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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.

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.

COVER ARTICLE

In-beam γ-rays of back-streaming white neutron source at China Spallation Neutron Source
Ren Jie, Ruan Xi-Chao, Chen Yong-Hao, Jiang Wei, Bao Jie, Luan Guang-Yuan, Zhang Qi-Wei, Huang Han-Xiong, Wang Zhao-Hui, An Qi, Bai Huai-Yong, Bao Yu, Cao Ping, Chen Hao-Lei, Chen Qi-Ping, Chen Yu-Kai, Chen Zhen, Cui Zeng-Qi, Fan Rui-Rui, Feng Chang-Qing, Gao Ke-Qing, Gu Min-Hao, Han Chang-Cai, Han Zi-Jie, He Guo-Zhu, He Yong-Cheng, Hong Yang, Huang Wei-Ling, Huang Xi-Ru, Ji Xiao-Lu, Ji Xu-Yang, Jiang Hao-Yu, Jiang Zhi-Jie, Jing Han-Tao, Kang Ling, Kang Ming-Tao, Li Bo, Li Chao, Li Jia-Wen, Li Lun, Li Qiang, Li Xiao, Li Yang, Liu Rong, Liu Shu-Bin, Liu Xing-Yan, Mu Qi-Li, Ning Chang-Jun, Qi Bin-Bin, Ren Zhi-Zhou, Song Ying-Peng, Song Zhao-Hui, Sun Hong, Sun Kang, Sun Xiao-Yang, Sun Zhi-Jia, Tan Zhi-Xin, Tang Hong-Qing, Tang Jing-Yu, Tang Xin-Yi, Tian Bin-Bin, Wang Li-Jiao, Wang Peng-Cheng, Wang Qi, Wang Tao-Feng, Wen Jie, Wen Zhong-Wei, Wu Qing-Biao, Wu Xiao-Guang, Wu Xuan, Xie Li-Kun, Yang Yi-Wei, Yi Han, Yu Li, Yu Tao, Yu Yong-Ji, Zhang Guo-Hui, Zhang Lin-Hao, Zhang Xian-Peng, Zhang Yu-Liang, Zhang Zhi-Yong
2020, 69 (17): 172901. doi: 10.7498/aps.69.20200718
Abstract +
The back-streaming neutron beam line (Back-n) was built in the beginning of 2018, which is part of the China Spallation Neutron Source (CSNS). The Back-n is the first white neutron beam line in China, and its main application is for nuclear data measurement. For most of neutron-induced nuclear reaction measurements based on white neutron facilities, the beam of gamma rays accompanied with neutron beam is one of the most important experimental backgrounds. The back streaming neutron beam is transported directly from the spallation target to the experimental station without any moderator or shielding, the flux of the in-beam gamma rays in the experimental station is much larger than those of these facilities with neutron moderator and shielding. Therefore, it is necessary to consider the influence of in-beam gamma rays on the experimental results. Studies of the in-beam gamma rays are carried out at the back-n. Monte-Carlo simulation is employed to obtain the energy distribution and the time structure of the in-beam gamma rays. According to the simulation results, when the neutron flight time is longer than 1.0 μs the energy distribution of the in-beam gamma rays does not vary with flight time. Therefore, the time structure of these gamma rays can be measured without the correction of the detection efficiency. In this work, the time structure of the in-beam gamma rays in the low neutron energy region is measured by both direct and indirect methods. In the direct measurement, a 6Li loaded ZnS(Ag) scintillator is located on the neutron beam line and the time of flight method is used to determine the time structure of neutrons and gamma rays. The gamma rays are separated from neutrons with pulse-shape discrimination. The black filter method is used to verify the particle discrimination results. In the indirect measurement, the C6D6 scintillation detectors are used to measure the gamma rays scattered off a Pb sample on the way of the neutron beam. The time structure of the in-beam gamma rays is derived from that of the scattered gamma rays. The experimental results are in good agreement with the simulations with the time-of-flight between 12 μs and 2.0 ms. Besides, according to the simulation results, the intensity of the in-beam gamma rays is 1.21 × 106 s–1·cm–2 in the center of the experimental station 2 of Back-n, which is 76.5 m away from the spallation target of CSNS.

