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1.
Opt Lett ; 44(9): 2197-2200, 2019 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-31042182

RESUMO

We report on the high-performance nanoporous (NP) GaN-based metal-insulator-semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with a thermal oxidized ß-Ga2O3insulating layer. The devices show a high responsivity of 4.5×105 A/W and maximum external quantum efficiency of 1.55×108% at 360 nm under a 10 V applied bias, which are attributed to the trap-assisted tunneling induced internal gain mechanism. Correspondingly, a specific detectivity of 8.27×1015 Jones and excellent optical switching repeatability are also observed in our fabricated PDs. The NP-GaN/ß-Ga2O3 MIS UV PD may act as an excellent candidate for the application in UV photodetection due to the high performance and simple fabrication process.

2.
Micromachines (Basel) ; 10(5)2019 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-31086010

RESUMO

ß-Ga2O3, characterized with high n-type conductivity, little lattice mismatch with III-Nitrides, high transparency (>80%) in blue, and UVA (400-320 nm) as well as UVB (320-280 nm) regions, has great potential as the substrate for vertical structure blue and especially ultra violet LEDs (light emitting diodes). Large efforts have been made to improve the quality of III-Nitrides epilayers on ß-Ga2O3. Furthermore, the fabrication of vertical blue LEDs has been preliminarily realized with the best result that output power reaches to 4.82 W (under a current of 10 A) and internal quantum efficiency (IQE) exceeds 78% by different groups, respectively, while there is nearly no demonstration of UV-LEDs on ß-Ga2O3. In this review, with the perspective from materials to devices, we first describe the basic properties, growth method, as well as doping of ß-Ga2O3, then introduce in detail the progress in growth of GaN on (1 0 0) and (-2 0 1) ß-Ga2O3, followed by the epitaxy of AlGaN on gallium oxide. Finally, the advances in fabrication and performance of vertical structure LED (VLED) are presented.

3.
Adv Mater ; 31(29): e1901624, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31140651

RESUMO

For III-nitride-based devices, such as high-brightness light-emitting diodes (LEDs), the poor heat dissipation of the sapphire substrate is deleterious to the energy efficiency and restricts many of their applications. Herein, the role of vertically oriented graphene (VG) nanowalls as a buffer layer for improving the heat dissipation in AlN films on sapphire substrates is studied. It is found that VG nanowalls can effectively enhance the heat dissipation between an AlN film and a sapphire substrate in the longitudinal direction because of their unique vertical structure and good thermal conductivity. Thus, an LED fabricated on a VG-sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction. Moreover, the introduction of VG nanowalls does not degrade the quality of the AlN film, but instead promotes AlN nucleation and significantly reduces the epilayer strain that is generated during the cooling process. These findings suggest that the VG nanowalls can be a good buffer layer candidate in III-nitride semiconductor devices, especially for improving the heat dissipation in high-brightness LEDs.

4.
Adv Mater ; 31(23): e1807345, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-30993771

RESUMO

The growth of single-crystal III-nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN-derived deep-ultraviolet light-emitting diodes (DUV-LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi-van der Waals epitaxial (QvdWE) growth of high-quality AlN films on graphene/sapphire substrates is reported and their application in high-performance DUV-LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror-smooth single-crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as-fabricated DUV-LED shows a low turn-on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.

5.
Nanotechnology ; 30(4): 045604, 2019 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-30485254

RESUMO

Here, we demonstrate the growth of horizontal GaN nanowires (NWs) on silicon (111) by a surface-directed vapor-liquid-solid growth. The influence of the Au/Ni catalysts migration and coalescence on the growth of the NWs has been systematically studied. 2D root-like branched NWs were gown spontaneously through catalyst migration. Furthermore, a novel phenomenon that a catalyst particle is embedded in a horizontal NW was observed and attributed the destruction of growth steady state due to the catalysts coalescence. The transmission electron microscopy and photoluminescence, cathodoluminescence measurement demonstrated that the horizontal NWs exhibit single crystalline structures and good optical properties. Our work sheds light on the horizontal NWs growth and should facilitate the development of highly integrated III-V nanodevices on silicon.

