Hybrid silicon/graphene and related materials and devices
编者按：
碳是一种神奇的元素。除了大家熟知的钻石， 还可以呈现从零维（富勒烯）、一维(碳纳米管)到二维（石墨烯）的稳定结构。这些结构的核心是二维单层石墨片结构，对其基本电子态结构的理论可以追溯到20世纪40年代（Wallace P R 1947 Phys. Rev. 71 622）。单层石墨片或石墨烯的应用也至少可以追溯到20世纪70年代。单层石墨片作为电镜样品的完美支撑膜，对20世纪七八十年代高分子电子显微成像技术的发展也做出过巨大贡献。但单层石墨片被单独作为一种材料，并开始广泛影响物理学、化学等学科随后十余年的发展则始于21世纪初，两位俄裔英国学者Andre Geim 和Konstantin Novoselov于2004年成功将单层石墨片分离到绝缘基底，并对其输运性质进行了系统的测试，特别是在室温观察到量子霍尔效应，一举获得了2010年的诺贝尔物理奖。近十年，石墨烯研究已成为发展势头最为迅猛的领域，2012年以石墨烯为主题的论文发表数量首次超过一万篇，2016年已达39373篇。在这迅猛发展的研究之后，是对石墨烯及相关材料在未来信息技术中所能起作用的巨大期盼。然而，本征的石墨烯材料没有能隙，并不适合数字电路应用。二维石墨烯类材料，例如半导体过渡金属硫化物等应运而生，相关材料和器件研究也快速发展成为热点研究领域。此外，虽然摩尔定律发展已经接近其尽头，但是硅基信息处理技术仍牢牢处于信息产业的霸主地位，结合硅基技术发展石墨烯类材料及其器件也顺理成章地成为了未来该领域研究的主流和本专题的主题。
专题收录了国内高校和研究院所相关研究人员的共19篇研究论文和综述，涉及到从石墨烯类材料的（1）生长： 金属衬底上高质量大面积石墨烯的插层及其机制（郭辉、路红亮、黄立、王雪艳、林晓、王业亮、杜世萱、高鸿钧）；介电层表面直接生长石墨烯的研究进展（杨慧慧、高峰、戴明金、胡平安）；双层石墨烯的化学气相沉积法制备及其光电器件（杨云畅、武斌、刘云圻）；（2）结构表征、加工与性能调控：锯齿形石墨烯反点网络加工与输运性质研究（张婷婷、成蒙、杨蓉、张广宇）；石墨烯纳米结构的制备及带隙调控研究（张慧珍、李金涛、吕文刚、杨海方、唐成春、顾长志、李俊杰)；二维原子晶体的低电压扫描透射电子显微学研究（黎栋栋、周武）；硼烯和碱土金属硼化物二维纳米材料的制备、结构、物性及应用研究（郭泽堃、田颜、甘海波、黎子娟、张彤、许宁生、陈军、陈焕君、邓少芝、刘飞）；石墨烯纳米带的制备与电学特性调控（张辉、蔡晓明、郝振亮、阮子林、卢建臣、蔡金明）；（3）输运：多端口石墨烯系统中的非局域电阻（王孜博、江华、谢心澄）； 到（4）类石墨烯：类石墨烯锗烯研究进展（秦志辉）；硅烯的化学功能化（杨硕、程鹏、陈岚、吴克辉）；（5）器件：基于双极性二维晶体的新型p-n结（张增星、李东）；基于六角氮化硼二维薄膜的忆阻器（吴全潭、时拓、赵晓龙、张续猛、伍法才、曹荣荣、龙世兵、吕杭炳、刘琦、刘明）；基于毫米级单晶石墨烯的倍频器性能研究（高庆国、田猛串、李思超、李学飞、吴燕庆)；硅基底石墨烯器件的现状及发展趋势（武佩、胡潇、张健、孙连峰）；石墨烯射频器件研究进展（卢琪、吕宏鸣、伍晓明、吴华强、钱鹤）；二维半导体过渡金属硫化物的逻辑集成器件（李卫胜、周健、王瀚宸、汪树贤、于志浩、黎松林、施毅、王欣然）；和（6）硅基混合系统：石墨烯-硅基混合光子集成电路（肖廷辉、于洋、李志远）；高性能石墨烯霍尔传感器（黄乐、张志勇、彭练矛）。专题覆盖了石墨烯类材料及其器件所涉及的几乎所有方面，是本领域不可多得的珍贵参考资料。相信专题的发表能够进一步促进这个重要的研究领域的发展，促生相关产业的诞生。
2017, 66 (21): 216103.
