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COVER ARTICLE

A review of key technologies for epitaxy and chip process of micro light-emitting diodes in display application
Pan Zuo-Jian, Chen Zhi-Zhong, Jiao Fei, Zhan Jing-Lin, Chen Yi-Yong, Chen Yi-Fan, Nie Jing-Xin, Zhao Tong-Yang, Deng Chu-Han, Kang Xiang-Ning, Li Shun-Feng, Wang Qi, Zhang Guo-Yi, Shen Bo
2020, 69 (19): 198501. doi: 10.7498/aps.69.20200742
Abstract +
The continuous miniaturization and integration of pixelated devices have become a main trend in the field of display. Micro light-emitting diode (micro-LED) display is composed of an array of LEDs that are sub-50-micrometers in length. It has huge advantages in brightness, resolution, contrast, power consumption, lifetime, response speed and reliability compared with liquid crystal display (LCD) and organic LED (OLED) display. Consequently, micro-LED display is regarded as the next-generation display technology with high potential applications, such as virtual reality (VR), augmented reality (AR), mobile phones, tablet computers, high-definition TVs and wearable devices. Currently, the combination of commercial 5G communication technology with VR/AR display, ultra high definition video technologies will further prompt the development of micro-LED display industry. However, some basic scientific and technological problems in micro-LED display remain to be resolved. As the chip size shrinks to below 50 μm, some problems that are not serious for large-sized LEDs appear for micro-LEDs. These problems include crystalline defects, wavelength uniformity, full-color emmision, massively tranferring and testing, etc. In the past two decades, various solutions to those problems have been proposed, which have greatly promoted the progress of micro-LED display. In this paper, an overview of micro-LED display since 2000 is given firstly, which includes the main research results and application achievements. Secondly the issues involved in the wafer epitaxy and chip process of micro-LEDs and possible solutions are discussed based on the display application in detail. The surface state induced by the dangling bonds and dry etching damages are concerned for the nonradiative recombination at a low injection level. The remedies are provided for those surface states, such as atomic-layer deposition and neutral beam etching. Some methods to reduce the threading dislocation and suppress the polarization field are summarized for micro-LED epitaxial growth. Moreover, the GaN-based LEDs on Si (100) substrate are also introduced for the future integration of micro-LEDs into the Si-based integrated circuits. As to the wavelength uniformity, the MOCVD equipment and growth technology including the laser treatment are discussed. In the chip processing part, the full-color display, mass transfer and effective inspection technology are discussed. Assembling RGB individual LEDs, quantum dot phosphor material and nanocoloumn LEDs are different routes for full-color display. Their trends in the future are provided. The pick and place, laser lift-off technologies, are strengthened in the massively transferring for micro-LEDs. In the massively and rapidly inspection technologies, the photoluminscence combined with Raman scattering, the electroluminescence combined with digital camera are discussed. Finally, the summary and outlook in these issues are also provided.

INVITED REVIEW

  

INVITED REVIEW

Recent research progress of non-noble metal based surface-enhanced Raman scattering substrates
Liu Xiao-Hong, Jiang Shan, Chang Lin, Zhang Wei
2020, 69 (19): 190701. doi: 10.7498/aps.69.20200788
Abstract +
Surface-enhanced Raman scattering (SERS) is of great importance in analytical science, the noble-metal such as gold and silver are widely used in SERS research and applications. However, noble-metal based substrates are hampered in practical application. As for comparison, the Non-noble metal especially the semiconductor materials are the emerging SERS research frontier. Non-noble metal (such as C, Ti, Zn, Cu, Mo, W, etc.) nanomaterials based SERS substrate have been widely studied and applied due to their superior stability, selectivity, biocompatibility and low cost comparing to noble metal materials. As the chemical enhancement dominate its total SERS signals, it also provides an ideal platform for the investigation of chemical enhancement mechanism. In this review, we explored the development of non-noble metal SERS substrates, focusing on its enhancement mechanism and SERS performance of different materials as well as the future development direction.

INVITED REVIEW

  

INVITED REVIEW

On glass formation thermodynamics: Enthalpy vs. Entropy
Wang Li-Min, Liu Ri-Ping, Tian Yong-Jun
2020, 69 (19): 196401. doi: 10.7498/aps.69.20200707
Abstract +
Glass formation thermodynamics usually concerns the liquid-crystal Gibbs free energy difference. But, in practice, its efficiency in predicting the occurrence of the glass transition of materials and guiding the composition design is quite quantitative. In particular, it remains to be clarified to understand the relationship between and the contributions to the two fundamental quantities of enthalpy and entropy involved herein. In this paper, we study the relation between the enthalpy and the entropy involved in glass formation of various materials, and find that they are strongly correlated with each other. Theoretical and experimental analyses indicate the intrinsic correlation of the entropy of fusion with other key parameters associated with glass formation like melting viscosity and enthalpy of mixing, which confirms the close relation between the entropy of fusion and glass formation. Close inspection finds that the low entropy of fusion benefits the glass formation. Owing to the fact that the two glass-formation key variables of viscosity and enthalpy can be addressed by the entropy of fusion, we propose that the entropy of fusion be able to serve as a representative thermodynamic quantity to understand the glass formation in materials. The reliability in understanding the glass formation in terms of entropy of fusion is further verified. The studies provide a new reference for developing the glass formation thermodynamics.

