Excitons have significant impacts on the properties of semiconductors.They exhibit significantly different properties when a direct semiconductor turns in to an indirect one by doping.Huybrecht variational method is a...Excitons have significant impacts on the properties of semiconductors.They exhibit significantly different properties when a direct semiconductor turns in to an indirect one by doping.Huybrecht variational method is also found to influence the study of exciton ground state energy and ground state binding energy in Al_(x)Ga_(1−x)As semiconductor spherical quantum dots.The Al_(x)Ga_(1−x)As is considered to be a direct semiconductor at AI concentration below 0.45,and an indirect one at the concentration above 0.45.With regards to the former,the ground state binding energy increases and decreases with AI concentration and eigenfrequency,respectively;however,while the ground state energy increases with AI concentration,it is marginally influenced by eigenfrequency.On the other hand,considering the latter,while the ground state binding energy increases with AI concentration,it decreases with eigenfrequency;nevertheless,the ground state energy increases both with AI concentration and eigenfrequency.Hence,for the better practical performance of the semiconductors,the properties of the excitons are suggested to vary by adjusting AI concentration and eigenfrequency.展开更多
The nonlinear absorption properties of direct (GaN) and indirect (CdI2) band gap crystals have been studied by using an open aperture Z-scan technique under fundamental (1064 nm) and frequency doubled (532 nm) wavelen...The nonlinear absorption properties of direct (GaN) and indirect (CdI2) band gap crystals have been studied by using an open aperture Z-scan technique under fundamental (1064 nm) and frequency doubled (532 nm) wavelength respectively with 10 ns or 60 ps pulse durations. Direct band gap crystal exhibits two and three photon absorption at all input irradiances. On the other hand, at low input irradiance the indirect band gap crystal exhibits saturable absorption (SA). At higher input irradiances two and three photon absorption becomes dominant. A monotonic increase of the nonlinear absorption coefficients with increasing laser pulse duration from 60 ps to 10 ns is observed for GaN and CdI2 crystals.展开更多
Beam splitting upon refraction in a triangular sonic crystal composed of aluminum cylinders in air is experimentally and numerically demonstrated to occur due to finite source size, which facilitates circumvention of ...Beam splitting upon refraction in a triangular sonic crystal composed of aluminum cylinders in air is experimentally and numerically demonstrated to occur due to finite source size, which facilitates circumvention of a directional band gap. Experiments reveal that two distinct beams emerge at crystal output, in agreement with the numerical results obtained through the finite-element method. Beam splitting occurs at sufficiently-small source sizes comparable to lattice periodicity determined by the spatial gap width in reciprocal space. Split beams propagate in equal amplitude, whereas beam splitting is destructed for oblique incidence above a critical incidence angle.展开更多
Band gap anomaly is a well-known issue in lead chalcogenides PbX （X = S, Se, Te, Po）. Combining ab initio calculations and tight-binding （TB） method, we have studied the band evolution in PbX, and found that the b...Band gap anomaly is a well-known issue in lead chalcogenides PbX （X = S, Se, Te, Po）. Combining ab initio calculations and tight-binding （TB） method, we have studied the band evolution in PbX, and found that the band gap anomaly in PbTe is mainly related to the high on-site energy of Te 5s orbital and the large s-p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that PbPo is an indirect band gap （6.5 meV） semiconductor with band inversion at L point, which clearly indicates that PbPo is a topological crystalline insulator （TCI）. The calculated mirror Chern number and surface states double confirm this conclusion.展开更多
The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influen...The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influence on optical properties of In1-xGaxN/P,where(x=0.0,0.25,0.50,0.75,and 1.0)alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method(FP-LAPW)and modified Becke and Johnson potentials(TB-mBJ).The direct bandgaps turn from 0.7 eV to 3.44 eV,and 1.41 eV to 2.32 eV for In1-xGaxN/P alloys,which increases their potentials for optoelectronic devices.The optical properties are discussed such as dielectric constants,refraction,absorption,optical conductivity,and reflection.The light is polarized in the low energy region with minimum reflection.The absorption and optical conduction are maxima in the visible region,and they are shifted into the ultraviolet region by Ga doping.Moreover,static dielectric constant e1(0)is in line with the bandgap from Penn’s model.展开更多
