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The optical properties of blue-violet InGaN light-emitting diodes under normal and reversed polarizations are numerically studied. The best light-emitting performance under normal and reversed polarization is obtained in a single quantum-well structure and double quantum-well structure, respectively. The key factors responsible for these phenomena are presumably the carrier concentration distribution and the amount of carriers in quantum wells. The turn-on voltage of light-emitting diodes under reversed polarization is lower than that of light-emitting diodes under normal polarization, due mainly to lower potential heights for electrons and holes in the active region.

The effect of polarization-matched AlGaInN electron-blocking layer and barrier layer on the optical performance of blue InGaN light-emitting diodes is numerically investigated. The polarization-matched AlGaInN electron-blocking layer and barrier layer are employed in an attempt to reduce the polarization effect inside the active region of the light-emitting diodes. The simulation results show that the polarization-matched AlGaInN electron-blocking layer is beneficial for confining the electrons inside the quantum well region. With the use of both polarization-matched AlGaInN electron-blocking layer and barrier layer, the optical performance of blue InGaN light-emitting diodes is greatly improved due to the increased overlap of electron and hole wavefunctions. The method proposed in this paper can also be applied to the light-emitting diodes operating in other spectral range.

The optical properties of blue InGaN LEDs that emit in a spectral range from 410 to 445 nm are theoretically investigated by using the APSYS simulation program. It is found that the light performance can be enhanced effectively when the conventional GaN barrier layers are replaced by In0.02Ga0.98N and In0.05Ga0.95N barrier layers. The numerical results indicate that the output power of LEDs with In0.02Ga0.98 N barrier layers is improved gradually above the emission wavelength of 410 nm. However, when the In0.05Ga0.95N barrier layers are used, the emitting power of LEDs varies significantly when the emission wavelength changes. When the emission wavelength is 410 nm, the use of GaN and In0.02Ga0.98N barrier layers can lead to higher output power. However, if the emission wavelength is 445 nm, the use of In0.05Ga0.95N barrier layers is beneficial for maintaining high output power.

The Auger recombination is recently proposed as one of the possible origins for the deteriorated internal quantum efficiency of InGaN light-emitting diodes. The Auger recombination behavior is quite different under widely varied Auger coefficients. The effect of Auger coefficient on the efficiency and output power is investigated numerically. The simulation results indicate that the Auger recombination with large Auger coefficient greatly decreases the efficiency in the whole current range under study. It is found that the electron current leakage and nonuniform hole distribution are the possible mechanisms responsible for the efficiency droop at high injection current.

Theoretical analysis for InGaP/AlGaInP laser diodes with different ridge waveguide structures is performed to investigate the lateral mode behavior using advanced device simulation. The internal physical mechanisms including temperature-induced changes in the refractive index profile, spatial hole burning effect, lateral carrier distribution, and gain profile variation with increasing input current are discussed by theoretical calculation to analyze the effects of different ridge structures on the lateral mode behavior of 660-nm AlGaInP laser diodes. The simulation results show that the use of narrow and shallow ridge geometry is the approach to obtaining single mode operation. Furthermore, it is found that the different values of the ridge height cause the lateral carrier distribution within the active region to be varied, which is also an important factor in determining the emergence of the first order lateral mode in addition to the geometry-dependent waveguide cutoff condition.

Polarization is a crucial issue for electrical and optical characteristics of the UV AlGaInN light-emitting diodes. The effect of spontaneous and piezoelectric polarization on optical characteristics of the UV AlGaInN light-emitting diodes is investigated numerically. The simulation results indicate that the polarization-related effect for the UV AlGaInN light-emitting diodes is dominated by the piezoelectric polarization. When the UV AlGaInN light-emitting diodes are without piezoelectric polarization, the optical performance is enhanced effectively due to improved overlap of electron and hole wavefunctions, reduced electron leakage current, and increased density of holes in the active region.