COVER ARTICLE

The g-factor anisotropy of trapped excitons in CH3NH3PbBr3 perovskite
Song Fei-Long, Wang Yu-Nuan, Zhang Feng, Wu Shi-Yao, Xie Xin, Yang Jing-Nan, Sun Si-Bai, Dang Jian-Chen, Xiao Shan, Yang Long-Long, Zhong Hai-Zheng, Xu Xiu-Lai
2020, 69 (16): 167102. doi: 10.7498/aps.69.20200646
Abstract +
Hybrid organic-inorganic perovskites show large potential applications in solar cells, light emitting diodes and low threshold lasers because of the high tolerance of defects compared with other semiconductor materials. Normally they have been synthesized by dilution method, generating a device with high performance, but they also introduce lots of defects. So far, investigations have been done intensively on ensemble defects both in theory and experiment, but single-defect related trapped excitons are yet to be explored. In this work, we prepared high-quality CH3NH3PbBr3 perovskite nanowires with the length of about 1 μm and the width of several hundred nanometers by “reverse” ligand assisted reprecipitation method, and performed the magneto-photoluminescence measurement of different trapped excitons in single perovskite nanowires at a low temperature with a standard confocal microscopic system. The photoluminescence (PL) peak with narrow linewidth has been observed from trapped excitons with high luminescence intensity and the trapped excitons can be coupled with phonons in different ways. Both Zeeman splittings and diamagnetic effects have been observed in single trapped excitons under the magnetic field, and we found that the different trapped excitons have different Zeeman splittings and diamagnetic effects which is caused by the different defects near the trapped excitons. At the same time, we have extracted the g-factor of the trapped excitons under different magnetic field angles. The extracted exciton g-factors show anisotropic, which can be ascribed to the limitation of the lattice structure of the perovskite and the trapped exciton wave-function anisotropy under a vector magnetic field. Our results demonstrate that trapped excitons with narrow linewidth have very good luminescence properties and studying the magneto-optical properties from single trapped excitons can provide a deep understanding of trapped excitons in perovskites for applications in quantum light sources and spintronics. Furthermore, our results can also provide a possibility to control the electron spin in single-trapped-excitons-based hybrid organic-inorganic perovskites by manipulating the g-factor through an applied vector magnetic field, which promotes the application of the perovskite-based spintronics.

SPECIAL TOPIC—Optical metamaterials

  

COVER ARTICLE

Influence of bias magnetic field direction on band structure of magnetic photonic crystal and its role in constructing topological edge states
Xi Xiang, Ye Kang-Ping, Wu Rui-Xin
2020, 69 (15): 154102. doi: 10.7498/aps.69.20200198
Abstract +
The topological transitions in two-dimensional photonic crystals (PCs) originate from the opening-closing-reopening of the bandgap, accompanied with the band order inversion. The topological bandgap in magnetic PC can be created by applying a bias magnetic field or deforming the geometry structure of the PC. In this paper, we demonstrate that the direction of the bias magnetic field also plays a key role in modifying the band structure in a two-dimensional magnetic PC. The results show that by reversing the direction of the bias magnetic field, the eigenstates with the same parity may exchange their orders in the band structure. We investigate this type of band order exchange in the applications of constructing topological edge states and its influence on the properties of edge states. We find, for example, reversing the direction of the bias magnetic field can create two almost degenerated topological edge modes, which propagate in the same direction but have opposite orbital angular momenta. The edge modes and their characteristics can be determined by the schematics of the band orders for the photonic crystals on the two sides of the boundary. The relative relationship of the band orders determines the emergence of the topological edge states, the number of edge states, and edge modes’ properties such as the orbital angular momentum and group velocity. Also, it affects the transmission efficiency of the electromagnetic wave on the boundary. The direction effect of the bias magnetic field on band order exchange presented in this paper provides us with a new way to change the feature of topological edge states and helps us to better understand the influence of band order on topological phases of photonic crystals. It may have potential applications, such as in pseudo-spin splitter and reflection-free one-way optical switch.