The monolayer arsenic in the puckered honeycomb structure was recently predicted to be a stable two-dimensional layered semiconductor and therefore named arsenene. Unfortunately, it has an indirect band gap, which lim...The monolayer arsenic in the puckered honeycomb structure was recently predicted to be a stable two-dimensional layered semiconductor and therefore named arsenene. Unfortunately, it has an indirect band gap, which limits its practical application. Using first-principles calculations, we show that the band gaps of few-layer arsenic have an indirect-direct transition as the number of arsenic layers(n) increases from n=1 to n=2. As n increases from n=2 to infinity, the stacking of the puckered honeycomb arsenic layers forms the orthorhombic arsenic crystal ??-As, arsenolamprite), which has a similar structure to the black phosphorus and also has a direct band gap. This indirect-direct transition stems from the distinct quantum-confinement effect on the indirect and direct band-edge states with different wavefunction distribution. The strain effect on these electronic states is also studied, showing that the in-plane strains can induce very different shift of the indirect and direct band edges, and thus inducing an indirect-direct band gap transition too. The band gap dependence on strain is non-monotonic, with both positive and negative deformation potentials. Although the gap of arsenene opens between As p-p bands, the spin-orbit interaction decreases the gap by only 0.02 e V, which is much smaller than the decrease in Ga As with an s-p band gap. The calculated band gaps of arsenene and ?-As using the hybrid functional are 1.4 and 0.4 e V respectively, which are comparable to those of phosphorene and black phosphorus.展开更多
A novel two-dimensional（2D） Te Se_2 structure with high stability is predicted based on the first-principles calculations. As a semiconductor, the results disclose that the monolayer Te Se_2 has a wide-band gap of 2...A novel two-dimensional（2D） Te Se_2 structure with high stability is predicted based on the first-principles calculations. As a semiconductor, the results disclose that the monolayer Te Se_2 has a wide-band gap of 2.392 e V. Interestingly, the indirect-band structure of the monolayer Te Se_2 transforms into a direct-band structure under the wide biaxial strain（0.02–0.12）. The lower hole effective mass than monolayer black phosphorus portends a high carrier mobility in Te Se_2 sheet. The optical properties and phonon modes of the few-layered Te Se_2 were characterized. The few-layer Te Se_2 shows a strong optical anisotropy. Specially, the calculated results demonstrate that the multilayer Te Se_2 has a wide range of absorption wavelength. Our result reveals that Te Se_2 as a novel 2D crystal possesses great potential applications in nanoscale devices, such as high-speed ultrathin transistors, nanomechanics sensors, acousto-optic deflectors working in the UV-vis red region and optoelectronic devices.展开更多
Using hybrid-functional first-principles calculation combined with the supercell method and band unfolding technique we investigate the band structure of non-strained Ge1-xSnx alloys with various Sn concentrations. Th...Using hybrid-functional first-principles calculation combined with the supercell method and band unfolding technique we investigate the band structure of non-strained Ge1-xSnx alloys with various Sn concentrations. The calculations show that at the Sn concentration of^3.1 mol% the GeSn alloy presents a direct band gap. The variation of the band structure are ascribed to the weaker electro-negativity of Sn atoms and a slight charge transfer from Sn atoms to Ge atoms.展开更多
In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic...In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory （DFT） with the generalized gradient approximation （GGA） are carried out to investigate the energy band gap structure on silicon （Si） and germanium （Ge） nanofilms. Simulation results show that the band gaps in Si （100） and Ge （111） nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and 7.25 nm respectively, but the band gaps of Si （111） and Ge （110） nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si （110） and Ge （100） nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si（1-x）/ZGexSi（1-x）/2 sandwich structure become the direct-gap structure in a certain area whether （111） or （100） surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects.展开更多
Germanium （Ge） pin photodiodes show clear direct band gap emission at room temperature, as grown on bulk silicon in both photoluminescence （PL） and electro- luminescence （EL）. PL stems from the top contact layer...Germanium （Ge） pin photodiodes show clear direct band gap emission at room temperature, as grown on bulk silicon in both photoluminescence （PL） and electro- luminescence （EL）. PL stems from the top contact layer with highly doped Ge because of strong absorption of visible laser light excitation （532 nm）. EL stems from the recombination of injected carriers in the undoped intrinsic layer. The difference in peak positions for PL （0.73 eV） and EL （0.80 eV） is explained by band gap narrowing from high doping in n＋-top layer. A superlinear increase of EL with current density is explained by a rising ratio of direct/ indirect electron densities when quasi Fermi energy level rises into the conduction band. An analytical model for the direct/indirect electron density ratio is given using simplifying assumptions.展开更多
基金supported by the National Natural Science Foundation of China(Nos.12164032 and 11964026)the Natural Science Foundation of Inner Mongolia(No.2019MS01010)+3 种基金Scientific Research Projects in Colleges and Universities in Inner Mongolia(No.NJZZ19145)Graduate Science Innovative Research Projects(No.S20210281Z)the Natural Science Foundation of Inner Mongolia(No.2022MS01014)Doctor Research Start-up Fund of Inner Mongolia Minzu University(No.BS625).