doi: 10.7498/aps.66.216103
Abstract +
Graphene nanostructures with defined edges are proposed as a promising platform for the realization of nano-electronics and spin-electronics. However, patterned graphene nanostructure can lead to extra damage and drastically reduce its charge carrier mobility due to the edge disorder. The high flexibility of a top-down patterning method with edge smoothness is extremely desirable. Hydrogen plasma enhanced anisotropic etching graphene is demonstrated to be an efficient method of fabricating zigzag-edge graphene nanostructures. In addition, boron nitride is shown to be an excellent substrate for graphene due to its atomic flatness. Here in this work, we fabricate zigzag edge graphene antidot lattices on a boron nitride substrate via dry transfer method and traditional electron beam lithography, and reactive ion etching followed by hydrogen anisotropic etching approach. At low magnetic fields, weak localization is observed and its visibility is enhanced by intervalley scattering on antidot edges. We observe commensurate features in magnetotransport properties which stem from carriers around one antidot, signifying the high quality of our patterned samples. At high magnetic field, crossover from Shubnikov-de Haas oscillation to quantum Hall effect can be clearly observed due to the high mobility of our zigzag edge graphene antidot lattices. The transport properties of our patterned samples suggest that our fabrication method paves the way for achieving high quality graphene antidot lattices. High quality zigzag edge graphene antidot lattice might be a great platform to study the transport properties of lateral superlattice potential modulation graphene.
2017, 66 (21): 216802.
doi: 10.7498/aps.66.216802
Abstract +
With tremendous progress of graphene and with the consideration of the compatibility with semiconductor industry, the construction of analogous two-dimensional crystalline systems-new two-dimensional honeycomb and layered materials composed of elements other than carbon, the group IV (Si, Ge) analogs of graphene and the investigation of their fascinated electronic properties have become the frontier topics of condensed matter physics. Theoretical calculation predicts that unlike the planar structure of graphene, the germanene has stable, two-dimensional, low-buckled, honeycomb structure similar to that of silicene, but has much higher spin-orbit band gap than silicene, which is certainly of crucial importance in future electronics. The influences of atomic structures and the buckling of the low-buckled geometry on local electronic structure of the fabricated germanene are also reviewed from the atomic point of view. As theoretical studies on germanene are rapidly increasing, now the major challenge in this field is the preparation of high-quality germanene. Compared with silicene, the germanene has larger Ge-Ge interatomic distance which can weaken the orbital overlaps, resulting in the big difficulty in constructing germanene. In this work we review the recent progress of experimental epitaxial growth of germanene on surfaces, with emphasis on metal surfaces. The growth of quasi-freestanding germanene and its potential applications in nanoelectronics in the future are also discussed.
2017, 66 (21): 216803.
doi: 10.7498/aps.66.216803
Abstract +
Graphene, a two-dimensional material with honeycomb lattice, has attracted great attention from the communities of fundamental research and industry, due to novel phenomena such as quantum Hall effect at room temperature, Berry phase, and Klein tunneling, and excellent properties including extremely high carrier mobility, high Young's modulus, high thermal conductivity and high flexibility. Some key issues hinder graphene from being used in electronics, including how to integrate it with Si, since Si based technology is widely used in modern microelectronics, and how to place high-quality large area graphene on semiconducting or insulating substrates. A well-known method of generating large-area and high-quality graphene is to epitaxially grow it on a single crystal metal substrate. However, due to the strong interaction between graphene and metal substrate, the intrinsic electronic structure is greatly changed and the conducting substrate also prevents it from being directly used in electronics. Recently, we have developed a technique, which intercalates silicon between epitaxial graphene and metal substrate such as Ru (0001) and Ir (111). Experimental results from Raman, angle-resolved photoemission spectroscopy, and scanning tunneling spectroscopy confirm that the intercalation layer decouples the interaction between graphene and metal substrate, which results in the recovery of its intrinsic band structure. Furthermore, we can use this technique to intercalate thick Si beyond one layer and intercalate Si between graphene and metal film, which indicates the possibility of integrating both graphene and Si device and vast potential applications in industry by reducing its cost. Besides Si, many other metal elements including Hf, Pb, Pt, Pd, Ni, Co, Au, In, and Ce can also be intercalated between graphene and metal substrate, implying the universality of this technique. Considering the versatility of these elements, we can expect this intercalation technique to have wide applications in tuning graphene properties. We also investigate the intercalation mechanism in detail experimentally and theoretically, and find that the intercalation process is composed of four steps:creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of intercalation layers. The intercalation of versatile elements with different structures by this technique provides a new route to the construction of graphene heterostructures, espectially van der Waals heterostructure such as graphene/silicene and graphene/hafnene, and also opens the way for placing graphene on insulating substrate for electronic applications if the intercalation layer can be oxidized by further oxygen intercalation.
