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Strain is one of the effective ways to modulate the band structure of monolayer transition metal dichalcogenides (TMDCs), which has been reported in theoretical and steady-state spectroscopic studies. However, the strain effects on the charge transfer processes in TMDC heterostructures have not been experimentally addressed thus far. Here, we systematically investigate the strain-mediated transient spectral evolutions corresponding to excitons at band-edge and higher energy states for monolayer MoS2 and monolayer WSe2. It is demonstrated that Γ and K valleys in monolayer WSe2 and monolayer MoS2 present different strain responses, according to the broadband femtosecond pump-probe experimental results. It is further observed that the resulting band offset changes tuned by applied tensile strains in MoS2-WSe2 heterostructures would not affect the band-edge electron transfer profiles, where only monolayer WSe2 is excited. From a flexible optoelectronic applications perspective, the robust charge transfer under strain engineering in TMDC heterostructures is very advantageous.

Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS2 QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS2 nanosheets, the selected monolayer WS2 QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin–orbit coupling. This suggests a complete change of the band structure for monolayer WS2 QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS2 nanosheets at room temperature, which is not observed for monolayer WS2 QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS2 QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.

Understanding and controlling the charge transfer processes of two-dimensional (2D) materials are fundamental for the optimized device performance based on 2D semiconductors and heterostructures. The charge transfer rate is very robust in transition metal disulfide (TMD) heterostructures with type II band alignments, which can be manipulated by intercalating a dielectric layer like hBN to isolate the donor and acceptor monolayers. This study shows that there is an alternative way to change the electron transfer and recombination rates in the case of nLMoS2/mLWSe2 multilayer heterostructures, where the donor-acceptor distance is maintained, but the rate of electron transfer is strongly layer dependent and shows asymmetry for the layer number of donor and acceptor monolayers. Especially, the 1LMoS2/2LWSe2 heterostructure slows electron transfer and charge recombination rates ∼2.3 and ∼12 times that of the 1LMoS2/1LWSe2 heterostructure, respectively, which have been competitive with that in the 1LMoS2/hBN/1LWSe2 heterostructure. From an application perspective, the noninterfacial electron transfer in which photogenerated electrons should across more than one atomically thin layer is not favorable due to the built-in electric field established by the initial interfacial electron transfer.

The static and dynamic electrical characteristics of GaN-based blue light-emitting diodes with different numbers of quantum wells were investigated by software Silvaco technology computer-aided design numerically, which were essential for visible light communication study. As a factor impacts on the response time of light-emitting diodes, the capacitance, varying with applied bias, with modulation frequency, and with the numbers of quantum wells, was analyzed detailedly. Besides that, the response time of light-emitting diodes, including the optical rise time and fall time, was also firstly simulated in Silvaco, and then, based on these, the pure − 3 dB bandwidth of the light-emitting diodes without building-out circuit could be calculated, so as to self-design semiconductor devices like light-emitting diodes mentioned in this study with the optimum bandwidth used for visible light communication.

A novel package structure has been developed to fabricate quantum-dot (QD) white light-emitting diodes (WLEDs) by dispersing QDs into the backlight lens and integrating them with blue light-emitting diode to form white light. For this in-lens encapsulation structure, the luminous efficiency and lifetime of QD-WLEDs can reach 90 lm/W and 1000 h, which is 25 times longer than that of QD-WLEDs with on-chip encapsulation structure. Furthermore, a 32-inch prototype liquid crystal display TV shows a wide color gamut of 108% under the National Television Systems Committee standard with this in-lens encapsulation structure. The improvement performance should be attributed to providing a hermetic encapsulation environment preventing oxygen and water from reacting with the QDs and reducing the temperature of QDs due to in-lens package structure. Our results demonstrate that the in-lens solution can be a promising application in display backlight.

