Renli Liang

Enhanced Optical and Thermal Performance of Eutectic Flip-Chip Ultraviolet Light-Emitting Diodes via AlN-Doped-Silicone Encapsulant

This paper investigated the optical and thermal performance of the nitride-based ultraviolet light-emitting diodes fabricated by the eutectic flip-chip method. A new packaging structure was proposed by introducing a thin encapsulation layer doped with 0.4 wt% AlN nanoparticles (NPs) and uniform quartz lens simultaneously. Experimental results showed that the packaging structure proposed in this paper could significantly enhance the light output power, reduce the junction temperature, and increase the emission angle compared with the encapsulation layer consisting silicone only. When the NPs concentration increased from 0.1 to 0.4 wt%, the light output power increased from 7.6% to 17.4% at the forward current of 800 mA. Meanwhile, the junction temperature decreased by 5.7 °C, while the emission angle increased by 11.3°. What is more, it was found that the enhancement of light output power depended on the NPs concentration and showed the maximum at the concentration of 0.4 wt%. The enhanced light output power was attributed to the additional light scattering and the increased average refractive index resulted from the NPs introduced in the proposed package structure.

Ag-Decorated Localized Surface Plasmon-Enhanced Ultraviolet Electroluminescence from ZnO Quantum Dot-Based/GaN Heterojunction Diodes by Optimizing MgO Interlayer Thickness

We demonstrate the fabrication and characterization of localized surface plasmon (LSP)-enhanced n-ZnO quantum dot (QD)/MgO/p-GaN heterojunction light-emitting diodes (LEDs) by embedding Ag nanoparticles (Ag-NPs) into the ZnO/MgO interface. The maximum enhancement ration of the Ag-NP-decorated LEDs in electroluminescence (EL) is 4.3-fold by optimizing MgO electron-blocking layer thickness. The EL origination was investigated qualitatively in terms of photoluminescence (PL) results. Through analysis of the energy band structure of device and carrier transport mechanisms, it suggests that the EL enhancement is attributed to the increased rate of spontaneous emission and improved internal quantum efficiency induced by exciton-LSP coupling.

Improvement of Interface Thermal Resistance for Surface-Mounted Ultraviolet Light-Emitting Diodes Using a Graphene Oxide Silicone Composite

In this study, based on silicone composites with graphene oxide (GO) as a filler, a novel packaging strategy was proposed to reduce the interface thermal resistance of surface-mounted ultraviolet light-emitting diodes (UV-LEDs) and provide a potentially effective way for enhancing the long-term stability of devices. The 4 wt % GO-based composite showed an excellent performance in the thermal conductivity, and the interface thermal resistance was reduced by 34% after embedding the 4 wt % GO-based composite into the air gaps of bonding interfaces in the UV-LEDs, leading to a reduction of junction temperature by 1.2 °C under the working current of 1000 mA. Meanwhile, a decrease of thermal stress in bonding interfaces was obtained based on the finite element analysis. What is more, it was found that the lifetime of UV-LEDs with the proposed structure could be obviously improved. It is believed to provide a simple and effective approach for improving the performance of surface-mounted UV-LEDs.

Experimental Study on the Effects of Eutectic Voids on the Thermal Performance Within Flip-Chip Ultraviolet Light-Emitting Diodes

In this paper, the effects of eutectic voids in bonding area on the thermal resistance and light output power of flip-chip ultraviolet light-emitting diode (UV-LED) devices have been studied by experiments in detail. It has been demonstrated that the eutectic voids effectively influenced the thermal resistance, the junction temperature, and the light output power of the UV-LEDs. The UV-LEDs with lower eutectic voidage exhibited a superior thermal performance based on the thermal resistance analysis. The thermal resistance was greatly reduced by 48% and the junction temperature by 3.5 °C as the driving current was 800 mA when the eutectic voidage decreased from 30% to 3%. As a consequence, a maximal enhancement of 13.3% of light output power was obtained when the LED was driven by a current of 1500 mA. These results confirmed that a better performance of eutectic flip-chip UV-LED can be obtained with lower eutectic voidage for the reason that the thermal resistance and junction temperature can be decreased effectively.

Pure ultraviolet emission from ZnO quantum dots-based/GaN heterojunction diodes by MgO interlayer

We demonstrate the fabrication and characterization of ZnO/GaN-based heterojunction light-emitting diodes (LEDs) by using air-stable and solution-processable ZnO quantum dots (QDs) with a thin MgO interlayer acting as an electron blocking layer (EBL). The ZnO QDs/MgO/p-GaN heterojunction can only display electroluminescence (EL) characteristic in reverse bias regime. Under sufficient reverse bias, a fairly pure ultraviolet EL emission located at 370 nm deriving from near band edge of ZnO with a full width at half maximum (FWHM) of 8.3 nm had been obtained, while the deep-level emission had been almost totally suppressed. The EL origination and corresponding carrier transport mechanisms were investigated qualitatively in terms of photoluminescence (PL) results and energy band diagram.