2017, 66 (21): 217301.
doi: 10.7498/aps.66.217301
Abstract +
Graphene has potential applications in future microelectronics due to its novel electronic and mechanical properties. However, the lack of the bandgap in graphene poses a challenge and hinders its applications. In order to be able to work in ambient condition, gap engineering of graphene with nanostructure needs about sub-10 nm characteristic size, which increases the difficulty of fabrication and leads to less driving current that can be borne. In this paper, a new method to fabricate sub-10 nm graphene nanostructures is developed. With PMMA/Cr bilayer structure, sub-10 nm graphene nanostructures can be obtained precisely and repeatedly through controlling the etching time. Meanwhile, a new device based on graphene nanoconstrictions connected in parallel is designed and fabricated, whose band gap is bigger than that of graphene nanoribbon and whose characteristic width is the same as that of graphene nanoribbon. With the graphene nanoconstrictions connected in parallel, the band gap of the graphene can be adjusted effectively and the driving current can be significantly increased, which is very important for future practical applications of graphene.
2017, 66 (21): 217302.
doi: 10.7498/aps.66.217302
Abstract +
Two-dimensional (2D) materials have a unique crystal structure and excellent properties, which renders it possible to be used to construct novel artificial nanostructures and design novel nanodevices, thereby achieving a breakthrough in the semiconductor field. In this review paper, the basic behaviors of the ambipolar 2D crystals and the fabrication method of the van der Waals heterostructures are first introduced. We mainly summarize the applications of the ambipolar 2D crystals for novel electrical-field-tunable 2D p-n junctions and p-n heterojunctions (field-effect p-n heterojunction transistor) and non-volatile storable p-n junctions, and other aspects of the relevant structural design, electronic and optoelectronic properties. Then we further introduce their potential applications of logic rectifiers, field-effect optoelectronic transistors, multi-mode non-volatile memories, rectifier memories, optoelectronic memories, photovoltaics, etc. Finally, we provide an outlook of the future possible studies of this new type of p-n junctions in the relevant fields.
2017, 66 (21): 217303.
doi: 10.7498/aps.66.217303
Abstract +
Two-dimensional (2D) materials, such as graphene and transition-metal dichalcogenide monolayers, have unique properties that are distinctly different from those of their bulk counterparts, and hopefully possess a wide range of applications in 2D semiconductor device. Structural defects are known to have profound influences on the properties of crystalline materials; thus, correlating the defect structure with local properties in 2D material is of fundamental importance. However, electron microscopy studies of 2D materials on an atomic scale have become a challenge as most of these materials are susceptible to electron beam irradiation damage under high voltage and high dose experimental conditions. The development of low voltage aberration-corrected scanning transmission electron microscopy (STEM) has made it possible to study 2D materials at a single atom level without damaging their intrinsic structures. In addition, controllable structural modification by using electron beam becomes feasible by controlling the electron beam-sample interaction. New nanostructures can be created and novel 2D materials can be fabricated in-situ by using this approach. In this article, we review some of our recent studies of graphene and transition-metal dichalcogenides to showcase the applications of low voltage aberration corrected STEM in 2D material research.
2017, 66 (21): 218102.
doi: 10.7498/aps.66.218102
Abstract +
Graphene, a two-dimensional sheet of sp2-hybridized carbon material, possesses excellent properties, such as high carrier mobility, high electrical conductivity, high thermal conductivity, strong mechanical strength and quantum anomalous Hall effect. So graphene quickly lights the enthusiasm for its research and application due to its superior performance. The silicon-based graphene devices are compatible with traditional silicon-based semiconductor technology. The combination of silicon-based graphene devices and silicon-based devices can greatly improve the overall performances of semiconductor devices. With the optimization of graphene preparation process and transfer technology, graphene devices using silicon as the substrate will show promising potential applications.