Three kinds of InGaN/GaN multiple-quantum-well light-emitting diode with different electron-blocking layers (EBLs) have been investigated numerically. The energy band diagrams, light–current curves, voltage–current curves, electrostatic fields, carrier concentrations in the quantum wells, electron current densities, radiative recombination rates in active region, and internal quantum efficiency (IQE) are reported. The results show that, when the conventional AlGaN EBL is replaced by a p-AlGaN/InGaN superlattice EBL or graded p-AlyGa1−yN/InxGa1−xN superlattice EBL, respectively, the light output power at 200 mA can be increased by 46.0% and 52.8%, the turn-on voltage can be decreased from 3.2 V to 3.0 V, the IQE can be increased by 45.6% and 53.8% at 200 mA, and the efficiency droop can be reduced from 31.2% to 21.6%. These improvements are mainly attributed to the properly modified energy band structures which favor carrier injection.

In this study, the structure, morphology, electrical and optical properties of Si and Ta co-doped gallium oxide (Ga2O3) films deposited on (1 0 0) Ga2O3, (0 0 0 1) sapphire and quartz substrates by pulsed laser deposition are analyzed. The Si and Ta co-doped Ga2O3 film on Ga2O3 substrate possessed the best crystal quality with a high carrier concentration of 1.12 × 1020 cm−3 and a conductivity of 2.13 S·cm−1 compared to the sapphire and quartz substrates. The Si and Ta co-doped Ga2O3 films on different substrates exhibited high transmittance (>73%) above 300 nm and smooth film surface. The optical gap of Si and Ta co-doped Ga2O3 films on Ga2O3, sapphire and quartz substrates decreases from 4.89 to 4.85 eV, and the corresponding Urbach energy increases from 0.396 to 0.704 eV, so the homoepitaxial Si and Ta co-doped Ga2O3 film possess the best quality.

In this work, segmented silver nanowires (AgNWs) with an average diameter of 60 nm have been successfully synthesized by a typical polyol method without any templates and seeds. The synthesized segmented AgNWs were strongly dependent on the reaction temperature and time. It was found from high-resolution transmission electron microscopy and selected area electron diffraction measurements that the connection node of segmented AgNWs was in the form of a twinned crystal. We speculated that these segmented AgNWs were possibly derived from end-to-end self-connection and self-concrescence of two neighbouring Ag nanorods or nanowires at a suitable reaction temperature and time, which is further confirmed by the secondary growth of AgNWs. In addition, segmented AgNWs were blended into hole transporting layers to enhance the performance of polymer solar cells (PSCs) by utilizing their localized surface plasmon resonance and optical scattering effects. As a result, the power conversion efficiency (PCE) and short-circuit current density (Jsc) of PSCs with segmented AgNWs increased from 2.81% and 8.99 mA cm−2 to 3.30% and 9.95 mA cm−2, respectively.

In this study, a new design of quantum barriers has been investigated numerically with the purpose of improving the optical performance of blue InGaN light-emitting diodes (LEDs). Through analysis of energy band diagrams, carrier concentrations, carrier current densities, Auger recombination rates, and radiative recombination rates, we obtain simulation results showing that the proposed structure with a mix of AlGaN and InGaN quantum barriers can significantly improve the light output power and internal quantum efficiency (IQE), being mainly attributed to successful enhancement of the hole injection efficiency and suppression of the electron leakage current. Moreover, the efficiency droop of the LEDs is markedly improved by using the newly designed structure.

In this study, three kinds of electron-blocking layer (EBL) in blue InGaN light-emitting diodes (LEDs) are investigated numerically. They are conventional AlGaN EBL, AlInGaN EBL and gradual Al and In composition p-AlInGaN EBL. Through the analysis of the output power, internal quantum efficiency, turn-on voltage, energy band diagrams, carrier current densities and radiative recombination rates, we have got the simulation results that the LED with gradual Al and In composition p-AlInGaN EBL exhibits a higher output power, a lower electron leakage, a better hole injection efficiency and a more peaceable efficiency droop than the LED with a conventional AlGaN EBL or with a AlInGaN EBL.