The optical properties of InGaN multi-quantum-well laser diodes with different polarization-matched AlInGaN barrier layers have been investigated numerically by employing an advanced device simulation program. The use of quaternary polarization-matched AlInGaN barrier layers enhances the electron–hole wave function overlap due to the compensation of polarization charges between InGaN quantum well and AlInGaN barrier layer. According to the simulation results, it is found that, among the polarization-matched quantum-well structures under study, lower threshold current and higher slope efficiency can be achieved simultaneously when the aluminum composition in AlInGaN barrier layers is about 10–15%. The optimal polarization-matched InGaN/AlInGaN laser diode shows lower threshold current and higher slope efficiency compared to conventional InGaN/InGaN laser diodes.

In this work, the multiple-quantum-well InGaAsN laser structures with indirect-GaAsP and direct-GaAsN barriers are investigated by using LASTIP simulation program. We vary the quantum-well number, from 1 to 5, to find appropriate barrier material for InGaAsN laser structures. The simulation results show that InGaAsN laser structure has higher characteristic temperature regardless of what quantum-well number is if the indirect-GaAsP barrier is utilized. Furthermore, for InGaAsN laser structure, the usage of indirect-GaAsP barrier is beneficial for reducing the threshold current when the quantum-well number is from 1 to 2 and the usage of direct-GaAsN barrier is beneficial for reducing the threshold current when the quantum-well number is from 3 to 5.

Theoretical analysis for different active layer structures is performed to minimize the laser threshold current of the ultraviolet GaN/AlGaN multiple-quantum-well laser diodes by using advanced device simulation. The simulation results show that the lower threshold current can be obtained when the number of quantum wells is two or three and the aluminum composition in the barrier layer is about 10%-12%. This result is attributed to several different effects including electron leakage current, nonuniform carrier distribution, interface charge density induced by spontaneous and piezoelectric polarization, and optical confinement factor. These internal physical mechanisms are investigated by theoretical calculation to analyze the effects of quantum-well number and different aluminum compositions in barrier layer on laser threshold properties. Furthermore, the effect of quantum-well thickness is discussed as well. It is found that the optimal quantum-well thickness is about 3 nm due to the balance of the advantages of a large confinement factor against the disadvantages of significant quantum-confined Stark effect (QCSE).

InGaP/AlGaInP 660-nm resonant-cavity light-emitting diodes (RCLEDs) with stable temperature characteristics have been achieved by extending the resonant cavity length from one optical wavelength (1 lambda) to three optical wavelengths (3 lambda) and tripling the number of quantum wells. When the operation temperature increases from 25degC to 95degC, the degree of power variation at 20 mA is reduced from -2.1 dB to -0.6 dB for the conventional 1- lambda cavity RCLEDs and 3- lambda cavity RCLEDs, respectively. In order to interpret the temperature-dependent experimental results, advanced device simulation is applied to model the RCLEDs with different cavity designs. According to the numerical simulation results, we deduce that the stable temperature-dependent output performance should originate from the reduction of electron leakage current and thermally enhanced hole transport for the 3- lambda cavity AlGaInP RCLEDs.

Effects of built-in polarization and carrier overflow on InGaN quantum-well lasers with a ternary AlGaN or a quaternary AlInGaN electronic blocking layer (EBL) have been numerically investigated by employing an advanced device-simulation program. The simulation results indicate that the characteristics of InGaN quantum-well lasers can be improved by using the quaternary AlInGaN EBL. When the aluminum and indium compositions in the AlInGaN EBL are appropriately designed, the built-in charge density at the interface between the InGaN barrier and the AlInGaN EBL can be reduced. Under this circumstance, the electron leakage current and the laser threshold current can obviously be decreased as compared with the laser structure with a conventional AlGaN EBL when the built-in polarization is taken into account in the calculation. Furthermore, the AlInGaN EBL also gives a higher refractive index than the AlGaN EBL, which is a benefit for a higher quantum-well optical confinement factor in laser operations.

The zincblende In x Ga 1− x N alloys are studied by numerical analysis based on first-principles calculations. A 64-atom supercell is used to model these alloys. Our results indicate that the ternary In x Ga 1− x N alloys have an average band gap bowing parameter of 2.08 eV. The simulation results also suggest that the composition-dependent bowing parameter of the ternary In x Ga 1− x N alloys can be expressed by a third-order polynomial equation, b ( x ) = −4.7422 x 3 + 6.9592 x 2 − 2.3136 x + 2.1301 (eV).

