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

2D Embedded Ultrawide Bandgap Devices for Extreme Environment Applications

Ultrawide bandgap semiconductors such as AlGaN, AlN, diamond, and β-Ga2O3 have significantly enhanced the functionality of electronic and optoelectronic devices, particularly in harsh environment conditions. However, some of these materials face challenges such as low thermal conductivity, limited P-type conductivity, and scalability issues, which can hinder device performance under extreme conditions like high temperature and irradiation. In this review paper, we explore the integration of various two-dimensional materials (2DMs) to address these challenges. These materials offer excellent properties such as high thermal conductivity, mechanical strength, and electrical properties. Notably, graphene, hexagonal boron nitride, transition metal dichalcogenides, 2D and quasi-2D Ga2O3, TeO2, and others are investigated for their potential in improving ultrawide bandgap semiconductor-based devices. We highlight the significant improvement observed in the device performance after the incorporation of 2D materials. By leveraging the properties of these materials, ultrawide bandgap semiconductor devices demonstrate enhanced functionality and resilience in harsh environmental conditions. This review provides valuable insights into the role of 2D materials in advancing the field of ultrawide bandgap semiconductors and highlights opportunities for further research and development in this area.

Atomic Evolution Mechanism and Suppression of Edge Threading Dislocations in Nitride Remote Heteroepitaxy

The majority of dislocations in nitride epilayers are edge threading dislocations (TDs), which diminish the performance of nitride devices. However, it is extremely difficult to reduce the edge TDs due to the lack of available slip systems. Here, we systematically investigate the formation mechanism of edge TDs and find that besides originating at the coalescence boundaries, these dislocations are also closely related to geometrical misfit dislocations at the interface. Based on this understanding, we propose a novel strategy to reduce the edge TD density of the GaN epilayer by nearly 1 order of magnitude via graphene-assisted remote heteroepitaxy. The first-principles calculations confirm that the insertion of graphene dramatically reduces the energy barrier required for interfacial sliding, which promotes a new strain release channel. This work provides a unique approach to directly suppress the formation of edge TDs at the source, thereby facilitating the enhanced performance of photoelectronic and electronic devices.

Recent advances in batch production of transfer-free graphene

Large-area transfer-free graphene films prepared via chemical vapor deposition have proved appealing for various applications, with exciting examples in electronics, photonics, and optoelectronics. To achieve their commercialisation, batch production is a prerequisite. Nevertheless, the prevailing scalable synthesis strategies that have been reported are still obstructed by production inefficiencies and non-uniformity. There has also been a lack of reviews in this realm. We present herein a comprehensive and timely summary of recent advances in the batch production of transfer-free graphene. Primary issues and promising approaches for improving the graphene growth rate are first addressed, followed by a discussion of the strategies to guarantee in-plane and batch uniformity for graphene grown on planar plates and wafer-scale substrates, with the design of the target equipment to meet productivity requirements. Finally, potential research directions are outlined, aiming to offer insights into guiding the scalable production of transfer-free graphene.

Dimension-Confined Growth of a Crack-Free PbS Microplate Array for Infrared Image Sensing

Epitaxy of semiconductors is a necessary step toward the development of electronic devices such as lasers, detectors, transistors, and solar cells. However, the lattice ordering of semiconductor functional films is inevitably disrupted by excessive concentrated stress due to the mismatch of the thermal expansion coefficient. Herein, combined with the first-principles calculation, we find that a rigid film/substrate bilayer heterostructure with a large thermal expansion mismatch upon cooling to room temperature from growth is free of surface cracks when the rigid film exhibits a dimension smaller than the critical condition for the breaking energy. The principle has been verified in a PbS/SrTiO3 bilayer system that is crack free on PbS single-crystalline microplate arrays through the designing of a dimension-confined growth (DCG) method. Interestingly, this crack-free, large-scale PbS microplate array exhibits exceptional uniformity in morphology, dimensions, thickness, and photodetection properties, enabling a broad-band infrared image sensing. This work provides a new perspective to design materials and arrays that demand smooth and continuous surfaces, which are not limited only to semiconductor electronics but also include mechanical structures, optical materials, biomedical materials, and others.

Pulsed-Mode Metalorganic Vapor-Phase Epitaxy of GaN on Graphene-Coated c-Sapphire for Freestanding GaN Thin Films

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off by using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices.

