Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

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.

Epitaxial growth of 1D GaN-based heterostructures on various substrates for photonic and energy applications

GaN is an important III-V semiconductor for a variety of applications owing to its large direct band gap. GaN nanowires (NWs) have demonstrated significant potential as critical building blocks for nanoelectronics and nanophotonic devices, as well as integrated nanosystems. We present a comprehensive analysis of the vapor-liquid-solid (VLS) as a general synthesis technique for NWs on a variety of substrates, the morphological and structural characterization, and applications of GaN NWs in piezoelectric nanogenerators, light-emitting diodes, and solar-driven water splitting. We begin by summarizing the overall VLS growth process of GaN NWs, followed by the growth of NWs on several substrates. Subsequently, we review the various uses of GaN NWs in depth.

Lattice Orientation Heredity in the Transformation of 2D Epitaxial Films

The ability to control lattice orientation is often an essential requirement in the growth of both 2D van der Waals (vdW) layered and nonlayered thin films. Here, a unique and universal phenomenon termed "lattice orientation heredity" (LOH) is reported. LOH enables product films (including 2D-layered materials) to inherit the lattice orientation from reactant films in a chemical conversion process, excluding the requirement on the substrate lattice order. The process universality is demonstrated by investigating the lattice transformations in the carbonization, nitridation, and sulfurization of epitaxial MoO₂ , ZnO, and In₂ O₃ thin films. Their resultant compounds all inherit the mono-oriented crystal feature from their precursor oxides, including 2D vdW-layered semiconductors (e.g., MoS₂ ), metallic films (e.g., MXene-like Mo₂ C and MoN), wide-bandgap semiconductors (e.g., hexagonal ZnS), and ferroelectric semiconductors (e.g., In₂ S₃ ). Using LOH-grown MoN as a seeding layer, mono-oriented GaN is achieved on an amorphous quartz substrate. The LOH process presents a universal strategy capable of growing epitaxial thin films (including 2D vdW-layered materials) not only on single-crystalline but also on noncrystalline substrates.

Two-dimensional group-III nitrides and devices: a critical review

As third-generation semiconductors, group-III nitrides are promising for high power electronic and optoelectronic devices because of their wide bandgap, high electron saturation mobility, and other unique properties. Inspired by the thickness-dependent properties of two-dimensional (2D) materials represented by graphene, it is predicted that the 2D counterparts of group-III nitrides would have similar properties. However, the preparation of 2D group-III nitride-based materials and devices is limited by the large lattice mismatch in heteroepitaxy and the low rate of lateral migration, as well as the unsaturated dangling bonds on the surfaces of group-III nitrides. The present review focuses on theoretical and experimental studies on 2D group-III nitride materials and devices. Various properties of 2D group-III nitrides determined using simulations and theoretical calculations are outlined. Moreover, the breakthroughs in their synthesis methods and their underlying physical mechanisms are detailed. Furthermore, devices based on 2D group-III nitrides are discussed accordingly. Based on recent progress, the prospect for the further development of the 2D group-III nitride materials and devices is speculated. This review provides a comprehensive understanding of 2D group-III nitride materials, aiming to promote the further development of the related fields of nano-electronic and nano-optoelectronics.

Fabrication of gallium nitride and nitrogen doped single layer graphene hybrid heterostructures for high performance photodetectors

Gallium nitride (GaN) was epitaxially grown on nitrogen doped single layer graphene (N-SLG) substrates using chemical vapour deposition (CVD) technique. The results obtained using x-ray diffractometer (XRD) revealed the hexagonal crystal structure of GaN. Photoluminescence (PL) spectroscopy, energy dispersive x-ray (EDX) spectroscopy and x-ray photoelectron (XPS) spectroscopy revealed traces of oxygen, carbon and nitrogen occurring either as contamination or as an effect of doping during the GaN growth process. In addition, PL revealed a weak yellow luminescence peak in all the samples due to the presence of N-SLG. From the obtained results it was evident that, presence of N-SLG underneath GaN helped in improving the material properties. It was seen from the current-voltage (I-V) response that the barrier height estimated is in good agreement with the Schottky-Mott model, while the ideality factor is close to unity, emphasizing that there are no surface and interface related inhomogeneity in the samples. The photodetector fabricated with this material exhibit high device performances in terms of carrier mobility, sensitivity, responsivity and detectivity. The hall measurement values clearly portray that, the GaN thus grown possess high electron contents which was beneficial in attaining extraordinary device performance.

Transferable High-Quality Inorganic Perovskites for Optoelectronic Devices by Weak Interaction Heteroepitaxy

Transferable semiconductors with superior light-emitting properties are important for developing flexible and integrated optoelectronics. However, finding such a qualified candidate remains challenging. Here, we report the fabrication of transferable high-quality CsPbBr₃ single crystals on a highly oriented pyrolytic graphite (HOPG) substrate via weak interaction heteroepitaxy for the first time. Semi-quantitative kinetic analysis based on the classical nucleation theory well accounts for the van der Waals (vdW) epitaxial growth process of perovskite on the HOPG substrate. The density functional theory calculations illustrate the bonding nature of the interface and predict the Volmer-Weber growth mode in vdW epitaxy, which is consistent with our experimental observations. Importantly, the extremely weak vdW interaction between the perovskite and HOPG not only enables the high quality of the crystals but also endows them with the facile transferability to any foreign substrate by the mechanical exfoliation technique. Leveraging on the transferred CsPbBr₃ single crystals, the low-threshold microlasers and monolithic perovskite light-emitting diode devices are demonstrated. Our results represent a significant step toward advanced optoelectronic devices relying on the emerging perovskite semiconductors.

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.