Xiaoyan Yi

Efficiency droop in InGaN/GaN multiple-quantum-well blue light-emitting diodes grown on free-standing GaN substrate

In this work, the dislocation-related efficiency droop in InGaN/GaN blue light-emitting diodes (LEDs) was investigated by comparing the external quantum efficiency (EQE) of GaN grown on c-plane sapphire and free-standing GaN substrate over a wide range of operation conditions. The values of A, B, and C coefficients had been iteratively obtained by fitting quantum efficiency in the rate equation model. Analysis revealed that threading dislocation density was strongly related to the decrease in EQE of InGaN LEDs at elevated currents by introducing a number of acceptor-like levels with the energy EA lying within the band gap.

Enhanced performance of GaN based light-emitting diodes with a low temperature p-GaN hole injection layer

A hole injection layer (HIL) is designed in GaN-based light emitting diodes (LEDs) between multiple quantum wells and p-AlGaN electron blocking layer (EBL). Based on numerical simulation by apsys, the band diagram is adjusted by HIL, leading to the improved hole-injection efficiency. The designed HIL is a p-GaN buffer layer grown at low temperature (LT_pGaN) on last quantum barrier before p-AlGaN EBL. The output power of the fabricated GaN-based LED device with LT_pGaN HIL is enhanced by 128% at 100 A/cm2, while the efficiency droop is reduced by 33% compared to the conventional LED.

Two distinct carrier localization in green light-emitting diodes with InGaN/GaN multiple quantum wells

The effect of carrier localization in InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes is investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements. PL results show that two peaks obtained by Gaussian fitting both relate to the emission from localized states. By fitting the TRPL lifetimes at various emission energies, two localization depths corresponding to the In-rich regions and quasi-MQWs regions are obtained. Using a model we proposed, we suggest that compositional fluctuations of In content and variation of well width are responsible for carrier localization in In-rich regions and quasi-MQWs regions, respectively.

Effects of light extraction efficiency to the efficiency droop of InGaN-based light-emitting diodes

Light extraction efficiency (LEE) droop as an important factor contributing to the efficiency droop of InGaN-based light-emitting diodes (LEDs) has been demonstrated and investigated in detail. The LEE droop effect is induced by the spatial dependence of the extraction efficiency of photons inside of the LED devices and the aggravating crowding effect of the injection electrons around n-type electrodes as injection current increases. A current blocking layer is introduced to alleviate the LEE droop effect. And the light output power of the LEDs is also improved by 43% at an injection current of 350 mA.

Annealed InGaN green light-emitting diodes with graphene transparent conductive electrodes

Multi-layer graphene (MLG) films were transferred onto p-GaN layer as transparent conductive electrodes in InGaN green light-emitting diodes (MLG-GLEDs), and their optoelectronic properties were investigated. The interdiffusion between metal atoms from metal pads and Ga atoms from p-GaN had a strong effect on the contact barrier at graphene/p-GaN interface, resulting in substantial changes in transport characteristics of MLG-GLEDs and deterioration of the electrical contact between graphene and p-GaN. A high temperature annealing treatment was employed to improve the light-emitting performance of MLG-GLEDs.

Light extraction efficiency improvement by multiple laser stealth dicing in InGaN-based blue light-emitting diodes.

We report a multiple laser stealth dicing (multi-LSD) method to improve the light extraction efficiency (LEE) of InGaN-based light-emitting diodes (LEDs) using a picosecond (Ps) laser. Compared with conventional LEDs scribed by a nanosecond (Ns) laser and single stealth-diced LEDs, the light output power (LOP) of the LEDs using multi-LSD method can be improved by 26.5% and 11.2%, respectively. The enhanced LOP is due to the increased side emission from the large-area roughened sidewalls of the sapphire substrates fabricated in the multi-LSD process. Numerical simulation results show that the multi-LSD process has little thermal damages to the multiple quantum wells (MQWs) of the LEDs.

Partially sandwiched graphene as transparent conductive layer for InGaN-based vertical light emitting diodes

InGaN-based vertical structure light emitting diodes (VLEDs) with multi-layer graphene transparent electrodes with higher optical output have been fabricated and tested. High temperature annealing introduced inter-diffusion of metal atoms and Ga atoms and generated the partially sandwiched graphene structure, which contributed to the performance improvement of VLEDs.

Phosphor-Free, Color-Tunable Monolithic InGaN Light-Emitting Diodes

We have demonstrated phosphor-free color-tunable monolithic GaN-based light-emitting diodes (LEDs) by inserting an ultrathin 1-nm-thick InGaN shallow quantum well (QW) between deep InGaN QWs and GaN barriers. Without using any phosphors, this monolithic LED chip can be tuned to realize wide-range multicolor emissions from red to yellow under different injection currents. In partical, when the injection current reaches an upper level above 100 mA, the LEDs will achieve white emission with a very high color rending index (CRI) of 85.6. This color-tunable characteristic is attributed to the carrier redistribution in the shallow/deep QWs and the energy band filling effect as well.

Quantum Efficiency Enhancement of 530 nm InGaN Green Light-Emitting Diodes with Shallow Quantum Well

InGaN-based green light-emitting diodes (LEDs) with low-indium-composition shallow quantum well (SQW) inserted before the InGaN emitting layer are investigated theoretically and experimentally. Numerical simulation results show an increase of the overlap of electron–hole wave functions and a reduction of electrostatic field within the active region of the SQW LED, compared to those of the conventional LED. Photoluminescence (PL) measurements exhibit reduced full width at half maximum (FWHM) and increased PL intensity for the SQW LED. A 28.9% enhancement of output power at 150 mA for SQW LED chips of 256×300 µm2 size is achieved.

Enhancing the performance of green GaN-based light-emitting diodes with graded superlattice AlGaN/GaN inserting layer

Green InGaN/GaN multiple quantum wells light-emitting diodes with graded superlattice (GSL) AlGaN/GaN inserting layer are investigated numerically and experimentally. Our simulation results indicate that GSL inserting layer can decrease the effective barrier height of holes by 57 meV, which makes holes more easily inject into the quantum wells. The piezoelectric polarization field near the last barrier is suppressed effectively by introducing of the GSL inserting layer. As a result, the efficiency droop radio is improved from 35.8% to 19.4% at current density of 100 A/cm−2.