Lattice modulation strategies for 2D material assisted epitaxial growth

As an emerging single crystals growth technique, the 2D-material-assisted epitaxy shows excellent advantages in flexible and transferable structure fabrication, dissimilar materials integration, and matter assembly, which offers opportunities for novel optoelectronics and electronics development and opens a pathway for the next-generation integrated system fabrication. Studying and understanding the lattice modulation mechanism in 2D-material-assisted epitaxy could greatly benefit its practical application and further development. In this review, we overview the tremendous experimental and theoretical findings in varied 2D-material-assisted epitaxy. The lattice guidance mechanism and corresponding epitaxial relationship construction strategy in remote epitaxy, van der Waals epitaxy, and quasi van der Waals epitaxy are discussed, respectively. Besides, the possible application scenarios and future development directions of 2D-material-assisted epitaxy are also given. We believe the discussions and perspectives exhibited here could help to provide insight into the essence of the 2D-material-assisted epitaxy and motivate novel structure design and offer solutions to heterogeneous integration via the 2D-material-assisted epitaxy method.

Understanding the 2D-material and substrate interaction during epitaxial growth towards successful remote epitaxy: a review

Remote epitaxy, which was discovered and reported in 2017, has seen a surge of interest in recent years. Although the technology seemed to be difficult to reproduce by other labs at first, remote epitaxy has come a long way and many groups are able to consistently reproduce the results with a wide range of material systems including III-V, III-N, wide band-gap semiconductors, complex-oxides, and even elementary semiconductors such as Ge. As with any nascent technology, there are critical parameters which must be carefully studied and understood to allow wide-spread adoption of the new technology. For remote epitaxy, the critical parameters are the (1) quality of two-dimensional (2D) materials, (2) transfer or growth of 2D materials on the substrate, (3) epitaxial growth method and condition. In this review, we will give an in-depth overview of the different types of 2D materials used for remote epitaxy reported thus far, and the importance of the growth and transfer method used for the 2D materials. Then, we will introduce the various growth methods for remote epitaxy and highlight the important points in growth condition for each growth method that enables successful epitaxial growth on 2D-coated single-crystalline substrates. We hope this review will give a focused overview of the 2D-material and substrate interaction at the sample preparation stage for remote epitaxy and during growth, which have not been covered in any other review to date.

Effect of illuminance and colour temperature of LED lighting on asthenopia during reading

PURPOSE

A self-controlled study to determine the influence of illuminance and correlated colour temperature (CCT) of light-emitting diode (LED) lighting on asthenopia.

METHODS

Twenty-two healthy postgraduates (nine women) were recruited to read under eight LED lighting conditions with four illuminances (300 lx, 500 lx, 750 lx and 1000 lx) and four CCTs (2700, 4000, 5000 and 6500 K) for 2 h. A subjective asthenopia questionnaire, the optical quality analysis system (OQAS) and an inflammatory cytokine assay were used to assess the levels of asthenopia.

RESULTS

Increased asthenopia was observed after reading, but the degree varied with lighting conditions. There were significant differences among the groups in terms of subjective symptoms (inattention, eye pain, dry eye and total score), optical performance parameters (modulation transfer function [MTF] cut-off frequency, Strehl ratio [SR], objective scattering index [OSI], mean OSI and accommodative amplitude [AA]) as well as inflammatory cytokines in the tears (epidermal growth factor [EGF], transforming growth factor [TGF]-α, interleukin [IL]-6, IL-8, macrophage inflammatory protein [MIP]-1β, tumour necrosis factor [TNF]-α, TNF-β and vascular endothelial growth factor [VEGF]-A). All of the subjective and objective measurements collectively suggested that asthenopia was lessened for the 500 lx-4000 K condition. However, asthenopia was significantly worse for 300 lx-2700 K and 1000 lx-6500 K in terms of subjective symptoms and objective optical performance, respectively.

CONCLUSIONS

LED illuminance and CCT do have a significant effect on asthenopia during reading. 500 lx-4000 K lighting resulted in the lowest level of asthenopia. Conversely, low illuminance at low CCT (300 lx-2700 K) and high illuminance at high CCT (1000 lx-6500 K) promoted more severe asthenopia.