COVER ARTICLE

Ground state of spin-orbit coupled rotating ferromagnetic Bose-Einstein condensate in toroidal trap
Li Ji, Liu Bin, Bai Jing, Wang Huan-Yu, He Tian-Chen
2020, 69 (14): 140301. doi: 10.7498/aps.69.20200372
Abstract +
Realization of spinor Bose-Einstein condensate in an optical trap has made it possible to create a variety of topological nontrivial structures, due to the vector character of the order parameter. Recently, artificial spin-orbit coupling in the spinor Bose-Einstein condensate, owing to coupling between the spin and the center-of-mass motion of the atom, provides an unprecedented opportunity to search for novel quantum states. As is well known, the potential well in the Bose-Einstein condensate is adjustable. The toroidal trap is an important model potential because of its simplicity and richness in physics. In particular, the spinor Bose-Einstein condensate under the toroidal trap has brought an ideal platform for studying fascinating properties of a superfluid, such as persistent flow and symmetry-breaking localization. For the case of the spin-orbit-coupled Bose-Einstein condensate, the previous studies of the toroidal trap mainly focused on the two-component or antiferromagnetic case. However, in the presence of a toroidal trap, there remains an open question whether the combined effects of the spin-orbit coupling and rotation can produce previously unknown types of topological excitations in the ferromagnetic Bose-Einstein condensate. In this work, by using quasi two-dimensional Gross-Pitaevskii equations, we study the ground state structure of spin-orbit coupled rotating ferromagnetic Bose-Einstein condensate in the toroidal trap. We concentrate on the effects of the spin-orbit coupling and the rotation on the ground states. The numerical results show that in the presence of a toroidal trap, the ground state structure is displayed as half-skyrmion chain with circular distribution. Adjusting the strength of spin-orbit coupling not only changes the number of half-skyrmion in the system, but also controls the symmetry of half-skyrmion with circular distribution. As the rotation frequency increases, the system undergoes the transitions from the plane wave to the half-skyrmion chain with circular distribution, and eventually developing the half-skyrmion phase of triangular lattice. Next, we examine the effect of spin-independent interaction on spin-orbit coupled rotating spinor Bose-Einstein condensate. As the spin-independent interaction increases, the topological defects in the condensate increase due to the variation of the local magnetic order. We also discuss the influence of well shape on the ground state structure. These topological structures can be detected via the time-of-flight absorption imaging technique. The spin-orbit coupled spinor Bose-Einstein condensate in the toroidal trap is an important quantum platform, which not only opens up a new avenue for exploring the exotic topological structures, but also is crucial for realizing the transitions among different ground states. This work paves the way for futureexploring the topological defects and the corresponding dynamical stability in quantum system subjected to the toroidal trap.

COVER ARTICLE

Efficient and stable blue perovskite light emitting diodes based on defect passivation
Wu Hai-Yan, Tang Jian-Xin, Li Yan-Qing
2020, 69 (13): 138502. doi: 10.7498/aps.69.20200566
Abstract +
Solution-processable metal halide perovskites materials have many advantages, such as adjustable band gap, high photoluminescence quantum yield (PLQY), high color purity, high carrier mobility, low temperature solution process, excellent charge transport property and so on. These make them potential application in the display field. In the past few years, the device performance of perovskite light emitting devices (PeLEDs) have been greatly improved by manipulating the perovskite microstructures through various strategies, such as stoichiometry control, dimensional engineering, defect passivation and so on. At present, except for blue PeLEDs, the external quantum efficiencies (EQEs) over 20% have been achieved for green, red, and near-infrared PeLEDs. The low efficiency of blue PeLEDs is retarding their potential applications in full-color display and solid-state lighting. The main reasons in blue PeLEDs are the poor film coverage of blue perovskite materials and the spectral instability during device operation. In order to improve the quality of perovskite film and device performance, the quasi two-dimensional perovskite materials phenylethylammonium cesium lead bromide chloride (PEAxCsPbBr3–yCly) are used as the main perovskite emission material, by partially replacing Br with Cl to enlarge their bandgap to achieve the blue emission. The Lewis base polyethyleneglycol (PEG) is introduced to passivate the surface trapping defects and improve perovskite film coverage. The potassium bromide (KBr) is introduced to reduce perovskite grain size, suppress mobile ion migration and exhibit excellent spectral stability. Dual additives PEG and KBr are incorporated into the quasi-2D blue perovskite for inhibiting the nonradiative losses by passivating the traps in the perovskite films. Eventually, the PEAxCsPbBr3–yCly + PEG + KBr based blue PeLEDs with the emission peak of 488 nm are accompanied, which maximum brightness, current efficiency, and external quantum efficiency reached 1049 cd·m–2, of 5.68 cd·A–1, and of 4.6%, respectively, with high color purity (the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates is (0.0747, 0.2570)) and the narrow full width at half maximum (FWHM) of 20 nm. Compare to the devices without additives, the efficiency has increased by nearly 3 times. Furthermore, the devices also show better spectral stability and operation lifetime. This work provides an effective method of blue PeLEDs toward the practical applications.