With the scaling of device, the heat dissipation, power consumption and other issues impede the integration of silicon-based devices. Graphene provides a possible solution to these problems. In this paper, we summarize the graphene application in field effect transistor. The bandgap of graphene is zero, which will have adverse effect on the switching ratio of the device. In order to solve this problem, a variety of methods are used to open its bandgap, such as the quantum confinement method, the chemical doping method, the electric field regulation method, and the introduction stress method. In the field of optoelectronic devices, graphene can evenly absorb light at all frequencies, and its photoelectric properties have also been widespread concerned, such as photoelectric detector, photoelectric modulator, solar cell, etc. At the same time, graphene, as a typical two-dimensional material, possesses superior electrical properties and ultra-high specific surface area, and becomes the hottest material in high sensitivity sensors.
2017, 66 (21): 218501.
doi: 10.7498/aps.66.218501
Abstract +
The state-of-the-art graphene Hall elements and integrated circuits are reviewed. By optimizing the growth and transfer of graphene and the micro-fabrication process of Hall sensor, graphene Hall elements and integrated circuits outperform conventional Hall sensors in many aspects. Graphene Hall elements exhibit better sensitivities, resolutions, linearities and temperature stabilities than commercialized Hall elements. Through developing a set of passivation processes, the stabilities of graphene Hall elements are improved. Besides, the flexible magnetic sensing and multifunctional detection applications based on graphene are demonstrated. In addition, graphene/silicon hybrid Hall integrated circuits are realized. By developing a set of low temperature processes (below 180℃), graphene Hall elements are monolithically integrated onto the passivation layer of silicon complementary metal oxide semiconductor chip. This work demonstrates that graphene possesses significant performance advantages in Hall magnetic sensing and potentially practical applications.
2017, 66 (21): 218502.
doi: 10.7498/aps.66.218502
Abstract +
Graphene, the first realized two-dimensional material, has received much attention in electronic applications in recent years. With ultra-high carrier mobility and one atom thick structure, graphene becomes a promising semiconductor candidate for solving the problem of short channel effect in nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET), and exploring its applications in radio frequency devices. How to develop the advantages of graphene transistor in radio frequency is an attractive research area. The first step is to obtain high quality graphene material. In this article we summarize the graphene growth methods commonly used in electronic field, including chemical vapor deposition on metal substrates and epitaxial method on wide bandgap semiconductor and insulator substrates. Another key factor to improve graphene transistor performance is to carefully design the device structure and process flow. Multi-finger gate and T-shaped gate are widely used in MOSFET. These two structures can significantly reduce gate resistance, and result in a better radio frequency performance. Inverted process is introduced for graphene FET fabrication, which is compatible with silicon-based back-end-of-line technology. It can reduce the damages to graphene during fabrication. Another improved self-aligned gate deposition process can lead to a good gate coupling and less parasitic parameters. These newly developed process play a prominent part in increasing the cut-off frequency and maximum oscillation frequency of graphene radio frequency devices. In addition, single crystal graphene is helpful in eliminating carriers scattering and improving the radio frequency properties of graphene transistor. So far, the highest cut-off frequency of graphene transistor reaches over 300 GHz by a few groups, but the maximum oscillation frequency remains low. Record-high maximum oscillation frequency is 200 GHz when gate length is 60 nm. Further improvement of maximum oscillation frequency needs to be tried out. Several graphene radio frequency circuits are also discussed in the paper. Some of the circuits have similar structures to silicon-based circuits, and others are designed based on the unique property of graphene transistor, like ambipolar transport properties. The new concept circuits have simpler structures than conventional circuits. With the rapid development of graphene growth and related integrating technology, the potential to use graphene in radio frequency field will be further increased.
2017, 66 (21): 216804.