In this study, a new design on last quantum barrier (LQB) is investigated numerically with the purpose of improving the optical performance of InGaN light-emitting diodes (LEDs). Through the analysis of the energy band diagrams, electrostatic fields, carrier concentrations, carrier current densities, and radiative recombination rates, we have got the simulation results that the proposed undoped-InGaN/AlInGaN superlattice (SL) LQB can significantly improve the output power and internal quantum efficiency, which is mainly attributed to the successful improvement in hole injection efficiency and suppression of electron current leakage. Moreover, the efficiency droop of the LEDs is improved slightly by using u-InGaN/AlInGaN SL as last barrier.

The characteristics of InGaN-based blue light-emitting diodes (LEDs) with an AlGaN/GaN superlattice (SL) electron-blocking layer (EBL) of gradual Al molar fraction below the active region are analyzed numerically. The output power, internal quantum efficiency, electrostatic fields, energy band diagrams, carrier concentrations, radiative recombination, and electron leakage current are investigated. The results indicate that the LED with n-type AlGaN/GaN SL EBL of gradual Al molar fraction has smaller electrostatic fields and lower electron leakage in its active region than the LED with a rectangular p-AlGaN EBL or n-AlGaN EBL. These result in a markedly reduced efficiency droop.

The advantages of GaN based light-emitting diodes (LEDs) with strain-compensated p-AlGaN/InGaN superlattice (SL) last quantum barrier (LQB) are investigated numerically. The simulation results indicate that the output power and internal quantum efficiency have been improved significantly by replacing the last barrier of the conventional u-GaN and p-GaN with p-AlGaN/InGaN SL. These improvements are mainly attributed to the improvement of electron confinement and hole injection efficiency caused by mitigating the polarization-induced band bending of last barrier with the new designed structure. Moreover, the efficiency droop of the LEDs is markedly improved by using p-AlGaN/InGaN SL as last barrier.

With the purpose to increase the uniformity of carrier distribution without sacrificing the enhancement of carrier injection efficiency, the light-emitting diodes (LEDs) without an electron-blocking layer (EBL) by using AlGaN step-like barriers (SLBs) is proposed and investigated numerically. The simulation results show that the enhanced electron confinement and hole injection efficiency are mainly attributed to the mitigated downward band bending induced by polarization field at the interface of the last barrier and EBL and the increased carrier distribution uniformity is due to step-like potential height for carrier of the new designed LEDs. In addition, the distribution of radiative recombination rate and the efficiency droop are markedly improved when the conventional GaN barriers are replaced by AlGaN SLBs and the EBL is removed.

The characteristics of blue InGaN light-emitting diodes (LEDs) with InGaN barriers and dip-shaped last barrier are investigated numerically. The simulation results show that the newly designed LEDs have a better performance over the conventional InGaN/GaN and InGaN/InGaN counterparts attributed to the enhancement of carriers confinement induced by the improved potential barrier height for electrons and holes, and the amelioration of electron-hole spatial overlap caused by the reduced polarization effect between the well and barrier. In addition, the efficiency droop and the radiative recombination rate are markedly improved by employing the LEDs with InGaN barriers and dip-shaped last barrier.

The blue InGaN light-emitting diodes (LEDs), employing a lattice-compensated p-AlGaN/InGaN superlattice (SL) interlayer to link the last quantum barrier and electron blocking layer (EBL), are proposed and investigated numerically. The simulation results indicate that the newly designed LEDs have better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region over the conventional LEDs mainly attributed to the mitigated polarization-induced downward band bending. Furthermore, the markedly improved output power and efficiency droop are also suggested when the conventional LEDs corresponding to experiment data are replaced by the newly designed LEDs.

Based on spin-polarized density function theory, Electronic structure and magnetism of V-doped AlN are investigated. 6.25% V-doped AlN is found half-metallic with 100% carrier spin polarization of the conduction carriers. Per supercell has a net magnetic moment of 2 μ B , which are mostly localized within the VN 4 tetrahedron. V-doped AlN favors ferromagnetic ground state which can be explained by double exchange mechanism. To study the effects of electron correlation, GGA+ U calculations have been performed. Besides, formation energy is good for V doping. Our results suggest that the V-doped AlN may present a promising material and has potential applications in semiconductor spintronics devices.