文摘Excitons have significant impacts on the properties of semiconductors.They exhibit significantly different properties when a direct semiconductor turns in to an indirect one by doping.Huybrecht variational method is also found to influence the study of exciton ground state energy and ground state binding energy in Al_(x)Ga_(1−x)As semiconductor spherical quantum dots.The Al_(x)Ga_(1−x)As is considered to be a direct semiconductor at AI concentration below 0.45,and an indirect one at the concentration above 0.45.With regards to the former,the ground state binding energy increases and decreases with AI concentration and eigenfrequency,respectively;however,while the ground state energy increases with AI concentration,it is marginally influenced by eigenfrequency.On the other hand,considering the latter,while the ground state binding energy increases with AI concentration,it decreases with eigenfrequency;nevertheless,the ground state energy increases both with AI concentration and eigenfrequency.Hence,for the better practical performance of the semiconductors,the properties of the excitons are suggested to vary by adjusting AI concentration and eigenfrequency.
文摘The nonlinear absorption properties of direct (GaN) and indirect (CdI2) band gap crystals have been studied by using an open aperture Z-scan technique under fundamental (1064 nm) and frequency doubled (532 nm) wavelength respectively with 10 ns or 60 ps pulse durations. Direct band gap crystal exhibits two and three photon absorption at all input irradiances. On the other hand, at low input irradiance the indirect band gap crystal exhibits saturable absorption (SA). At higher input irradiances two and three photon absorption becomes dominant. A monotonic increase of the nonlinear absorption coefficients with increasing laser pulse duration from 60 ps to 10 ns is observed for GaN and CdI2 crystals.
基金Project supported by Akdeniz University Scientific Research Projects Coordination Unit
文摘Beam splitting upon refraction in a triangular sonic crystal composed of aluminum cylinders in air is experimentally and numerically demonstrated to occur due to finite source size, which facilitates circumvention of a directional band gap. Experiments reveal that two distinct beams emerge at crystal output, in agreement with the numerical results obtained through the finite-element method. Beam splitting occurs at sufficiently-small source sizes comparable to lattice periodicity determined by the spatial gap width in reciprocal space. Split beams propagate in equal amplitude, whereas beam splitting is destructed for oblique incidence above a critical incidence angle.
基金Project supported by the National Natural Science Foundation of China(Grant No.11204359)the National Basic Research Program of China(Grant No.2013CB921700)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB07020100)
文摘Band gap anomaly is a well-known issue in lead chalcogenides PbX （X = S, Se, Te, Po）. Combining ab initio calculations and tight-binding （TB） method, we have studied the band evolution in PbX, and found that the band gap anomaly in PbTe is mainly related to the high on-site energy of Te 5s orbital and the large s-p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that PbPo is an indirect band gap （6.5 meV） semiconductor with band inversion at L point, which clearly indicates that PbPo is a topological crystalline insulator （TCI）. The calculated mirror Chern number and surface states double confirm this conclusion.
文摘The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influence on optical properties of In1-xGaxN/P,where(x=0.0,0.25,0.50,0.75,and 1.0)alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method(FP-LAPW)and modified Becke and Johnson potentials(TB-mBJ).The direct bandgaps turn from 0.7 eV to 3.44 eV,and 1.41 eV to 2.32 eV for In1-xGaxN/P alloys,which increases their potentials for optoelectronic devices.The optical properties are discussed such as dielectric constants,refraction,absorption,optical conductivity,and reflection.The light is polarized in the low energy region with minimum reflection.The absorption and optical conduction are maxima in the visible region,and they are shifted into the ultraviolet region by Ga doping.Moreover,static dielectric constant e1(0)is in line with the bandgap from Penn’s model.
