study of thermoelectric (TE) effects in graphene based systems. TE devices are finding applications in power generation and solid state refrigeration. This study involves analyzing the electronic, thermal and electrical transport properties of these systems. Electronic thermal conductivity, of graphene based systems (κe), is found to be negligible as compared to its phonon-induced lattice thermal conduction (κp). Variations in κp of graphene and GN Rs are evaluated as a function of their width and length of their edges, chiralities, temperature, and number of layers. The interdependence of transport parameters, i.e., electrical conductivity (σ), thermoelectric power (TEP) or Seebeck coefficient (S), and κ of graphene are discussed. The thermoelectric performance of these materials is determined mainly by a parameter called Figure-of-Merit. Effective methods to optimize the value of Figure-of-Merit are explored. Reducing the thermal conductivity and increasing the power factor of these systems are found to improve the Figure-of-Merit significantly. This involves correlation of structure and transport properties. Effects of doping on σ, κ and Hall coefficient are discussed

Solid State Circuits Technologies

The evolution of solid-state circuit technology has a long history within a relatively short period of time. This technology has lead to the modern information society that connects us and tools, a large market, and many types of products and applications. The solid-state circuit technology continuously evolves via breakthroughs and improvements every year. This book is devoted to review and present novel approaches for some of the main issues involved in this exciting and vigorous technology. The book is composed of 22 chapters, written by authors coming from 30 different institutions located in 12 different countries throughout the Americas, Asia and Europe. Thus, reflecting the wide international contribution to the book. The broad range of subjects presented in the book offers a general overview of the main issues in modern solid-state circuit technology. Furthermore, the book offers an in depth analysis on specific subjects for specialists. We believe the book is of great scientific and educational value for many readers. I am profoundly indebted to the support provided by all of those involved in the work. First and foremost I would like to acknowledge and thank the authors who worked hard and generously agreed to share their results and knowledge. Second I would like to express my gratitude to the Intech team that invited me to edit the book and give me their full support and a fruitful experience while working together to combine this book

Hybrid Nanomaterials

Two of the hottest research topics today are hybrid nanomaterials and flexible electronics. As such, this book covers both topics with chapters written by experts from across the globe. Chapters address hybrid nanomaterials, electronic transport in black phosphorus, three-dimensional nanocarbon hybrids, hybrid ion exchangers, pressure-sensitive adhesives for flexible electronics, simulation and modeling of transistors, smart manufacturing technologies, and inorganic semiconductors

Charge and Thermoelectric Transport in Semicrystalline Conjugated Polymers and Single-Walled Carbon Nanotube Networks