6.
Materials (Basel) ; 11(12)2018 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-30518146

RESUMO

Van der Waals epitaxy (vdWE) has drawn continuous attention, as it is unlimited by lattice-mismatch between epitaxial layers and substrates. Previous reports on the vdWE of III-nitride thin film were mainly based on two-dimensional (2D) materials by plasma pretreatment or pre-doping of other hexagonal materials. However, it is still a huge challenge for single-crystalline thin film on 2D materials without any other extra treatment or interlayer. Here, we grew high-quality single-crystalline AlN thin film on sapphire substrate with an intrinsic WS2 overlayer (WS2/sapphire) by metal-organic chemical vapor deposition, which had surface roughness and defect density similar to that grown on conventional sapphire substrates. Moreover, an AlGaN-based deep ultraviolet light emitting diode structure on WS2/sapphire was demonstrated. The electroluminescence (EL) performance exhibited strong emissions with a single peak at 283 nm. The wavelength of the single peak only showed a faint peak-position shift with increasing current to 80 mA, which further indicated the high quality and low stress of the AlN thin film. This work provides a promising solution for further deep-ultraviolet (DUV) light emitting electrodes (LEDs) development on 2D materials, as well as other unconventional substrates.

7.
Materials (Basel) ; 11(12)2018 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-30486245

RESUMO

High density of defects and stress owing to the lattice and thermal mismatch between nitride materials and heterogeneous substrates have always been important problems and limit the development of nitride materials. In this paper, AlGaN light-emitting diodes (LEDs) were grown directly on a single-layer graphene-covered Si (111) substrate by metal organic chemical vapor deposition (MOCVD) without a metal catalyst. The nanorods was nucleated by AlGaN nucleation islands with a 35% Al composition, and included n-AlGaN, 6 period of AlGaN multiple quantum wells (MQWs), and p-AlGaN. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) showed that the nanorods were vertically aligned and had an accordant orientation along the [0001] direction. The structure of AlGaN nanorod LEDs was investigated by scanning transmission electron microscopy (STEM). Raman measurements of graphene before and after MOCVD growth revealed the graphene could withstand the high temperature and ammonia atmosphere in MOCVD. Photoluminescence (PL) and cathodoluminescence (CL) characterized an emission at ~325 nm and demonstrated the low defects density in AlGaN nanorod LEDs.

8.
J Am Chem Soc ; 140(38): 11935-11941, 2018 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-30175921

RESUMO

We study the roles of graphene acting as a buffer layer for growth of an AlN film on a sapphire substrate. Graphene can reduce the density of AlN nuclei but increase the growth rate for an individual nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AlN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in AlN and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AlN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.

9.
Adv Mater ; 30(30): e1801608, 2018 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-29883036

RESUMO

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

10.
Nanoscale ; 10(13): 5888-5896, 2018 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-29521388

RESUMO

Recently, horizontal nanowires (NWs) have attracted much attention due to their increased compatibility with NW-based integrated nanoelectronic and nanophotonic systems. However, it still remains challenging to synthesize horizontal NWs efficiently. Here we introduce a novel method towards controllable growth of horizontal GaN NWs using HVPE with an Au/Ni thin film as the catalyst. By simply flipping the substrate, horizontal GaN NWs with various growth directions and cross sections have been obtained on a sapphire substrate with various facet orientations. Benefiting from the high decomposition frequency of GaCl precursors, the growth rate for the horizontal NWs is as fast as 400 µ h-1. Our results show that the facing orientation of the loaded substrate affects the flow of the local precursor, which determines the growth mode of the GaN NWs, i.e., no matter whether the substrate is facing downward or upwards. Photoluminescence measurements of the horizontal NWs show a finite blue shift of the band edge-related emission. It indicates the presence of compressed stress and is confirmed by the geometrical phase analysis (GPA) further. Our work opens up a new route and sheds light on the horizontal GaN NWs and will advance the development of horizontal NW-based nanoelectronic and nanophotonic devices and systems.