基金supported by the Special Funds for Major State Basic Research of China(Grant Nos.2012CB921401,2014CB921104)National Natural Science Foundation of China(Grant Nos.61106087,91233121 and 61125403)+1 种基金Shanghai Rising-Star Program(Grant No.14QA1401500)the Program for Changjiang Scholars and Innovative Research Team in University,and the computer center of East China Normal University
文摘The monolayer arsenic in the puckered honeycomb structure was recently predicted to be a stable two-dimensional layered semiconductor and therefore named arsenene. Unfortunately, it has an indirect band gap, which limits its practical application. Using first-principles calculations, we show that the band gaps of few-layer arsenic have an indirect-direct transition as the number of arsenic layers(n) increases from n=1 to n=2. As n increases from n=2 to infinity, the stacking of the puckered honeycomb arsenic layers forms the orthorhombic arsenic crystal ??-As, arsenolamprite), which has a similar structure to the black phosphorus and also has a direct band gap. This indirect-direct transition stems from the distinct quantum-confinement effect on the indirect and direct band-edge states with different wavefunction distribution. The strain effect on these electronic states is also studied, showing that the in-plane strains can induce very different shift of the indirect and direct band edges, and thus inducing an indirect-direct band gap transition too. The band gap dependence on strain is non-monotonic, with both positive and negative deformation potentials. Although the gap of arsenene opens between As p-p bands, the spin-orbit interaction decreases the gap by only 0.02 e V, which is much smaller than the decrease in Ga As with an s-p band gap. The calculated band gaps of arsenene and ?-As using the hybrid functional are 1.4 and 0.4 e V respectively, which are comparable to those of phosphorene and black phosphorus.
基金supported by the National Natural Science Foundation of China (21376199, 51002128 and 51401176)the Scientific Research Foundation of Hunan Provincial Education Department (17A205 and 15B235)
文摘A novel two-dimensional（2D） Te Se_2 structure with high stability is predicted based on the first-principles calculations. As a semiconductor, the results disclose that the monolayer Te Se_2 has a wide-band gap of 2.392 e V. Interestingly, the indirect-band structure of the monolayer Te Se_2 transforms into a direct-band structure under the wide biaxial strain（0.02–0.12）. The lower hole effective mass than monolayer black phosphorus portends a high carrier mobility in Te Se_2 sheet. The optical properties and phonon modes of the few-layered Te Se_2 were characterized. The few-layer Te Se_2 shows a strong optical anisotropy. Specially, the calculated results demonstrate that the multilayer Te Se_2 has a wide range of absorption wavelength. Our result reveals that Te Se_2 as a novel 2D crystal possesses great potential applications in nanoscale devices, such as high-speed ultrathin transistors, nanomechanics sensors, acousto-optic deflectors working in the UV-vis red region and optoelectronic devices.
基金Project supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars of the State Education Ministry of China(Grant No.[2015]-1098)the Open Project of the State Key Laboratory of Surface Physics of Fudan University,the Natural Science Foundation of Guangdong Province of China(Grant No.2016A030307038)the University Innovating and Strengthening Project of Department of Education of Guangdong Province,China(Grant No.2015KTSCX090)
文摘Using hybrid-functional first-principles calculation combined with the supercell method and band unfolding technique we investigate the band structure of non-strained Ge1-xSnx alloys with various Sn concentrations. The calculations show that at the Sn concentration of^3.1 mol% the GeSn alloy presents a direct band gap. The variation of the band structure are ascribed to the weaker electro-negativity of Sn atoms and a slight charge transfer from Sn atoms to Ge atoms.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11264007 and 61465003)
文摘In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory （DFT） with the generalized gradient approximation （GGA） are carried out to investigate the energy band gap structure on silicon （Si） and germanium （Ge） nanofilms. Simulation results show that the band gaps in Si （100） and Ge （111） nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and 7.25 nm respectively, but the band gaps of Si （111） and Ge （110） nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si （110） and Ge （100） nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si（1-x）/ZGexSi（1-x）/2 sandwich structure become the direct-gap structure in a certain area whether （111） or （100） surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects.
文摘Germanium （Ge） pin photodiodes show clear direct band gap emission at room temperature, as grown on bulk silicon in both photoluminescence （PL） and electro- luminescence （EL）. PL stems from the top contact layer with highly doped Ge because of strong absorption of visible laser light excitation （532 nm）. EL stems from the recombination of injected carriers in the undoped intrinsic layer. The difference in peak positions for PL （0.73 eV） and EL （0.80 eV） is explained by band gap narrowing from high doping in n＋-top layer. A superlinear increase of EL with current density is explained by a rising ratio of direct/ indirect electron densities when quasi Fermi energy level rises into the conduction band. An analytical model for the direct/indirect electron density ratio is given using simplifying assumptions.