INVITED REVIEW

  

INVITED REVIEW

Recent progresses of thermal conduction in two-dimensional materials
Wu Xiang-Shui, Tang Wen-Ting, Xu Xiang-Fan
2020, 69 (19): 196602. doi: 10.7498/aps.69.20200709
Abstract +
The two-dimensional (2D) materials represented by graphene and boron nitride provide an excellent platform for the study of thermal conduction and the interfacial thermal resistance in low-dimensional system. Recent studies recover exotic physics behind the novel thermal transport properties of 2D materials, such as length effect, dimensional effect, isotopic effect, anisotropic effect, etc. In this review, we introduce the recent progress of thermal properties in 2D materials in the last decade. The principle and development of thermal conduction measurement technologies used in 2D materials are introduced, followed by the experimental progress of thermal conduction and interfacial thermal resistance. Special attention is paid to the abnormal thermal transport and relevant physical problems. Finally, we present thermal management and heat dissipation in 2D electronic devices, summarize and point out the problems and bottlenecks, and forecast the future research directions and foregrounds.

EDITOR'S SUGGESTION

Excited state proton transfer processes of pyranine studied by femtosecond stimulated Raman spectroscopy
Wang Zi-Yu, Wei Jing-Le, Xu Wen-Qi, Jiang Jia-Ming, Huang Yi-Fan, Liu Wei-Min
2020, 69 (19): 198201. doi: 10.7498/aps.69.20200230
Abstract +
As one of the most fundamental processes, proton transfer reaction plays an important role in chemical and biological process, and to reveal the choreography of the proton motion intra- and intermolecularly, a spectroscopic technique capable of capturing molecular structural dynamics of excited-state proton transfer motions on an intrinsic time scale is needed. In this study, We utilize wavelength-tunable femtosecond stimulated Raman spectroscopy with a time resolution of ~100 fs, spectral resolution of 15 cm–1 and spectral range of 400 cm–1—1800 cm–1, combined with traditional transient absorption spectroscopywith a time window between 0 and 600 ps to simultaneously achieve reaction dynamics for transient reactant and product of the photoacid pyranine (8-hydroxypyrene1, 3, 6-trisulfonic acid, HPTS) molecules undergoing excited-state proton transfer reaction in complex with water and acetate molecules. Marker bands attributed to the deprotonated form of HPTS in a frequency range from 400 cm–1 to 1700 cm–1 are obtained under the excitation of 400 nm laser pulses. The marker band at 1516 cm–1, which is assigned to phenolic ring carbon carbon double band stretching accompanied with carbon hydrogen in-plane rocking motions, exhibits complex rise and decay dynamics. The simultaneously observed excited-state Raman mode at 920 cm–1 which is assigned to the excited carbon-carbon single bond stretch mode in the protonated acetic acid root molecule, helps us to clearly resolve the reaction rates of excited-state proton transfer. Based on the multi-exponential fitting results, the dynamics of excited-state Raman mode at 920 cm–1 exhibits bi-exponential processes with time constants of ~470 fs and ~3 ps. The ultrafast time component indicates that the excited-state proton transfer originates from an HPTS-acetate complex, indicating that part of the ground-state HPTS molecules are in the “tight” hydrogen bonding configuration that can quickly shift the excited-state proton charge toward the acetate acceptor molecule through a direct hydrogen bond. The second slower time component implies a significant subpopulation of HPTS in the ground state, i.e. hydrogen bonds to an acetate ion via an intervening water molecule, and upon photo excitation, the proton transfers to the water solvent before proton is picked up by the acetate ion.

