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Open AccessArticle

Impact of Temperature and Substrate Type on the Optical and Structural Properties of AlN Epilayers: A Cross-Sectional Analysis Using Advanced Characterization Techniques

by Wenwang Wei, Yi Peng, Yuefang Hu, Xiuning Xu and Quanwen Xie

Molecules 2024, 29(22), 5249; https://doi.org/10.3390/molecules29225249 - 6 Nov 2024

Cited by 2 | Viewed by 1950

Abstract

AlN, with its ultra-wide bandgap, is highly attractive for modern applications in deep ultraviolet light-emitting diodes and electronic devices. In this study, the surface and cross-sectional properties of AlN films grown on flat and nano-patterned sapphire substrates are characterized by a variety of [...] Read more.

AlN, with its ultra-wide bandgap, is highly attractive for modern applications in deep ultraviolet light-emitting diodes and electronic devices. In this study, the surface and cross-sectional properties of AlN films grown on flat and nano-patterned sapphire substrates are characterized by a variety of techniques, including photoluminescence spectroscopy, high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Raman spectroscopy. The results indicate that different sapphire substrates have minimal impact on the photoluminescence spectrum of the epitaxial films. As the temperature increased, the radius of curvature of the AlN films increased, while the warpage decreased. The AlN films grown on nano-patterned substrates exhibited superior quality with less surface oxidation. During the growth of AlN thin films on different types of substrates, slight shifts in the energy bands occurred due to differences in the introduction of carbon-related impurities and intrinsic defects. The Raman shift and full width at half maximum (FWHM) of the E₂(low), A₁(TO), E₂(high), E₁(TO), and E₁(LO) phonon modes for the cross-sectional AlN films varied with the depth and temperature. The stress state within the film was precisely determined with specific depths and temperatures. The FWHM of the E₂(high) phonon mode suggests that the films grown on nano-patterned substrates exhibited better crystalline quality. Full article

(This article belongs to the Special Issue Applied Chemistry in Asia)

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11 pages, 4972 KB

Open AccessArticle

Promoting Light Extraction Efficiency of Ultraviolet Light Emitting Diodes by Nanostructure Optimization

by Biaomin Li, Shihong Xia, Wei Guo, Zhenhai Yang, Yuheng Zeng, Zhizhong Yuan and Jichun Ye

Crystals 2022, 12(11), 1601; https://doi.org/10.3390/cryst12111601 - 10 Nov 2022

Cited by 5 | Viewed by 2857

Abstract

Ultraviolet (UV) light-emitting diodes (LEDs), as one of the more promising optoelectronic devices, are intrinsically limited by poor light extraction efficiencies (LEEs). To unlock the full potential of UV-LEDs, we propose a simple and effective strategy to promote the LEEs of UV-LEDs by [...] Read more.

Ultraviolet (UV) light-emitting diodes (LEDs), as one of the more promising optoelectronic devices, are intrinsically limited by poor light extraction efficiencies (LEEs). To unlock the full potential of UV-LEDs, we propose a simple and effective strategy to promote the LEEs of UV-LEDs by screening and tailoring suitable optical structures/designs through rigorous numerical simulations. The photonic crystals (PCs) and/or nano-patterned sapphire substrates (NPSSs) equipped with the nano-pillar, nano-cone, nano-oval, and their derivates, are particularly investigated. The simulated results show that individual PC with an average transmittance of 28% is more efficient than that of individual NPSS (24.8%). By coupling PC and NPSS structures, a higher LEE with an average transmittance approaching 29% is obtained, much higher than that of the flat one (23.6%). The involved mechanisms are clarified and confirm that the promotion of optical performance of the nanostructured devices should be attributed to the widened response angles (from 0 to 60°), rather than the enhanced transmittances in the small angles within 30°. Full article

(This article belongs to the Special Issue Preparation and Characterization of Optoelectronic Functional Films)

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11 pages, 2647 KB

Open AccessArticle

Study of Defects and Nano-patterned Substrate Regulation Mechanism in AlN Epilayers

by Wenwang Wei, Yi Peng, Yanlian Yang, Kai Xiao, Mudassar Maraj, Jia Yang, Yukun Wang and Wenhong Sun

Nanomaterials 2022, 12(22), 3937; https://doi.org/10.3390/nano12223937 - 8 Nov 2022

Cited by 8 | Viewed by 3016

Abstract

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains [...] Read more.

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10⁻³ cm⁻² for flat substrate AlN and 0.11 × 10⁻³ cm⁻² for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 10⁸ cm⁻² for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method. Full article

(This article belongs to the Special Issue Advances in Nanostructured Semiconductors and Heterojunctions)

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15 pages, 5393 KB

Open AccessArticle

Temperature Dependence of Stress and Optical Properties in AlN Films Grown by MOCVD

by Wenwang Wei, Yi Peng, Jiabin Wang, Muhammad Farooq Saleem, Wen Wang, Lei Li, Yukun Wang and Wenhong Sun

Nanomaterials 2021, 11(3), 698; https://doi.org/10.3390/nano11030698 - 10 Mar 2021

Cited by 40 | Viewed by 8107

Abstract

AlN epilayers were grown on a 2-inch [0001] conventional flat sapphire substrate (CSS) and a nano-patterned sapphire substrate (NPSS) by metalorganic chemical vapor deposition. In this work, the effect of the substrate template and temperature on stress and optical properties of AlN films [...] Read more.