Biogenic synthesis of reduced graphene oxide from Ziziphus spina-christi (Christ's thorn jujube) extracts for catalytic, antimicrobial, and antioxidant potentialities

In the current work, various concentrations of the aqueous extract of Ziziphus spina-christi were employed for the phytoreduction of graphene oxide (GO). The green synthesized reduced graphene oxide (rGO) was characterized through UV-Vis spectrometry, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy, and energy-dispersive X-ray spectroscopy (SEM-EDX). Gas chromatography-mass spectrometry (GC-MS) denoted the presence of numerous phytoconstituents including ketones, terpenoids, fatty acids, esters, and flavonoids, which acted as reducing and capping agents. The obtained results indicated the increase in rGO yield and shape with increasing the extract concentration. The optimized rGO was instantaneously ~100% removed methylene blue (MB) from the water at 5 mg L-1. However, the removal efficiency was slightly declined to reach 73.55 and 65.1% at 10 and 15 mg L-1, respectively. A powerful antibacterial activity for rGO particularly against gram-negative bacteria with a high concentration of 2 × 108 CFU mL-1 was confirmed. Furthermore, rGO demonstrated promising and comparable antioxidant efficiency with vitamin C against DPPH free radical scavenging. While vitamin C recorded 13.45 and 48.4%, the optimized rGO attained 13.30 and 45.20% at 12 and 50 μg mL-1, respectively.

Continuous Single-Crystalline GaN Film Grown on WS2 -Glass Wafer

Use of 2D materials as buffer layers has prospects in nitride epitaxy on symmetry mismatched substrates. However, the control of lattice arrangement via 2D materials at the heterointerface presents certain challenges. In this study, the epitaxy of single-crystalline GaN film on WS₂ -glass wafer is successfully performed by using the strong polarity of WS₂ buffer layer and its perfectly matching lattice geometry with GaN. Furthermore, this study reveals that the first interfacial nitrogen layer plays a crucial role in the well-constructed interface by sharing electrons with both Ga and S atoms, enabling the single-crystalline stress-free GaN, as well as a violet light-emitting diode. This study paves a way for the heterogeneous integration of semiconductors and creates opportunities to break through the design and performance limitations, which are induced by substrate restriction, of the devices.

High Luminous Efficacy Phosphor-Converted Mass-Produced White LEDs Achieved by AlN Prebuffer and Transitional-Refraction-Index Patterned Sapphire Substrate

Constant advance in improving the luminous efficacy (ηL) of nitride-based light-emitting diodes (LEDs) plays a critical role for saving measurable amounts of energy. Further development is motivated to approach the efficiency limit for this material system while reducing the costs. In this work, strategies of using thin AlN prebuffer and transitional-refraction-index patterned sapphire substrate (TPSS) were proposed, which pushed up the efficiency of white LEDs (WLEDs). The AlN prebuffer was obtained through physical vapor deposition (PVD) method and TPSS was fabricated by dry-etched periodic silica arrays covered on sapphire. Devices in mass production confirmed that PVD AlN prebuffer was able to improve the light output power (φe) of blue LEDs (BLEDs) by 2.53% while increasing the productivity by ~8% through shortening the growth time. Additionally, BLEDs on TPSS exhibited an enhanced top ηext of 5.65% in contrast to BLEDs on the conventional PSS through Monte Carlo ray-tracing simulation. Consequently, φe of BLEDs was experimentally enhanced by 10% at an injected current density (Jin) of 40 A/cm2. A peak ηL of 295.2 lm/W at a Jin of 0.9 A/cm2 and the representative ηL of 282.4 lm/W at a Jin of 5.6 A/cm2 for phosphor-converted WLEDs were achieved at a correlated color temperature of 4592 K.

Vertical 1D/2D Heterojunction Architectures for Self-Powered Photodetection Application: GaN Nanorods Grown on Transition Metal Dichalcogenides

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenide (TMD) materials have attracted the attention of researchers to conduct fundamental investigations on emerging physical phenomena and expanding diverse nano-optoelectronic devices. Herein, the quasi-van der Waals epitaxial (QvdWE) growth of vertically aligned one-dimensional (1D) GaN nanorod arrays (NRAs) on TMDs/Si substrates is reported, and their vdW heterojunctions in the applications of high-performance self-powered photodetection are demonstrated accordingly. Such 1D/2D hybrid systems fully combine the advantages of the strong light absorption of 1D GaN nanoarrays and the excellent electrical properties of 2D TMD materials, boosting the photogenerated current density, which demonstrates a light on/off ratio above 10⁵. The device exhibits a competitive photovoltaic photoresponsivity over 10 A W^-1 under a weak detectable light signal without any external bias, which is attributed to the efficient photogenerated charge separation under the strong built-in potential from the type-II band alignment of GaN NRAs/TMDs. This work presents a QvdWE route to prepare 1D/2D heterostructures for the fabrication of self-powered photodetectors, which shows promising potentials for practical applications of space communications, sensing networks, and environmental monitoring.

