First-principles theory of spin dynamics in semiconductorsNew progress has been made
The team of the "Laboratory of Low-Dimensional Physics and Devices" of the School of Physics and its international cooperation team have made new progress in the first-principles theoretical research of semiconductor spin dynamics. The research results were published in Nature Communications (Nature Communications 15, 188, 2024) under the title "How Spin Relaxes and Dephases in Bulk Halide Perovskites". Perovskites halide have excellent optoelectronic properties and have received extensive attention in the research fields of solar cells and light-emitting devices. At the same time, due to the efficient spin generation, long spin lifetime and highly adjustable spin-orbit coupling field, these materials are considered to have great potential for semiconductor spintronics applications, and many related experimental and theoretical studies have been carried out.
The spin lifetime of halide perovskites, as a key parameter that determines its application prospects in the field of spintronics, has been widely measured, but has not been fully studied on first principles. It is necessary to systematically model the spin lifetime of halide perovskites using an accurate first-principles approach to fully understand their spin relaxation and dephasing mechanisms and to identify the key factors affecting their spin lifetimes.
Figure 1Spin relaxation and dephasing of bulk halide perovskite cspbbr3. (a) Schematic diagram of the structure of the orthorhombic system CSPBBR3;(b) Comparison of theoretical results and experimental data of spin relaxation time t1 with temperature under different carrier concentrations(c), (d), and (e) are the lande g factors for electrons and holes. The g-factor determines the motion of the spin in an external magnetic field. Among them, (c) is the g-factor of different k points near the edge of the energy band, (d) is the average g-factor, and (e) is the fluctuation range of the g-factor(f) Comparison of the theoretical results and experimental data of the reciprocal change of the spin dephase time (t2*) of the ensemble with the transverse external magnetic field under different carrier concentrations.
The team used a self-developed first-principles density matrix master equation method to simulate the spin relaxation time (T1, see Fig. 1b) and the spin dephase time (T2*, Fig. F) of a typical halide perovskite, CSBBR3 (Fig. 1A). The method accurately takes into account self-consistent spin-orbit coupling (SoC) and contains a quantum description of the electron-phonon scattering process. Therefore, the team accurately estimated the intrinsic spin lifetime of the material (see Figure 1b), thereby setting the upper limit of the spin lifetime and investigating the dependence of the spin lifetime on temperature, external field, carrier density, and defects. The team further identified the spin relaxation mechanism of CSPBB3, in which the contribution of the Frohlich electroacoustic interaction, which dominates the carrier relaxation, to the spin relaxation is negligible. This phenomenon originates from the weak correlation of spins in the Frohlich electroacoustic interactions. The team also implemented a first-principles simulation of the Lande G-factor in solids (see Fig. 1c, d, e) and introduced it into the spin dynamics simulation, which allowed the team to accurately simulate the spin dephase phenomenon in the outer field (see Fig. 1f). Finally, the team investigated the effect of spatial inversion symmetry breaking on spin lifetime. The theoretical results show that the so-called "permanent spin helix" can increase the spin lifetime when the spin cleavage is large, but the Rashba spin-orbit coupling field can reduce the spin lifetime. The team's theoretical approach provides a new way to optimize the spin and carrier transport properties of halide perovskite materials.
Hefei University of Technology is the first signatory unit. Professor Xu Junqing of the School of Physics of Hefei University of Technology is the first author, and Yuan Ping, associate professor of the University of Wisconsin, R**Ishankar Sundararaman, associate professor of Rensselaer Polytechnic Institute, and Valy Vardeny, professor of the University of Utah, are the co-corresponding authors. The research was funded by the construction fund of talent introduction conditions of Hefei University of Technology and the ** project of natural sciences in the United States.
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Important progress has been made in the field of high-sensitivity silicon-based ultra-narrowband detectors
Associate Professor Wang Li and Professor Luo Linbao from the Advanced Semiconductor Devices and Optoelectronic Integration Laboratory of the School of Microelectronics have successfully developed an ultra-sensitive near-infrared narrowband photodetector based on a single p-type silicon Schottky junction. The results were published in IEEE Electron Device Letters, a well-known journal in the field of semiconductor devices, as "Ultra-sensitive narrow-band p-si schottky photodetector with good w**elength selectivity and low driving voltage" as the cover article**. 
Figure 1Cover of IEEE Electron Device Letters 2024 Issue 1
Because narrowband photodetectors are only sensitive to the target wavelength and can effectively suppress the interference of background noise light, they have important application value in the fields of machine vision, specific band imaging, optical communication and biomaterial identification. However, the existing narrow-band detection mechanisms, such as the addition of filters, the narrowing of charge collection, or the thermoelectron effect, generally have the problem of low quantum efficiency. To improve the sensitivity of narrowband detection, the researchers implemented the photomultiplication effect within the device by introducing a charge trap into the active layer for interface tunneling injection, or by using a field-enhanced exciton ionization process. However, these mechanisms often require higher voltages of tens of volts to be activated, resulting in the performance degradation of narrowband detectors and high operating energy consumption.