AlN epilayers were grown on a 2-inch [0001] conventional flat sapphire substrate (CSS) and a nano-patterned sapphire substrate (NPSS) by metalorganic chemical vapor deposition. In this work, the effect of the substrate template and temperature on stress and optical properties of AlN films has been studied by using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible spectrophotometer and spectroscopic ellipsometry (SE). The AlN on NPSS exhibits lower compressive stress and strain values. The biaxial stress decreases from 1.59 to 0.60 GPa for AlN on CSS and from 0.90 to 0.38 GPa for AlN on NPSS sample in the temperature range 80–300 K, which shows compressive stress. According to the TEM data, the stress varies from tensile on the interface to compressive on the surface. It can be deduced that the nano-holes provide more channels for stress relaxation. Nano-patterning leads to a lower degree of disorder and stress/strain relaxes by the formation of the nano-hole structure between the interface of AlN epilayers and the substrate. The low crystal disorder and defects in the AlN on NPSS is confirmed by the small Urbach energy values. The variation in bandgap (E_g) and optical constants (n, k) with temperature are discussed in detail. Nano-patterning leads to poor light transmission due to light scattering, coupling, and trapping in nano-holes. Full article

(This article belongs to the Special Issue Nanopatterned Functional Materials)

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14 pages, 3053 KB

Open AccessArticle

Enhancing GaN LED Efficiency through Nano-Gratings and Standing Wave Analysis

by Xiaomin Jin, Simeon Trieu, Gregory James Chavoor and Gabriel Michael Halpin

Nanomaterials 2018, 8(12), 1045; https://doi.org/10.3390/nano8121045 - 13 Dec 2018

Cited by 13 | Viewed by 5461

Abstract

Based on our recent work, this paper reviews our theoretical study on gallium nitride (GaN) light-emitting-diode (LED). The focus of the paper is to improve LED light extraction efficiency through various nano-grating designs. The gratings can be designed at different locations, such as [...] Read more.

Based on our recent work, this paper reviews our theoretical study on gallium nitride (GaN) light-emitting-diode (LED). The focus of the paper is to improve LED light extraction efficiency through various nano-grating designs. The gratings can be designed at different locations, such as at the top, the middle, and the bottom, on the LED. They also can be made of different materials. In this study, we first present a GaN LED error-grating simulation model. Second, nano Indium Tin Oxide (ITO) top gratings are studied and compared with conventional LED (CLED) using standing wave analysis. Third, we present results related to a patterned sapphire substrate (PSS), SiO₂ Nanorod array (NR), and Ag bottom reflection layer. Finally, we investigate the nano-top ITO grating performance over different wavelengths to validate our design simulation, which focusing on a single wavelength of 460 nm. Full article

(This article belongs to the Special Issue Optoelectronic Nanodevices)

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11 pages, 3927 KB

Open AccessArticle

Structural and Stress Properties of AlGaN Epilayers Grown on AlN-Nanopatterned Sapphire Templates by Hydride Vapor Phase Epitaxy

by Chi-Tsung Tasi, Wei-Kai Wang, Sin-Liang Ou, Shih-Yung Huang, Ray-Hua Horng and Dong-Sing Wuu

Nanomaterials 2018, 8(9), 704; https://doi.org/10.3390/nano8090704 - 10 Sep 2018

Cited by 9 | Viewed by 4931

Abstract

In this paper, we report the epitaxial growth and material characteristics of AlGaN (Al mole fraction of 10%) on an AlN/nanopatterned sapphire substrate (NPSS) template by hydride vapor phase epitaxy (HVPE). The crystalline quality, surface morphology, microstructure, and stress state of the AlGaN/AlN/NPSS [...] Read more.

In this paper, we report the epitaxial growth and material characteristics of AlGaN (Al mole fraction of 10%) on an AlN/nanopatterned sapphire substrate (NPSS) template by hydride vapor phase epitaxy (HVPE). The crystalline quality, surface morphology, microstructure, and stress state of the AlGaN/AlN/NPSS epilayers were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The results indicate that the crystal quality of the AlGaN film could be improved when grown on the AlN/NPSS template. The screw threading dislocation (TD) density was reduced to 1.4 × 10⁹ cm⁻² for the AlGaN epilayer grown on the AlN/NPSS template, which was lower than that of the sample grown on a flat c-plane sapphire substrate (6.3 × 10⁹ cm⁻²). As examined by XRD measurements, the biaxial tensile stress of the AlGaN film was significantly reduced from 1,187 MPa (on AlN/NPSS) to 38.41 MPa (on flat c-plane sapphire). In particular, an increase of the Al content in the overgrown AlGaN layer was confirmed by the TEM observation. This could be due to the relaxation of the in-plane stress through the AlGaN and AlN/NPSS template interface. Full article

