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1.
Opt Express ; 27(20): A1544-A1553, 2019 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-31684505

RESUMO

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.

2.
Opt Express ; 27(20): A1601-A1614, 2019 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-31684591

RESUMO

Optical polarization characteristics and light extraction behavior of deep-ultraviolet (DUV) light-emitting diode (LED) flip-chip with full-spatial omnidirectional reflector (FSODR) have been investigated. FSODR is fabricated to be simultaneously covered on the whole flip-chip, except the sapphire surface. It is found that the FSODR greatly enhance both transverse-electric (TE) and transverse-magnetic (TM) mode light extraction at every space angle, resulting in total enhancement of 73.1% and 79.8%, respectively. Moreover, the four individual ODR structures separated from FSODR, which are covered on the surface of n-AlGaN, the interface of p-GaN/p-AlGaN, the sidewall of mesa and the sidewall of n-AlGaN/AlN, respectively, show considerably different optical polarization characteristics and extraction behaviors between each other. The achievements of FSODR cannot be obtained by any separated ODR, and all of the individual ODRs can contribute to the FSODR. Especially, the synergy effect of TM extraction behavior obviously exists in FSODR. As a result, the light extraction efficiency (LEE) enhancement of FSODR is approximately 60% at a high current density of 140A/cm2. This study is significant for understanding and modulating the extraction behavior of polarized light to realize high efficiency AlGaN-based DUV LEDs.

3.
Nanoscale Res Lett ; 14(1): 347, 2019 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-31754922

RESUMO

AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) suffer from electron overflow and insufficient hole injection. In this paper, novel DUV LED structures with superlattice electron deceleration layer (SEDL) is proposed to decelerate the electrons injected to the active region and improve radiative recombination. The effects of several chirped SEDLs on the performance of DUV LEDs have been studied experimentally and numerically. The DUV LEDs have been grown by metal-organic chemical vapor deposition (MOCVD) and fabricated into 762 × 762 µm2 chips, exhibiting single peak emission at 275 nm. The external quantum efficiency of 3.43% and operating voltage of 6.4 V are measured at a forward current of 40 mA, indicating that the wall-plug efficiency is 2.41% of the DUV LEDs with ascending Al-content chirped SEDL. The mechanism responsible for this improvement is investigated by theoretical simulations. The lifetime of the DUV LED with ascending Al-content chirped SEDL is measured to be over 10,000 h at L50, due to the carrier injection promotion.

4.
Nanotechnology ; 30(43): 435202, 2019 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-31304918

RESUMO

AlGaN-based deep ultraviolet (DUV) multiple-quantum-wells (MQWs) incorporating strain-modulated nanostructures are proposed, demonstrating enhanced degree of polarization (DOP) and improved light extraction efficiency (LEE). The influence of Al composition and bi-axial strains on the optical behaviors of the DUV-MQWs were carefully examined. Compared with planar DUV-MQWs, strain-modulated nanostructure patterned MQWs show three times higher photoluminescence and increased DOP from -0.43 to -0.16. Moreover, nanostructure patterned DUV-MQWs under compressive strains further illustrate higher DOP and LEE values than those under tensile strains due to more efficient diffraction of the guided modes of the transverse electric (TE) polarized light. Our work demonstrates, for the first time, that a combination of compressive in-plane strain and surface nanostructure show unambiguous advantages in facilitating TE mode emission, thus have great promises in the design and optimization of highly efficient polarized DUV optoelectronic devices.

5.
ACS Appl Mater Interfaces ; 11(21): 19623-19630, 2019 May 29.
Artigo em Inglês | MEDLINE | ID: mdl-31056914

RESUMO

Enhancing the light extraction efficiency is a prevalent but vital challenge for most solid-state lighting technologies, especially for deep ultraviolet light-emitting diodes (DUV-LEDs). In this paper, inspired by the microstructure of the butterfly's eye, we propose and fabricate a flexible fluoropolymer film (FFP film) to tackle this issue for all-mode, full-wavelength light extraction enhancement for most solid-state lighting technologies compatibly. The experimental results demonstrate that compared with one mounted with a smooth FFP film, the light output power of DUV-LED is enhanced up to 26.7% by mounting the FFP film with 325 nm radius nanocones at a driving current of 200 mA. Importantly, thanks to the super-flexible feature of the FFP film, it can both cover the top surface and sidewalls of the DUV-LED chip, leading to the improvement of transverse electric and transverse magnetic mode light extraction by 20.5 and 21.8%, respectively. Finite element analysis (FEA) simulations of the electric field distribution of DUV-LEDs with the FFP film reveal the underlying physics. The present strategy is proposed from the view of the packaging level, which is cost-effective, able to be manufactured at a large scale, and compatible with the solid-state lighting technologies.