The gain properties and valence subbands of InGaAsN/GaAsN quantum-well structures are numerically investigated with a self-consistent LASTIP simulation program. The simulation results show that the InGaAsN/GaAsN has lower transparency carrier density than the conventional InGaAsP/InP material system for 1.3-μm semiconductor lasers. The material gain and radiative current density of InGaAsN/GaAsN with different compressive strains in quantum well and tensile strains in barrier are also studied. The material gain and radiative current density as functions of strain in quantum well and barrier are determined. The simulation results suggest that the laser performance and Auger recombination rate of the 1.3-μm InGaAsN semiconductor laser may be markedly improved when the traditional GaAs barriers are replaced with the AlGaAs graded barriers.

In reference to real devices fabricated in laboratories, the optical properties of AlGaInAs, InGaNAs, and InGaAsP semiconductor material systems for 1.3-μm semiconductor lasers are systematically studied. Simulation results show that both the AlGaInAs/InP and InGaNAs/GaAs material systems have better gain performance and smaller transparency carrier density than the InGaAsP/InP material system. For the AlGaInAs/InP material system, the characteristic temperature is improved by using compensating tensile strain in barrier. Specifically, for a 250-μm-long short-cavity AlGaInAs/InP laser, when the barrier is with a compensating tensile strain of 0.39%, the characteristic temperatures in 290–330 K and 330–350 K can be enhanced to 121.7 K and 58.9 K, respectively. For the InGaNAs/GaAs material system, simulation results suggest that the laser performance can be significantly improved when the laser is with strain-compensated GaNAs barriers.

The physical and optical properties of compressively strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers are numerically studied. The simulation results show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAsP quantum wells can be efficiently improved by employing a compressive strain of approximately 1.24% in the InGaAsP quantum wells. The simulation results suggest that the 850-nm InGaAsP/InGaP vertical-cavity surface-emitting lasers have the best laser performance when the number of quantum wells is one, which is mainly attributed to the non-uniform hole distribution in multiple quantum wells due to high valence band offset.

The reduced peak efficiency and the efficiency droop afterward, i.e. the degraded efficiency, of blue InGaN lightemitting diodes (LEDs) is investigated numerically. It is depicted that the joint effects of multiple factors, including the influences of polarization-induced electric field, the phenomenon of current crowding, and the Auger and ShockleyRead-Hall (SRH) recombinations, are responsible for the degraded efficiency. Among them, the severe SRH recombination due to the poor crystalline quality is the main cause of reduced peak efficiency, while the serious Auger recombination resulted from high Auger recombination coefficient and non-uniform carrier distribution of the active region is the major factor contributing to efficiency droop. It is shown that the strong built-in polarization field and the crowded current flow will result in the nonuniform carrier distribution, and thus enlarge the Auger losses and the efficiency droop.

For 365-nm ultraviolet light-emitting diodes (UV LEDs), an electron blocking layer (EBL) is usually utilized to mitigate electron overflow. However, using EBL might obstruct holes from injecting into the active region. Moreover, the large polarization field in conventional EBL might also pull down the effective barrier height for electrons, and thus the electrons could easily overflow to the p-side region. To solve the above drawbacks, in this study, the Al content and p-doping concentration of the EBL in typical 365-nm UV LEDs are investigated systematically. Specifically, designs of AlGaN/GaN superlattice EBL and Al-content-graded EBL are explored in detail.

Optical properties of the InGaN violet and ultraviolet multiple-quantum-well laser diodes are numerically studied with a self-consistent simulation program. Specifically, the performance of the laser diodes of various active region structures, operating in a spectral range from 370 to 410 nm, are investigated and compared. The feasibility of using multiquantum barriers and doped barriers to improve the laser performance is numerically evaluated. The characteristic temperatures for the laser diodes under study will be investigated. Optimization of the structures for InGaN violet and ultraviolet multiple-quantum-well laser diodes will be attempted.

