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

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1 eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Hexagonal Boron Nitride as an Intermediate Layer for Gallium Nitride Epitaxial Growth in Near-Ultraviolet Light-Emitting Diodes

We introduce the development of gallium nitride (GaN) layers by employing graphene and hexagonal boron nitride (h-BN) as intermediary substrates. This study demonstrated the successful growth of GaN with a uniformly smooth surface morphology on h-BN. In order to evaluate the crystallinity of GaN grown on h-BN, a comparison was conducted with GaN grown on a sapphire substrate. Photoluminescence spectroscopy and X-ray diffraction confirmed that the crystallinity of GaN deposited on h-BN was inferior to that of GaN grown on conventional GaN. To validate the practical applicability of the GaN layer grown on h-BN, we subsequently grew an NUV-LED structure and fabricated a device that operated well in optoelectrical performance experiments. Our findings validate the potential usefulness of h-BN to be a substrate in the direct growth of a GaN layer.

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.

Determination of the preferred epitaxy for III-nitride semiconductors on wet-transferred graphene

Transferred graphene provides a promising III-nitride semiconductor epitaxial platform for fabricating multifunctional devices beyond the limitation of conventional substrates. Despite its tremendous fundamental and technological importance, it remains an open question on which kind of epitaxy is preferred for single-crystal III-nitrides. Popular answers to this include the remote epitaxy where the III-nitride/graphene interface is coupled by nonchemical bonds, and the quasi-van der Waals epitaxy (quasi-vdWe) where the interface is mainly coupled by covalent bonds. Here, we show the preferred one on wet-transferred graphene is quasi-vdWe. Using aluminum nitride (AlN), a strong polar III-nitride, as an example, we demonstrate that the remote interaction from the graphene/AlN template can inhibit out-of-plane lattice inversion other than in-plane lattice twist of the nuclei, resulting in a polycrystalline AlN film. In contrast, quasi-vdWe always leads to single-crystal film. By answering this long-standing controversy, this work could facilitate the development of III-nitride semiconductor devices on two-dimensional materials such as graphene.

Surface etching and edge control of hexagonal boron nitride assisted by triangular Sn nanoplates

Hexagonal boron nitride (hBN) is an ideal insulating substrate and template for other two-dimensional (2D) materials. The combination of hBN and 2D materials of group IV atoms, such as graphene, is interesting, because it can offer attractive physical properties and promising applications. Here, we demonstrate the unique behavior of tin (Sn), one of the group IV elements, on multilayer hBN which was grown by chemical vapor deposition (CVD). At high temperatures, triangular nanoplates formed after thermal deposition of Sn on the hBN surface, with their orientations determined by the hBN lattice. The triangular Sn nanoplates moved on the hBN surface, leaving monolayer-deep nanotrenches. Low-energy electron microscopy (LEEM) revealed that the nanotrenches are aligned in the armchair directions of the hBN. Furthermore, an additional Ar annealing without supplying Sn vapor induced the structural change of the linear trenches to triangular pits, indicating the preferential formation of zigzag edges in the absence of Sn. Our work highlights the unique behavior of Sn on hBN and offers a novel route to engineer the hBN surface.

Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization

Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12 eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Interfacial Modulated Lattice-Polarity-Controlled Epitaxy of III-Nitride Heterostructures on Si(111)

Monolithic integration of wurtzite III-nitrides with nonpolar silicon (Si), the two most-produced semiconductor materials, is essential and critical for a broad range of applications in electronics, optoelectronics, quantum photonics, and renewable energy. To date, however, it has remained challenging to achieve III-nitride heterostructures on Si with controlled lattice-polarity. Herein, we show that such critical challenges of III-nitrides on Si can be fundamentally addressed through a unique interfacial modulated lattice-polarity-controlled epitaxy (IMLPCE). It is discovered that the lattice-polarity of aluminum nitride (AlN) grown on Si(111) is primarily determined by the AlSiN interlayer: N-polar and Al-polar AlN can be achieved by suppressing and promoting the AlSiN interlayer formation, respectively. Furthermore, we develop a unique active-nitrogen-free in situ annealing process to mitigate the AlSiN layer formation at the GaN/AlN interface, which can eliminate the inverted domain formation commonly seen in N-polar GaN on AlN/Si. This study provides an alternative approach for controlling the lattice-polarity of III-nitrides on Si substrates and will enable their seamless integration with the mature Si-based device technology.

Direct growth of hexagonal boron nitride on non-metallic substrates and its heterostructures with graphene

Hexagonal boron nitride (h-BN) and its heterostructures with graphene are widely investigated van der Waals (vdW) quantum materials for electronics, photonics, sensing, and energy storage/transduction. However, their metal catalyst-based growth and transfer-based heterostructure assembly approaches present impediments to obtaining high-quality and wafer-scale quantum material. Here, we have presented our perspective on the synthetic strategies that involve direct nucleation of h-BN on various dielectric substrates and its heterostructures with graphene. Mechanistic understanding of direct growth of h-BN via bottom-up approaches such as (a) the chemical-interaction guided nucleation on silicon-based dielectrics, (b) surface nitridation and N+ sputtering of h-BN target on sapphire, and (c) epitaxial growth of h-BN on sapphire, among others, are reviewed. Several design methodologies are presented for the direct growth of vertical and lateral vdW heterostructures of h-BN and graphene. These complex 2D heterostructures exhibit various physical phenomena and could potentially have a range of practical applications.

Demonstration of epitaxial growth of strain-relaxed GaN films on graphene/SiC substrates for long wavelength light-emitting diodes

Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C-N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

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.

Growth mechanism on graphene-regulated high-quality epitaxy of flexible AlN film

We report a novel diffusion–adsorption regulation growth method in the epitaxy of AlN on graphene for the high-quality and transferable large-size AlN film.