11.
Opt Express ; 25(2): 587-594, 2017 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-28157948

RESUMO

We demonstrate aluminum nitride (AlN) on sapphire as a novel platform for integrated optics. High-confinement AlN microring resonators are realized by adopting a partially etched (pedestal) waveguide to relax the required etching selectivity for exact pattern transfer. A wide taper is employed at the chip end facets to ensure a low fiber-to-chip coupling loss of ~2.8 dB/facet for both transverse-electric (TE) and transverse-magnetic (TM) modes. Furthermore, the intrinsic quality factors (Qint) recorded with a high-resolution linewidth measurement are up to ~2.5 and 1.9 million at telecom band for fundamental TE00 and TM00 modes, corresponding to a low intracavity propagation loss of ~0.14 and 0.2 dB/cm as well as high resonant buildup of 473 and 327, respectively. Such high-Q AlN-on-sapphire microresonators are believed to be very promising for on-chip nonlinear optics.

12.
Nanotechnology ; 28(11): 114003, 2017 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-28103586

RESUMO

Gallium nitride-based nanopyramid light-emitting diodes are a promising technology to achieve highly efficient solid-state lighting and beyond. Here, periodic nanopyramid light-emitting diode arrays on gallium nitride/sapphire templates were fabricated by selective-area metalorganic chemical vapor deposition and multiple-exposure colloidal lithography. The electric field intensity distribution of incident light going through polystyrene microspheres and photoresist are simulated using finite-different time-domain method. Nitrogen as the carrier gas and a low V/III ratio (ratio of molar flow rate of group-V to group-III sources) are found to be important in order to form gallium nitride nanopyramid. In addition, a broad yellow emission in photoluminescence and cathodoluminescence spectra were observed. This phenomena showed the potential of nanopyramid light-emitting diodes to realize long wavelength visible emissions.

13.
Sci Rep ; 6: 28620, 2016 06 24.
Artigo em Inglês | MEDLINE | ID: mdl-27340030

RESUMO

We report a novel method to fabricate high quality 2-inch freestanding GaN substrate grown on cross-stacked carbon nanotubes (CSCNTs) coated sapphire by hydride vapor phase epitaxy (HVPE). As nanoscale masks, these CSCNTs can help weaken the interface connection and release the compressive stress by forming voids during fast coalescence and also block the propagation of threading dislocations (TDs). During the cool-down process, thermal stress-induced cracks are initiated at the CSCNTs interface with the help of air voids and propagated all over the films which leads to full self-separation of FS-GaN substrate. Raman and photoluminescence spectra further reveal the stress relief and crystalline improvement of GaN with CSCNTs. It is expected that the efficient, low cost and mass-producible technique may enable new applications for CNTs in nitride optoelectronic fields.

14.
Opt Express ; 24(2): A44-51, 2016 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-26832596

RESUMO

Ni/Au electrodes with single, twined and triplet hole array patterns light-emitting diodes have been fabricated by multiple-exposure colloidal lithography. It is found that 45.6%, 83.6% and 15.5% improvement in light output at 350 mA has been achieved by patterning Ni/Au electrodes with single, twined, triplet hole arrays. In addition, patterned Ni/Au LEDs possess much larger view angles than non-patterned ones due to scattering effects of light around the holes, especially for triplet hole array patterned Ni/Au LEDs. Our proposed method for fabricating multiple holes structure would be very promising to improve light output power of LEDs when using advanced electrodes.

15.
Adv Colloid Interface Sci ; 228: 105-22, 2016 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-26732300

RESUMO

Highly ordered nanostructures have gained substantial interest in the research community due to their fascinating properties and wide applications.Micro-/nano-spherical lens photolithography (SLPL) has been recognized as an inexpensive, inherently parallel, and high-throughput approach to the creation of highly ordered nanostructures. SLPL based on monolayer colloidal crystals (MCCs) of self-assembled colloidal micro-/nano-spheres have recently made remarkable progress in overcoming the constraints of conventional photolithography in terms of cost, feature size, tunability, and pattern complexity. In this review, we highlight the current state-of-the-art in this field with an emphasis on the fabrication of a variety of highly ordered nanostructures based on this technique and their demonstrated applications in light emitting diodes, nano-patterning semiconductors, and localized surface plasmon resonance devices. Finally, we present a perspective on the future development of MCC-based SLPL technique, including a discussion on the improvement of the quality of MCCs and the compatibility of this technique with other semiconductor micromachining process for nanofabrication.