In this study, the carrier blocking effect on 850 nm InAlGaAs/AlGaAs vertical-cavity surface-emitting layers (VCSELs) was theoretically and experimentally investigated. By means of inserting a high-bandgap electron blocking layer, which was either 10 nm thick Al0.75Ga0.25As or 13 nm thick Al0.9Ga0.1As, on the p-side of a quantum-well active region, the laser output performance was theoretically found to be improved. VCSELs with and without an electron blocking layer were also experimentally demonstrated. It was found that the threshold current was reduced from 1.47 to 1.33 mA and the slope efficiency was increased from 0.37 to 0.53 mW mA −1 by inserting a 10 nm thick Al0.75Ga0.25As electron blocking layer. Also, the device became less sensitive to the device temperature, where the amount of increase in the threshold current at an elevated temperature of 95 ◦ Cw as only 0.27 mA and the slope efficiency dropped by only 24.5%. A peak frequency response of nearly 9 GHz at 5 mA, measured from relative intensity noise (RIN), was obtained in these VCSEL devices.

Blue InGaN micro-size light emitting diodes (LEDs) with diameters from 3 to 70 μm have been fabricated. An ion implantation technique and a 12 μm electro-ridge were used to simplify the fabrication processes. The 3–70 μm LEDs exhibiting a large emission of photon blue shift (40–240 meV) were observed in electro-luminescence (EL) spectra. The dependence of the blue shift on size is studied. The characteristics of the micro-size LEDs are also investigated numerically with the use of an advanced physical model of semiconductor devices (APSYS). The experimental measurements and simulation results are in close agreement for maximum blue shift. Base on the simulation, the difference between blue shift caused by band filling effect and red shift caused by lateral carrier confinement are approximately the same. Hence, the maximum blue shift increases when the size of micro-size LED increases because of decreased red shift resulted from thermal effect.

The valence subband structures, optical gain spectra, transparency carrier densities, and transparency radiative current densities of different compressively strained InGaAlAs quantum wells with Al 0.3 Ga 0.7 As barriers are systematically investigated using a 6 × 6 k · p Hamiltonian including the heavy hole, light hole, and spin–orbit splitting bands. The results of numerical calculations show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAlAs quantum wells can be enhanced by introducing more compressive strain in quantum wells. However, further improvement of the optical properties of InGaAlAs quantum wells becomes minimal when the compressive strain is higher than approximately 1.5%. The simulation results suggest that the compressively strained InGaAlAs quantum wells are of advantages for application in high-speed 850-nm vertical-cavity surface-emitting lasers.

An ultraviolet (UV) AlGaInN light-emitting diode (LED) with 370 nm emission is demonstrated. At room temperature (RT) UV power of 0.8 mW at 20 mA with 3.6 V operation voltage is achieved. It provides 4 mW output when driven at 125 mA under continuous-wave (CW) operation. Qualitative optimization of the Al composition in the AlGaN electron-block layer and the quaternary AlGaInN quantum well (QW) number of the UV LED is also investigated in this study. The numerical results fit with the experimentally demonstrated output performance of our AlGaInN UV LED. We find that the UV AlGaInN LED can provide better output characteristics when the Al composition in the AlGaN electron-block layer is in the range 19–21% and the AlGaInN QW number is in the range 5–7 by reducing the electron leakage current.

In this study, the gain-carrier characteristics of In/sub 0.02/Ga/sub 0.98/As and InAlGaAs quantum wells (QWs) of variant In and Al compositions with an emission wavelength of 838 nm are theoretically investigated. More compressive strain, caused by higher In and Al compositions in InAlGaAs QW, is found to provide higher material gain, lower transparency carrier concentration, and transparency radiative current density over the temperature range of 25-95/spl deg/C. To improve the output characteristics and high-temperature performance of 850-nm vertical-cavity surface-emitting laser (VCSEL), In/sub 0.15/Al/sub 0.08/Ga/sub 0.77/As/Al/sub 0.3/Ga/sub 0.7/As is utilized as the active region, and a high-bandgap 10-nm-thick Al/sub 0.75/Ga/sub 0.25/As electronic blocking layer is employed for the first time. The threshold current and slope efficiency of the VCSEL with Al/sub 0.75/Ga/sub 0.25/As at 25/spl deg/C are 1.33 mA and 0.53 W/A, respectively. When this VCSEL is operated at an elevated temperature of 95/spl deg/C, the increase in threshold current is less than 21% and the decrease in slope efficiency is approximately 24.5%. A modulation bandwidth of 9.2 GHz biased at 4 mA is demonstrated.