doi: 10.7498/aps.66.216804
Abstract +
As one of the most appealing materials, graphene possesses remarkable electric, thermal, photoelectric and mechanic characteristics, which make it extremely valuable both for fundamental researches and practical applications. Nowadays the synthesis of graphene is commonly achieved by growing on metal substrate via chemical vapor deposition. For the integration in micro-electric device, the as-grown graphene needs to be transferred onto target dielectric layer. However, wrinkles, cracks, damages, and chemical residues from the metal substrate and the auxiliary polymer are inevitably introduced to graphene during such a transfer process, which are greatly detrimental to the performances of the graphene devices. Therefore, the direct synthesis of graphene on dielectric layer is of great importance. Many researches about this subject have been carried out in the last few years. While only few papers have systematically reviewed the direct growth of graphene on dielectric layer. For the in-depth understanding and further research of it, a detailed overview is required. In this paper, we summarize the recent research progress of the direct syntheses of graphene on dielectric layers, and expatiate upon different growth methods, including metal assisted growth, plasma enhanced growth, thermodynamics versus kinetics tailored growth, et al. Then differences in property between graphenes grown on various dielectric and insulating layers which serve as growth substrates in the direct growing process are discussed, such as SiO2/Si, Al2O3, SrTiO3, h-BN, SiC, Si3N4 and glass. Some kinds of mechanisms for graphene to be directly grown on dielectric layers have been proposed in different reports. Here in this paper, we review the possible growth mechanisms and divide them into van der Waals epitaxial growth and catalytic growth by SiC nanoparticles or oxygen atoms. Detailed data including Raman signals, sheet resistances, transmittances, carrier motilities are listed for the direct comparison of the quality among the graphenes grown on dielectric layers. The research focus and major problems existing in this field are presented in the last part of this paper. We also prospect the possible developing trend in the direct syntheses of high quality graphenes on dielectric layers in the future.
2017, 66 (21): 216805.
doi: 10.7498/aps.66.216805
Abstract +
Silicene exhibits extraordinary physical properties especially Dirac fermion characteristics. However, the zero-gap band structure of silicene hinders its applications in nanoelectronic and optoelectronic devices. It is thus desirable to open a finite band gap in silicene. Chemical functionalization is a commonly used method to tailor the structures and electronic properties of two-dimensional materials. In this paper we review the recent 3-year progress of silicene, including its hydrogenation, oxidization, halogenation, and other methods to modify silicene.
2017, 66 (21): 217201.
doi: 10.7498/aps.66.217201
Abstract +
Since the nonlocal measurement is helpful in discovering nontrivial physics that is too difficult to detect directly, the nonlocal measurement has now become one of the research focuses in condensed matter physics. Recent experiments find the signal of the giant nonlocal resistance in an H-shaped multi-terminal graphene system. After excluding other possible transport mechanisms, such as the classic Ohmic diffusion and the edge states, researchers tend to believe that the nonlocal resistance signal originates from the spin/valley Hall effect existing in graphene sample. Based on the Landauer-Buttiker formula, the numerical results make a relatively perfect match with the experimental data in the same multi-terminal graphene system. However, though the theoretic research has made certain progress in explaining the existence of the nonlocal resistance, it is still difficult to understand some exotic behaviors of the nonlocal resistance, which exhibits properties even contradictory to the known classical theories. For instance, the nonlocal resistance decreases to zero much more rapidly than the local one, and the giant peak of the nonlocal resistance appears inside the energy gap of the graphene. In this review, the experiments focusing on the nonlocal resistance in multi-terminal graphene system are carefully reviewed. Besides, this review also shows the associated theoretic studies, and an overlook of the future study is also provided.
2017, 66 (21): 217304.
doi: 10.7498/aps.66.217304
Abstract +
Hexagonal boron nitride (h-BN) based resistive switching device is fabricated with the multilayer h-BN film serving as an active material. The device shows the coexistence of forming-free and self-compliance bipolar resistive switching behavior with reproducible switching endurance and long retention time. Moreover, the device in pulse mode shows analog resistive switching characteristics, i.e. the resistance states can be continuously tuned by successive voltage pulses. This suggests that the device is also capable of mimicking the synaptic weight changes in neuromorphic systems.
2017, 66 (21): 217305.