Due to their flexibility, solution-processability and continuously improving electronic performance, conjugated polymer semiconductors and single-walled carbon nanotube (SWCNT) networks are promising candidates for wearable electronics, flexible optoelectronic devices and thermoelectric generators. In the past two decades the development of high-mobility donor-acceptor copolymers outperforming amorphous silicon, employed in commercial display technologies, and the ability to tune the diameter distribution in SWCNT networks via selective dispersion with conjugated polymers and other sorting methods have been major breakthroughs for these material systems. This thesis provides an improved understanding of their charge and thermoelectric transport. In particular, the charge density and temperature dependence of their field-effect mobility and gated Seebeck coefficient are investigated. When a temperature difference is applied to a conducting system, a thermal voltage builds up as a response. The Seebeck coefficient is the ratio of the thermal voltage to the temperature difference and characterizes the entropy transported by a carrier divided by its charge. Consequently, it offers insights into the transport energetics and the density of states (DoS). It can be used to identify the prevailing transport mechanisms, such as phonon-assisted hopping between localized states or scattering-limited transport through delocalized states, scattering mechanisms and carrier-carrier interactions as well as the extent of charge carrier trapping. Firstly, it is demonstrated that charge transport in semicrystalline high-mobility copolymers is incompatible with disorder-based transport models that were developed for preceding, more disordered polymers. Instead the charge density and temperature dependence of the field-effect mobility and gated Seebeck coefficient of the semicrystalline n-type polymer P(NDI2OD-T2) with varying degrees of crystallinity provides direct evidence for low-disorder, narrow-band conduction. The inclusion of short-range electron-electron interactions and the consideration of a spatially inhomogeneous DoS allow to explain both the measured mobility and Seebeck coefficient. These findings outline the extension of crystalline domains as a mean for improved thermoelectric conversion efficiencies. Subsequently, the charge density and temperature-dependent field-effect mobility and gated Seebeck coefficient of polymer-sorted monochiral small diameter (6,5) (0.76 nm) and mixed large diameter SWCNT (1.17-1.55 nm) networks with different network densities and length distributions are reported. It is shown that charge and thermoelectric transport in SWCNT networks can be modelled by the Boltzmann transport formalism incorporating transport in heterogeneous media and fluctuation-induced tunneling. The charge density and temperature dependence of the Seebeck coefficient can be simulated via the consideration of the diameter-dependent one-dimensional DoS of the SWCNTs composing the network. Due to the carrier relaxation time being anti-proportional to energy, the simulations further point towards a more two-dimensional character of scattering, as opposed to one-dimensional acoustic and optical phonon scattering in single SWCNTs, as well as the potential necessity to consider scattering at SWCNT junctions. Trap-free, narrow DoS distribution, large diameter SWCNT networks, allowing low tunnel barriers and a large thermally accessible DoS, are proposed for both electronic and thermoelectric applications. Finally, the thermoelectric performance of the molecularly doped semicrystalline polymer PBTTT is presented in the high charge density limit, which is relevant for applications in thermoelectric generators. Using the recently reported ion-exchange doping routine charge densities on the order of one carrier per monomer repeat unit can be obtained, allowing to attain a highly conductive system. Ongoing investigations of the impact of polymer alignment on charge and thermoelectric transport in this regime are presented.EPSRC studentshi

A Double-Voltage-Controlled Effective Thermal Conductivity Model of Graphene for Thermoelectric Cooling

© 1963-2012 IEEE. Graphene provides a new opportunity for thermoelectric study based on its unique heat transfer behavior controllable by a gate voltage. In this paper, an effective thermal conductivity model of graphene for thermoelectric cooling is proposed. The model is based on a double-voltage-control mechanism. According to the law of Fourier heat conduction, an effective thermal conductivity model of the proposed thermoelectric cooling device is derived taking a tunable external voltage into account. Then, a gate voltage is used which can change graphene's thermoelectric characteristics. To verify the correctness and effectiveness of the proposed model, a circuit simulation model using HSPICE is built based on the thermoelectric duality. The simulation results from HSPICE and the calculated results from the mathematic model show good agreements with each other. This paper provides a novel precisely controlling method for thermoelectric cooling

