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Vol. 66, No. 21 (2017)

2017-11-05

EDITOR'S SUGGESTION

Fabrication of zigzag-edged graphene antidot lattice and its transport properties
Zhang Ting-Ting, Cheng Meng, Yang Rong, Zhang Guang-Yu
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.
Recent progress of graphene-like germanene
Qin Zhi-Hui
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.
Intercalation and its mechanism of high quality large area graphene on metal substrate
Guo Hui, Lu Hong-Liang, Huang Li, Wang Xue-Yan, Lin Xiao, Wang Ye-Liang, Du Shi-Xuan, Gao Hong-Jun
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.

EDITOR'S SUGGESTION

Fabrication of graphene nanostructure and bandgap tuning
Zhang Hui-Zhen, Li Jin-Tao, Lü Wen-Gang, Yang Hai-Fang, Tang Cheng-Chun, Gu Chang-Zhi, Li Jun-Jie
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.
Novel p-n junctions based on ambipolar two-dimensional crystals
Zhang Zeng-Xing, Li Dong
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.
Low voltage scanning transmission electron microscopy for two-dimensional materials
Li Dong-Dong, Zhou Wu
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.
Research status and development graphene devices using silicon as the subtrate
Wu Pei, Hu Xiao, Zhang Jian, Sun Lian-Feng
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.
High performance graphene Hall sensors
Huang Le, Zhang Zhi-Yong, Peng Lian-Mao
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.
Research progress of graphene radio frequency devices
Lu Qi, Lyu Hong-Ming, Wu Xiao-Ming, Wu Hua-Qiang, Qian He
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.
Research progress of direct synthesis of graphene on dielectric layer
Yang Hui-Hui, Gao Feng, Dai Ming-Jin, Hu Ping-An
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.
Chemical functionalization of silicene
Yang Shuo, Cheng Peng, Chen Lan, Wu Ke-Hui
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.
Nonlocal resistance in multi-terminal graphene system
Wang Zi-Bo, Jiang Hua, Xie Xin-Cheng
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.

EDITOR'S SUGGESTION

Two-dimensional hexagonal boron nitride based memristor
Wu Quan-Tan, Shi Tuo, Zhao Xiao-Long, Zhang Xu-Meng, Wu Fa-Cai, Cao Rong-Rong, Long Shi-Bing, Lü Hang-Bing, Liu Qi, Liu Ming
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.
Gigahertz frequency doubler based on millimeter-scale single-crystal graphene
Gao Qing-Guo, Tian Meng-Chuan, Li Si-Chao, Li Xue-Fei, Wu Yan-Qing
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.
Preparation, structure configuration, physical properties and applications of borophene and two-dimensional alkaline-earth metal boride nanomaterials
Guo Ze-Kun, Tian Yan, Gan Hai-Bo, Li Zi-Juan, Zhang Tong, Xu Ning-Sheng, Chen Jun, Chen Huan-Jun, Deng Shao-Zhi, Liu Fei
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.
Fabrication and electrical engineering of graphene nanoribbons
Zhang Hui, Cai Xiao-Ming, Hao Zhen-Liang, Ruan Zi-Lin, Lu Jian-Chen, Cai Jin-Ming
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.
Graphene-silicon hybrid photonic integrated circuits
Xiao Ting-Hui, Yu Yang, Li Zhi-Yuan
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.
Synthesis of bilayer graphene via chemical vapor deposition and its optoelectronic devices
Yang Yun-Chang, Wu Bin, Liu Yun-Qi
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.
Logical integration device for two-dimensional semiconductor transition metal sulfide
Li Wei-Sheng, Zhou Jian, Wang Han-Chen, Wang Shu-Xian, Yu Zhi-Hao, Li Song-Lin, Shi Yi, Wang Xin-Ran
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.