6.
Opt Lett ; 44(8): 1944-1947, 2019 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-30985781

RESUMO

We report on the demonstration of a 386 nm light emission and detection dual-functioning device based on nonpolar a-plane n-ZnO/i-ZnO/p-Al0.1Ga0.9N heterojunction under both forward and reverse bias. The electroluminescence intensity under reverse bias is significantly stronger than that under forward bias, facilitated by carrier tunneling when the valence band of p-AlGaN aligns with the conduction band of i-ZnO under reverse bias. Also amid reverse bias, the photodetection was observed and applied in a duplex optical communication device. Optical polarization of the light emission is studied for potential polarization-sensitive device applications. The proposed device provides an important pathway for the multifunctional devices operating in a UV spectrum.

7.
Opt Lett ; 43(11): 2684-2687, 2018 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-29856393

RESUMO

The distinct ultraviolet (UV) light absorption of indium tin oxide (ITO) limits the performance of GaN-based near-UV light-emitting diodes (LEDs). Herein, we report an Al-doped ITO with enhanced UV transmittance and low sheet resistance as the transparent conductive electrode for GaN-based 395 nm flip-chip near-UV LEDs. The thickness dependence of optical and electrical properties of Al-doped ITO films is investigated. The optimal Al-doped ITO film exhibited a transmittance of 93.2% at 395 nm and an average sheet resistance of 30.1 Ω/sq. Meanwhile, at an injection current of 300 mA, the forward voltage decreased from 3.14 to 3.11 V, and the light output power increased by 13% for the 395 nm near-UV flip-chip LEDs with the optimal Al-doped ITO over those with pure ITO. This Letter provides a simple and repeatable approach to further improve the light extraction efficiency of GaN-based near-UV LEDs.

8.
Nanotechnology ; 29(19): 195203, 2018 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-29469057

RESUMO

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.

9.
Opt Express ; 26(2): 680-686, 2018 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-29401950

RESUMO

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).

10.
ACS Appl Mater Interfaces ; 10(9): 8238-8244, 2018 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-29388430

RESUMO

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.

11.
ACS Omega ; 2(8): 5005-5011, 2017 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-31457777

RESUMO

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.

12.
Nanoscale Res Lett ; 11(1): 480, 2016 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-27797089

RESUMO

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.

13.
Nanotechnology ; 26(41): 415601, 2015 Oct 16.
Artigo em Inglês | MEDLINE | ID: mdl-26390848

RESUMO

An effective approach for growing large-scale, uniformly aligned ZnO nanorods arrays is demonstrated. The synthesis uses a GaN quantum dot (QD) template produced by a self-assembled Stranski-Krastanow mode in metal organic chemical vapor deposition, which serves as a nucleation site for ZnO owing to the QD's high surface free energy. The resultant ZnO nanorods with uniform shape and length align vertically on the template, while their density is easily tunable by adjusting the density of GaN QDs, which can be adjusted by simply varying growth interruption. By controlling the density of ZnO nanorod arrays, their optical performance can also be improved. This approach opens the possibility of combining one-dimensional (1D) with 0D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, and interconnects.

14.
Nanoscale Res Lett ; 10(1): 993, 2015 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-26156218

RESUMO

A solvothermal method has been employed to synthesize bismuth sulfide (Bi2S3) with three-dimensional (3D) hierarchical architectures. The influences of different types of surfactants and Cl(-) species on the size and morphology were investigated. A possible formation mechanism was also proposed on the basis of time-dependent experiments. The photoresponse properties show that the conductivity of Bi2S3 micro-flowers is significantly enhanced and the photocurrent is approximately two orders of magnitude larger than the dark current. The response and decay times are estimated to be 142 and 151 ms, respectively. It is expected that hierarchical architectures Bi2S3 may provide a new pathway to develop advanced nanomaterial for high-speed and high-sensitivity photoelectrical switches and photodetecting devices.

15.
Nanoscale Res Lett ; 9(1): 341, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-25136276

RESUMO

With an appropriate high anneal temperature under H2 atmosphere, GaN quantum dots (QDs) have been fabricated via GaN thermal decomposition in metal organic chemical vapor deposition (MOCVD). Based on the characterization of atomic force microscopy (AFM), the obtained GaN QDs show good size distribution and have a low density of 2.4 × 10(8) cm(-2). X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the GaN QDs were formed without Ga droplets by thermal decomposition of GaN.

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