Theoretical analysis for different active layer structures under the same waveguide confinement is conducted to minimize the electron overflow from the active layer to the p-cladding layer for the AlGaInP laser diode. An active layer with five quantum wells and a (AlxGa1-x)InP barrier with an x composition of 0.5 has found to be the optimal structure for the AlGaInP laser diode suitable for DVD-ROM and DVD player. Experimental results have confirmed that the characteristic temperature can be as high as 110 K at far field angles of 29°/9° for this optimized AlGaInP laser diode.

Numerical calculation based on first-principles is applied to study the structural characteristics and the band-energy properties of wurtzite AlxGa1-xN. The lattice constants obtained from the equilibrium energy are larger than those obtained from the Vegard’s law. The deviation parameter is 0.040±0.005 A for the a lattice constant and 0.125±0.009 A for the c lattice constant. The band gap energy is overestimated with the Vegard’s law. The bowing parameter of direct (indirect) band gap energy of 0.752±0.069 eV (0.889±0.057 eV) is obtained with the equilibrium lattice constants, and 0.352±0.046 eV (0.271±0.056 eV) is obtained with the lattice constants derived from the Vegard’s law.

Optical properties of a 1.3-μm AlGaInAs/InP strained multiple quantum-well structure with an AlInAs electron stopper layer, which is located between the active region and the p-type graded-index separate confinement heterostructure layer, are studied numerically with a LASTIP simulation program. Specifically, the effect of the electron stopper layer on the characteristic temperature and the temperature dependence of the slope efficiency are investigated. Various physical parameters at different operating temperatures are adjusted so that the threshold currents of the simulated laser structure can be matched to the experimental results of an identical laser structure fabricated by Selmic et al. The simulation results suggest that, with the use of a p-type Al0.5In0.5As electron stopper layer and a strain-compensated active region consisting of Al0.175Ga0.095In0.73As (6 nm)/Al0.32Ga0.2In0.48As (10 nm), a characteristic temperature as high as 108.7 K can be achieved for a 250-μm-long AlGaInAs/InP laser.

Numerical simulation based on first-principle calculations is applied to study the wurtzite AlxIn1-xN. Simulation results suggest that the Vegard’s law deviation parameter is 0.063 ± 0.014 A for the a lattice constant, and -0.160 ± 0.015 A for the c lattice constant. The band gap bowing parameter is 3.668 ± 0.147 eV with the lattice constants by means of the minimized equilibrium energy, and 3.457 ± 0.152 eV with the lattice constants derived from Vegard’s law.

Numerical calculation based on first-principles is applied to study the structural characteristics and the band-energy properties of wurtzite AlxGa1-xN. The lattice constants obtained from the equilibrium energy are larger than those obtained from the Vegard’s law. The deviation parameter is 0.040±0.005 A for the a lattice constant and 0.125±0.009 A for the c lattice constant. The band gap energy is overestimated with the Vegard’s law. The bowing parameter of direct (indirect) band gap energy of 0.752±0.069 eV (0.889±0.057 eV) is obtained with the equilibrium lattice constants, and 0.352±0.046 eV (0.271±0.056 eV) is obtained with the lattice constants derived from the Vegard’s law.