SPECIAL TOPIC—Dielectric materials and physics

  

COVER ARTICLE

Effect of manganese doping on ferroelectric and piezoelectric properties of KNbO3 and (K0.5Na0.5)NbO3 lead-free ceramics
Xu Ze, Lou Lu-Yao, Zhao Chun-Lin, Tang Hao-Cheng, Liu Yi-Xuan, Li Zhao, Qi Xiao-Mei, Zhang Bo-Ping, Li Jing-Feng, Gong Wen, Wang Ke
2020, 69 (12): 127705. doi: 10.7498/aps.69.20200277
Abstract +
Potassium sodium niobate ((K0.5Na0.5)NbO3)-based lead-free piezoelectric ceramics are excellent ferroelectric materials and have been demonstrated to have many practical applications. Recent studies have revealed that chemical doping plays a crucial role in optimizing the electromechanical coupling properties of (K0.5Na0.5)NbO3-based piezoelectric ceramics. In this paper, MnO2 is doped into potassium niobate (KNbO3) and (K0.5Na0.5)NbO3 piezoelectric ceramics prepared by the conventional solid-state reaction method. The influences of doped Mn cation on KNbO3 and (K0.5Na0.5)NbO3 piezoelectric ceramics including microstructure and macroscopic electrical properties are systematically investigated. The doping effects of Mn cation on the KNbO3 and (K0.5Na0.5)NbO3 piezoelectric ceramics are significantly different from each other. For the Mn-doped KNbO3 piezoelectric ceramics, the sizes of ferroelectric domains are reduced. Meanwhile, the diffused orthorhombic-tetragonal phase transition is observed, which is accompanied by reducing dielectric loss and Curie temperature, and broadening vibration peaks in Raman spectrum. It is known that the oxygen vacancy can be formed to compensate for the charges created by the acceptor doping of Mn into the B site of perovskite, and thus forming a defect dipole with the acceptor center. From the ferroelectric measurement, a double hysteresis loop (P-E curve) and a recoverable electric-field-induced strain due to the formation of defect dipole are observed. On the contrary, for the Mn-doped (K0.5Na0.5)NbO3 piezoelectric ceramics, the sizes of ferroelectric domains are not reduced. Meanwhile, the Curie temperature and vibration peaks in Raman spectrum are not changed. A rectangular hysteresis loop (P-E curve) and an unrecoverable electric-field-induced strain are observed in the ferroelectric measurement. The difference between these systems might originate from the greater ionic disorder and lattice distortion in (K0.5Na0.5)NbO3 piezoelectric ceramics. The difference in ionic radius between Na+ and K+ can affect the migration and distribution of oxygen vacancies, which makes it difficult to form stable defect dipoles in the Mn-doped (K0.5Na0.5)NbO3 piezoelectric ceramics. The results will serve as an important reference for preparing high-performance (K0.5Na0.5)NbO3-based piezoelectric ceramics via chemical doping.

SPECIAL TOPIC—Majorana in condensed matter

  

