graphene-assisted-quasi-van-der-waals-epitaxy-aln-film-on-nano

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers ...

Chinese Academy of Sciences

University of Chinese Academy of Science

We demonstrated for the first time the formation and study of semi-polar AlGaN multiple-quantum-wells (MQWs) grown on highly regular hexagonal AlN nanopyramids. The AlN nanopyramids were obtained by a metal-organic chemical vapor phase deposition regrowth method on a well-ordered AlN nanorod array prepared by a top-down etching process. The growth mechanism of the AlN nanopyramids was ascribed to the slow growth of the (101[combining macron]1) semi-polar plane, which resulted from hydrogen passivation. Beneath the semi-polar facets, air voids were formed. This was attributed to the insufficient delivery of gas reactants to the bottom of the nanorods during the growth process. The polarization effect in semi-polar AlGaN MQWs was numerically calculated. The results showed that the internal electric field (IEF) in the semi-polar MQWs was remarkably reduced by 80% in comparison with c-plane MQWs. Power dependent photoluminescence indicated that the semi-polar AlGaN MQWs had negligible wavelength shifts that resulted from the reduced IEF, which was in accordance with theoretical predictions. In addition, epitaxial strain was greatly relieved in the AlN regrowth layer, which was revealed from the peak shift of the E2(high) phonon using micro-Raman spectroscopy. The advantages of AlGaN-based hexagonal nanopyramid semi-polar three dimensional nanostructures would lead to a large improvement of output power in UV-LEDs. 

Formation and characteristics of AlGaN-based three-dimensional hexagonal nanopyramid semi-polar multiple quantum wells.

We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p-n junction core/shell nitride nanowires. GaN nanowires containing seven radial InGaN/GaN quantum wells were grown by metal-organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.

Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors.

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.

Ultrafast growth of horizontal GaN nanowires by HVPE through flipping the substrate.

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 In Ga 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.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

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.

Fast Growth of Strain-Free AlN on Graphene-Buffered Sapphire.

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.

Horizontal GaN nanowires grown on Si (111) substrate: the effect of catalyst migration and coalescence.

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.

Direct Growth of AlGaN Nanorod LEDs on Graphene-Covered Si.

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.

Direct van der Waals Epitaxy of Crack-Free AlN Thin Film on Epitaxial WS₂.

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.

Improved Epitaxy of AlN Film for Deep-Ultraviolet Light-Emitting Diodes Enabled by Graphene.

β-GaO, 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 β-GaO. 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 β-GaO. In this review, with the perspective from materials to devices, we first describe the basic properties, growth method, as well as doping of β-GaO, then introduce in detail the progress in growth of GaN on (1 0 0) and (-2 0 1) β-GaO, followed by the epitaxy of AlGaN on gallium oxide. Finally, the advances in fabrication and performance of vertical structure LED (VLED) are presented.

Epitaxy of III-Nitrides on β-GaO and Its Vertical Structure LEDs.

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in ultraviolet regions. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of 128 terahertz and high conversion efficiency. This process relies on adiabatic quadratic frequency translation of a near-visible supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where chirp-modulated taper waveguides are patterned to ensure a broad phase matching. The heterodyne characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain high coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.

Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide.

Deep ultraviolet light-emitting diodes (DUV LEDs) (<280 nm) have been important light sources for broad applications in, e.g., sterilization, purification, and high-density storage. However, the lack of excellent transparent electrodes in the DUV region remains a challenging issue. Here, we demonstrate an architectural engineering scheme to flexibly tune the work function of Cu@shell nanowires (NWs) as top transparent electrodes in DUV LEDs. By fast encapsulation of shell metals on Cu NWs and a shift of electron binding energy, the electronic work function could be widely tailored down to 4.37 eV and up to 5.73 eV. It is revealed that the high work function of Cu@Ni and Cu@Pt NWs could overcome the interfacial barrier to p-AlGaN and achieve direct ohmic contact with high transparency (91%) in 200-400 nm. Completely transparent DUV LED chips are fabricated and successfully lighted with sharp top emission (wall-plug efficiency reaches 3%) under a turn-on voltage of 6.4 V. This architectural strategy is of importance in providing highly transparent ohmic electrodes for optoelectronic devices in broad wavelength regions.

Article	Year
Formation and characteristics of AlGaN-based three-dimensional hexagonal nanopyramid semi-polar multiple quantum wells. Nanoscale\| PubMed ID: 27174102	2016
Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors. ACS photonics\| PubMed ID: 27331079	2016
Ultrafast growth of horizontal GaN nanowires by HVPE through flipping the substrate. Nanoscale\| PubMed ID: 29521388	2018
High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 29883036	2018
Fast Growth of Strain-Free AlN on Graphene-Buffered Sapphire. Journal of the American Chemical Society\| PubMed ID: 30175921	2018
Horizontal GaN nanowires grown on Si (111) substrate: the effect of catalyst migration and coalescence. Nanotechnology\| PubMed ID: 30485254	2019
Direct Growth of AlGaN Nanorod LEDs on Graphene-Covered Si. Materials (Basel, Switzerland)\| PubMed ID: 30486245	2018
Direct van der Waals Epitaxy of Crack-Free AlN Thin Film on Epitaxial WS₂. Materials (Basel, Switzerland)\| PubMed ID: 30518146	2018
Improved Epitaxy of AlN Film for Deep-Ultraviolet Light-Emitting Diodes Enabled by Graphene. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 30993771	2019
Epitaxy of III-Nitrides on β-GaO and Its Vertical Structure LEDs. Micromachines\| PubMed ID: 31086010	2019
Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide. Nature communications\| PubMed ID: 31278261	2019
Enhanced Emission of Deep Ultraviolet Light-Emitting Diodes through Using Work Function Tunable Cu Nanowires as the Top Transparent Electrode. The journal of physical chemistry letters\| PubMed ID: 32141757	2020

Jianchang Yan

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