16.
Opt Express ; 23(15): A957-65, 2015 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-26367696

RESUMO

In this paper, the high performance GaN-based light-emitting diodes (LEDs) on carbon-nanotube-patterned sapphire substrate (CNPSS) by metal-organic chemical vapor deposition (MOCVD) are demonstrated. By studying the mechanism of nucleation, we analyze the reasons of the crystal quality improvement induced by carbon nanotubes (CNTs) in different growth process. Combining with low temperatures photoluminescence (PL) measurements and two-dimensional (2D) finite difference time-domain (FDTD) simulation results, we conclude that the improvement of optical properties and electrical properties of CNPSS mainly originates from the improvement of the internal quantum efficiency (IQE) due to decreased dislocation density during nano-epitaxial growth on CNPSS. Additionally, in order to reduce the light absorption characteristics of CNTs, different time annealing under the oxygen environment is carried out to remove part of CNTs. Under 350 mA current injections, the light output power (LOP) of CNPSS-LED annealed 2 h and 10 h exhibit 11% and 6% enhancement, respectively, compared to that of the CNPSS-LED without annealing. Therefore, high temperature annealing can effectively remove parts of CNTs and further increase the LOP, while overlong annealing time has caused degradation of the quantum well resulting in the attenuation of optical power.

17.
Small ; 11(37): 4910-21, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26179658

RESUMO

Large-area polystyrene (PS) colloidal monolayers with high mechanical strength are created by a combination of the air/water interface self-assembly and the solvent vapor annealing technique. Layer-by-layer (LBL) stacking of these colloidal monolayers leads to the formation of (2+1)D photonic crystal superlattice with enhanced crystalline integrity. By manipulating the diameter of PS spheres and the repetition period of the colloidal monolayers, flexible control in structure and stop band position of the (2+1)D photonic crystal superlattice has been realized, which may afford new opportunities for engineering photonic bandgap materials. Furthermore, an enhancement of 97.3% on light output power of a GaN-based light emitting diode is demonstrated when such a (2+1)D photonic crystal superlattice employed as a back reflector. The performance enhancement is attributed to the photonic bandgap enhancement and good angle-independence of the (2+1)D photonic crystal superlattice.

18.
Opt Express ; 22 Suppl 5: A1284-91, 2014 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-25322183

RESUMO

We reported that the peak efficiency together with the efficiency droop in InGaN-based light emitting diodes could be effectively modified through a simple and low-cost etch-regrown process in n-GaN layer. The etched n-GaN template contained pyramid arrays with inclined side planes. The following lateral overgrowth process from the etched n-GaN template substantially reduced the edge dislocation density and residential compressive strain in epilayers. The efficiency droop of LED samples thus could be modified due to the reduced polarization field, resulting from the strain relaxation in epilayers. What is more, the peak efficiency and reverse current leakage were also modified due to the reduction of dislocations.

19.
Opt Express ; 22 Suppl 4: A1093-100, 2014 Jun 30.
Artigo em Inglês | MEDLINE | ID: mdl-24978072

RESUMO

Homoepitaxially grown InGaN/GaN light emitting diodes (LEDs) with SiO2 nanodisks embedded in n-GaN and p-GaN as photonic crystal (PhC) structures by nanospherical-lens photolithography are presented and investigated. The introduction of SiO2 nanodisks doesn't produce the new dislocations and doesn't also result in the electrical deterioration of PhC LEDs. The light output power of homoepitaxial LEDs with embedded PhC and double PhC at 350 mA current is increased by 29.9% and 47.2%, respectively, compared to that without PhC. The corresponding light radiation patterns in PhC LEDs on GaN substrate show a narrow beam shape due to strong guided light extraction, with a view angle reduction of about 30°. The PhC LEDs are also analyzed in detail by finite-difference time-domain simulation (FDTD) to further reveal the emission characteristics.

20.
Opt Express ; 22 Suppl 3: A1001-8, 2014 May 05.
Artigo em Inglês | MEDLINE | ID: mdl-24922364

RESUMO

Polarization-reversed electron-blocking structure, which had negative polarization charges localized at the interface between the last quantum barrier (LQB) and electron-blocking layer (EBL), was demonstrated to remarkably improve the light-emitting efficiency of GaN-based blue light-emitting diodes (LEDs) numerically and experimentally. The improvement was attributed to the enhanced electron-blocking effectiveness by the elevated conduction band nearby the LQB/EBL interface. Nevertheless, the efficiency droop was not mitigated because the decrease of electron-leakage was accompanied by the increase of Auger recombination.

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