Graphene as the 2D material with extraordinary properties has attracted the interest of research communities to master the synthesis of this remarkable material at a large scale without sacrificing the quality. Although Top-Down and Bottom-Up approaches produce graphene of different quality, chemical vapour deposition (CVD) stands as the most promising technique. This review details the leading CVD methods for graphene growth, including hot-wall, cold-wall and plasma-enhanced CVD. The role of process conditions and growth substrates on the nucleation and growth of graphene film are thoroughly discussed. The essential characterisation techniques in the study of CVD-grown graphene are reported, highlighting the characteristics of a sample which can be extracted from those techniques. This review also offers a brief overview of the applications to which CVD-grown graphene is well-suited, drawing particular attention to its potential in the sectors of energy and electronic devices.

Chemical Vapour Deposition of Graphene—Synthesis, Characterisation, and Applications: A Review

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.

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.

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

2D III-nitride materials have been receiving considerable attention recently due to their excellent physicochemical properties, such as high stability, wide and tunable bandgap, and magnetism. Therefore, 2D III-nitride materials can be applied in various fields, such as electronic and photoelectric devices, spin-based devices, and gas detectors. Although the developments of 2D h-BN materials have been successful, the fabrication of other 2D III-nitride materials, such as 2D h-AlN, h-GaN, and h-InN, are still far from satisfactory, which limits the practical applications of these materials. In this review, recent advances in the properties, growth methods, and potential applications of 2D III-nitride materials are summarized. The properties of the 2D III-nitride materials are mainly obtained by first-principles calculations because of the difficulties in the growth and characterizations of these materials. The discussion on the growth of 2D III-nitride materials is focused on 2D h-BN and h-AlN, as the developments of 2D h-GaN and h-InN are yet to be realized. Therefore, applications have been realized mostly based on the 2D h-BN materials; however, many potential applications are cited for the entire range of 2D III-nitride materials. Finally, future research directions and prospects in this field are also discussed.

2D III-Nitride Materials: Properties, Growth, and Applications.

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.

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

The growth mechanism of AlN materials on a sapphire substrate with a hexagonal BN overlayer (hBN/sapphire) by metal–organic chemical vapor deposition (MOCVD) is reported for the first time. Here, we have grown high-quality AlN films on sapphire with a hBN layer by MOCVD. A growth model has been proposed to clarify the growth mechanism of AlN on hBN. To facilitate the nucleation of AlN on hBN/sapphire, we have introduced some artificial dangling bonds for hBN through O2 plasma treatment (hBN-O2). The change in atom connective structure of hBN materials after the treatment has been inferred according to the results of Raman spectroscopy and X-ray photoelectron spectroscopy. The total dislocation density of AlN on hBN-O2/sapphire is much lower than that of AlN on hBN/sapphire, which has evidenced the high crystalline quality of epitaxial films and the effect of the O2 treatment. Moreover, an AlGaN-based deep-ultraviolet light emitting diode (DUV LED) structure on hBN-O2/sapphire was achieved. The electroluminescence results have exhibited strong emissions with a peak wavelength of 290 nm, which further confirms the high quality of the AlN. This work provides a possible solution for further developing efficient DUV LEDs on 2D materials as well as other unconventional substrates.

Growth mechanism of AlN on hexagonal BN/sapphire substrate by metal–organic chemical vapor deposition

We experimentally investigated the possibility of using multilayer graphene to solve large mismatch problems between sapphire and nitride and further studied the effects of a multilayer graphene interlayer on the optical and electrical properties of LEDs. For the subsequent growth of 3-µm-thick GaN on AlN, multilayer graphene helps release stress and effectively removes cracks. In addition, multilayer graphene increases the diffraction of the substrate surface as determined from the increase in optical transmittance spectra in the wavelength range of 400–900 nm. Although the crystalline quality of GaN with multilayer graphene is slightly decreased, LEDs grown on multilayer graphene still show a higher output power than those grown on conventional sapphire. The present findings showed that the multilayer graphene layer is attractive as a potential substrate for the epitaxial growth of III–nitride to reduce stress and it could improve back light extraction as a rough layer to increase external quantum efficiency.

https://iopscience.iop.org/article/10.7567/JJAP.56.085506/pdf

Van der Waals epitaxy of GaN-based light-emitting diodes on wet-transferred multilayer graphene film

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Growth mechanism of AlN on hexagonal BN/sapphire substrate by metal–organic chemical vapor deposition

Van der Waals epitaxy of GaN-based light-emitting diodes on wet-transferred multilayer graphene film