GENERAL
Denoising of contaminated chaotic signals based on collaborative filtering
Chen Yue, Liu Xiong-Ying, Wu Zhong-Tang, Fan Yi, Ren Zi-Liang, Feng Jiu-Chao
2017, 66 (21): 210501. doi: 10.7498/aps.66.210501
Abstract +
Reconstructing chaotic signals from noised data plays a critical role in many areas of science and engineering. However, the inherent features, such as aperiodic property, wide band spectrum, and extreme sensitivity to initial values, present a big challenge of reducing the noises in the contaminated chaotic signals. To address the above issues, a novel noise reduction algorithm based on the collaborative filtering is investigated in this paper. By exploiting the fractal self-similarity nature of chaotic attractors, the contaminated chaotic signals are reconstructed subsequently in three steps, i.e., grouping, collaborative filtering, and signal reconstruction. Firstly, the fragments of the noised signal are collected and sorted into different groups by mutual similarity. Secondly, each group is tackled with a hard threshold in the two-dimensional (2D) transforming domain to attenuate the noise. Lastly, an inverse transformation is adopted to estimate the noise-free fragments. As the fragments within a group are closely correlated due to their mutual similarity, the 2D transform of the group should be sparser than the one-dimensional transform of the original signal in the first step, leading to much more effective noise attenuation. The fragments collected in the grouping step may overlap each other, meaning that a signal point could be included in more than one fragment and have different collaborative filtering results. Therefore, the noise-free signal is reconstructed by averaging these collaborative filtering results point by point. The parameters of the proposed algorithm are discussed and a set of recommended parameters is given. In the simulation, the chaotic signal is generated by the Lorenz system and contaminated by addictive white Gaussian noise. The signal-to-noise ratio and the root mean square error are introduced to measure the noise reduction performance. As shown in the simulation results, the proposed algorithm has advantages over the existing chaotic signal denoising methods, such as local curve fitting, wavelet thresholding, and empirical mode decomposition iterative interval thresholding methods, in the reconstruction accuracy, improvement of the signal-to-noise ratio, and recovering quality of the phase portraits.
Directed weighted complex networks based on time series symbolic pattern representation
Zeng Ming, Wang Er-Hong, Zhao Ming-Yuan, Meng Qing-Hao
2017, 66 (21): 210502. doi: 10.7498/aps.66.210502
Abstract +
Complex networks are capable of modeling different kinds of complex systems in nature and technology, which contain a large number of components interacting with each other in a complicated manner. Quite recently, various approaches to analyzing time series by means of complex networks have been proposed, and their great potentials for uncovering valuable information embedded in time series, especially when nonlinear dynamical systems are incapable of being described by theoretical models have been proven. Despite the existing contributions, up to now, mapping time series into complex networks is still a challenging problem. In order to more effectively dig out the structural characteristics of time series (especially the nonlinear time series) and simplify the computational complexity of time series analysis, in this paper we present a novel method of constructing a directed weighted complex network based on time series symbolic pattern representation combined with sliding window technique. The proposed method firstly implements symbolic procession according to the equal probability segment division and then combines with the sliding window technique to determine the symbolic patterns at different times as nodes of the network. Next, the transition frequency and direction of symbolic patterns are set as the weights and directions of the network edges, thus establishing the directed weighted complex network of the analyzed time series. The results of test using the Logistic system with different parameter settings show that the topological structures of the directed weighted complex network can not only intuitively distinguish the periodic time series and chaotic time series, but also accurately reflect the subtle changes of two types of time series. These results are superior to those from the classical visibility graph method which can be only roughly classified as two types of signals. Finally, the proposed technique is used to investigate the natural wind field signals collected at an outdoor open space in which nine high precision two-dimensional (2D) ultrasonic anemometers are deployed in line with 1 m interval. The topological parameters of the network analysis include the network size, weighted clustering coefficient, and average path length. The corresponding results of our approach indicate that the values of three network parameters show consistent increase or decrease trend with the spatial regular arrangement of the nine anemometers. While the results of the visibility graph network parameters are irregular, and cannot accurately predict the spatial deployment relationship of nine 2D ultrasonic anemometers. These interesting findings suggest that topological features of the directed weighted complex network are potentially valuable characteristics of wind signals, which will have broad applications in researches such as wind power prediction, wind pattern classification and wind field dynamic analysis.
