Bo-Ting Liou

Improvement in output power of a 460 nm InGaN light-emitting diode using staggered quantum well

Staggered quantum well structures are studied to eliminate the influence of polarization-induced electrostatic field upon the optical performance of blue InGaN light-emitting diodes (LEDs). Blue InGaN LEDs with various staggered quantum wells which vary in their indium compositions and quantum well width are theoretically studied and compared by using the APSYS simulation program. According to the simulation results, the best optical characteristic is obtained when the staggered quantum well is designed as In0.20Ga0.80N (1.4 nm)–In0.26Ga0.74N (1.6 nm) for blue LEDs. Superiority of this novelty design is on the strength of its enhanced overlap of electron and hole wave functions, uniform distribution of holes, and suppressed electron leakage in the LED device.

Composite discrete-ordinate solutions for radiative transfer in a two-layer medium with fresnel interfaces

Abstract The discrete-ordiaate method using composite quadrature is extended to analyze radiative transfer in a two-layer scattering medium with Fresnet interfaces. Since the discrete ordinates used in the present method depend on the critical angles at the interfaces, the discrete ordinates for each of the two layers may be different. Thus a technique adopting the formal solution of the radiation intensity leaving the interface from one layer to determine the intensity on the discrete ordinates of the other is presented. We apply the technique to obtain the hemispherical reflectivity and transmissivity of a two-layer medium. The effects of albedos, refractive indices, and various reflecting models at the lower surface of the medium are investigated.

Numerical investigation on the enhanced carrier collection efficiency of Ga-face GaN/InGaN p-i-n solar cells with polarization compensation interlayers

The impact of the polarization compensation InGaN interlayer between the heterolayers of Ga-face GaN/InGaN p-i-n solar cells is investigated numerically. Because of the enhancement of carrier collection efficiency, the conversion efficiency is improved markedly, which can be ascribed to both the reduction of the polarization-induced electric field in the InGaN absorption layer and the mitigation of potential barriers at heterojunctions. This beneficial effect is more remarkable in situations with higher polarization, such as devices with a lower degree of relaxation or devices with a higher indium composition in the InGaN absorption layer.

Vegard's law deviation in band gaps and bowing parameters of the wurtzite III-nitride ternary alloys

The wurtzite AlxGa1-xN, InxGa1-xN, and AlxIn1-xN alloys are studied by numerical simulation based on first-principles calculations. For AlxGa1-xN the Vegards law deviation parameter is 0.018 ± 0.001 Å for the a lattice constant and -0.036 ± 0.005 Å for the c lattice constant. For InxGa1-xN that is 0.047 ± 0.011 Å for the a lattice constant and -0.117 ± 0.026 Å for the c lattice constant. For AlxIn1-xN that is 0.063 ± 0.014 Å for the a lattice constant and -0.160 ± 0.015 Å for the c lattice constant The results indicate that the band gap bowing parameters obtained with the equilibrium lattice constant and with the lattice constants derived from the Vegards law are 0.341 ± 0.035 eV and 0.351 ± 0.043 eV respectively for AlxGa1-xN, 1.782 ± 0.076 eV and 1.916 ± 0.068 eV respectively for InxGa1-xN, and 3.668 ± 0.147 eV and 3.457 ± 0.152 eV respectively for AlxIn1-xN.

Discrete-ordinate solutions for radiative transfer in a cylindrical enclosure with Fresnel boundaries

Abstract This work considers radiative heat transfer in a 2-D cylindrical scattering medium with Fresnel boundaries. The present analysis divides the radiative intensity into the attenuated incident and in-scattering components, solves the resulting problem of the former analytically, and that of the latter by the discreteordinate method (DOM). In addition, the present analysis rationalizes the distribution of the quadrature points used by the DOM to treat relevantly the strongly angular dependence of radiative intensity around the critical angles. Comparisons of the results obtained by the different methods show that the above techniques can improve the discrete-ordinate solutions. The present results reveal that the gradient of radiative heat flux in a cylindrical medium with Fresnel boundaries may vary abruptly at some locations.

Numerical Investigation of High-Efficiency InGaN-Based Multijunction Solar Cell

A four-junction InGaN-based multijunction solar cell structure is proposed theoretically. The simulation results show that, with the use of appropriately designed compositional grading layers, the performance of InGaN-based multijunction solar cell can be maintained without the cost in performance degradation caused by the polarization-induced electric field and the potential barriers resulting from the heterointerfaces. After the optimization in thicknesses for current matching, a high conversion efficiency of 46.45% can be achieved under 1000-sun AM1.5D illumination, in which the short-circuit current density, open-circuit voltage, and fill factor are 12.2×103 mA/cm2, 4.18 V, and 0.77, respectively. The simulation results suggest that, in addition to the detrimental effects caused by the built-in electric polarization and potential barriers, the issue of crystalline quality is another critical factor influencing the performance of multijunction solar cells.

Numerical study for 1.55-μm AlGaInAs/InP semiconductor lasers

Referred to the laser structure and its experimental results obtained by Selmic et al. and Liu et al., optimized active structure for the 1.55-μm quantum well lasers based on AlGaInAs material system is investigated. A structure with 1.2% compressive-strained wells and a p-type AlInAs electron stopper layer of 20 nm thickness and 5×1023 m-3 doping concentration is suggested. Using this structure the threshold current is reduced to 17.8 mA, and the electron overflow percentage is decreased to 1.74% at 330 K. Furthermore, the characteristic temperatures of threshold currents are enhanced to 55.6 K, 67.0 K, and 43.3 K in operating temperature ranges of 300 K~350 K, 300 K~330 K, and 330 K~350 K, respectively.

Reduced efficiency droop in blue InGaN light-emitting diodes by thin AlGaN barriers

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.

Polarization engineering in III-nitride based ultraviolet light-emitting diodes

In this study, the polarization effect in III-nitride based ultraviolet (UV) light-emitting diodes (LEDs) has been investigated theoretically. Some specific designs in active region are proposed to reduce the polarization effect and, hence, improve the device performance. Simulation results show that by utilizing properly designed quaternary AlInGaN material in active region, the hole injection efficiency can be enhanced due to the reduction of polarization mismatch between hetero-layers. On the other hand, the electron leakage is suppressed owing to that the effective potential height for electrons is increased. Therefore, the performance of UV LEDs is significantly improved by the polarization engineering in active region.

Numerical investigation of blue InGaN light-emitting diodes with staggered quantum wells

Effect of polarization on optical characteristics of blue InGaN LEDs with staggered QW are numerically investigated in this article by using APSYS simulation program. Specifically, band diagram, carrier distribution, and output power have been discussed. According to the simulation results, the structure of staggered QW is proposed to reduce the polarization-related effect; furthermore, the staggered QW structure together with thinner well width is beneficial for improvement of the output power of the blue InGaN SQW LEDs. In this work, the best optical performance is obtained when the quantum-well structure is designed as In0.20Ga0.80N (0.9 nm)-In0.26Ga0.74N (1.1 nm) owing mainly to the enhanced overlap of electron and hole wavefunctions inside the QW.