doi: 10.7498/aps.66.217305
Abstract +
Graphene shows great potential applications in ultrahigh speed electronics due to its high carrier mobility and velocity. Nowadays, many radio frequency circuits based on graphene have been realized. For example, graphene frequency doubler is a promising option for signal generation at high frequencies. Graphene frequency doubler can achieve excellent spectral purity, because of its ambipolar transport and highly symmetric transfer characteristics. Here, we present high performance graphene frequency doublers based on millimeter-scale single-crystal graphene on HfO2 and Si substrates. We achieve a high spectral purity degree of larger than 94% without any filtering and the conversion gain is -23.4 dB at fin=1 GHz. The high conversion gain and spectral purity can be attributed to the high-quality millimeter-scale single-crystal graphene and high-quality high- substrates. Furthermore, we investigate the relation of conversion gain to source-drain voltage Vd and input signal power Pin. The results show that the conversion gain increases with source-drain voltage increasing, and the conversion gain also increases with input signal power increasing. The dependence of conversion gain on Vd and Pin can be attributed to the transconductance increasing with Vd and Pin. We compare the conversion gains and spectral purity degrees of graphene frequency doublers with different transconductances and electron-hole symmetries at different frequencies. The result shows that the conversion gain is larger for device with higher transconductance and the spectral purity has a moderate tolerance for the electron-hole symmetry of the graphene transistor at fin=1 GHz. As the working frequency increases to 4 GHz, the spectral purity of the device with weak electron-hole symmetry decreases dramatically, while the spectral purity of the device with better electron-hole symmetry is kept around 85%. We attribute this phenomenon to the different carrier transit times and different electron-hole symmetries of graphene transistors. In conclusion, the short channel graphene transistor with ultrathin gate dielectric and high electron-hole symmetry is needed in order to achieve high performance graphene frequency doubler.
2017, 66 (21): 217702.
doi: 10.7498/aps.66.217702
Abstract +
With the rise of graphene, two-dimensional nanomaterials have been significantly developed in recent years. As novel two-dimensional nanostructures, borophene and alkaline-earth metal boride two-dimensional materials have received much attention because of their unique physical and chemical properties, such as high Fermi velocities, high electron mobilities, large Young's moduli, high transparencies, negative Poisson's ratios and high chemical stabilities. This paper focuses on the researches of the fabrication techniques, structure configurations, properties and applications of borophene and two-dimensional alkaline-earth metal boride nanomaterials. Firstly, the current preparation methods and structure configurations of borophene are summarized. Secondly, the possible structures and fabrication techniques of two-dimensional alkaline-earth metal boride nanomaterials are introduced in detail. Thirdly, the physical properties of borophene and two-dimensional alkaline-earth metal boride nanomaterials are investigated. Finally, the most promising application areas of borophene and two-dimensional alkaline-earth metal boride nanomaterials in the future are predicted.
2017, 66 (21): 218103.
doi: 10.7498/aps.66.218103
Abstract +
Graphene, as a typical representative of advanced materials, exhibits excellent electronical properties due to its unique and unusual crystal structure. The valence band and conduction band of pristine graphene meet at the corners of the Brillouin zone, leading to a half-metal material with zero bandgap. However, although the extraordinary electronical properties make graphene possess excellent electrical conductivity, it also restricts its applications in electronic devices, which usually needs an appropriate bandgap. Therefore, opening and tuning the bandgap of graphene has aroused great scientific interest. To date, many efforts have been made to open the bandgap of graphene, including defects, strain, doping, surface adsorptions, structure tunning, etc. Among these methods, graphene nanoribbon, the quasi-one-dimensional strips of graphene with finite width ( 10 nm) and high aspect ratios, possesses a band gap opening at the Dirac point due to the quantum confinement effects. Thus, graphene nanoribbon has been considered as one of the most promising candidates for the future electronic devices due to its unique electronic and magnetic properties. Specifically, the band gap of graphene nanoribbons is strongly dependent on the lateral size and the edge geometry, which has attracted tremendous attention. Furthermore, it has been reported that armchair graphene nanoribbons possess gaps inversely proportional to their width, and numerous efforts have been devoted to fabricating the graphene nanoribbons with different widths by top-down or bottom-up approaches. Moreover, based on the on-surface reaction, the bottom-up approach shows the capability of controlling the width and edge structures, and it is almost contamination-free processing, which is suitable to performing further characterizations. Ultra-high-vacuum scanning tunneling microscope is a valid tool to fabricate and characterize the graphene nanorribons, and it can also obtain the band structure information when combined with the scanning tunneling spectroscopy. Taking the advantage of the bottom-up synthetic technique, the nearly perfect graphene nanoribbons can be fabricated based on the organic molecule reaction on surface, which is a promising strategy to study the original electronic properties. To precisely tuning the band engineering of graphene nanoribbons, the researchers have adopted various effective methods, such as changing the widths and topological morphologies of graphene nanoribbons, doping the graphene nanoribbons with heteroatoms, fabricating the heterojunctions under a controlable condition. The precise control of graphene synthesis is therefore crucial for probing their fundamental physical properties. Here we highlight the methods of fabricating the graphene nanoribbons and the precise tuning of graphene bandgap structure in order to provide a feasible way to put them into application.