EDITOR'S SUGGESTION

Cellular automata ray tracing in two-dimensional aero-optical flow fields
Luo Liang, Xia Hui, Liu Jun-Sheng, Fei Jia-Le, Xie Wen-Ke
2020, 69 (19): 194201. doi: 10.7498/aps.69.20200532
Abstract +
For the supersonic flow field with large density fluctuation produced by the unsteady flow and turbulent large-scale structures, an effective method to obtain the beam path is to solve the ray equation. Then the optical path difference (OPD), Strehl ratio (SR), optical transmission function (OTF), etc. can be obtained to analyze the optical distortion, and the correction of aero optics effects can be realized to improve the optical system performance. Generally, when the refractive index distribution is arbitrary, the ray equation analytic solution is difficult to obtain. Cellular automata (CA) ray tracing algorithm is proposed in this paper for aero-optical calculation in the 2D discrete flow fields. Unlike numerically solving the ray equation (NSRE), the coordinate value and the offset angle are calculated according to the position and direction transformation rules in CA algorithm. The position transformation rule is used to obtain the end point of the beam vector and determine whether the offset angle needs calculating at each iteration, the direction transformation rule is to calculate the offset angles. Then the refractive index field is integrated along the beam path to obtain the optical path length (OPL). The OPD is calculated from OPL. In this paper, aero-optical calculation is based on two types of flow fields. The supersonic shear layer including supersonic mixing layer and boundary layer 2D density distribution is measured by the nano-tracer-based planar laser scattering (NPLS) technique. The supersonic flow field surrounding the optical dome is simulated based on detached-eddy simulation (DES).The OPDrms and program running time quantitatively verify the calculation accuracy and high efficiency of CA. The results show that for the 2D supersonic NPLS flow field and the 2D supersonic flow field surrounding the optical dome, the calculation accuracy of CA is approximately equal to NSRE. Moreover, we find that the program running time of NSRE is about four times that of the CA algorithm, so the efficiency of ray tracing is effectively improved. Thus, the CA algorithm provides a new scheme for aero-optical calculation in the 2D supersonic flow field. Meanwhile, it also provides a guideline for the research on the ray tracing in 3D discrete aero-optical flow field.

EDITOR'S SUGGESTION

Effect of functional groups on crumpling behavior and structure of graphene oxide
Chen Chao, Duan Fang-Li
2020, 69 (19): 193102. doi: 10.7498/aps.69.20200651
Abstract +
Graphene has a wide range of applications in the fields of electricity, chemistry, biomedicine, and lubrication. But the strong van der Waals interaction between graphene sheets makes it easy to aggregate in preparation process, difficult to produce and put into practical applcation on a large-scale. There are many methods to prevent the graphene sheets from aggregating, such as reducing the size of sheets, adjusting the interaction between solvent and graphene, and using dispersant. Another possible method is to turn the sheet graphene into a three-dimensional structure like the crumpled paper. Compared with sheet graphene, the crumpled graphene ball has excellent aggregation-resistant. The current research on crumpled graphene ball mainly focuses on the effect of the initial structure of graphene sheet on the structure stability of the crumpled ball, but rarely involves the effect of functional groups. In this paper, ReaxFF molecular dynamics is used to simulate the crumpling process of graphene oxide sheet. The effect of functional groups (hydroxyl, epoxy) on the crumpling behavior and the stability of the crumpled ball of graphene oxide are studied. Graphene sheet oxidized by hydroxyl exhibits a push-up crumpling behavior. Graphene sheet oxidized by epoxy exhibits a layer-to-layer fitted crumpling behavior. Different crumpling behavior will lead to the difference in final crumpled ball structure. By analyzing the relationship between the atomic level potential energy incremental distribution and the distribution of broken and formed C—C bonds, we find that the broken and formed C—C bonds mainly occur in areas with a large degree of deformation, and the epoxy group has a stronger weakening effect on the C—C bond connected to it than the hydroxyl group. The release process of graphene oxide crumpled ball is simulated to study its structural stability. The stability of graphene oxide crumpled ball depends on the number of the broken and formed C—C bonds, that is, the more the number of broken and formed C—C bonds, the more stable the structure is, and under the same oxidation rate, the stability of the crumpled ball structure increases with the proportion of epoxy groups increasing. This study shows that the stability of graphene oxide crumpled ball structure can be controlled by changing the relative proportion of functional groups.