Flexible High-Stability Self-Variable-Voltage Monolithic Integrated System Achieved by High-Brightness LED for Information Transmission

The emergence of visible light information transmission systems is profoundly affecting the future of the Internet of Things (IoT) technology. The complex sensing and driving circuits of the IoT have become the key factor to hinder signal conversion and processing. Herein, a high-performance self-variable-voltage light information transmission integrated system (SVV-LTS) is reported and its application potential in low-power, self-powered optical communication transmission systems is demonstrated. Diffusion-adsorption regulation growth method and laser induction technology are innovatively used to realize high-brightness light-emitting diode (LED) and flexible micro-supercapacitor (MSC) on graphene. Meanwhile, MSC realizes the dual functions of supplying power to the system, realizing pressure signal response, and converting pressure signals into electrical signals. Finally, the MSC as power, sensor and LED as signal transmitter are integrated into an SVV-LTS. The response time of SVV-LTS is 80 ms and the luminous wavelength fluctuation of the LED is stable at 1.2 nm. This study will provide a new approach to realize low-power optical communication transmission systems affecting the IoT technology.

Dimension- and position-controlled growth of GaN microstructure arrays on graphene films for flexible device applications

This paper describes the fabrication process and characteristics of dimension- and position-controlled gallium nitride (GaN) microstructure arrays grown on graphene films and their quantum structures for use in flexible light-emitting device applications. The characteristics of dimension- and position-controlled growth, which is crucial to fabricate high-performance electronic and optoelectronic devices, were investigated using scanning and transmission electron microscopes and power-dependent photoluminescence spectroscopy measurements. Among the GaN microstructures, GaN microrods exhibited excellent photoluminescence characteristics including room-temperature stimulated emission, which is especially useful for optoelectronic device applications. As one of the device applications of the position-controlled GaN microrod arrays, we fabricated light-emitting diodes (LEDs) by heteroepitaxially growing In_xGa_1−xN/GaN multiple quantum wells (MQWs) and a p-type GaN layer on the surfaces of GaN microrods and by depositing Ti/Au and Ni/Au metal layers to prepare n-type and p-type ohmic contacts, respectively. Furthermore, the GaN microrod LED arrays were transferred onto Cu foil by using the chemical lift-off method. Even after being transferred onto the flexible Cu foil substrate, the microrod LEDs exhibited strong emission of visible blue light. The proposed method to enable the dimension- and position-controlled growth of GaN microstructures on graphene films can likely be used to fabricate other high-quality flexible inorganic semiconductor devices such as micro-LED displays with an ultrahigh resolution.

High-Quality Transferred GaN-Based Light-Emitting Diodes through Oxygen-Assisted Plasma Patterning of Graphene

Two-dimensional (2D) release layers are commonly used to realize flexible nitride films. Here, high-quality, large-area, and transferable nitride films can be precisely controlled grown on O₂-plasma-assisted patterned graphene. The first-principles calculation indicates that the patterned graphene introduced by O₂ plasma changes the original wettability of sapphire and the growth behavior of Al atoms is related with layer number of graphene, which is consistent with experimental results. The as-fabricated violet GaN-based light-emitting diodes (LEDs) show high stability and high light output power (LOP). This work provides a general rule for the growth of high-quality and transferable III-nitride films on graphene from the atomic scale and provide actual demonstration in LED. The advantages of the proposed new growth method can supply new ways for electronic and optoelectronic flexible devices of group III nitride semiconductors.

Van der Waals epitaxy of nearly single-crystalline nitride films on amorphous graphene-glass wafer

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interfacial layer. The epitaxial GaN-based light-emitting diode structures, with a record internal quantum efficiency, can be readily lifted off, becoming large-size flexible devices. Without the effects of the potential field from a single-crystalline substrate, we expect this approach to be equally applicable for high-quality growth of nitrides on arbitrary substrates. Our work provides a revolutionary technology for the growth of high-quality semiconductors, thus enabling the hetero-integration of highly mismatched material systems.

The stability of graphene and boron nitride for III-nitride epitaxy and post-growth exfoliation

A challenging approach, but one providing a key solution to material growth, remote epitaxy (RE)-a novel concept related to van der Waals epitaxy (vdWE)-requires the stability of a two-dimensional (2-D) material. However, when graphene, a representative 2-D material, is present on substrates that have a nitrogen atom, graphene loss occurs. Although this phenomenon has remained a hurdle for over a decade, restricting the advantages of applying graphene in the growth of III-nitride materials, few previous studies have been conducted. Here, we report the stability of graphene on substrates containing oxygen or nitrogen atoms. Graphene has been observed on highly decomposed Al2O3; however, graphene loss occurred on decomposed AlN at temperatures over 1300 °C. To overcome graphene loss, we investigated 2-D hexagonal boron nitride (h-BN) as an alternative. Unlike graphene on AlN, it was confirmed that h-BN on AlN was intact after the same high-temperature process. Moreover, the overgrown AlN layers on both h-BN/AlN and h-BN/Al2O3 could be successfully exfoliated, which indicates that 2-D h-BN survived after AlN growth and underlines its availability for the vdWE/RE of III-nitrides with further mechanical transfer. By enhancing the stability of the 2-D material on the substrate, our study provides insights into the realization of a novel epitaxy concept.

