Yi-An Chang

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.

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.

Diode-pumped multi-frequency Q-switched laser with intracavity cascade Raman emission.

A diode-pumped actively Q-switched mixed Nd:Y(0.3)Gd(0.7)VO(4) laser with an intracavity KTP crystal is developed to produce cascade SRS emission up to the fourth order. With an incident pump power of 14 W and a repetition rate of 50 kHz, the average output powers at the first, second, third and fourth Stokes modes are approximately 0.05 W, 0.61 W, 0.25 W, and 0.11 W, respectively. The maximum peak power is greater than 2 kW.

Highly efficient CdS-quantum-dot-sensitized GaAs solar cells

We demonstrate a hybrid design of traditional GaAs-based solar cell combined with colloidal CdS quantum dots. With anti-reflective feature at long wavelength and down-conversion at UV regime, the CdS quantum dot effectively enhance the overall power conversion efficiency by as high as 18.9% compared to traditional GaAs-based device. A more detailed study showed an increase of surface photoconductivity due to UV presence, and the fill factor of the solar cell can be improved accordingly.

Efficiency improvement of single-junction InGaP solar cells fabricated by a novel micro-hole array surface texture process

In this study, single-junction InGaP solar cells fabricated by a novel micro-hole array surface texture process are presented. The characteristics of the single-junction InGaP solar cells with and without the micro-hole array surface texture are studied. An increase of 10.4% in short-circuit current is found when a single-junction InGaP solar cell is fabricated by the micro-hole array surface texture process. The conversion efficiency measured under one-sun air mass 1.5 global illumination at room temperature can also be improved from 13.8% to 15.9% when the size of the micro-holes is 5.3 μm and the period of micro-hole array is designed to 5 μm. (Some figures in this article are in colour only in the electronic version)

Investigation of green InGaN light-emitting diodes with asymmetric AlGaN composition-graded barriers and without an electron blocking layer

In this study, a green InGaN light-emitting diode with asymmetric AlGaN composition-graded barriers and without the use of an AlGaN electron blocking layer is presented to possess markedly enhanced optical and electrical performance. The simulation results show that the output power is increased by 10.0% and 33.2%, which corresponds to an increment of 7% and 29.4% in internal quantum efficiency, at 100 mA when the conventional GaN barriers are replaced by the asymmetric AlGaN composition-graded barriers and the commonly used AlGaN electron blocking layer is removed. The simulation results suggest that the improved device performance is due mainly to the markedly enhanced injection of holes into the active region.

Theoretical and experimental analysis on InAlGaAs/AlGaAs active region of 850-nm vertical-cavity surface-emitting lasers

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.

Single-mode 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP quantum well vertical cavity surface emitting lasers

The 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75/spl deg/C. The VCSELs continuously operate up to 105/spl deg/C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of /spl sim/5.25GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C. We also accumulated life test data up to 1000 h at 70/spl deg/C/10 mA.

Simulation of High-Efficiency GaN/InGaN p-i-n Solar Cell With Suppressed Polarization and Barrier Effects

The photovoltaic characteristics of Ga-face GaN/InGaN p-i-n solar cells are investigated numerically. The severe polarization and barrier effects induced by the GaN/InGaN hetero-interfaces are demonstrated to be detrimental for the carrier collection. The conversion efficiency could be degraded to be out of application when the degree of polarization and/or indium composition are high. To efficiently eliminate both critical issues, the solar cell structure with appropriate band engineering is introduced. In the proposed structure, the photovoltaic characteristics not only show high-grade performance but also become insensitive to the degree of polarization, even in the situation of high indium composition.

Investigation of InGaN green light-emitting diodes with chirped multiple quantum well structures

The effect of using chirped multiple quantum-well (MQW) structures in InGaN green light-emitting diodes (LEDs) is numerically investigated. An active structure, which is with both thick QWs with low indium composition on the p-side and thin QWs with high indium composition next to the n-region, is presented in this study. The thickness and indium composition in each single QW is specifically tuned to emit the same green emission spectrum. Comparing with conventional active structure design of green LEDs, which is using uniform MQWs, the output power is increased by 27% at 20 mA, and by 15% at 100 mA current injections. This improvement is mainly attributed to the enhanced efficiency of carrier injection into QWs and the improved capability of carrier transport.