Continuous-wave (CW) mode operation InGaAsN/GaAsN double-quantum-well lasers with a laser wavelength of 1.295 µm are demonstrated by metal-organic chemical vapour deposition (MOCVD). With the use of a high-bandgap GaAs0.9P0.1 into the active region before the growth of p-type layers, a room temperature (RT) threshold current of 99 mA and the characteristic temperature (T0) values of 155 K in a temperature range of 25–95 °C and 179 K in a temperature range of 25–85 °C are obtained from a 4 × 1000 µm2 ridge waveguide uncoated laser diode. The T0 value of the conventional structure without the high-bandgap GaAs0.9P0.1 is 118 K in a temperature range of 25–95 °C. High-temperature performance is improved and the results of numerical analysis suggest that it may be attributed to the reduced electronic leakage current.

In this letter, the photovoltaic characteristics of InGaN/AlGaN superlattice solar cells with In=21% and 40% in InGaN multiple quantum wells, and Al=14% and 20% in AlGaN barriers are investigated. The results obtained numerically show that an increment of 18.2% in short-circuit current density is achieved by introducing p-type doped (holenconcentration = 3×1017 cm-3) Al0.2Ga0.8N barriers in In0.4Ga0.6N superlattice solar cell when compared with undoped Al0.14Ga0.86N barriers. This improvement is mainly attributed to the increased electric field in the main absorption region, which is beneficial for the holes in the valence band to move towards the p-region. The capability of carrier transport is efficiently improved, thus increasing the carrier-collection efficiency and the conversion efficiency.

The characteristics of the violet–blue InGaN quantum-well lasers with an emission wavelength of 400–480 nm when the band-offset ratio of the In x Ga 1− x N/In y Ga 1− y N heterojunction is 3/7, which was most frequently used before, and 7/3, which is accepted recently by most researchers, are studied numerically. Specifically, the characteristics of the electronic current overflow, stimulated recombination rate, and laser performance are discussed. Meanwhile, the relationship between the threshold current and the number of quantum wells in the InGaN quantum-well laser is investigated. Simulation results suggest that, when the band-offset ratio is 7/3, the lowest threshold current of the violet–blue InGaN quantum-well lasers is obtained when the number of InGaN well layers is two if the emission wavelength is shorter than 450 nm, and one if the emission wavelength is longer than 450 nm.

For InGaN laser diodes with emission wavelengths longer than 435 nm, the threshold current density usually increases with the number of InGaN well layers. This phenomenon could be attributed to the dissociation of the high indium content InGaN well layer at a high growth temperature of 750 °C due to a high InGaN dissociation pressure. In this article, the laser performance of the blue InGaN laser diode structures have been numerically investigated with a laser technology integrated program simulation program. The simulation results suggest that the inhomogeneous hole distribution in the quantum wells also plays an important role in the laser performance as a function of the number of InGaN well layers. In addition to the inhomogeneous hole distribution in the quantum wells, the phenomenon and resolution of the electronic current overflow problem in the blue InGaN quantum-well lasers are also investigated.

Efficiency enhancement of blue InGaN/GaN light-emitting diodes (LEDs) with shallow first well is investigated. Energy band diagrams, carrier concentrations in quantum wells, spontaneous emission spectra, light-current-voltage performance curves, and internal quantum efficiency are studied. Simulation results show that the LED is markedly improved when regular first well is replaced by shallow first well due to reduced polarization field and thereby enhanced injection efficiency of carriers in quantum wells. The proposed structure is beneficial for the blue InGaN LED even at a high level of current injection.

In this letter, the effect of slightly-doped step-like electron-blocking layer (EBL) in blue InGaN multiple-quantum well light-emitting diodes is numerically investigated. Results from the simulation analyses indicate that under a low p-doping concentration, the structure with step-like EBL has better optical performance than its counterpart with conventional AlGaN EBL because of the appropriately modified energy band diagrams, which are favorable for the injection of holes and confinement of electrons.

The influence of spontaneous and strain-induced piezoelectric polarizations on the photovoltaic characteristics of an Al0.14Ga0.86N/In0.21Ga0.79N superlattice (SL) solar cell is numerically investigated. The simulation results show that, compared with the conventional p-i-n structure, the Al0.14Ga0.86N/In0.21Ga0.79N SL solar cell has better photovoltaic characteristics and is less sensitive to the effect of internal polarization. Therefore, the use of an SL structure in the III-nitride solar cells is advantageous if both the polarization effect and the crystal quality are of major concern.