Graphene-Nanorod Enhanced Quasi-Van Der Waals Epitaxy for High Indium Composition Nitride Films

The nitride films with high indium (In) composition play a crucial role in the fabrication of In-rich InGaN-based optoelectronic devices. However, a major limitation is In incorporation requiring a low temperature during growth at the expense of nitride dissociation. Here, to overcome this limitation, a strain-modulated growth method, namely the graphene (Gr)-nanorod (NR) enhanced quasi-van der Waals epitaxy, is proposed to increase the In composition in InGaN alloy. The lattice transparency of Gr enables constraint of in-plane orientation of nitride film and epitaxial relationships at the heterointerface. The Gr interlayer together with NRs buffer layer substantially reduces the stress of the GaN film by 74.4%, from 0.9 to 0.23 GPa, and thus increases the In incorporation by 30.7%. The first principles calculations confirm that the release of strain accounts for the dramatic improvement. The photoluminescence peak of multiple quantum wells shifts from 461 to 497 nm and the functionally small-sized cyan light-emitting diodes of 7 × 9 mil² are demonstrated. These findings provide an efficient approach for the growth of In-rich InGaN film and extend the applications of nitrides in advanced optoelectronic, photovoltaic, and thermoelectric devices.

The Mechanism of Graphene Vapor-Solid Growth on Insulating Substrates

Wafer-scale single-crystal graphene film directly grown on insulating substrates via the chemical vapor deposition (CVD) method is desired for building high-performance graphene-based devices. In comparison with the well-studied mechanism of graphene growth on transition metal substrates, the lack of understanding on the mechanism of graphene growth on insulating surfaces greatly hinders the progress. Here, by using first-principles calculation, we systematically explored the absorption of various carbon species CH_x (x = 0, 1, 2, 3, 4) on three typical insulating substrates [h-BN, sapphire, and quartz] and reveal that graphene growth on an insulating surface is dominated by the reaction of active carbon species with the hydrogen-passivated graphene edges and thus is less sensitive to the type of the substrate. The dominating gas phase precursor, CH₃, plays two key roles in graphene CVD growth on an insulating substrate: (i) to feed the graphene growth and (ii) to remove excessive hydrogen atoms from the edge of graphene. The threshold reaction barriers for the growth of graphene armchair (AC) and zigzag (ZZ) edges were calculated as 3.00 and 1.94 eV, respectively; thus the ZZ edge grows faster than the AC one. Our theory successfully explained why the circumference of a graphene island grown on insulating substrates is generally dominated by AC edges, which is a long-standing puzzle of graphene growth. In addition, the very slow graphene growth rate on an insulating substrate is calculated and agrees well with existing experimental observations. The comprehensive insights on the graphene growth on insulating surfaces at the atomic scale provide guidance on the experimental design for high-quality graphene growth on insulating substrates.

Strain-Reduced Micro-LEDs Grown Directly Using Partitioned Growth

Strain-reduced micro-LEDs in 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, 500 μm × 500 μm, and 1,000 μm × 1,000 μm sizes were grown on a patterned c-plane sapphire substrate using partitioned growth with the metal-organic chemical-vapor deposition (MOCVD) technique. The size effect on the optical properties and the indium concentration for the quantum wells were studied experimentally. Here, we revealed that the optical properties can be improved by decreasing the chip size (from 1,000 to 100 µm), which can correspondingly reduce the in-plane compressive stress. However, when the chip size is further reduced to 50 μm × 50 μm, the benefit of strain release is overridden by additional defects induced by the higher indium incorporation in the quantum wells and the efficiency of the device decreases. The underlying mechanisms of the changing output power are uncovered based on different methods of characterization. This work shows the rules of thumb to achieve optimal power performance for strain-reduced micro-LEDs through the proposed partitioned growth process.