In this work, GaN-based light-emitting diodes (LEDs) with a p-GaN/i-InGaN short-period superlattice (SPSL) structure, p-GaN and undoped GaN last quantum barrier (LQB) have been numerically investigated by using the APSYS simulation software. It has been found that the efficiency droop is significantly improved when the undoped GaN LQB in a typical blue LED is replaced by a p-GaN/i-InGaN SPSL structure. According to the simulation analysis, using the p-GaN/i-InGaN SPSL structure as LQB is beneficial to increasing the hole injection efficiency and decreasing the electron current leakage. Therefore, the radiative recombination and optical power are enhanced.

C mono-doped and C–In co-doped ZnO are investigated by the first-principles calculations. It is found that the C mono-doped ZnO is p-type with hole carriers locating nearby valence band maximum. Furthermore, a shallower C acceptor energy level appears in the band gap after incorporating In into C-doped ZnO system. Meantime, compared with C mono-doped ZnO, C–In co-doped ZnO has a lower formation energy, correspondingly a higher chemical stability, and thus to enhance the incorporation efficiency of C. These results suggest that C–In co-doping method provided an efficient technique for realizing p-type ZnO.

Using the first-principles method based on the density functional theory, we have studied magnetic and optical properties of V-doped GaN. For 12.5% V-doped GaN, total energy calculations show that the ferromagnetic state is 255 meV lower than the antiferromagnetic state and is thus predicted to be the ground state with a Curie temperature above room temperature. The magnetic moments are localized at the V atoms and ferromagnetic exchange interaction is short ranged. The analysis of optical properties shows that V-doped GaN is a promising dielectric material and has potential applications in optoelectronic devices.

Using the first-principles method based on the density functional theory, we study electronic and magnetic properties of GaN doped with 6.25% of V. The V dopants are found spin polarized and order ferromagnetically in GaN, with a magnetic moment of 2 μ B per supercell. The V-doped GaN favors ferromagnetic ground state which can be explained in terms of double-exchange mechanism, and a Curie temperature above room temperature can be expected. Also, incorporation of V is easy. These results suggest that the V-doped GaN may present a promising dilute magnetic semiconductor and find applications in the field of spintronics.

Designs of p-doped in quantum well (QW) barriers and specific number of vertically stacked QWs are proposed to improve the optical performance of GaN-based dual-wavelength light-emitting diodes (LEDs). Emission spectra, carrier concentration, electron current density, and internal quantum e±ciency (IQE) are studied numerically. Simulation results show that the e±ciency droop and the spectrum intensity at the large current injection are improved markedly by using the proposed design. Compared with the conventional LEDs, the uniform spectrum intensity of dual-wavelength luminescence is realized when a specific number of vertically stacked QWs is adopted. Suppression of electron leakage current and the promotion of hole injection e±ciency could be one of the main reasons for these improvements.

The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580″, can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 × 10 18 cm −3 has been obtained.

An InGaN/GaN blue-violet light-emitting diode (LED) structure and an InGaN/GaN blue LED structure were grown sequentially on the same sapphire substrate by metal-organic chemical vapor deposition (MOCVD). At the low injection current, the intensity ratio of blue-violet light to blue light was almost constant, while the blue light intensity increased gradually with increasing injection current when the latter was more than 40 mA. High color rendering has been realized for a Y3Al5O12:Ce3+ phosphor-converted white LED based on dual-blue-emitting active layers relative to a single blue-emitting active layer at the same injection current.