EDITOR'S SUGGESTION

Manipulations on mechanical properties of multilayer folded graphene by annealing temperature: a molecular dynamics simulation study
Li Xing-Xin, Li Si-Ping
2020, 69 (19): 196102. doi: 10.7498/aps.69.20200836
Abstract +
Annealing is a commonly used fabrication technology of graphene-assembled materials, which serves as an efficient method to control material properties. In graphene-assembled materials, the multilayer folded configuration of graphene has been widely observed due to the two dimensional characteristic of graphene. However, the manipulation on the mechanical properties of graphene-assembled materials by annealing has not been fully understood yet, especially considering the effect of folded microstructures. In this paper, we focus on the effect of annealing temperature on the mechanical properties of multilayer folded graphene. The dependences of elastic modulus, tensile strength, ultimate strain and fracture toughness on the annealing temperature have been systematically studied by molecular dynamics simulations. Moreover, the mechanisms behind the manipulations by annealing temperature have been revealed combining the structural evolutions obtained from the simulations. Our results indicate that the multilayer folded graphene after annealing under higher temperature exhibits significant reinforcement on its elastic modulus and tensile strength, while its ultimate strain drops instead. The fracture toughness is enhanced only within a certain range of annealing temperature. The controllable mechanical properties are attributed to the formation of interlayer covalent bonds between carbon atoms belonging to adjacent layers during the annealing processing. With the annealing temperature increases, more interlayer crosslinks are observed from simulations, which greatly strengthens the interlayer interaction. For the cases with lower annealing temperature, the folded graphene can be unfolded easily then finally flattened under tensile stretch, and the structural failure originates from the interlayer slippage in the folded area. However, for the cases with higher annealing temperature, the unfolding deformation is prevented since the folded graphene is blocked by much denser interlayer crosslinks, and the origins of structural failure transforms to the intralayer fracture in graphene plane. Considering the intralayer covalent bond interaction is far more powerful than the interlayer van der Waals interaction, the higher annealing temperature will bring higher elastic modulus and tensile strength due to the change on the structural failure mode, but it will sacrifice the ductility at the same time due to the blocked unfolding process of folded area. It is confirmed in our study that the annealing is an effective approach for the synthetic modulation on the stiffness, strength, ductility and toughness of multilayer folded graphene.

SPECIAL TOPIC—Nonlinear optics and devices of low-dimensional materials

  

COVER ARTICLE

Dual-wavelength self-starting mode-locking Er-doped fiber laser with MnPS3 saturable absorber
Yu Qiang, Guo Kun, Chen Jie, Wang Tao, Wang Jin, Shi Xin-Yao, Wu Jian, Zhang Kai, Zhou Pu
2020, 69 (18): 184208. doi: 10.7498/aps.69.20200342
Abstract +
As a member of the metal phosphorus trichalcogenide family, MPS3 is widely used in nonlinear optics and devices, which can be regarded as a significant benefit for the excellent photonic and optoelectronic properties. In this work, the MnPS3 nanosheet is prepared by the chemical vapor transport method and the MnPS3 saturable absorber is demonstrated by modifying mechanical exfoliation. To the best of our knowledge, the dual-wavelength self-starting mode-locking erbium-doped fiber laser with MnPS3 saturable absorber is demonstrated for the first time. The dual wavelength mode-locked laser with a pulse repetition rate of 5.102 MHz at 1565.19 nm and 1565.63 nm is proposed. Its maximum output power at the dual-wavelength is 27.2 MW. The mode-locked laser can self-start and stably run for more than 280 h.

YOUNG SCIENTISTS' FORUM

  

YOUNG SCIENTISTS' FORUM

Research progress of Rydberg many-body interaction
Zhang Zheng-Yuan, Zhang Tian-Yi, Liu Zong-Kai, Ding Dong-Sheng, Shi Bao-Sen
2020, 69 (18): 180301. doi: 10.7498/aps.69.20200649
Abstract +
The interaction of many-body quantum system is a critical problem to be solved in the field of quantum information science. Rydberg atoms have large dipole moment, enabling them to interact with others in a long range, thereby offering us a powerful tool for studying many-body quantum physics. Meanwhile, atoms in the ground state are stable, which makes it easy to manipulate them. Therefore, Rydberg-atom many-body system is an ideal platform for studying the interaction of many-body quantum system. Studies of Rydberg-atom many-body system may contribute to understanding the properties of many-body system and putting the interaction of many-body quantum system into practical applications. In this review, we introduce some studies of properties of interaction of Rydberg-atom many-body system, including the Rydberg excitation blockade, the variation of Rabi frequencies of the many-body system and special spatial distribution of Rydberg atoms in a many-body system. Firstly, the Rydberg excitation blockade, the most important property in the Rydberg-atom many-body system, indicates that atoms’ excitation will be suppressed in a certain range around one Rydberg excitation because the interaction between the Rydberg excitation and atoms leads the energy level to shift so that atoms cannot be excited by the same pulse. Secondly, there is a collective Rabi frequency in the system, which is proportional to the square of the number of atoms in the suppressed area. And additionally, because of the Rydberg blockade effect, Rydberg excitations in the ensemble cannot be at casual positions but a regular distribution is formed. Besides the studies of properties, several researches on the applications of interaction of Rydberg-atom many-body system are introduced, including single-photon source, quantum storage, single-atom imaging, quantum simulation, etc. These applications contribute to the development of quantum community and quantum computing, which may bring us a quantum-technology time. Finally, we discuss the future development of Rydberg-atom many-body system and its further applications. Further development includes the development of many-body system with a larger number of atoms, the development of many-body system of atoms with more than one electron, and some other specific subjects based on many-system, such as Rydberg dimer and topological phase. Also some promising applications such as in studying optimization problem by quantum annealing, may become true.
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