Shuti Li

Advantages of GaN based light-emitting diodes with a p-InGaN hole reservoir layer

A p-type InGaN hole reservoir layer (HRL) was designed and incorporated in GaN based light-emitting diodes (LEDs) to enhance hole injection efficiency and alleviate efficiency droop. The fabricated LEDs with p-type HRL exhibited higher light output power, smaller emission energy shift and broadening as compared to its counterpart. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the alleviated band bending in the last couple of quantum well and electron blocking layer, and thus better hole injection efficiency. Meanwhile, the efficiency droop can be effectively mitigated when the p-InGaN HRL was used.

Enhanced Performance of Blue Light-Emitting Diodes With InGaN/GaN Superlattice as Hole Gathering Layer

An InGaN/GaN superlattice (SL) with Mg-doped barriers was designed and inserted into the InGaN-based blue light-emitting diodes (LEDs) as a hole gathering layer (HGL) to promote hole injection into the active region. The fabricated LEDs with the SL HGL show 36.4% increase in light output power at an injection current of 200 mA. Meanwhile, the efficiency droop is also mitigated effectively, as compared to the traditional LEDs. The improved performance is attributed to increased hole injection efficiency and decreased electron leakage into the p-type region.

Influence of a p-InGaN/GaN short-period superlattice on the performance of GaN-based light-emitting diodes

We have investigated the performance of GaN-based blue light-emitting diodes (LEDs) with a p-InGaN/GaN short-period superlattice (SPS) contact layer, which were grown by metal organic chemical vapor deposition. It was found that dot-like features appeared on the surface when a p-InGaN/GaN SPS structure was grown. The LED operating voltage decreased from 3.52 V to 3.15 V and the electrostatic discharge properties of LEDs were improved by using such a SPS structure. The output powers of LEDs were also increased by adjusting the In mole fraction in the SPS. However, the lifetime of LEDs became shorter when such a SPS structure was used.

Study of InGaN/GaN Light Emitting Diodes With Step-Graded Electron Blocking Layer

Step-graded AlGaN electron blocking layers (EBLs) have been designed to replace the original AlGaN EBL in blue light emitting diodes (LEDs). The step-graded EBL with increasing Al composition along the growth direction (IEBL) leads to inferior optical performance in the fabricated LEDs, whereas the step-graded EBL with decreasing Al composition (DEBL) results in enhanced light output power and reduced efficiency droop, compared with the original EBL. The simulation results indicate enhanced hole injection and electron confinement by DEBL, which accounts for improved performance. On the contrary, insufficient electron blocking and hole injection by IEBL lead to inferior performance.

Ultrathin nickel oxide film as a hole buffer layer to enhance the optoelectronic performance of a polymer light-emitting diode

In order to promote a polymer LED (PLED), we fabricated and introduced an ultrathin nickel oxide (NiO) buffer layer (<10 nm) between the indium tin oxide (ITO) anode and the poly (3, 4-ethylenedioxythiophene) hole injection layer in the PLED. The NiO buffer layer was easily formed on the ITO anode by electron-beam deposition of a nickel (Ni) metal source and an oxygen plasma treatment process. As a result, the PLED device with the NiO buffer layer on its ITO anode had the same turn-on voltage as conventional PLED devices without the NiO buffer layer, and the luminance of the PLED device with the NiO buffer layer was doubled, compared with the conventional PLED devices without the NiO buffer layer. Improvement of the optoelectronic performance of the PLED can be attributed to the increase of the current driven into the diode, resulting from the NiO buffer layer, which can enhance the hole injection and balance the injection of the two types of carriers (holes and electrons). Thus it is an excellent choice to introduce the NiO buffer layer onto the ITO anode of the PLED devices in order to enhance the optoelectronic performance of PLED devices.

Advantages of GaN-Based Light-Emitting Diodes With Polarization-Reduced Chirped Multiquantum Barrier

The advantages of GaN-based light-emitting diodes with polarization-reduced chirped multiquantum barrier (PR-CMQB) as electron blocking layer (EBL) are investigated numerically and experimentally. Both simulation and experiment results indicate that the LED with PR-CMQB possesses higher internal quantum efficiency and light output power as compared with its counterparts with either conventional single AlGaN EBL or CMQB. These improvements are mainly attributed to the suppression of electron leakage and the enhancement of hole injection efficiency. Furthermore, the efficiency droop is markedly reduced when the PR-CMQB is employed.

Effect of Mg Doping in GaN Interlayer on the Performance of Green Light-Emitting Diodes

A light-emitting diode (LED) structure containing a low-temperature (LT) GaN interlayer between active region and AlGaN electron blocking layer is proposed to improve the performance of InGaN-based green LEDs. The experimental and simulated results show that, as the Mg doping depth in the LT-GaN interlayer increases, the hole injection efficiency gets improved and electron current leakage decreases, while defect-related nonradiative recombination increases. With an optimized Mg doping depth in the LT-GaN interlayer, a substantial suppression of efficiency droop can be achieved compared with the conventional LED.

The Advantages of AlGaN-Based Ultraviolet Light-Emitting Diodes With Al Content Graded AlGaN Barriers

A novel five-period AlGaN/AlGaN multiple quantum wells light-emitting diodes (LEDs) structure with Al content graded AlGaN barriers is designed in order to improve the electrical and optical performance of ultraviolet LEDs (UV-LEDs), and the effects are analyzed by using the APSYS simulation programs. The results show that the effective potential height for electrons is increased, and simultaneously the effective potential height for holes is decreased with the increased number of graded AlGaN barriers, which contributes to less electron leakage and better hole injection efficiency. Thus, the internal quantum efficiency and light output power are significantly improved, and the efficiency droop is also mitigated effectively as the number of graded AlGaN barriers increases. However, there is an undesired peak emission in the last barrier when it is replaced by the graded AlGaN barrier.

Develop-Free Solgel Fabrication of Low-Loss Embedded Channel Waveguide Array

We report on the fabrication of a 4-channel embedded waveguide array by a develop-free solgel process, adopted for the first time in waveguide fabrication to improve the sidewall roughness and simplify the fabrication procedures. A kind of novel hybrid HfO2/SiO2 solgel material, which can be used to form a crack-free thick film by a single spin-coating, was prepared to construct waveguide core and cladding, whose refractive index can be accurately set by adjusting ultraviolet (UV) exposure time. The average propagation losses of the fabricated single-mode channel waveguides were measured as low as 0.19 ± 0.02 dB/cm and 0.22 ± 0.03 dB/cm at a wavelength of 1.55 ¿m for TE and TM polarizations, respectively.

Blue InGaN light-emitting diodes with dip-shaped quantum wells

InGaN based light-emitting diodes (LEDs) with dip-shaped quantum wells and conventional rectangular quantum wells are numerically investigated by using the APSYS simulation software. It is found that the structure with dip-shaped quantum wells shows improved light output power, lower current leakage and less efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and the optical characteristics are attributed mainly to the alleviation of the electrostatic field in dip-shaped InGaN/GaN multiple quantum wells (MQWs).