비공유 화학적 도핑을 이용한 단일층 그래핀 소자의 전자특성 최적화

학위논문(박사) -- 서울대학교대학원 : 자연과학대학 화학부, 2022. 8. 홍병희.2004년 그래핀은 테이프를 이용한 (고배향 열분해성) 흑연(highly oriented pyrolytic graphite; HOPG)으로부터의 박리를 통해 최초 발견되었다. 이후 수많은 연구들에 의해 그래핀이 우수한 열적, 기계적, 전기적, 광학적 특성을 지녔음이 알려졌다. 2009년에 이르러 화학기상증착(chemical vapor deposition; CVD) 방식을 이용한 다결정 그래핀의 대면적 합성이 실험적으로 가능해졌고, 이로써 그래핀이 다양한 분야에 응용될 수 있는 발판이 마련되었다. 특히 그래핀의 응용분야 중 전기전자특성을 이용한 분야가 각광을 받고 있다. 그래핀은 높은 전자이동도, 전기전도도 및 열전도도를 지닌 재료이며, 밀접결합(tight-binding; TB) 근사 모형을 이용하여 계산한, 결함이 없는 단결정 단층 그래핀의 밴드갭(band gap)은 0임이 밝혀졌다. 재료의 전자특성 조절은 전자소자로의 응용에 필수적 공정이고, 도핑은 전자특성 조절에 주로 쓰이는 방법 중 하나이다. 그래핀에 도핑 처리를 함으로써 밴드갭, 전기전도도 및 일함수와 같은 전기전자특성을 조절할 수 있다. 그래핀에 대한 도핑 방법으로는 원자 치환, 전계 인가, 분자나 금속 나노입자 등의 물리적 흡착 등이 있다. 이 중 물리적 흡착 방식은 결함 없이 간단하고 우수한 도핑 효과를 얻을 수 있어 그래핀 도핑 방법으로 널리 사용되고 있다. 본 논문에서는 화학기상증착 방식으로 합성한 그래핀의 전자특성 최적화 방법 및 전자소자로의 응용에 관한 연구를 다루었다. 그래핀의 전자특성 최적화 방식으로 물리적 흡착을 통한 비공유 화학적 도핑을 택하였으며, 도핑된 그래핀의 전자소자로의 응용 가능성에 대하여 확인하였다. 제1장에서는 그래핀의 물리적 특성 중 전기전자특성에 초점을 맞춰 설명하였다. 또한, 연구에 사용한 도핑 방법과 도핑된 그래핀의 전하 이동현상에 관하여 소개하였다. 제2장에서는 그래핀의 합성, 전사 및 도핑 방법에 관하여 서술하였다. 연구에 사용된 그래핀은 화학기상증착 방식으로 합성되었으며, 합성된 그래핀은 구리 식각 및 전사 공정을 통해 소자 연구를 위한 시편으로 제작되었다. 그래핀은 자기조립단층(self-assembled monolayer; SAM)을 형성하는 분자 외 다양한 나노물질을 이용한 물리적 흡착 방식에 의해 화학적 도핑된다. 라만 분광분석을 통해 합성 및 도핑 직후의 그래핀 시편의 품질을 평가하였고, 3 전극 시스템을 이용한 전계효과 트랜지스터를 제작하여 그래핀의 전자특성을 분석하였다. 제3장에서는 화학기상증착 방식으로 합성한 그래핀에 다양한 나노물질을 차례로 제공함으로써 화학적 도핑 효과의 변화를 나타낸 전자소자 연구를 기술하였다. 그래핀 표면에 금 나노입자를 물리적 흡착 방식으로 도핑하여 비공유 기능화하고, 이를 이용한 그래핀을 전계효과 트랜지스터 소자로 제작하였다. 제작된 소자에 존재하는 금 나노입자에 4-머캅토벤조산(4-mercaptobenzoic acid; 4-MBA) 분자를 흡착시킴으로써 자기조립단층을 형성케 한다. 이때 수은 이온을 주입하면 자기조립단층을 형성한 4-MBA 분자의 카복시기(carboxyl group)가 리간드로 작용하여 수은 이온을 포획하면서 킬레이트(chelate) 복합체를 구성한다. 각 단계의 그래핀 전계효과 트랜지스터 소자의 전자특성 분석을 통해, 각 나노물질 요소에 의해 그래핀 표면의 도핑 효과가 미세 조정됨을 알 수 있다. 본 연구를 통해 그래핀 전계효과 트랜지스터의 화학적 기능화에 대한 가능성을 확인하였다. 제4장에서는 화학기상증착 방식으로 합성한 그래핀에 n-알킬아민(n-alkylamine; H2NCn) 분자를 도입함으로써, n형 도핑된 그래핀을 이용한 열전소자 성능의 향상에 관하여 기술하였다. n-알킬아민 분자는 그래핀 표면에서 자기조립단층을 형성하고 비공유 기능화를 통해 전자를 그래핀에 제공한다. 탄소사슬 길이가 각기 다른 n-알킬아민 분자를 이용하여 도핑한 그래핀을 3 전극 시스템을 통해 분석함으로써, 서로 다른 길이의 분자를 통해 그래핀 시편의 전하운반자 농도의 조절이 가능함을 확인하였다. n-알킬아민 분자의 자기조립단층이 형성된 각 그래핀 시편 위로 산화갈륨(Ga2O3) 박막층 및 갈륨-인듐 공융합금(eutectic Ga-In alloy; EGaIn) 벌크층을 차례로 적층하여 열전소자를 제작하였다. n-알킬아민 분자의 비공유 접합에 의해 유도 갭 상태(induced-gap state)가 그래핀 열전소자(SLG//H2NCn//Ga2O3/EGaIn)에 도입되었다. 금 박막층과 n-알케인싸이올레이트(n-alkanethiolates; SCn) 분자의 접합으로 구성된 종래의 열전소자(Au/SCn//Ga2O3/EGaIn)와의 비교를 통해, 상기한 방식으로 제작된 그래핀 열전소자가 우수한 열전특성을 지니고 있음을 증명하였다.Since its first discovery as a flake-form from mechanical exfoliation of highly-oriented pyrolytic graphite (HOPG) using tape in 2004, numerous studies have shown that graphene has outstanding and extraordinary thermal, mechanical, electrical, electronic and optical properties. In 2009, large-area synthesis of polycrystalline graphene using a chemical vapor deposition (CVD) method became experimentally possible, thereby establishing a foothold for the graphene to be applied to various fields. In particular, the field of application using electrical and electronic characteristics of graphene is in the spotlight. Graphene is a remarkable material with high electron mobility, electrical conductivity and thermal conductivity. Furthermore, the pristine single-layer graphene (SLG) has zero gap, a theoretical value calculated by a tight-binding (TB) approximation model. Engineering the electronic properties of materials is an essential process for application to electronic devices, and doping is one of the methods mainly used to control electronic properties. By doping graphene, electrical and electronic characteristics such as band gap, electrical conductivity, and work function (WF) can be modified and controlled. Doping methods for graphene include atomic substitution, applying electric field, physisorption (physical adsorption) of molecules and metal nanoparticles, etc. Among those methods, the physisorption is widely used as a graphene doping method because it can obtain a simple and superior doping effect without crystallographic defects. This paper describes researches on optimization methods of the electronic properties of graphene synthesized by CVD method and its applications of electronic devices. Noncovalent chemical doping by the physisorption was selected as the optimization method of the electronic properties of graphene, and the possibility of application of the doped graphene to an electronic device was verified. Chapter 1 delineates the physical properties