Layer-Scale and Chip-Scale Transfer Techniques for Functional Devices and Systems: A Review

Hetero-integration of functional semiconductor layers and devices has received strong research interest from both academia and industry. While conventional techniques such as pick-and-place and wafer bonding can partially address this challenge, a variety of new layer transfer and chip-scale transfer technologies have been developed. In this review, we summarize such transfer techniques for heterogeneous integration of ultrathin semiconductor layers or chips to a receiving substrate for many applications, such as microdisplays and flexible electronics. We showed that a wide range of materials, devices, and systems with expanded functionalities and improved performance can be demonstrated by using these technologies. Finally, we give a detailed analysis of the advantages and disadvantages of these techniques, and discuss the future research directions of layer transfer and chip transfer techniques.

Toward Large-Scale Ga2O3 Membranes via Quasi-Van Der Waals Epitaxy on Epitaxial Graphene Layers

Epitaxial growth using graphene (GR), weakly bonded by van der Waals force, is a subject of interest for fabricating technologically important semiconductor membranes. Such membranes can potentially offer effective cooling and dimensional scale-down for high voltage power devices and deep ultraviolet optoelectronics at a fraction of the bulk-device cost. Here, we report on a large-area β-Ga₂O₃ nanomembrane spontaneous-exfoliation (1 cm × 1 cm) from layers of compressive-strained epitaxial graphene (EG) grown on SiC, and demonstrated high-responsivity flexible solar-blind photodetectors. The EG was favorably influenced by lattice arrangement of SiC, and thus enabled β-Ga₂O₃ direct-epitaxy on the EG. The β-Ga₂O₃ layer was spontaneously exfoliated at the interface of GR owing to its low interfacial toughness by controlling the energy release rate through electroplated Ni layers. The use of GR templates contributes to the seamless exfoliation of the nanomembranes, and the technique is relevant to eventual nanomembrane-based integrated device technology.

Epitaxial growth mechanisms of single-crystalline GaN on single-crystalline graphene

Dangling bonds, an AlN nucleation layer and high selectivity of AlN nucleation are prerequisites for the epitaxy of single-crystalline GaN on a single-crystalline graphene template.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al_0.5Ga_0.5N/sapphire templates by propane (C₃H₈) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C₃H₈ flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp²/sp³ hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq^-1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Highly Conductive Nitrogen-Doped Vertically Oriented Graphene toward Versatile Electrode-Related Applications

The direct growth of vertically oriented graphene (VG) on low-priced, easily accessible soda-lime glass can propel its applications in transparent electrodes and energy-relevant areas. However, graphene deposited at low temperature (∼600 °C) on the catalysis-free insulating substrates usually presents high defect density, poor crystalline quality, and unsatisfactory electrical conductivity. To tackle this issue, we select high borosilicate glass as the growth substrate (softening point ∼850 °C), which can resist higher growth temperature and thus afford higher graphene crystalline quality, by using a radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) route. A nitrogen doping strategy is also combined to tailor the carrier concentration through a methane/acetonitrile-precursor-based synthetic strategy. The sheet resistance of as-grown nitrogen-doped (N-doped) VG films on high borosilicate glass can thus be lowered down to ∼2.3 kΩ·sq^-1 at a transmittance of 88%, less than half of the methane-precursor-based PECVD product. Significantly, this synthetic route allows the achievement of 30-inch-scale uniform N-doped graphene glass, thus promoting its applications as excellent electrodes in high-performance switchable windows. Additionally, such N-doped VG films were also employed as efficient electrocatalysts for electrocatalytic hydrogen evolution reaction.

Hexagonal BN-Assisted Epitaxy of Strain Released GaN Films for True Green Light-Emitting Diodes

Epitaxial growth of III-nitrides on 2D materials enables the realization of flexible optoelectronic devices for next-generation wearable applications. Unfortunately, it is difficult to obtain high-quality III-nitride epilayers on 2D materials such as hexagonal BN (h-BN) due to different atom hybridizations. Here, the epitaxy of single-crystalline GaN films on the chemically activated h-BN/Al2O3 substrates is reported, paying attention to interface atomic configuration. It is found that chemical-activated h-BN provides B-O-N and N-O bonds, where the latter ones act as effective artificial dangling bonds for following GaN nucleation, leading to Ga-polar GaN films with a flat surface. The h-BN is also found to be effective in modifying the compressive strain in GaN film and thus improves indium incorporation during the growth of InGaN quantum wells, resulting in the achievement of pure green light-emitting diodes. This work provides an effective way for III-nitrides epitaxy on h-BN and a possible route to overcome the epitaxial bottleneck of high indium content III-nitride light-emitting devices.