ATOMIC AND MOLECULAR PHYSICS
Autler-Townes splitting of ultracold cesium Rydberg atoms
Xue Yong-Mei, Hao Li-Ping, Jiao Yue-Chun, Han Xiao-Xuan, Bai Su-Ying,   Zhao Jian-Ming, Jia Suo-Tang
2017, 66 (21): 213201. doi: 10.7498/aps.66.213201
Abstract +
Autler-Townes (A-T) splitting, known as an AC Stark effect, shows a change of an absorption/emission spectral line of a probe beam when an oscillating field is tuned in resonance with the atomic or molecular transition. The A-T splitting is observed in different three-level atoms and widely investigated in a vapor cell and in a magneto-optical trap (MOT). The A-T splitting plays an important role in the atom-based microwave electric-field measurements where a cascade three-level system involving Rydberg state is adopted. In this work, an A-T splitting is observed in an ultracold cesium Rydberg gas, which is cooled down to about 100 pK and center density is about 1010 cm-3 in a conventional MOT by using the laser cooling technology. We present the A-T spectrum in a ladder three-level atomic system involving a 34D5/2 Rydberg state. The cesium ground state (6S1/2), excited state (6P3/2) and Rydberg state (34D5/2) constitute a Rydberg three-level system. A coupling laser, locked to the Rydberg transition by using a Rydberg electromagnetically induced transparency signal that is obtained from a cesium room-temperature vapor cell, couples 6P3/2 (F'=5) 34D5/2 Rydberg transition. A weak probe laser, stabilized to a ground-state transition by using a polarization spectroscopy, is swept, covering the transition 6S1/2 (F=4) 6P3/2 (F'=5) with a double-passed acousto-optic modulator. The probe and coupling lasers are counter-propagated through the MOT center. The power of probe light is 200 pW, corresponding Rabi frequency p=21.05 MHz. During the experiment, 50 s after turning off the trapping laser, both the coupling and probe beams are switched on and last 100 s. The A-T spectrum as a function of the probe detuning is detected with a single-photon counter module detector. We use Gaussian multiple peak fitting to obtain the positions of the A-T peaks and the A-T splitting. The measured A-T splitting is proportional to the Rabi frequency of the coupling light. We numerically solve the density matrix equations to obtain the A-T spectrum, and the calculations reproduce A-T spectra well. The measured A-T splitting shows good agreement with the theoretical calculation for Rabi frequency of the coupling light c29 MHz. The A-T splitting is less than the calculation for the case of c29 MHz, the deviation is mainly attributed to the increased dephasing rate induced by the strong interaction between Rydberg atoms, whose number increases with the coupling laser Rabi frequency. In this work, the adopted method for the cascade three-level system involving Rydberg state is also suitable for -and V-type cases.
