Hanling Long

Anisotropic optical polarization dependence on internal strain in AlGaN epilayer grown on Al x Ga1−x N templates

Anisotropic optical polarization of AlGaN has been one of the major challenges responsible for the poor efficiency of AlGaN-based ultraviolet light emitting diodes (UV LEDs). In this work, we experimentally investigated the effect of internal strain on the optical polarization of AlGaN epilayers which were pseudomorphically grown on Al x Ga1?x N templates with Al composition changing from 0.1 to 0.42. High-resolution x-ray diffraction and reciprocal space mapping were conducted to determine the crystal quality and strain status. Polarization-dependent photoluminescence (PL) measurement was performed to study the degree of polarization (DOP) of light emission from lateral facet of the AlGaN epilayer. The result showed that the DOP increased from???0.69 to???0.24 with the in-plane strain changing from tensile status (1.19%) to compressive status (?0.70%) and it exhibited a strong dependence of the DOP on the strain. These results demonstrated that the compressive in-plane strain could facilitate TE mode emission from AlGaN, which providing a potential way to enhance the surface light emission of AlGaN-based UV LEDs via strain management of the active region.

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.

Quantum confinement dependence of exciton localization in a-plane GaN/AlGaN multiquantum wells investigated by temperature dependent photoluminescence

The exciton localization effect in nonpolar a-plane GaN/AlGaN multiquantum wells (MQWs) structures with different quantum confinements (different well thicknesses or Al molar fractions of barrier) has been investigated by temperature dependent photoluminescence (PL). An “S-shaped” PL peak energy variation is observed in the spectra, indicating the existence of localized states. The exciton localization energy is larger in the MQWs with stronger quantum confinement. A good agreement of the localization energy is obtained by theoretical calculation assuming ± 5% fluctuation of well thickness, which demonstrates that potential minima caused by well thickness fluctuation are the major origin of exciton localization. In addition, the internal quantum efficiency shows more than 3 times enhancement with decreasing the well width from 8.1 to 2.7 nm due to the strong exciton localization which can prevent the carriers from trapping into the nonradiative recombination centers.

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).

Localized surface plasmon enhanced deep UV-emitting of AlGaN based multi-quantum wells by Al nanoparticles on SiO2 dielectric interlayer

In this paper, we report a 2.6-fold deep ultraviolet emission enhancement of integrated photoluminescence (PL) intensity in AlGaN-based multi-quantum wells (MQWs) by introducing the coupling of local surface plasmons from Al nanoparticles (NPs) on a SiO2 dielectric interlayer with excitons and photons in MQWs at room temperature. In comparison to bare AlGaN MQWs, a significant 2.3-fold enhancement of the internal quantum efficiency, from 16% to 37%, as well as a 13% enhancement of photon extraction efficiency have been observed in the MQWs decorated with Al NPs on SiO2 dielectric interlayer. Polarization-dependent PL measurement showed that both the transverse electric and transverse magnetic mode were stronger than the original intensity in bare AlGaN MQWs, indicating a strong LSPs coupling process and vigorous scattering ability of the Al/SiO2 composite structure. These results were confirmed by the activation energy of non-radiative recombination from temperature-dependent PL measurement and the theoretical three dimensional finite difference time domain calculations.

AlN gradient interlayer design for the growth of high-quality AlN epitaxial film on sputtered AlN/sapphire substrate

In this study, we proposed a novel method to grow high-quality AlN films on sputtered AlN/sapphire substrates by designing an AlN gradient interlayer (GIL-AlN). The epitaxial growth kinetics and growth mechanism were studied in detail, which are responsible for the effects of GIL-AlN layers on the morphology evolution, crystal quality and in-plane biaxial stress of AlN films by varying temperature and/or V/III ratio gradient. The results showed that the insertion of a temperature and V/III ratio gradient GIL-AlN promoted the initial coalescence of small sputtered AlN grains, which finally grew into larger and low-density AlN islands in consistent c-axis orientation, leading to less threading dislocations and biaxial tensile stress in the upper HT-AlN films. Finally, we obtained a 1.5 μm high-quality crack-free AlN film with an atomically smooth surface, and the full width at half-maximum values of (0002) and (102) rocking curves were 197 and 435 arcsec, respectively, in an almost stress-free state. This GIL-AlN method provides a possible way to improve the surface cracks and quality of AlN films deposited on sputtered AlN/sapphire substrates, which hold great promise for commercialization in AlN-based devices.

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.