GaN-Based LEDs Grown on Graphene-Covered SiO2/Si (100) Substrate

The growth of nitride on large-size and low-cost amorphous substrates has attracted considerable attention for applications in large-scale optoelectronic devices. In this paper, we reported the growth of GaN-based light-emitting diodes (LEDs) on amorphous SiO2 substrate with the use of nanorods and graphene buffer layers by metal organic chemical vapor deposition (MOCVD). The effect of different growth parameters on the morphology and vertical-to-lateral aspect ratio of nanorods was discussed by analyzing growth kinetics. Furthermore, we tuned nanorod coalescence to obtain continuous GaN films with a blue-LED structure by adjusting growth conditions. The GaN films exhibited a hexagonal wurtzite structure and aligned c-axis orientation demonstrated by X-ray diffractometer (XRD), Raman, and transmission electron microscopy (TEM) results. Finally, five-pair InGaN/GaN multi-quantum-wells (MQWs) were grown. The photoluminescence (PL) showed an intense emission peak at 475 nm, and the current–voltage (I-V) curve shows a rectifying behavior with a turn-on voltage of 5.7 V. This work provides a promising fabrication method for the large-area and low-cost GaN-based devices on amorphous substrates and opens up the further possibility of nitride integration with Si (100) complementary metal oxide semiconductor (CMOS) electronics.

Van der Waals Epitaxy of III-Nitrides and Its Applications

III-nitride semiconductors have wide bandgap and high carrier mobility, making them suitable candidates for light-emitting diodes (LEDs), laser diodes (LDs), high electron mobility transistors (HEMTs) and other optoelectronics. Compared with conventional epitaxy technique, van der Waals epitaxy (vdWE) has been proven to be a useful route to relax the requirements of lattice mismatch and thermal mismatch between the nitride epilayers and the substrates. By using vdWE, the stress in the epilayer can be sufficiently relaxed, and the epilayer can be easily exfoliated and transferred, which provides opportunities for novel device design and fabrication. In this paper, we review and discuss the important progress on the researches of nitrides vdWE. The potential applications of nitride vdWE are also prospected.

Enhancing carrier transport and carrier capture with a good current spreading characteristic via graphene transparent conductive electrodes in InGaN/GaN multiple-quantum-well light emitting diodes

In this work, InGaN/GaN multiple-quantum-wells light-emitting diodes with and without graphene transparent conductive electrodes are studied with current-voltage, electroluminescence, and time-resolved electroluminescence (TREL) measurements. The results demonstrate that the applications of graphene electrodes on LED devices will spread injection carriers more uniformly into the active region and therefore result in a larger current density, broader luminescence area, and stronger EL intensity. In addition, the TREL data will be further analyzed by employing a 2-N theoretical model of carrier transport, capture, and escape processes. The combined experimental and theoretical results clearly indicate that those LEDs with graphene transparent conductive electrodes at p-junctions will have a shorter hole transport time along the lateral direction and thus a more efficient current spreading and a larger luminescence area. In addition, a shorter hole transport time will also expedite hole capture processes and result in a shorter capture time and better light emitting efficiency. Furthermore, as more carrier injected into the active regions of LEDs, thanks to graphene transparent conductive electrodes, excessive carriers need more time to proceed carrier recombination processes in QWs and result in a longer carrier recombination time. In short, the LED samples, with the help of graphene electrodes, are shown to have a better carrier transport efficiency, better carrier capture efficiency, and more electron-hole recombination. These research results provide important information for the carrier transport, carrier capture, and recombination processes in InGaN/GaN MQW LEDs with graphene transparent conductive electrodes.

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp² -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

The influence of AlN buffer layer on the growth of self-assembled GaN nanocolumns on graphene

GaN nanocolumns were synthesized on single-layer graphene via radio-frequency plasma-assisted molecular beam epitaxy, using a thin migration-enhanced epitaxy (MEE) AlN buffer layer as nucleation sites. Due to the weak nucleation on graphene, instead of an AlN thin-film we observe two distinguished AlN formations which affect the subsequent GaN nanocolumn growth: (i) AlN islands and (ii) AlN nanostructures grown along line defects (grain boundaries or wrinkles) of graphene. Structure (i) leads to the formation of vertical GaN nanocolumns regardless of the number of AlN MEE cycles, whereas (ii) can result in random orientation of the nanocolumns depending on the AlN morphology. Additionally, there is a limited amount of direct GaN nucleation on graphene, which induces non-vertical GaN nanocolumn growth. The GaN nanocolumn samples were characterized by means of scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffraction, room temperature micro-photoluminescence, and micro-Raman measurements. Surprisingly, the graphene with AlN buffer layer formed using less MEE cycles, thus resulting in lower AlN coverage, has a lower level of nitrogen plasma damage. The AlN buffer layer with lowest AlN coverage also provides the best result with respect to high-quality and vertically-aligned GaN nanocolumns.