COVER ARTICLE

Emergent vortex Majorana zero mode in iron-based superconductors
Kong Ling-Yuan, Ding Hong
2020, 69 (11): 110301. doi: 10.7498/aps.69.20200717
Abstract +
During the recent years, the iron-based superconductors with a topological band structure have attracted intensive attention from the science community as a new and promising platform for emerging Majorana zero modes in their vortex core. These topological iron-based superconductors possess all of the desirable properties, i.e. single material, high-Tc superconductivity, strong electron-electron correlation and topological band structure, thus successfully avoiding the difficulties suffered by previous Majorana platforms, such as intrinsic topological superconductors and multiple types of proximitized heterostructures. So far, one has observed pristine vortex Majorana zero modes in several different compounds of iron-based superconductors. The systematic studies performed on those systems show that the vortex Majorana zero modes are quite evident experimentally and very clear theoretically, leading to a bright future in applications. The vortex cores of iron-based superconductors can become one of the major candidates for exploring topological quantum computing in the future. In this review article, we will focus on Fe(Te, Se) single crystal, to introduce the original ideas and research progress of the new emerging “iron home” for Majorana zero modes. Having elabrated the basic band structures and the experimental facts of the observed vortex zero modes in Fe(Te, Se), we will systematically summarize the main observations and fundamental physics of vortex Majorana zero modes in Fe(Te, Se). First of all, with the help of the observed behavior of Majorana wavefunction and quasiparticle poisioning, we will analyze the emerging mechanism of vortex Majorana zero modes in Fe(Te, Se). Then we will elaborate the measurements on Majorana symmetry and topological nature of vortex Majorana zero modes, assisted by several existing Majorana theories. After that, we will switch our view angle from quantum physics to quantum engineering, and comprehensively analyze the fate of vortex Majorana zero modes in a real material under a real environment, which may benefit the potential engineering applications in the future. This review article follows the physical properties of vortex Majorana zero modes, and emphasizes the link between theories and experiments. Our goal is to bridge the gap between the classical Majorana theories and the new emerging Majorana platform in iron-based superconductors, and help the readers to understand the experimental observations of the newly discovered “iron home” for Majoranas.

COVER ARTICLE

Preparation and properties for X-ray scintillation screen based on ZnO:In nanorod arrays
Li Qian-Li, Hu Ya-Hua, Ma Yi-Fan, Sun Zhi-Xiang, Wang Min, Liu Xiao-Lin, Zhao Jing-Tai, Zhang Zhi-Jun
2020, 69 (10): 102902. doi: 10.7498/aps.69.20200282
Abstract +
X-ray scintillation screens as the core component of X-ray imaging detectors have widespread applications in the medical imaging, security inspection, high energy physics, radiochemistry, and so on. For a long time, the development of X-ray scintillation screen mainly focuses on improving the light yield in order to enhance its detection efficiency. However, a novel tendency has recently emerged for ultrafast time performance of the X-ray imaging detector. The indium doping zinc oxide (ZnO:In) with high radiation hardness, higher light yield(>10000 photons/MeV) and subnanosecond decay time is a promising scintillation material for ultrafast detections. In order to satisfy the requirements of X-ray scintillation screens with ultrafast and high-spatial-resolution in the existing and upcoming high energy physics experiments, the ZnO:In nanorod arrays have been prepared on a 100-nm-thick ZnO-seeded substrate by hydrothermal reaction method and then treated by hydrogen plasma in present work. The results of SEM demonstrate the average diameter and length of the ZnO:In nanorods are about 0.5 and 12 μm, respectively. The XRD shows the ZnO:In nanorods are highly aligned perpendicular to the substrate along c-axis direction. The X-ray excited luminescence spectra show that two luminescence bands are observed, i.e. an ultraviolet emission peak located at about 395 nm and a visible emission band at 450–750 nm. It is particularly important to point out that hydrogen plasma treatment can enhance the ultraviolet emission of ZnO:In nanorod arrays and suppress its visible emission. The reason is attributed to the formation of shallow donors through hydrogen entering the ZnO and the combination of VO and Oi. In addition, the fluorescence decay times of the ultraviolet and visible emissions for the ZnO:In nanorod arrays are subnanosecond and nanosecond, respectively, satisfying the demand of the fast X-ray imaging. The spatial resolution of ZnO:In nanorod arrays has been characterized in X-ray imaging beamline at the Shanghai Synchrotron Radiation Facility. Under excitation of the X-ray beam with the energy of 20 keV, a system spatial resolution of 1.5 μm could be achieved by using an 12 μm thickness ZnO:In nanorod arrays as the scintillation screen, which is exceeded the highest level had ever been reported on ZnO:In nanorod arrays scintillation screen. In conclusion, this present work shows that it is a feasible solution for X-ray detection and imaging with high temporal and spatial resolution by using ZnO:In nanorod arrays as the X-ray scintillation screen.
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