Internal strain induced significant enhancement of deep ultraviolet light extraction efficiency for AlGaN multiple quantum wells grown by MOCVD

In this work, combined analysis of internal strain effects on optical polarization and internal quantum efficiency (IQE) were conducted for the first time. Deep ultraviolet light extraction efficiency of AlGaN multiple quantum wells (MQWs) have been investigated by means of polarization-dependent photoluminescence (PD-PL) and temperature-dependent photoluminescence (TD-PL). With the increase of compressive internal strain applied to the MQWs by an underlying n-AlGaN layer, the degree of polarization (DOP) of the sample was improved from -0.26 to -0.06 leading to significant enhancement of light extraction efficiency (LEE) as the PL intensity increased by 29.2% even though the internal quantum efficiency declined by 7.7%. The results indicated that proper management of the internal compressive strain in AlGaN MQWs can facilitate the transverse electric (TE) mode and suppress the transverse magnetic (TM) mode which could effectively reduce the total internal reflection (TIR) and absorption. This work threw light upon the promising application of compressively strained MQWs to reduce the wave-guide effect and improve the LEE of deep ultraviolet light emitting diodes (DUV LEDs).

Interface Anchored Effect on Improving Working Stability of Deep Ultraviolet Light-Emitting Diode Using Graphene Oxide-based Fluoropolymer Encapsulant

The graphene oxide (GO)-based fluoropolymer is first proposed as an interface encapsulant to improve the light extraction efficiency and achieve the ultralong working stability of deep ultraviolet light-emitting diodes (DUV-LEDs), benefitting from its superior interface performance based on an anchored effect. For the GO-based fluoropolymer composite, the anchored structure is designed to effectively and tightly rivet the quartz lens on the DUV-LED chip by using the interface reaction between GO embedded in fluoropolymer and 3-aminopropyltriethoxy-silane grafted on the surfaces. Experimental results show that on the basis of the interface anchored effect, the air voids in the interface layer of DUV-LED are reduced by 84%, leading to an improvement of the light output power by 15% and a decrease of the junction temperature by 5%, by virtue of the sealing characteristics of the 0.10 wt % GO-based fluoropolymer. In addition, the steady working time is dramatically improved by 660% and it was attributed to the good interface anchored bonding of the 0.10 wt % GO-based fluoropolymer. This novel graphene oxide-based fluoropolymer is believed to provide a feasible and effective interface encapsulant to improve the performance of DUV-LEDs.

Thermal investigation of high-power UV-LEDs using graphene oxide silicone encapsulant

In this paper, a new packaging structure was proposed to reduce the heat accumulation of high-power UV-LEDs using silicone encapsulant with high thermal conductivity. The simulations for the thermal behavior of the traditional and proposed structures had been performed using finite element analysis (FEA). It was obtained that the improvement of temperature distribution was obviously enhanced as the thermal conductivity of encapsulant increased to 6.0 W/(m·k). Therefore, the traditional structure and proposed structure using silicone encapsulant with 4 wt% graphene oxide (GO) as filler were fabricated and investigated by measurements. And the thermal conductivity of 4 wt% GO-embedded silicone was measured at 6.1 W/(m·K). Accordingly, the interface thermal resistance was reduced by 1.2 K/W, and the surface temperature was reduced by 1.4 °C. This approach is believed to provide a simple and effective strategy for improving the performance of UV-LEDs.

Investigation on thermal characteristics and fabrication of DUV-LEDs using copper filled thermal hole

For deep ultraviolet light emitting diodes (DUV-LEDs) packaging, the choice of substrate directly affected its performance and reliability. In this paper, a structure was proposed to promote thermal management and lifespan of DUV-LEDs by introducing the ceramic substrate with copper filled thermal hole. The AlN ceramic substrates with different number of copper filled thermal holes were fabricated by electroplating process. And modeling and thermal simulation using finite element analysis(FEA) is developed by considering the geometrical model of AlN ceramic substrate with 0, 2×2, 3×3, 4 ×4 thermal holes. Meanwhile, to validate the simulation, the thermal parameters of DUV-LEDs were determined and measured by a thermal transient tester. It was found that thermal resistance and junction temperature decreased with the number of thermal holes increasing. Compared with traditional structure, the thermal resistance of DUV-LED based 4 ×4 thermal holes was reduced by 23.04%. This novel approach is believed to provide a simple and effective strategy for improving the heat dissipation and thermal reliability of DUV-LEDs.

Strain dependent anisotropy in photoluminescence of heteroepitaxial nonpolar a-plane ZnO layers

Nonpolar a-plane ZnO layers with anisotropic in-plane strains were prepared on the three substrates of r-sapphire, a-GaN, and a-Al0.08GaN templates via a pulsed laser deposition system, to investigate the distinguishing anisotropic photoluminescence properties of a-ZnO grown on foreign substrates. The optical anisotropy of nonpolar ZnO grown on GaN and AlGaN templates was investigated via polarization-dependent photoluminescence (PL) measurement and polarization transmission spectra measurement. The 0.3 μm a-ZnO layer grown on the a-GaN template has significant anisotropic optical properties with a degree of polarization (DOP) of the photoluminescence (PL) spectrum of about 0.8907, larger than 0.8786 of ZnO on a-Al0.08GaN or 0.8408 of a-ZnO on r-sapphire, revealing that the a-GaN may be the best candidate for the fabrication of modulators and that the increase of the Al component x of p-AlxGa1-xN will attenuate the anisotropic properties of the heteroepitaxial a-ZnO layer, providing a valuable basis for the choice of appropriate substrate for nonpolar a-plane ZnO based polarized optoelectronic devices. Moreover, the relationship between crystal quality anisotropy and optical anisotropy was proposed.