H2 O-Etchant-Promoted Synthesis of High-Quality Graphene on Glass and Its Application in See-Through Thermochromic Displays

Direct growth of graphene on glass can bring an innovative revolution by coupling the complementary properties of traditional glass and modern graphene (such as transparency and conductivity), offering brand new daily-life related applications. However, preparation of high-quality graphene on nonmetallic glass is still challenging. Herein, the direct route of low sheet resistance graphene on glass is reported by using in situ-introduced water as a mild etchant and methane as a carbon precursor via chemical vapor deposition. The derived graphene features with large domain sizes and few amorphous carbon impurities. Intriguingly, the sheet resistance of graphene on glass is dramatically lowered down to ≈1170 Ω sq^-1 at the optical transmittance ≈93%, ≈20% of that derived without the water etchant. Based on the highly conductive and optical transparent graphene on glass, a see-through thermochromic display is thus fabricated with transparent graphene glass as a heater. This work can motivate further investigations of the direct synthesis of high-quality graphene on functional glass and its versatile applications in transparent electronic devices or displays.

Solid-State Carbon-Doped GaN Schottky Diodes by Controlling Dissociation of the Graphene Interlayer with a Sputtered AlN Capping Layer

Carbon-doped GaN (GaN:C) Schottky diodes are prepared by controlling the destruction status of the graphene interlayer (GI) on the substrate. The GI without a sputtered AlN capping layer (CL) was destroyed because of ammonia precursor etching behavior in a high-temperature epitaxy. The damaged GI, like nanographite as a solid-state carbon doping source, incorporated the epitaxial growth of the GaN layer. The secondary ion mass spectroscopy depth profile indicated that the carbon content in the GaN layer can be tuned further by optimizing the sputtering temperature of AlN CL because of the better capping ability of high crystalline quality AlN CL on GI being achieved at higher temperature. The edge-type threading dislocation density and carbon concentration of the GaN:C layer with an embedded 550 °C-grown AlN CL on a GI substrate can be significantly reduced to 2.28 × 10⁹ cm^-2 and ∼2.88 × 10¹⁸ cm^-3, respectively. Thus, a Ni-based Schottky diode with an ideality factor of 1.5 and a barrier height of 0.72 eV was realized on GaN:C. The series resistance increased from 28 kΩ at 303 K to 113 kΩ at 473 K, while the positive temperature coefficient (PTC) of series resistance was ascribed to the carbon doping that induced the compensation effect and lattice scattering effect. The decrease of the donor concentration was confirmed by temperature-dependent capacitance-voltage (C-V-T) measurement. The PTC characteristic of GaN:C Schottky diodes created by dissociating the GI as a carbon doping source should allow for the future use of high-voltage Schottky diodes in parallel, especially in high-temperature environments.

Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene

The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction 31 × 31 R ± 9 ° of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm² V^-1 s^-1 . The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.

Atomic mechanism of strong interactions at the graphene/sapphire interface

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene–sapphire substrate.

Understanding the atomic arrangement and binding nature of 2D materials with substrates is crucial to understand their properties and utilize their functions. Here, authors report that at high temperature graphene and α-Al₂O₃ substrate form a C-O-Al bond, having strong interactions, while interfacial structural relaxations of sapphire remain suppressed.

Carrier lifetime enhancement in halide perovskite via remote epitaxy

Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the ‘defects-tolerant’ halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics.

Crystallographic dislocation has proven harmful to the carrier dynamics in conventional semiconductors but it is unexplored in metal halide perovskites. Here Jiang et al. grow remote epitaxial perovskite films on graphene with density-controlled dislocations and confirm their negative impact.

Enhancement of Heat Dissipation in Ultraviolet Light-Emitting Diodes by a Vertically Oriented Graphene Nanowall Buffer Layer

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.

Direct CVD Growth of Graphene on Traditional Glass: Methods and Mechanisms

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large-area, high-quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO₂ /Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so-called "super graphene glass," which has attracted great interest from the viewpoints of both fundamental research and daily-life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten-bed CVD growth, metal-catalyst-assisted growth, and plasma-enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of "super graphene glass" production are provided.

Realization of low dislocation density AlN on a small-coalescence-area nano-patterned sapphire substrate

Growth behaviors of AlN on hexagonal configuration hole-type and truncated-cone-pillar-type nano-patterned sapphire substrates (NPSSs) have been investigated.