Ya-Hsuan Shih

Improvement in Electron Overflow of Near-Ultraviolet InGaN LEDs by Specific Design on Last Barrier

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.

Numerical Study of the Effects of Hetero-Interfaces, Polarization Charges, and Step-Graded Interlayers on the Photovoltaic Properties of (0001) Face GaN/InGaN p-i-n Solar Cell

The photovoltaic properties of (0001) face GaN/InGaN p-i-n solar cell are studied numerically. The simulation results show that the detrimental effects of hetero-interfaces and polarization charges will seriously degrade the solar cell performance, especially when the indium composition is high. If these effects are not eliminated or diminished, the photovoltaic properties would not be good enough for practical applications even if a high-quality crystal could be obtained. For this purpose, the step-graded interlayers between the GaN-InGaN interfaces are introduced in order to overcome both aforementioned critical effects. The impacts of the thickness and p-type doping concentration of the step-graded interlayers are also investigated in detail.

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.

Numerical Investigation on the Carrier Transport Characteristics of AlGaN Deep-UV Light-Emitting Diodes

Carrier transport characteristics of AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) are theoretically investigated. Simulation results reveal that hole transport/injection may be severely obstructed by the large potential barrier at the p-electron-blocking layer/p-GaN interface. Under this circumstance, the slope efficiency degrades and electron leakage increases accordingly. By inserting the AlGaN interlayers to form band-engineered staircase p-region, both the transport/injection of holes and I-V characteristic are improved. Moreover, the LED characteristics become less sensitive to the polarization field, which is beneficial for obtaining high LED performance with the LED of high crystalline quality.

Simulation and Experimental Study on Barrier Thickness of Superlattice Electron Blocking Layer in Near-Ultraviolet Light-Emitting Diodes

The optical performance and relevant physical properties of near-ultraviolet (NUV) GaN-based light-emitting diodes (LEDs) are investigated. Specifically, the influence of traditional AlGaN bulk electron blocking layer (EBL) and AlGaN/GaN superlattice (SL) EBL with various thicknesses of AlGaN layers on NUV LEDs is explored. It is indicated from the band diagrams, electrostatic field profile, electron reflecting and hole transmitting spectra, and carrier concentrations profile that the use of a thin AlGaN layer of AlGaN/GaN SL EBL is beneficial to the electron confinement and hole injection in the active region, which results in the high internal quantum efficiency and low efficiency droop at high injection current. Moreover, the experimental results show that replacing the traditional AlGaN bulk EBL with the AlGaN/GaN SL EBL can markedly improve the optical performance. When compared with the NUV LED with traditional AlGaN bulk EBL, the output power of the NUV LED with the proposed AlGaN/GaN SL EBL increases from 13.5 to 48.7 mW at 100 mA.

Polarization Effect in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

The polarization effect in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated, which is critical for the development of DUV LEDs, because the basal material, epitaxial structure, and polarization characteristics are very distinct to those of the well-developed (In)GaN-based near-ultraviolet and visible light emitters. In this paper, the influence of the polarization effect in multi-quantum well active region and p-type layers on the characteristics of DUV LEDs with Ga-face or N-face polarization is explored. Simulation results show that the severe band bending of the p-type layers induced by the polarization field markedly affects the optical and electrical performance. A band-engineered DUV LED structure with compositional grading electron-blocking layer and p-interlayer is proposed to enhance the electron confinement and hole injection with the mitigation of polarization effect. The device performance of the proposed LED structure with Ga-face or N-face polarization is comparable with that of the DUV LED without polarization.

Efficient Carrier Confinement in Deep-Ultraviolet Light-Emitting Diodes With Composition-Graded Configuration

Characteristics of deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated. DUV LEDs possess severe electron current leakage due to insufficient carrier confinement of the active region. Simply increasing the Al composition of quantum barriers (QBs) or electron-blocking layer (EBL) to enlarge the relevant potential barrier height would arise unexpected detrimental effects, such as extra polarization effect or more obstruction for hole injection. In this paper, band-engineered composition-graded QBs and EBL are proposed to resolve this issue. Simulation results show that, with appropriate designs, the leakage current of DUV LEDs could be effectively diminished with just slight side effects.

Auger Recombination in Monolithic Dual-Wavelength InGaN Light-Emitting Diodes

Auger recombination of dual-wavelength emission in monolithic InGaN light-emitting diodes (LEDs) is numerically investigated. Simulation results show that effective suppression of Auger recombination plays an important role toward the realization of dual-wavelength emission in InGaN LEDs. With appropriate design, the carriers in active region can be spatially and spectrally balanced and thereby effective dual-wavelength emission can be achieved.

Structural design and optimization of near-ultraviolet light-emitting diodes with wide wells

The characteristics of the near-ultraviolet (NUV) light-emitting diodes (LEDs) with wide (14-nm-thick) and narrow (2-nm-thick) wells under the situations of different numbers of wells and degree of polarization are systematically investigated. The simulation results show that the Auger recombination can be efficiently suppressed with the increase of number of wells in NUV LEDs. For the LEDs with wide wells, the quantum-confined Stark effect and Shockley–Read–Hall recombination play an important role when the number of wells increases, especially when the LED is under low current injection or high degree of polarization. In order to take the advantage of using wide wells, it is proposed that the quaternary Al0.1In0.05Ga0.85N barriers be used in wide-well NUV LEDs along with the use of Al0.3Ga0.7N/Al0.1Ga0.9N superlattice electron-blocking layer to mitigate the polarization effect and electron overflow. With this band-engineering structural design, the optical performance of the wide-well NUV LEDs is much b...