(This article belongs to the Special Issue Synthesis and Modification of Nanostructured Thin Films)

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10 pages, 2051 KB

Open AccessArticle

Performance of InGaN/GaN Light Emitting Diodes with n-GaN Layer Embedded with SiO₂ Nano-Particles

by Hong-Seo Yom, Jin-Kyu Yang, Alexander Y. Polyakov and In-Hwan Lee

Appl. Sci. 2018, 8(9), 1574; https://doi.org/10.3390/app8091574 - 6 Sep 2018

Cited by 3 | Viewed by 6380

Abstract

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO₂ nano-particles (NPs) and subsequent epitaxial [...] Read more.

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO₂ nano-particles (NPs) and subsequent epitaxial overgrowth. The structure exhibits enhanced output power compared to similarly grown reference conventional LED without the air-void layer. This change in growth procedure contributes to the increase of internal quantum efficiency (IQE) and light extraction efficiency (LEE) resulting in a 13.5% increase of light output. LEE is 2 times more affected than IQE in the modified structure. Simulation demonstrates that the main effect causing the LEE changes is due to the emitted light being confined within the upper space above the air-void layer and thus enhancing the light scattering by the SiO₂ NPs and preferential light via front surface. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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12 pages, 3348 KB

Open AccessArticle

A Systematic Study on the Structural and Optical Properties of Vertically Aligned Zinc Oxide Nanorods Grown by High Pressure Assisted Pulsed Laser Deposition Technique

by Priyanka Karnati, Ariful Haque, M. F. N. Taufique and Kartik Ghosh

Nanomaterials 2018, 8(2), 62; https://doi.org/10.3390/nano8020062 - 25 Jan 2018

Cited by 44 | Viewed by 6733

Abstract

In this study, we synthesize high quality vertically aligned ZnO (VAZO) nanorods on silicon, sapphire, and indium tin oxide (ITO) substrates by using pulsed laser deposition (PLD) technique at high growth pressure (0.3 Torr). Systematic changes in structural and optical properties of VAZO [...] Read more.

In this study, we synthesize high quality vertically aligned ZnO (VAZO) nanorods on silicon, sapphire, and indium tin oxide (ITO) substrates by using pulsed laser deposition (PLD) technique at high growth pressure (0.3 Torr). Systematic changes in structural and optical properties of VAZO nanorods are studied by varying the substrate temperature (500–600 °C) and number of pulsed laser shots during the deposition. ZnO nanoparticles deposited at high pressure act as nucleation sites, eliminating requirement of catalyst to fabricate VAZO nanorods. Two sharp ZnO peaks with high intensity correspond to the (0002) and (0004) planes in X-ray diffraction pattern confirm the growth of ZnO nanorods, oriented along the c-axis. Scanning Electron Microscopy (SEM) images indicate a regular arrangement of vertically aligned hexagonal closed pack nano-structures of ZnO. The vertical alignment of ZnO nanorods is also supported by the presence of E₂ (high) and A₁ (LO) modes in Raman spectra. We can tune the diameter of VAZO nanorods by changing growth temperature and annealing environments. Photoluminescence spectroscopy illustrates reduction in defect level peak intensities with increase in diameter of VAZO nanorods. This study signifies that high pressure PLD technique can be used more efficiently for controlled and efficient growth of VAZO nanorods on different substrates. Full article

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12 pages, 7505 KB

Open AccessArticle

Scale-Dependent Light Scattering Analysis of Textured Structures on LED Light Extraction Enhancement Using Hybrid Full-Wave Finite-Difference Time-Domain and Ray-Tracing Methods

by Tsung-Xian Lee and Ching-Chia Chou

Energies 2017, 10(4), 424; https://doi.org/10.3390/en10040424 - 23 Mar 2017

Cited by 13 | Viewed by 7164

Abstract

A multiscale model that enables quantitative understanding and prediction of the size effect on the scattering properties of micro- and nanostructures is crucial for the design of light-emitting diode (LED) surface textures optimized for high light extraction efficiency (LEE). In this paper, a [...] Read more.

A multiscale model that enables quantitative understanding and prediction of the size effect on the scattering properties of micro- and nanostructures is crucial for the design of light-emitting diode (LED) surface textures optimized for high light extraction efficiency (LEE). In this paper, a hybrid process for combining full-wave finite-difference time-domain simulation and a ray-tracing technique based on a bidirectional scattering distribution function model is proposed. We apply this method to study the influence of different pattern sizes of a patterned sapphire substrate on GaN-based LED light extraction from the micro-scale to the nano-scale. The results show that near-wavelength–scale patterns with strong diffraction are not expected to enhance the LEE. By contrast, micro-scale patterns with optical diffusion behavior have the highest LEE at a specific aspect ratio, and subwavelength-scale patterns that have antireflection properties show a marked enhancement of the LEE for a wide range of aspect ratios. Full article

(This article belongs to the Special Issue Solid State Lighting)

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