Based on the in-depth analysis of the above problems, the research team proposed and implemented a highly sensitive narrowband photodetector that can operate at low driving voltage. By using a double-layer Schottky electrode and increasing the transition time difference between photogenerated electrons and holes, the photoelectric conversion efficiency of the device is greatly improved under the premise of ensuring high wavelength selectivity. The detector only has a detection peak around 1050 nm and has almost no response to UV and visible light. The specific detection rate of the device is 414 1012 Jones with a linear dynamic range of about 128dB. When the operating bias voltage is increased from 0V to -3V, the external quantum efficiency of the device can range from 96The 2% is significantly increased to 6939%, while the half-height and width of the probe peak remain unchanged at about 74 nm. This achievement provides a new idea for the realization of ultra-high sensitivity narrowband photodetectors that can operate at low driving voltages, and is expected to be widely used in the field of optoelectronics.
Figure 2(a) Simulation results of light intensity distribution in the device at zero bias;(b) Current-voltage curves of the device under different wavelengths;(c) Linear dynamic range;(d) Wavelength curves of the external quantum efficiency of the device at different bias voltages.
The above work has been funded by the National Natural Science Program, the Anhui Provincial Key R&D Program, the Anhui Provincial Natural Science Program, and the Special Fund for Fundamental Scientific Research in Universities.
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New progress has been made in the design of multifunctional two-dimensional ferromagnetic semiconductor materials
The research group of Professor Li Zhongjun of the low-dimensional semiconductor material design and device simulation team of the School of Physics used first-principles calculations and molecular dynamics simulation methods to cleverly introduce nitrogen atoms (N) to connect magnetic chromium atoms (Cr) to produce synergistic effects, and designed a two-dimensional van der Waals ferromagnetic semiconductor CR3(CN3)2 with high Curie temperature, high carrier mobility, high optical absorption and high stability. The results were published in Nano Letters under the title "Enhanced Direct Exchange Interaction and Hybridization by Single-Atom Linkers for High Curie Temperature and Superior Visible-Light Harvesting in CR3(CN3)2". Two-dimensional van der Waals ferromagnetic semiconductors show important application potential in the fields of spintronics and spin optoelectronics. However, there are still key problems to be solved in realizing high-performance spin devices based on such materials. Firstly, the D-p-D superexchange between magnetic metal atoms of two-dimensional van der Waals ferromagnetic materials has been reported, and the Curie temperature is much lower than room temperature, which seriously limits the application of materials in room temperature environment. Secondly, the electronic structure of these two-dimensional van der Waals ferromagnetic materials near the Fermi level has obvious flat-band characteristics, resulting in large effective mass and low mobility of the carriers, and the spin charge transport exhibits obvious localization. In addition, these materials usually have large or small bandgaps and poor light absorption.
Figure 1(a) The band centers of the spin-polarized orbitals of Cr, C, and N atoms in two-dimensional Cr3(CN3)2;(b, c) Schematic diagram of the mechanism of magnetic exchange.
In order to solve the above problems, the development of two-dimensional van der Waals ferromagnetic semiconductors with high Curie temperature, high carrier mobility, high light absorption and high stability, different from the reported studies using molecular systems as linkers, this work proposes to use a single N atom to connect magnetic metal CR atoms to construct a metal-organic framework CR3(CN3)2 based on two-dimensional Kagome structure. The results show that two-dimensional Cr3(CN3)2 is a ferromagnetic semiconductor with 106EV's direct bandgap and out-of-plane magnetic-prone shaft. Using a single atom N as the linking unit, on the one hand, the distance between the magnetic Cr atoms can be minimized, and on the other hand, the dyz DXZ and Pz orbitals of the Cr and N atoms have excellent matching characteristics in energy and symmetry (Figure 1), and the two aspects synergistically enhance the magnetic exchange interaction. Monte Carlo simulations based on the Heisenberg model show that the Curie temperature of the system is as high as 943 K (Fig. 2), which is the highest recorded for metal-organic framework ferromagnetic semiconductors so far. In addition, the excellent matching characteristics of the Cr and N atomic orbitals in terms of energy and symmetry also make the system exhibit strong orbital hybridization, enhance the energy band dispersion, and obtain a carrier mobility of 420 cm2·v 1·s 1. In addition, the moderate direct band gap of two-dimensional Cr3(CN3)2 and the symmetry-matched valence band and conduction band make the system exhibit strong light absorption characteristics, which are significantly enhanced in the visible range under the regulation of in-plane stress. In this work, a two-dimensional Cr3(CN3)2-based sandwich heterojunction with graphene as the electrode and hexagonal boron nitride as the protective layer was constructed, which verified its feasibility as a spin optoelectronic device.
Figure 2The geometry, carrier mobility, Curie temperature, and light absorption coefficient of two-dimensional Cr3(CN3)2.
This work reveals the synergistic effect and mechanism of N atoms attaching magnetic Cr atoms, and shows that two-dimensional Cr3(CN3)2 is an ideal ferromagnetic semiconductor, and the research results provide a new idea for the design and fabrication of multifunctional two-dimensional ferromagnetic semiconductors with high Curie temperature, high carrier mobility, high optical absorption and high stability.
Hefei University of Technology is the first signatory, Dr. Liu Xiaofeng and Associate Researcher Wang Haidi of Hefei University of Technology are the co-first authors, Professor Li Zhongjun of Hefei University of Technology, researcher Hu Wei of University of Science and Technology of China and Professor Zeng Xiaocheng of City University of Hong Kong are the co-corresponding authors. The above research work is supported by the Key R&D Program of the Ministry of Science and Technology, the National Natural Science of China, the Fundamental Research Funds of Hefei University of Technology, and the Supercomputing Center of Hefei University of Technology.
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* Faculty of Physics, Faculty of Microelectronics.
Edited by Zhang Qinghao.
Editor-in-charge: Wei Tingting.
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