Yen-Kuang Kuo

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.

Enhancement in hole-injection efficiency of blue InGaN light-emitting diodes from reduced polarization by some specific designs for the electron blocking layer

Some specific designs on the electron blocking layer (EBL) of blue InGaN LEDs are investigated numerically in order to improve the hole injection efficiency without losing the blocking capability of electrons. Simulation results show that polarization-induced downward band bending is mitigated in these redesigned EBLs and, hence, the hole injection efficiency increases markedly. The optical performance and efficiency droop are also improved, especially under the situation of high current injection.

Tunable Cr/sup 4+/:YSO Q-switched Cr:LiCAF laser

Tunable passive Q-switching (781 nm to 806 nm at 300 K) of a flash-lamp pumped Cr/sup 3+/:LiCaAlF/sub 6/ (Cr:LiCAF) laser with a Cr/sup 4+/:Y/sub 2/SiO/sub 5/ (Cr/sup 4+/:YSO) broad-band solid-state saturable absorber has been realized. Typical pulse widths of the Q-switched laser output ranged from 40 ns to 80 ns, depending on the lasing wavelength. Spectral narrowing and reduced beam diameter with the use of the saturable absorber were observed. The ground state and the excited state absorption cross sections of the Cr/sup 4+/:YSO absorber were found by bleaching experiments to be (7.0/spl plusmn/1.4)/spl times/10/sup -19/ cm/sup 2/ and (2.3/spl plusmn/0.5)/spl times/10/sup -19/ cm/sup 2/ at 694 nm, respectively. Numerical simulation was utilized to simulate the Cr:LiCAF passive Q-switching with Cr/sup 4+/:YSO solid-state saturable absorber. >

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.

Effects of electronic current overflow and inhomogeneous carrier distribution on InGaN quantum-well laser performance

Laser performance of several InGaN quantum-well (QW) lasers with an emission wavelength of 392-461 nm are numerically studied with a LASTIP simulation program. Specifically, the effects of electronic current overflow and inhomogeneous carrier distribution on the laser performance of InGaN QW lasers operating at different wavelengths are investigated. Simulation results indicate that the use of an AlGaN blocking layer can help reduce the electronic current overflow and, in addition to the dissociation of the InGaN well layer at a high growth temperature during crystal growth, the inhomogeneous carrier distribution in the QWs also plays an important role in the laser performance. From the simulation results, we conclude that the lowest threshold current density is obtained when the number of InGaN well layers is two if the emission wavelength is shorter than 427 nm and one if the emission wavelength is longer than 427 nm, which are in good agreement with the results observed by Nakamura et al. in their experiments.

Advantages of blue InGaN light-emitting diodes with AlGaN barriers

The advantages of blue InGaN light-emitting diodes (LEDs) with AlGaN barriers are studied numerically. The performance curves, energy band diagrams, electrostatic fields, and carrier concentrations are investigated. The simulation results show that the InGaNAlGaN LED has better performance than its conventional InGaNGaN counterpart owing to the increase of hole injection and the enhancement of electron confinement. The simulation results also suggest that the efficiency droop is markedly improved when the traditional GaN barriers are replaced by AlGaN barriers.

Advantages of InGaN light-emitting diodes with GaN-InGaN-GaN barriers

The advantages of InGaN light-emitting diodes with GaN-InGaN-GaN barriers are studied. The energy band diagrams, carrier concentrations in the quantum wells, radiative recombination rate in the active region, light-current performance curves, and internal quantum efficiency are investigated. The simulation results show that the InGaN/GaN-InGaN-GaN light-emitting diode has better performance over its conventional InGaN/GaN and InGaN/InGaN counterparts due to the appropriately modified energy band diagrams which are favorable for the injection of electrons and holes and uniform distribution of these carriers in the quantum wells.

Effects of Built-In Polarization and Carrier Overflow on InGaN Quantum-Well Lasers With Electronic Blocking Layers

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.