Sheng-Horng Yen

Advantages of blue InGaN multiple-quantum well light-emitting diodes with InGaN barriers

The advantages of blue InGaN light-emitting diodes (LEDs) with InGaN barriers are studied. The L-I curves, carrier concentrations in the quantum wells, energy band diagrams, and internal quantum efficiency are investigated. The simulation results show that the InGaN/InGaN LED has better performance over its conventional InGaN/GaN counterpart due to the enhancement of electron confinement, the reduced polarization effect between the barrier and well, and the lower potential barrier height for the holes to transport in the active region. The simulation results also suggest that the efficiency droop is markedly improved when the traditional GaN barriers are replaced by InGaN barriers.

Effect of P-Type Last Barrier on Efficiency Droop of Blue InGaN Light-Emitting Diodes

P-type doping in the last barrier is proposed to improve the efficiency droop of the blue InGaN light-emitting diodes (LEDs). The light-current curves, energy band diagrams, carrier concentrations, radiative recombination efficiency, and internal quantum efficiency of the blue LEDs under study are investigated. The simulation results show that the efficiency droop is significantly improved when the last undoped GaN barrier in a typical blue LED is replaced by a p-type GaN barrier. The simulation results suggest that the improvement in efficiency droop is mainly due to the decrease of electron current leakage and increase of hole injection efficiency.

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.

Effect of N-Type AlGaN Layer on Carrier Transportation and Efficiency Droop of Blue InGaN Light-Emitting Diodes

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.

Polarization-dependent optical characteristics of violet InGaN laser diodes

The polarization-dependent optical characteristics of violet InGaN laser diodes, such as band diagrams, emission wavelength, and threshold current, under different operation temperatures have been investigated numerically. Specifically, the normal and reversed polarizations are presented when the laser diodes with wurtzite structure are grown along Ga-face and N-face orientations, respectively. The simulation results show that the lowest threshold current is obtained for the double-quantum-well laser diode with normal polarization, while it is obtained for the single-quantum-well laser diode with reversed polarization. The main physical explanation for the phenomenon is due to effectively reduced electron leakage current, increased hole current density, and reduced Shockley–Read–Hall recombination rate within the active region as the idea of reversed polarization is considered.

Impact of active layer design on InGaN radiative recombination coefficient and LED performance

The relative roles of radiative and nonradiative processes and the polarization field on the light emission from blue (∼425 nm) InGaN light emitting diodes (LEDs) have been studied. Single and multiple double heterostructure (DH) designs have been investigated with multiple DH structures showing improved efficiencies. Experimental results supported by numerical simulations of injection dependent electron and hole wavefunction overlap and the corresponding radiative recombination coefficients suggest that increasing the effective active region thickness by employing multiple InGaN DH structures separated by thin and low barriers is promising for LEDs with high efficiency retention at high injection. The use of thin and low barriers is crucial to enhance carrier transport across the active region. Although increasing the single DH thickness from 3 to 6 nm improves the peak external quantum efficiency (EQE) by nearly 3.6 times due to increased density of states and increased emitting volume, the internal qua...

Numerical Study of Blue InGaN Light-Emitting Diodes With Varied Barrier Thicknesses

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.

Enhancement of Light Power for Blue InGaN LEDs by Using Low-Indium-Content InGaN Barriers

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 In_0.02Ga_0.98N and In_0.05Ga_0.95N barrier layers. The numerical results indicate that the output power of LEDs with In_0.02Ga_0.98 N barrier layers is improved gradually above the emission wavelength of 410 nm. However, when the In_0.05Ga_0.95N 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 In_0.02Ga_0.98N barrier layers can lead to higher output power. However, if the emission wavelength is 445 nm, the use of In_0.05Ga_0.95N barrier layers is beneficial for maintaining high output power.

Deep-ultraviolet light-emitting diodes with gradually increased barrier thicknesses from n-layers to p-layers

In this work, the structure with gradually increased barrier thicknesses from the n-layers to p-layers is proposed to replace the traditional structure with equal barrier thickness in deep-ultraviolet AlGaN light-emitting diodes. Simulation approach yields to a result that, when increased barrier thicknesses are used, the distribution of electron and hole carriers inside the active region becomes quite uniform, which leads to efficient recombination of electrons and holes and thereby a significant enhancement in output power.

Investigation of Optical Performance of InGaN MQW LED With Thin Last Barrier

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.