of graphene, focusing on the electrical and electronic properties. In addition, the doping method used in the study and the charge transfer phenomenon of doped graphene were introduced. Chapter 2 gives a detailed description of the procedure such as the synthesis, transfer, and doping methods of graphene. Graphene used in these researches was synthesized by CVD method, and the synthesized graphene was manufactured as electronic device specimens through copper etching and transfer processes. Graphene is chemically doped by the physisorption method using various nanomaterials such as molecules forming self-assembled monolayers (SAM). Through Raman spectroscopy, the quality of graphene specimens immediately after synthesis and doping process was evaluated. Moreover, the electronic properties of graphene were analyzed by a 3-electrode system using field-effect transistor (FET) devices Chapter 3 depicts a study on electronic devices showing changes in chemical doping effects by sequentially providing various nanomaterials to graphene synthesized by CVD method. Gold nanoparticles were used as dopants on the surface of graphene by physisorption for a noncovalent functionalization, and the doped graphene was manufactured as FET devices. SAM is formed by adsorbing 4-mercaptobenzoic acid (4-MBA) molecules onto gold nanoparticles on the manufactured graphene device. And then, if mercury ions are injected, a carboxyl group of 4-MBA molecules constructing SAM acts as a ligand to capture mercury ions, thereby assembling a chelate complex. Through the analyses of the electronic properties of the graphene FET devices in each step, it can be seen that the doping effect of the graphene surface is finely adjusted by each nanomaterial element. Through this study, the possibility of chemical functionalization of graphene FET devices was exactly clarified. Chapter 4 describes the improvement in the performance of graphene thermoelectric devices using n-type doping by introducing n-alkylamine (H2NCn) molecules onto SLG film synthesized by CVD method. The n-alkylamine molecules form SAM on the surface of graphene and provide electrons to graphene through noncovalent functionalization. Graphene doped by n-alkylamine molecules with different lengths of carbon chain was manufactured as FET devices and analyzed by a 3-electrode system. Graphene FET devices were proved clearly that the concentration of charge carriers of graphene specimens could be regulated by chemical doping method using each molecule. Graphene thermoelectric devices was manufactured by sequentially stacking a gallium oxide (Ga2O3) thin film layer and a eutectic gallium-indium alloy (EGaIn) bulk layer onto the n-alkylamine SAM formed on each graphene specimen. An induced-gap state was introduced into the graphene layer in graphene thermoelectric devices (SLG//H2NCn//Ga2O3/EGaIn) by noncovalent junctions of n-alkylamine molecules. Through comparison with thermoelectric devices with a conventional structure (Au/SCn/Ga2O3/EGaIn) composed of the junction of gold thin film layer and n-alkanethiolates (SCn) molecules, it was shown that the graphene thermoelectric devices produced by the above method have improved and outstanding thermoelectric properties.Cover 1 Abstract 3 Table of Contents 6 List of Tables 8 List of Figures 9 Chapter 1. Introduction to Graphene 12 1. 1. Discovery and Advancement of Graphene 12 1. 2. Crystal Structure of Graphene 16 1. 3. Band Structure of Graphene 25 1. 4. Group Theory to Analyze Graphene 40 1. 5. Chemical Doping of Graphene 47 1. 6. Properties of Doped Graphene 50 Chapter 2. Experimental 54 2. 1. Graphene Synthesis by Chemical Vapor Deposition 54 2. 2. Pre-treatment Process for Graphene Transfer 65 2. 3. Graphene Transfer Process 66 2. 4. Graphene Doping by Physisorption 69 2. 5. Raman Spectroscopic Analyses for Graphene 70 2. 6. Electronic Analyses for Graphene Field-effect Transistor 80 Chapter 3. Gold Nanoparticle-Mediated Noncovalent Functionalization of Graphene for Field-Effect Transistors 94 3. 1. Abstract 94 3. 2. Introduction 95 3. 3. Experimental 96 3. 4. Results and Discussion 101 3. 5. Conclusion 123 Chapter 4. Enhanced Thermopower of Saturated Molecules by Noncovalent Anchor-Induced Electron Doping of Single-Layer Graphene 124 4. 1. Abstract 124 4. 2. Introduction 125 4. 3. Experimental 128 4. 4. Results and Discussion 138 4. 5. Conclusion 157 Bibliography 158 Abstract in Korean 178박