Theoretical research on an efficient population transfer based on two different laser pulse sequences
Zhang Lu, Yan Lu-Yao, Bao Hui-Han, Chai Xiao-Qian, Ma Dan-Dan, Wu Qian-Nan, Xia Ling-Chen, Yao Dan, Qian Jing
2017, 66 (21): 213301. doi: 10.7498/aps.66.213301
Abstract +
A quantum gas of ultracold molecules, with long-range and anisotropic interactions, will enable a series of fundamental studies in physics and chemistry. In particular, samples of ground-state molecules at ultralow temperatures and high number densities will facilitate the explorations of a large number of many-body physical phenomena and applications in quantum information processing. However, due to the lack of efficiently cooling techniques such as laser cooling for atomic gases, high number densities for ultracold molecular samples are not readily attainable. Associating ultracold atoms to weakly bound dimer molecules via Feshbach resonance and subsequently transferring them to a wanted molecular ro-vibronic ground state by a stimulated Raman adiabatic passages (STIRAP) have proved to be an effective way in producing ideal ultracold molecular samples. As a typical illustration, in a recent study (2010 Nat. Phys. 6 265) Danzl et al. experimentally realized the preparation of Cs2 molecule into its ro-vibronic ground state via two different multi-level STIRAPs:one is based on a single conversion route and the others are based on a cascade-connected route (labeled by 4p-STIRAP and s-STIRAP, respectively). In this work, we present a theoretical study for these two STIRAP schemes, focusing on the differences in physical principle and realistic performance between them. On the one hand, according to the theoretical approach of quasi-dark eigenstates, we conclude that a highly efficient population transfer is achievable in both schemes. On the other hand, by systematically studying the influences of the relevant parameters, including the spontaneous decays and the detunings from the intermediate states, and the temporal sequence and the amplitude of the laser pulses, we disclose their respective advantages and weaknesses in the realistic implementation. We theoretically predict that for both schemes their maximal conversion efficiencies each can attain 0.97 as long as the spontaneous decays from the intermediate excited states are sufficiently suppressed. Yet considering the fact that the already implemented efficiency is only around 0.6 for both schemes, there is still room for optimization, e.g. using stable Rydberg energy levels in future experiment. Furthermore, the success of these two schemes can provide a new route to the controllable entanglement preparation, opening more applications in the fields of quantum logic gate and so on.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER,CLASSICAL MECHANICS, AND FLUID DYNAMICS
Reverse time migration Lamb wave imaging based on mode separation
Zhang Hai-Yan, Yang Jie, Fan Guo-Peng, Zhu Wen-Fa, Chai Xiao-Dong
2017, 66 (21): 214301. doi: 10.7498/aps.66.214301
Abstract +
Frequency domain reverse time migration method is used to reconstruct damages in isotropic and anisotropic plates. Considering multimode overlapping, the Lamb wave signals scattered by the defects may result in artifacts in defect imaging. The scattering signals are thus pre-processed by using a mode separation method based on the vibration symmetry difference between the fundamental guided modes. Based on the multi-element array ultrasonic technique, a numerical study is carried out for defect imaging of aluminum and composite plates by using the frequency reverse time migration method. This paper is organized as follows. Firstly, in order to capture multi-directional information about damages, scattering Lamb wave signals caused by the defects are numerically collected by an annular array of transducers through using the finite element simulation. Secondly, after the pre-processing of mode separation, the separated scattering signals are time-reversed and used to stimulate the corresponding receivers. The Green's function is utilized to back-propagate the scattering Lamb signals in frequency domain, so that the back-propagated acoustic field information of monitored area can be obtained. Finally, the defect images are reconstructed by the cross-correlation between the incident acoustic field and the back-propagated acoustic field. To illustrate the influence of mode separation, the numerical experiments are carried out on an aluminum plate with single defect and on another composite plate with two adjacent identical defects. The reconstructed results from frequency domain reverse time migration method with and without mode separation are compared. The comparison indicates the importance of mode separation. Furthermore, the method is extended to detecting the double adjacent defects with different depths in the composite plate. The imaging result illustrates that the presupposed two adjacent defects with different depths are successfully identified. Numerical results demonstrate that the pre-processing of mode separation helps to effectively remove the artifacts resulting from the multimode interference in the imaging process. The proposed frequency reverse time migration method presents a strong potential for detecting and imaging defects in isotropic and anisotropic plates, which is capable of accurately measuring multi-site defects with information about geometry, size and depth.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
One-dimensional numerical investigation on the formation of Z-pinch dynamic hohlraum using the code MULTI
Wu Fu-Yuan, Chu Yan-Yun, Ye Fan, Li Zheng-Hong, Yang Jian-Lun, Rafael Ramis, Wang Zhen, Qi Jian-Min, Zhou Lin, Liang Chuan