The advantages of a (0001) face GaN/InGaN p-i-n solar cell with compositional grading configuration between i-InGaN/p-GaN layers are studied numerically. With the use of the grading layer, the conversion efficiency is markedly promoted due to the reduction of potential barrier height for holes and due to the decrease of polarization. Optimized conversion efficiency is obtained when the thickness of the grading layer increases to a critical value. This critical thickness is strongly influenced by the polarization charges and doping concentration of the grading layer. When the density of the polarization charges is high or the doping concentration is low, a thick grading layer is required to achieve high efficiency.

Monolithic stacked InGaN light-emitting diode (LED) connected by a polarization-enhanced GaN/AlN-based tunnel junction is demonstrated experimentally in this study. The typical stacked LEDs exhibit 80% enhancement in output power compared with conventional single LEDs because of the repeated use of electrons and holes for photon generation. The typical operation voltage of stacked LEDs is higher than twice the operation voltage of single LEDs. This high operation voltage can be attributed to the non-optimal tunneling junction in stacked LEDs. In addition to the analyses of experimental results, theoretical analysis of different schemes of tunnel junctions, including diagrams of energy bands, diagrams of electric fields, and current-voltage relation curves, are investigated using numerical simulation. The results shown in this paper demonstrate the feasibility in developing cost-effective and highly efficient tunnel-junction LEDs.

Specific designs on the last barrier of near-ultraviolet InGaN light-emitting diodes are investigated numerically in order to diminish the electron leakage current without sacrificing the injection efficiency of holes. Due to the reduction of electron leakage current, the recombination of electrons and holes in the p-layers is decreased and, thus, more holes can be injected into the active region. The simulation results show that the optical performance and internal quantum efficiency are markedly improved when the last GaN barrier near the p-layers is partially replaced by In0.01Ga0.99N layer and intentionally p-doped.

The influence of piezoelectric polarization on the performance of p-on-n (0001)-face GaN/InGaN p-i-n solar cells is investigated. Simulation results show that the energy band is tilted into the direction detrimental for carrier collection due to the polarization-induced electric field. When the indium composition of InGaN layer increases, this unfavorable effect becomes more serious which, in turn, deteriorates the device performance. This discovery demonstrates that, besides the issue of crystal quality, the problem caused by the polarization effect needs to be overcome for the development of GaN-based solar cells.

A compositional grading layer between the p-In0.5Ga0.5P emitter layer and p-In0.5Al0.5P window layer in the p+-n In0.5Ga0.5P solar cell is investigated numerically. With the insertion of the grading layer, the short-circuit current density and conversion efficiency are improved due to the enhancement of carrier-collection efficiency, which can be ascribed to the reduction of potential barrier height in the valance band and the existence of internal quasi-electric field in the conduction band. An optimized value of conversion efficiency can be obtained by appropriately adjusting the thickness of the grading layer.

High-efficiency top-emitting organic light-emitting diodes (OLEDs) with step-doped emission layers are numerically investigated. The simulation results demonstrate that the OLEDs with step-doped emission layers have better electrical performance and higher emitting intensity than those without step-doped design. Compared to conventional OLED devices, step-doped OLEDs with Fabry-Perot microcavity not only have fixed wavelength but also have narrow emission spectral width. The simulation results suggest that the OLED structure with step-doped design in a Fabry-Perot microcavity has superior performance and hence is beneficial for the display applications.

This letter demonstrates the outcomes of numerical investigation of the InGaN light-emitting diodes with varied barrier thicknesses. Compared with the original structure with equal barrier thickness, the analyses focus on hole injection efficiency, carrier distribution, electron leakage, and radiative recombination. Simulation approach yields to a result that, when varied barrier thicknesses are used, more than one quantum well contributes to radiative recombination at high injection current which leads to the improvement of efficiency droop. Further analysis indicates that the thinner barrier located close to the p-side layers is beneficial for increasing hole injection, which leads to the reduction of electron leakage; moreover, holes can be confined in more quantum wells in such condition as well.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.

