Junjie Kang

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.

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.

Enhanced performance of GaN-based light-emitting diodes with graphene-Ag nanowires hybrid films

Incorporating Ag nanowires with graphene resulted in improved electrical conductivity and enhanced contact properties between graphene and p-GaN. The graphene/AgNWs hybrid films exhibited high transmittance and lower sheet resistance compared to bare graphene. The specific contact resistance between graphene and p-GaN reduced nearly an order of magnitude with the introduction of AgNWs. As a result, light emitting diodes based on the hybrid films showed 44% lower forward voltage and 2-fold higher light output power. The enhanced performance was attributed to the bridging by AgNWs of cracks, grain boundaries in graphene and the reduction of Schottky barrier height at graphene/ p-GaN interface.

The fabrication of GaN-based nanorod light-emitting diodes with multilayer graphene transparent electrodes

GaN-based nanorod light-emitting diodes (LEDs) with multilayer graphene (MLG) transparent electrodes have been fabricated. Two types of nano-LEDs with graphene on and under the metal pads are fabricated and their performances are investigated. And LEDs with graphene on the metal-pads exhibiting lower forward voltage and higher electroluminescence intensity are obtained. Using scanning electron microscope and Raman spectroscopy, we have demonstrated that graphene transferred after the metal deposition remains intact and has much less damages than graphene under the metal during the fabrication of LEDs with nanorods.

Pyramid Array InGaN/GaN Core--Shell Light Emitting Diodes with Homogeneous Multilayer Graphene Electrodes

Pyramid array InGaN/GaN core–shell light-emitting diodes (LEDs) were fabricated by using a highly homogeneous multilayer graphene transparent conducting electrode. This novel electrode exhibited excellent optical, structural, and electrical properties. In this design, graphene connected each pyramid array as a top window electrode. The current-driven pyramid array InGaN/GaN core–shell LED was operated at a low current injection and exhibited bright electroluminescence. Our results suggest that graphene offers excellent current spreading and transparent conductive properties for nano- or microscale devices.

Analysis Model for Efficiency Droop of InGaN Light-Emitting Diodes Based on Reduced Effective Volume of Active Region by Carrier Localization

Carrier localization can be modeled as a parameter of reduced effective volumes of the active region within the efficiency equation to describe efficiency droop of InGaN light-emitting diodes (LEDs). Reduced effective volume due to carrier localized in the potential minima of In-rich areas results in an increase of carrier density, which accelerates the saturation of radiative recombination as well as the loss of Auger recombination and carrier overflow. Wavelength-dependent droop can be well modeled with different reduced effective volumes of the active region.

Strong carrier localization effect in carrier dynamics of 585 nm InGaN amber light-emitting diodes

Temperature dependence and time-resolved photoluminescence (TRPL) have been carried out to study carrier dynamics for 585 nm InGaN amber light-emitting diodes (LEDs). It is found that in InGaN amber LEDs, peak emission energy only shows a slight blueshift from 588 to 575 nm, as temperature increased from 10 K to 300 K. Moreover, radiative recombination lifetime has demonstrated independent of temperature based TRPL results. These two features indicate that a strong carrier localization effect plays a dominant role in carrier dynamics for InGaN amber LEDs. Also, activation energy of 40.3 meV is obtained through Arrhenius plot of PL intensity versus temperature.