2017, 66 (21): 215201. doi: 10.7498/aps.66.215201
Abstract +
Z-pinch dynamic hohlraum can effectively convert Z-pinch plasma kinetic energy into radiation field energy, which has a potential to implode a pellet filled with deuterium-tritium fuel to fusion conditions when the drive current is sufficiently large. To understand the formation process of Z-pinch dynamic hohlraum on JULONG-I facility with a typical drive current of 8-10 MA, a new radiation magneto-hydrodynamics code is developed based on the program MULTI-IFE. MULTI-IFE is a one-dimensional, two-temperature, multi-group, open-source radiation hydrodynamic code, which is initially designed for laser and heavy ion driven fusion. The original program is upgraded to simulate Z-pinch related experiments by introducing Lorentz force, Joule heating and the evolution of magnetic field into the code. Numerical results suggest that a shock wave and a thermal wave will be launched when the high speed plasma impacts onto the foam converter. The thermal wave propagates much faster than shock wave, making the foam become hot prior to the arrival of shock wave. For the load parameters and drive current of shot 0180, the calculated propagation speed of thermal wave and shock wave are about 36.1 cm/s and 17.6 cm/s, respectively. The shock wave will be reflected when it arrives at the foam center and the speed of reflected shock wave is about 12.9 cm/s. Calculations also indicate that the plastic foam will expand obviously due to the high temperature radiation environment (~30 eV) around it before the collision between tungsten plasma and foam converter. The evolution of radial radiation temperature profile shows that a pair of bright strips pointing to the foam center can be observed by an on-axis streak camera and the radiation temperature in the foam center achieves its highest value when the shock arrives at the axis. A bright emission ring moving towards the foam center can also be observed by an on-axis X-ray frame camera. The best time to capture the bright strips and bright emission rings is before the thermal wave reaches the foam center. Even though some amount of X-ray radiation in the foam is expected to escape from the hohlraum via radiation transport process, simulation results suggest that the tungsten plasma can serve as a good hohlraum wall. The radiation temperature is about 80 eV when the dynamic hohlraum is created and can rise more than 100 eV before the shock arrives at the foam center. Most of the X-rays emitted by the wire-array plasma surface have energies below 1000 eV. In this paper, the physical model of the code MULTI-IFE and the simulation results of array implosions on Saturn facility are presented as well.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Phase-field crystal modelling the nucleation processes of graphene structures on different substrates
Gu Ji-Wei, Wang Jin-Cheng, Wang Zhi-Jun, Li Jun-Jie, Guo Can, Tang Sai
2017, 66 (21): 216101. doi: 10.7498/aps.66.216101
Abstract +
Two-dimensional materials with unique and excellent physical and chemical properties have attracted much attention in recent years. Among the two-dimensional materials, graphene or grapheme-like materials with honeycomb structure can be mainly prepared by the chemical vapor deposition (CVD) method. The key of this method is to select the substrates and control the nucleation and growth process of honeycomb structures. Graphene prepared by CVD contains many structure defects and grain boundaries, which mainly arise from nucleation process. However, the nucleation mechanism of graphene prepared by CVD method is not very clear. In addition, more than ten kinds of metal substrates can be used as substrate materials in CVD methods, such as Cu and Ni, which have nearly always face-centered cubic (FCC) structures and similar functions in the preparation process. In order to better describe the nucleation of graphene and understand the influences of metal substrates, we introduce the structural order parameter into the three-mode phase-field crystal model to distinguish the low-density gas phase from condensed phases. Nucleation processes of graphene on substrates with different symmetries are studied at an atomic scale by using the three-mode phase-field crystal model, which can simulate transitions between highly correlated condensed phases and low-density vapor phases. Simulation results indicate that no matter whether there is a substrate in the nucleation process, firstly gaseous atoms gather to form amorphous transitional clusters, and then amorphous transitional clusters gradually transform into ordered graphene crystals, with continuous accumulation of new gaseous atoms and position adjustment of atoms. In the nucleation process, five membered ring structures act as a transitional function. When grown on the substrate with a good geometric match with the honeycomb lattice, such as (111) plane of FCC metals, the graphene island has small structural defects. However, when grown without a substrate or on the substrate with a bad geometric match, such as (100) plane of FCC metals, the graphene island contains many structural defects and grain boundaries, which are not conducive to the preparation of high quality graphene. Compared with the (100) crystal plane of the tetragonal cell, the (110) crystal plane of the rectangular cell is favorable for the preparation of graphene single crystals with less defects. Therefore, the appropriate metal substrate can promote the nucleation process of graphene and reduce the formation of distortions and defects during the nucleation and growth of graphene.
Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction
He Ju-Sheng, Zhang Meng, Zou Ji-Jun, Pan Hua-Qing, Qi Wei-Jing, Li Ping
2017, 66 (21): 216102. doi: 10.7498/aps.66.216102
Abstract +
Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.
First-principles study on the adsorption of oxygen at NiTi (110) surface
Liu Kun, Wang Fu-He, Shang Jia-Xiang
2017, 66 (21): 216801. doi: 10.7498/aps.66.216801
Abstract +
NiTi alloys with equiatomic compositions have been widely used as structural materials in aerospace, aviation and other fields due to their shape memory effects and good mechanical performances. At the same time, they are considered as excellent biomedical materials for their biocompatibilities and high fatigue resistances. As structural materials, the oxidation resistance of NiTi alloy should be improved. However, as biomedical materials, the formation of dense TiO2 layers on the surface of NiTi alloy is required to suppress the release of Ni ions in body liquid. As a result, it is of great significance to study the oxidation mechanism of NiTi alloy. In this work, while the total number of Ti is kept the same as that of Ni atoms in the whole system, a series of defected c(22)-NiTi (110) surfaces with antisite of Ti are constructed to further understand the oxidation mechanism of NiTi alloy. The adsorption of oxygen atom at the NiTi (110) surface is investigated by the first-principles calculations. The calculated results show that the stability of the oxygen adsorption is strongly related to the enrichment of Ti atoms on the surface. The higher the enrichment of Ti atoms on the surface, the stronger the adsorption of oxygen atoms is. When the coverage of oxygen is high enough, the adsorption of oxygen atoms on the surface could cause the antisite of Ti atoms on the surface by the exchange of Ni atoms in the first layer with Ti atoms in other layers. Under the O-rich conditions (O -9.35 eV), it is the most stable that the oxygen atoms adsorbed on Ti antisite surface, with the whole Ni atoms in the first surface layer exchanged with the whole Ti atoms in the third surface layer. With the increase of the adsorbed oxygen atoms on the surface, the heights of Ti atoms in the surface layers are raised by the adsorption of oxygen. The TiO2 layer is formed by the expansive growth, while Ni atoms are enriched beneath the TiO2. As a result, the reason why the TiO2 layer is formed on the NiTi alloy surface in the experimental conditions is well explained.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL,MAGNETIC, AND OPTICAL PROPERTIES
Growth of graphene on Al2O3 (0001) surface
Li Jin-Jin, Li Duo-Sheng, Hong Yue, Zou Wei, He Jun-Jie
2017, 66 (21): 217101. doi: 10.7498/aps.66.217101
Abstract +
At present, high quality graphene is synthesized mainly by chemical vapor deposition. It is crucial to decompose and adsorb methane (CH4) on the surface of substrate before CH4 grows into graphene. The graphene is grown mainly on metal substrate due to the catalytic effect of metal. It is difficult to grow graphene thin film on the surface of non-metallic substrate, especially on the surface of -Al2O3 (0001). In this paper, the density functional theory based generalized gradient approximation method is applied to simulating the nucleation of graphene on -Al2O3 (0001) surface, synthesized by chemical vapor deposition. First, we establish a scientific -Al2O3 (0001) surface model, then simulate the decomposition process of CH4 on -Al2O3 (0001) surface by calculating the adsorption sites and adsorption configurations of groups and atoms. Finally, we investigate the groups of CH4 decomposition and atom coupling process on -Al2O3 (0001) surface. The results show that the CH3 groups, C and H atoms are preferentially adsorbed at the top of the O atoms, and the adsorption energies are -2.428 eV,-4.903 eV, and -4.083 eV, respectively. The CH2 and CH groups are preferentially adsorbed on the bridge between O and Al atoms with the adsorption energies of -4.460 eV and -3.940 eV, respectively. The decomposition of CH4 on -Al2O3 (0001) surface is an endothermic process. It requires higher energy and cross reactive energy barrier for CH4 to be completely decomposed into C atom, which makes it difficult that the C atom stays on the substrate surface. The coupling process among CH groups on the surface of -Al2O3 (0001) is an exothermic process. When CH and CH groups are coupled, the energy of the system decreases by 4.283 eV. When (CH)2 and CH groups are coupled, the energy of the system decreases by 3.740 eV. The (CH)x can be obtained by continuous migration and coupling between the CH groups on the surface of the -Al2O3 (0001), and (CH)x group is a precursor of graphene growth. The energy of the system decreases in the process. The above results show that the activated atom or group of graphene nucleation is not C atom but CH group. The CH group migration and aggregation on the surface of -Al2O3 (0001) give priority to the formation of lower energy (CH)x structure. In order to better understand the microscopic growth process of graphene on sapphire, it is important to study the role of (CH)x in the surface of sapphire for revealing the nucleation mechanism of graphene.