Tantalum-doped indium tin oxide ( Ta -doped ITO) transparent conductive films are deposited on glass substrates by electron-beam evaporation. The effects of different Ta concentrations and annealing temperatures on the structural, morphologic, electrical, and optical properties of Ta -doped ITO films are investigated by X-ray diffraction (XRD), atomic force microscope (AFM), Hall measurement, and optical transmission spectroscopy. The obtained films are polycrystalline with a cubic bixbyite structure of indium oxide and preferentially oriented in the (222) crystallographic direction. The minimum resistivity of 1.54×10-4 Ω ·cm is obtained from the ITO film containing 0.2 wt% tantalum annealed at 500°C and the average optical transmittance is over 95% from 425 nm to 460 nm.

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.

The effect of growth rate in the early stage of high temperature (HT) GaN layer growth on the quality of GaN by MOCVD has been investigated. The properties of GaN layers were measured by double crystal X-ray diffraction (DCXRD), Hall method, photoluminescence, and atomic force microscopy, respectively. The results reveal that the quality of GaN layers is influenced by the growth rate of HT GaN growth in the early stage. The properties of GaN layers are obviously improved by decreasing the growth rate in the early stage of HT GaN growth properly. However, when the growth rate decreases to some extent, the quality of GaN layer deteriorates instead. The reasons behind this phenomenon were discussed.

The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells (MQWs) light-emitting diode wafers was studied by means of electrochemical capacitance-voltage (ECV) and photoluminescence (PL). Compared with the sample unannealed, the hole carrier concentration of p-GaP layer increased from 5.5x10^1^8 to 6.5x10^1^8cm^-^3, and the hole carrier concentration of p-AlGaInP layer increased from 6.0x10^1^7 to 1.1x10^1^8cm^-^3, after wafer was annealed at 460^oC for 15min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature varied from 460 to 700^oC. However, after the sample was annealed under 780^oC for 15min, the hole carrier concentration of p-GaP layer and p-AlGaInP layer decreased to 8x10^1^7 and 1.7x10^1^7cm^-^3, respectively. At the same time, the diffusion of Mg atoms was observed.

The blue InGaN light-emitting diodes (LEDs) with specific designs of p-InGaN/GaN superlattice (SL) last quantum barrier are investigated numerically and experimentally. The proposed SL with a graded indium mole fraction from 0% to 5% shows improved efficiency droop and superior optical characteristics in comparison with the conventional LEDs. As indicated by the simulation results, the promotion of hole injection and the reduction of electron leakage play important roles in these improvements. Fabricated LEDs with this specific design exhibit stronger emission intensity, smaller forward voltage, and larger light output power compared to its counterparts.

In this study, the influence of multiple interruptions with trimethylindium (TMIn)-treatment in InGaN/GaN multiple quantum wells (MQWs) on green light-emitting diode (LED) is investigated. A comparison of conventional LEDs with the one fabricated with our method shows that the latter has better optical properties. Photoluminescence (PL) full-width at half maximum (FWHM) is reduced, light output power is much higher and the blue shift of electroluminescence (EL) dominant wavelength becomes smaller with current increasing. These improvements should be attributed to the reduced interface roughness of MQW and more uniformity of indium distribution in MQWs by the interruptions with TMIn-treatment.

Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, People’s Republic of China Abstract: The GaP layers grown on GaN layer by Metalo-organic chemical vapour deposition (MOCVD) were studied. The results show that the GaP layers are single crystals when the growth temperature varied from 350 to 500 . However, the crystal quality of GaP grown at low temperature is not good. Using a 40 nm GaP buffer layer grown at 400 , the crystal quality of GaP grown at 680 improves evidently. The p-type GaP layers with hole carrier concentration of 4.2™ 10

The energy band structures, density of states, and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O (IIIA = Al, Ga, In) are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O:IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein—Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O:IIIA are heavier than those of Mg0.25Zn0.75O, which is in agreement with the previous experimental result. The formation energies of MgZnO:Al and MgZnO:Ga are smaller than that of MgZnO:In, while their optical bandgaps are larger, so MgZnO:Al and MgZnO:Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet (UV) and deep UV optoelectronic devices.

The characteristics of a blue light-emitting diode (LED) with a p-InAlGaN hole injection layer (HIL) is analyzed numerically. The simulation results indicate that the newly designed structure presents superior optical and electrical performance such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-InAlGaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.

