Zhicong Li

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.

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.

Influence of Si doping on optical characteristics of cubic GaN grown on (001) GaAs substrates

The photoluminescence (PL) properties of Si-doped cubic GaN with different carrier concentrations were investigated at room temperature. The epilayers were grown on GaAs (001) by radio-frequency molecular-beam epitaxy. It was found that when the carrier concentration is increased from 5×1015 to 2×1018 cm−3, the PL peak shifted towards low energy, from 3.246 to 3.227 eV, and the PL linewidth increased from 77.1 to 121 meV. The PL peak shift is explained by the band gap narrowing effect due to the high doping concentration. The PL linewidth includes two parts: one is doping concentration independent, which is caused by the imperfection of samples and phonon scattering; the other is doping concentration dependent. We assign the second part to the broadening by the microscopic fluctuation of the doping concentration. The experimental measurements are in good agreement with the model.

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.

Low threading dislocation density in GaN films grown on patterned sapphire substrates

The growth process of three-dimensional growth mode (3D) switching to two-dimensional growth mode (2D) is investigated when GaN films are grown on cone-shaped patterned sapphire substrates by metal-organic chemical vapor deposition. The growth condition of the 3D-2D growth process is optimized to reduce the threading dislocation density (TDD). It is found that the condition of the 3D layer is critical. The 3D layer keeps growing under the conditions of low V/III ratio, low temperature, and high pressure until its thickness is comparable to the height of the cone-shaped patterns. Then the 3D layer surrounds the cone-shaped patterns and has inclined side facets and a top (0001) plane. In the following 2D-growth process, inclined side facets coalesce quickly and the interaction of TDs with the side facets causes the TDs to bend over. As a result, the TDD of GaN films can decrease to 1 10 8 cm 2 , giving full-width at half maximum values of 211 and 219 arcsec for (002) and (102) omega scans, respectively.

Photoluminescence study of Si doping cubic GaN grown on (001) GaAs substrates by molecular beam epitaxy

The photoluminescence (PL) properties of Si-doped cubic GaN with different carrier concentrations were investigated at room temperature. The epilayers were grown on GaAs (0 0 1) by radio-frequency molecular beam epitaxy. It was found that when the carrier concentration is increased fi om 5 x 10(15) to 2 x 10(18) cm(-3), the PL peak shifted towards low energy, from 3.246 to 3.227 eV, and the PL linewidth increased from 77.1 to 121 meV. The PL peak shift is explained by the bandgap narrowing effect due to the high doping concentration. The PL linewidth includes two parts: one is doping concentration independent, which is caused by the imperfection of samples and phonon scattering; the other is doping concentration dependent. We assign the second part to the broadening by the microscopic fluctuation of the doping concentration. The experimental measurements are in good agreement with the model

Excellent ESD Resistance Property of InGaN LEDs With Enhanced Internal Capacitance

The excellent electrostatic discharge (ESD) resistance of InGaN-light-emitting diodes is achieved by enhancing the internal capacitance. By inserting three pairs of 140-/40-nm u/n-GaN (5 × 1018 cm-3) layers on the low doped n-spacer layer before the active region, the internal capacitance was raised from 50 to 103 pF, while the human body model ESD pass yield at -8000 V was increased from 40% to 98%. A lower energy dispassion on the devices due to the enhanced internal capacitance leads to the excellent ESD resistance.

Improved ESD characteristic of GaN-based blue light-emitting diodes with a low temperature n-type GaN insertion layer

We demonstrate the improvement of the electrostatic discharge (ESD) characteristic of GaN-based blue light-emitting diodes (LEDs) by inserting a low-temperature n-type GaN (LT-nGaN) layer between the n-type GaN layer and InGaN/GaN multiple quantum wells (MQWs). The ESD endurance voltage > 4000 V pass yield is in- creased from 9.9% to 74.7% when the LT-nGaN insertion layer is applied to the GaN/sapphire-based LEDs. The LT-nGaN plays a role of buffer layer for MQWs, which reduces the strain of MQWs and improves the interface quality. Moreover, we also demonstrate that ESD characteristics of the LEDs with LT-nGaN insertion layer growth in N2 are much better than that in H2, which further confirm that the improvement of ESD characteristics is due to the strain relaxation in MQWs. Optoelectrical measurements show that there is no deterioration of the electrical properties of LEDs and the light output power of LEDs at an injection current of 20 mA is improved by 13.9%.

Epitaxial growth and characterization of GaN films on (001) GaAs substrates by radio-frequency molecular beam epitaxy

The intermediate nucleation layer effects on the crystal structure of GaN epitaxial layers grown on GaAs (0 0 1) substrates by solid-sourer molecular beam epitaxy using RF-N-2 plasma as a nitrogen source were investigated. The crystal structure of GaN grown on (0 0 1) GaAs substrates was critically influenced by the nucleation layer, that is, mainly cubic GaN was grown directly on the GaAs substrate with the epitaxial relationship of GaN (0 0 1)//GaAs(0 0 1) and GaN[1 1 0]//GaAs[1 1 0], while hexagonal GaN was frown on a very thin AlAs intermediate layer with the epitaxial relationship of GaN(0 0 0 1)//GaAs(0 0 1) and GaN[1 1 (2) over bar 0]//GaAs[1 1 0]. X-ray diffraction and transmission-electron-microscope are used to analyze the crystal structure of the two kinds of epilayers