Panpan 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.

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.

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.

Steplike magnetocapacitance and dielectric relaxation in spin frustrated Ca3Co2O6

The dielectric relaxation and magnetocapacitance effect of one-dimensional spin frustrated compound Ca(3)Co(2)O(6) are investigated. The steplike magnetocapacitance effect is observed and one to one corresponds to the steplike magnetization. We explain this phenomenon from the spin configuration dependent dielectric response. The simulation results using the Monte Carlo method are in good agreement with experimental data at low temperature. The close correspondence between the magnetic and dielectric properties indicates that the coupling is the intrinsic character of Ca(3)Co(2)O. The steplike magnetocapacitance effect may find potential applications in data storage and sensors

Broadband full-color monolithic InGaN light-emitting diodes by self-assembled InGaN quantum dots

We have presented broadband full-color monolithic InGaN light-emitting diodes (LEDs) by self-assembled InGaN quantum dots (QDs) using metal organic chemical vapor deposition (MOCVD). The electroluminescence spectra of the InGaN QDs LEDs are extremely broad span from 410 nm to 720 nm with a line-width of 164 nm, covering entire visible wavelength range. A color temperature of 3370 K and a color rendering index of 69.3 have been achieved. Temperature-dependent photoluminescence measurements reveal a strong carriers localization effect of the InGaN QDs layer by obvious blue-shift of emission peak from 50 K to 300 K. The broadband luminescence spectrum is believed to be attributed to the injected carriers captured by the different localized states of InGaN QDs with various sizes, shapes and indium compositions, leading to a full visible color emission. The successful realization of our broadband InGaN QDs LEDs provide a convenient and practical method for the fabrication of GaN-based monolithic full-color LEDs in wafer scale.

High Quantum Efficiency and Low Droop of 400-nm InGaN Near-Ultraviolet Light-Emitting Diodes Through Suppressed Leakage Current

High quantum efficiency and low efficiency droop of 400-nm InGaN near-ultraviolet (NUV) light-emitting diodes (LEDs) are achieved using Al_0.05Ga_0.95N quantum barriers and 3-nm thin Al_0.3Ga_0.7N insertion layer on last barrier before p-Al_0.15Ga_0.85N electron blocking layer. At 100 A/cm², for the fabricated 0.1-mm²-size LEDs with special designed barriers, the external quantum efficiency increases from 23.8% to 40.7%, and the efficiency droop ratio decreases from 40.5% to as low as 10.3%, as compared with the conventional NUV LEDs. APSYS simulations reveal that a significant suppressed leakage current is the main reason for the performance enhancement.

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.