In this study, a new design on last quantum barrier (LQB) is investigated numerically with the purpose of improving the optical performance of InGaN light-emitting diodes (LEDs). Through the analysis of the energy band diagrams, electrostatic fields, carrier concentrations, carrier current densities, and radiative recombination rates, we have got the simulation results that the proposed undoped-InGaN/AlInGaN superlattice (SL) LQB can significantly improve the output power and internal quantum efficiency, which is mainly attributed to the successful improvement in hole injection efficiency and suppression of electron current leakage. Moreover, the efficiency droop of the LEDs is improved slightly by using u-InGaN/AlInGaN SL as last barrier.

The efficiency enhancement of an InGaN light-emitting diode (LED) with an AlGaN/InGaN superlattice (SL) electron-blocking layer (EBL) is studied numerically, which involves the light-current performance curve, internal quantum efficiency electrostatic field band wavefunction, energy band diagram carrier concentration, electron current density, and radiative recombination rate. The simulation results indicate that the LED with an AlGaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular AlGaN EBL or a normal AlGaN/GaN SL EBL because of the appropriately modified energy band diagram, which is favorable for the injection of holes and confinement of electrons. Additionally, the efficiency droop of the LED with an AlGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.

Blue InGaN light-emitting diodes (LEDs) with a conventional electron blocking layer (EBL), a common n-AlGaN hole blocking layer (HBL), and an n-AlGaN HBL with gradual Al composition are investigated numerically, which involves analyses of the carrier concentration in the active region, energy band diagram, electrostatic field, and internal quantum efficiency (IQE). The results indicate that LEDs with an n-AlGaN HBL with gradual Al composition exhibit better hole injection efficiency, lower electron leakage, and a smaller electrostatic field in the active region than LEDs with a conventional p-AlGaN EBL or a common n-AlGaN HBL. Meanwhile, the efficiency droop is alleviated when an n-AlGaN HBL with gradual Al composition is used.

The characteristics of a blue light-emitting diode (LED) with an AlInN/GaN superlattice (SL) electron-blocking layer (EBL) are analyzed numerically. The carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results suggest that the LED with an AlInN/GaN SL EBL has better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region than the LED with a conventional rectangular AlGaN EBL or a AlGaN/ GaN SL EBL. The results also indicate that the efficiency droop is markedly improved when an AlInN/GaN SL EBL is used.

Using the first-principles method based on the density functional theory, we have calculated electronic structure of zinc blende AlN doped with 6.25% of V. The V dopants are found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The ferromagnetic ground state in V-doped AlN can be explained in terms of double-exchange mechanism, and a Curie temperature above room temperature can be expected. These results suggest that the V-doped AlN may present a promising dilute magnetic semiconductor and find applications in the field of spintronics.

InGaN-based light-emitting diodes with p-GaN and p-AlGaN hole injection layers are numerically studied using the APSYS simulation software. The simulation results indicate that light-emitting diodes with p-AlGaN hole injection layers show superior optical and electrical performance, such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-AlGaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.

The performance of InGaN blue light-emitting diodes (LEDs) with different kinds of electron-blocking layers is investigated numerically. We compare the simulated emission spectra, electron and hole concentrations, energy band diagrams, electrostatic fields, and internal quantum efficiencies of the LEDs. The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer (EBL) has a strong spectrum intensity, mitigates efficiency droop, and possesses higher output power compared with the LEDs with the other three types of EBLs. These advantages could be because of the lower electron leakage current and more effective hole injection. The optical performance of the specifically designed LED is also improved in the case of large injection current.

InGaN based light-emitting diodes (LEDs) with dip-shaped quantum wells and conventional rectangular quantum wells are numerically investigated by using the APSYS simulation software. It is found that the structure with dip-shaped quantum wells shows improved light output power, lower current leakage and less efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and the optical characteristics are attributed mainly to the alleviation of the electrostatic field in dip-shaped InGaN/GaN multiple quantum wells (MQWs).