2017, 66 (21): 217802.
doi: 10.7498/aps.66.217802
Abstract +
Silicon photonics is considered as a promising technology to realize high-performance photonic integrated circuits, owing to its complementary metal oxide semiconductor-compatibility which is applicable for large-scale integration at low cost. However, due to the limitation of optoelectronic properties of silicon, the challenge to the realization of high-performance active device on the silicon integrated platform still exists. The recent development of graphene-silicon hybrid photonic integrated circuit provides a practical solution to this problem, because graphene, as a superior two-dimensional material, possesses many advantageous optoelectronic properties, such as high mobility, high electro-optical coefficient, and broadband absorption, which can be fully exploited to break through the material limitation of silicon. Moreover, compared with other active integrated materials such as germanium and compound semiconductors, graphene is cost-effective and can be conveniently integrated with silicon photonic device. Here, we review some important research progress of graphene-silicon hybrid photonic integrated circuits that include optical sources, optical waveguides, optical modulators, and photodetectors. The challenges and prospects of these devices are also analyzed, which are expected to be beneficial to the relevant research communities.
2017, 66 (21): 218101.
doi: 10.7498/aps.66.218101
Abstract +
Due to its unique properties, graphene is a promising two-dimensional material in optoelectronic and energy applications. While the mobility of single layer graphene is extremely high, it has a zero bandgap. This feature restricts various applications of graphene in the field of semiconductor devices. Bilayer graphene, despite the nature of zero bandgap in its pristine form, can be tuned to open bandgap via a dual-gated vertical electrical field in a controlled manner. However, the size and layer number of mechanically exfoliated and liquid phase exfoliated graphene are poorly controlled. Controllable synthesis of large-sized bilayer graphene is an important research direction.
This review summarizes a series of work including the controlled synthesis of bilayer graphene by chemical vapor deposition method and bilayer graphene devices. Specifically, growth mechanism of bilayer graphene is dependent on the type of supporting substrate and experimental condition. In the case of Ni substrate, bilayer graphene is grown along the segregation route. On the other hand, graphene growth on Cu is a surface-mediated process due to the extremely low solubility of C in Cu bulk. Depending on the concentration ratio between CH4 and H2, the growth mode of bilayer graphene can be tuned to be similar to Volmer-Weber or Stranski-Krastanov mode, in which the second layer is either grown under or above the first graphene layer. The dynamic growth of bilayer graphene can be further understood by a chemical gate effect and the process in a confined space. Moreover, here in this paper we present several approaches to realize the better control of bilayer graphene growth by modulating the experimental conditions.
In terms of device applications for bilayer graphene, in this review we mention two typical applications including field-effect-transistors and hot-electron bolometers. Compared with conventional silicon-based hot-electron bolometer, the bilayer graphene based hot-electron bolometer has a small heat capacity and weak electron-phonon coupling, leading to high sensitivity, fast response, and small thermal noise-equivalent power. Such a bilayer graphene bolometer shows an exceptionally low noise-equivalent power and intrinsic speed three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.
Finally, the outlook and challenge for future research are also given. While significant progress has been made in the past several years, the controlled growth of bilayer or multi-layer graphene is still a key challenge, and the growth mechanism of bilayer graphene is not yet understood clearly. There is still much room for controlling graphene layer numbers, twisted angles, size, quality, and yield by optimizing the conditions. On the other hand, for the device applications of bilayer graphene, it is highly desired to develop high-performance bilayer graphene-based electronic devices.
2017, 66 (21): 218503.
doi: 10.7498/aps.66.218503
Abstract +
The semiconductor industry has experienced exponential growth for more than 50 years, following the Moore's Law. However, traditional microelectronic devices are currently facing challenges such as high energy consumption and the short-channel effect. As an alternative, two-dimensional layered materials show the ability to restrain the carriers in a 1 nm physical limit, and demonstrate high electron mobility, mutable bandgap, and topological singularity, which will hopefully give birth to revolutionary changes in electronics. The transition metal dichalcogenide (TMDC) is regarded as a prospective candidate, since it has a large bandgap (typically about 1-2 eV for a monolayer) and excellent manufacture compatibility. Here in this paper, we review the most recent progress of two-dimensional TMDC and achievements in logic integration, especially focusing on the following key aspects:charge transport, carrier mobility, contact resistance and integration. We also point out the emerging directions for further research and development.