Photo Thermal Effect Graphene Detector Featuring 105 Gbit s-1 NRZ and 120 Gbit s-1 PAM4 Direct Detection

The challenge of next generation datacom and telecom communication is to increase the available bandwidth while reducing the size, cost and power consumption of photonic integrated circuits. Silicon (Si) photonics has emerged as a viable solution to reach these objectives. Graphene, a single-atom thick layer of carbon5, has been recently proposed to be integrated with Si photonics because of its very high mobility, fast carrier dynamics and ultra-broadband optical properties. Here, we focus on graphene photodetectors for high speed datacom and telecom applications. High speed graphene photodetectors have been demonstrated so far, however the most are based on the photo-bolometric (PB) or photo-conductive (PC) effect. These devices are characterized by large dark current, in the order of milli-Amperes , which is an impairment in photo-receivers design, Photo-thermo-electric (PTE) effect has been identified as an alternative phenomenon for light detection. The main advantages of PTE-based photodetectors are the optical power to voltage conversion, zero-bias operation and ultra-fast response. Graphene PTE-based photodetectors have been reported in literature, however high-speed optical signal detection has not been shown. Here, we report on an optimized graphene PTE-based photodetector with flat frequency response up to 65 GHz. Thanks to the optimized design we demonstrate a system test leading to direct detection of 105 Gbit s-1 non-return to zero (NRZ) and 120 Gbit s-1 4-level pulse amplitude modulation (PAM) optical signal