Three-dimensional magnetization dynamics in majority gate studied by using multiferroic nanomagnet
Wei Bo, Cai Li, Yang Xiao-Kuo, Li Cheng
2017, 66 (21): 217501. doi: 10.7498/aps.66.217501
Abstract +
The scaling of traditional complementary metal oxide semiconductor (CMOS) device is reaching its physical limit, and alternative emerging devices are being explored as possible CMOS substitutes. One of the most promising device technologies is nano-magnetic logic (NML), which is an energy-efficient computing paradigm. The inherent nonvolatility and low energy consumption make NML device possess wide application perspectives. The basic element of multiferroic NML is a sub-100 nm sized single domain magnet. Generally, the x-y plane determines the in-plane dimension, while the z direction indicates the thickness of nanomagnet. Classical binary logic states 0 and 1 are encoded in two stable magnetization orientations along the easy axis (major axis) of the elliptical nanomagnet, while the hard axis (minor axis) refers to null logic. In order to propagate logic bits between the neighbor nanomagnets, one requires a clock that periodically flips every magnet's magnetization along the hard axis simultaneously, and the dipole-dipole interaction between the neighbors will force the magnet into the correct orientation along the easy axis, and thus the logic bit propagates unidirectionally. In multiferroic NML, the majority gate is a basic element of nanomagnet logcal circuit. In this paper, the three-dimensional switching dynamic model of a multiferroic nanomagnetic majority gate is established, and its magnetization dynamics is simulated by solving the Landau-Lifshitz-Gilbert equation with considering the thermal fluctuation effects. The majority gate is implemented with dipole-coupled two-phase (magnetostrictive/piezoelectric) multiferroic elements and is simulated by using different strain clocks and changing the input. It is found that the majority gate works efficiently and correctly when receiving new input. It is also found that the optimal time interval of stress releasing between central nanomagnet and output nanomagnet is 0.1-0.2 ns. Removing stress earlier will reduce the success rate of the majority gate operation while the work frequency increases. The reason behind the phenomenon may be that removing stress earlier results in weak dipole-coupled interaction, which cannot overcome the shape anisotropy. These findings are beneficial to the design of multiferroic logic circuit.