The phenomenon of efficiency droop in blue InGaN light-emitting diodes (LEDs) is studied numerically. Simulation results indicate that the severe Auger recombination is one critical mechanism corresponding to the degraded efficiency under high current injection. To solve this issue, LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed. The purpose of the strain-compensation AlGaN barriers is to mitigate the strain accumulation in a multiquantum well (MQW) active region in this thin-barrier structure. With the proposed LED structure, the hole injection and transportation of the MQW active region are largely improved. The carriers can thus distribute/disperse much more uniformly in QWs, and the Auger recombination is suppressed accordingly. The internal quantum efficiency and the efficiency droop are therefore efficiently improved.

In this work, the optical performance of the blue InGaN light-emitting diodes (LEDs) with varied last barrier thickness is investigated. The experimental measurement shows that the optical power of the InGaN LED with thinner last barrier is apparently improved. According to simulation analysis, thinner last barrier is beneficial for increasing the hole injection efficiency and holes can inject into more quantum wells within the active region. With better hole injection efficiency, the leakage electrons from active region to p-side layers are depressed correspondingly. Therefore, the radiative recombination and optical power are enhanced accordingly when the thinner last barrier is utilized.

In this letter, the situation when the barriers are partially p-doped in selected regions is considered in order to avoid the diffusion of Mg into the quantum well during crystal growth. Moreover, to increase the hole injection and improve the carrier distribution across the multiple quantum wells, the three barriers near the p-layers are p-doped with a gradually increased doping concentration in a blue InGaN light-emitting diode. According to the simulation results, when the stepwise p-doping profile is used in the selected barrier regions, the output power and internal quantum efficiency markedly improve due to the increased hole injection efficiency and decreased electron leakage.

The effect of an n-type AlGaN layer on the physical properties of blue InGaN light-emitting diodes (LEDs) is investigated numerically. The p-type AlGaN electron-blocking layer is usually used in blue LEDs to reduce the electron leakage current. However, the p-type AlGaN layer also retards the injection of holes, which leads to the degradation of efficiency at high current. To improve the efficiency droop of blue InGaN LEDs at high current, an n-type AlGaN layer below the active region is proposed to replace the traditional p-type AlGaN layer. The simulation results show that the improvement in efficiency droop is due mainly to the sufficiently reduced electron leakage current and more uniform distribution of holes in the quantum wells.

The peak material gains of 1.3 µm semiconductor material systems are numerically studied with a LASTIP simulation program. Specifically, the optical properties of the AlGaInAs and InGaAsN material systems are investigated. Simulation results suggest that, using a p-type AlInAs electron stopper layer, improved temperature dependence of slope efficiency in the operating temperature range from 25 to 85 °C can be obtained for a ridge-waveguide AlGaInAs/InP laser structure. On the other hand, using a strain-compensated active region consisting of In0.36Ga0.64As0.99N0.01 (6 nm)/GaAs0.99N0.01 (10 nm), a high laser performance and stimulated emission characteristics can be achieved for a ridge-waveguide InGaAsN/GaAs laser structure.

The dc characteristics of an InGaP/InGaAs/GaAs pseudomorphic high-electron-mobility transistor (HEMT) with a p-type δ-doped InGaP barrier are numerically investigated with the ISE-TCAD simulation program. The simulation results indicate that a HEMT with such a structure has a higher gate turn-on voltage, better carrier confinement that results in a lower voltage-dependent transconductance, and a larger breakdown voltage when compared with the typical HEMT. The simulation results also suggest that this structure is beneficial for linear and large-signal application.

In traditional III-nitride solar cells, the polarization-induced charges and potential barrier in the hetero-interfaces are demonstrated to be harmful for carrier collection. To solve these challenges, the elimination or mitigation of the abrupt hetero-interfaces should be efficient. In this study, various kinds of solar cell structures are investigated numerically. The structures under various situations of indium composition and degree of polarization are systematically explored. Specifically, the photovoltaic performance, energy band diagrams, electrostatic fields, and recombination rates are analyzed. Then, according to the simulation results, the appropriate solar cell structure which possesses high conversion efficiency is proposed.