Monte Carlo simulation of time-dependent dielectric breakdown of oxide caused by migration of oxygen vacancies
Li Ping, Xu Yu-Tang
2017, 66 (21): 217701. doi: 10.7498/aps.66.217701
Abstract +
In this article, the Monte Carlo method is used to study the formation and migration of oxygen vacancies in metal oxide dielectric. The time-dependent breakdown of the dielectric is simulated. In the direction of the electric field across the metal oxide, the migration barrier and migration work function of oxygen vacancies are found to be reduced by the applied electric field. This finding provides a good foundation for further studying the breakdown mechanism and evaluating the reliability of high gate dielectric. The Monte Carlo process is described as follows. Firstly, a three-dimensional metal oxide dielectric layer is built with two-dimensional symmetrical grid, where the thickness of the oxide layer is set to be 9 lattice points and the oxygen vacancies can migrate to the adjacent 8 arbitrary lattice positions in this simulation. Secondly, the possibilities of formation and migration of oxygen vacancies are calculated according to the distribution of oxygen vacancies. Finally, the Monte Carlo method is used to simulate the new distribution of oxygen vacancies. Therefore, we simulate the breakdown process of the metal oxide dielectric layer with different oxygen vacancy migration functions (Ea=1.15, 1.35 eV) at the interface. And we obtain the results as follows. 1) When the migration function is small, many oxygen vacancies accumulate largely at the forming interface. And the vacancies would migrate from the interface to the dielectric, forming a conductive channel. The breakdown time is determined by the migration barrier of oxygen vacancies in the dielectric. 2) When the migration function of the oxygen vacancies at the interface becomes larger, the formed oxygen vacancies will rapidly migrate to the other interface, and the reverse propagation of the conductive channel causes the dielectric breakdown. Therefore, larger migration function of the oxygen vacancies at the interface can effectively improve its reliability. 3) The original defects within the dielectric will seriously influence the migration of oxygen vacancies, and the breakdown is easier to occur with more primary defects. 4) The simulation shows that the oxygen vacancy migration function can be improved by optimizing the interface formation process. And the breakdown time could also be prolonged. Therefore, this simulation tool can be applied to the research of metal-oxide-semiconductor transistor gate dielectric breakdown and the assessment of its reliability accurately.
Fabrication of two-dimensional micro-nano photonic structures by symmetry-lost beams interference
Lü Hao, You Kai, Lan Yan-Yan, Gao Dong, Zhao Qiu-Ling, Wang Xia
2017, 66 (21): 217801. doi: 10.7498/aps.66.217801
Abstract +
Micro-nano photonic structures, such as meta-materials and photonic crystals, having special effects on light generation, transmission, detection and sensing on a submicron scale, play an increasingly significant role in optical information fields. Micro-nano photonic structures have great potential applications in surface laser emission, optical waveguide and high-Q laser. There are several methods to fabricate micro-nano photonic structures, including laser direct writing, electron beam direct writing, electrochemical corrosion, and holographic lithography and so on. Holographic lithography employs multi-beam interference to generate periodic patterns and records them on photosensitive materials to form typical structures. What is more, it has advantages of low cost, large area and high efficiency. However, there are still some challenges in fabricating typical micro-nano photonic structures, especially the precise optical alignment, beam polarization and control of the details of local interference pattern. A specially designed prism is employed in this work and we propose a compact symmetry-lost setup with the rapid adjustment of beam configuration and polarization. Based on the theory of multi-beam interference, symmetry-lost four-and five-beam interference with different polarizations are simulated. By changing the combination of beam configuration and polarization, novel two-dimensional micro-nano photonic structures can be achieved. The variations of azimuthal angle and polarization of beam in symmetry-lost system affect the wave vector difference, and thus changing the lattice shape and structure contrast. Branch-like and wave-like structures are generated by symmetry-lost four beams with polarizations of (90, 90, 90, 90) and five beams with polarizations of (90, 90, 90, 90, 0), respectively. An appropriate threshold is selected in simulation, such that the intensity data larger than the threshold are removed, while the smaller data are remained, which is transformed into an optical lattice pattern. The interference structures show different morphologies and structural contrasts, and have a period of several hundred nanometers. In experimental fabrication, a top-cut hexagonal prism is used to generate two-dimensional micro-nano photonic structure on CHP-C positive photoresist by single exposure. The intensity of each beam is about 18-20 mW, and the incident angle is 42. The beam polarization is adjusted by rotating a half waveplate inside the holes of the mask and structure volume fraction is determined by exposure dose controlled by beam intensity and exposure time. The scanning electron microscope images of the samples show good agreement with simulation results. This study could provide an effective method of fabricating novel photonic structures, which can be used as templates of fabricating different types of metal lattice structures, thereby promoting the development and applications of novel photonic devices.