voltage, high mobility, high luminous efficiency, long service life, and tunable bandgap energy by adjusting the aluminum and indium composition covering from ultraviolet to infrared. However, there are drawbacks for GaN. For example, growing GaN on a sapphire or silicon (111) substrate generates dislocation due to lattice mismatch between the GaN epitaxial layer and the substrate. In addition, the strong chemical bond at the interface makes it difficult to separate the epitaxial layer from the substrate, which limits the ability to apply GaN with excellent properties in various fields. Recently, several studies have sought to overcome these problems by adopting 2D materials between the epitaxial layer and the substrate.Xu and co-workers reported the reduced dislocation density in GaN/graphene/GaN structure using self-organized graphene as a nanomask at high-temperature in hydrogen (H 2 ) ambient with many graphene defects. [1,2] Liu and co-workers also reported that when aluminum nitride was grown on plasma-treated graphene, the dislocation density was reduced compared to when directly grown on sapphire. [3] Hong and co-workers proved that the embedded graphene oxide could be used to improve the heat dissipation in GaN light-emitting diodes. [4,5] In addition, other researchers are making efforts to separate epitaxially grown III−V films from the substrate with the help of the 2D material making the weak chemical bond between the epitaxial layer and the substrate that is transferred to the substrate such as graphene/silicon dioxide, [6] graphene/silicon carbide, [7,8] graphene/ gallium arsenide (GaAs), [9] and boron nitride/sapphire [10] structures. Very recently, Kim and co-workers reported demonstration results using various semiconductors that showed that the polarity of 2D materials and bulk play a crucial role in remote epitaxy. The article also provides the necessary general understanding of single-crystalline growth on 2D-material-coated substrates and subsequent exfoliation. [11] However, the earlyreported graphene-loss issue, particularly in GaN growth in metal-organic chemical vapor deposition (MOCVD), remains unclear and warrants investigation to take advantages of graphene in III−V compound semiconductor growth.Here, we investigated how the decomposition of substrates that support the graphene layer affect the graphene layer by Graphene has been adopted in III−V material growth since it can reduce the threading dislocations and the III−V epilayer can easily be separated from the substrate due to the weak chemical bond. However, depending on the substrate supporting the graphene, some substrates decompose in the III−V material growth environment, which results in the problem that no graphene remains. In this study, the influence of temperature-dependent substrate decomposition on graphene through an annealing process that resembles conventional growth conditions in metal-organic chemical vapor deposition (MOCVD) is investigated. It is also confirmed that trimethylgallium, hydrogen, and ammonia gases d...

In recent years, research into implementing microdisplays for use in virtual reality and augmented reality has been actively conducted worldwide. Specifically, inorganic light-emitting diodes (LED) have many advantages in microdisplays, so much effort has been made to implement them in various ways. However, it is still challenging to realize a display with high resolution using only inorganic LEDs without color conversion layers because a typical LED chip is designed to emit only one color on a single wafer. In this study, we integrated high-efficiency red, green, and blue (RGB) LED material systems on the same sapphire substrate. Since the blue and green LED structures comprised the same GaN semiconductor, a metalorganic chemical vapor deposition method was used to integrate them. Meanwhile, the red LEDs made from another semiconductor were incorporated into the blue/green LEDs using a wafer-bonding technique. The fabricated hybrid RGB LEDs were able to cover a wide color space. In addition, the RGB LED material systems consisting of a high-quality single crystal were stably combined on the sapphire substrate without any structural defects. We show the possibility of their use in displays by integrating the RGB LEDs on one chip and suggest that their utilization could range from large-area LED displays to ultrahigh-resolution small displays.

Challenging, but providing a key solution on material growth, Remote Epitaxy (RE)—the next concept of van der Waals Epitaxy (vdWE)—requires the stability of two-dimensional (2-D) material. However, when graphene, a...

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

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Quantum dot (QD)-based luminescent down-shifting (LDS) layers were deposited on Cu2ZnSn(S,Se)4 (CZTSSe) solar cells via the drop-casting method. The LDS layers can easily widen the narrow absorption wavelength regions of single-junction solar cells and enable improvement of the short-circuit current. The optical properties of LDS layers deposited on glass and containing different QD contents were analyzed based on their transmittance, reflectance, and absorbance. The absorber films to be used in the CZTSSe solar cells were determined by X-ray diffraction measurements and Raman spectroscopy to determine their crystal structures and secondary phases, respectively. The completed CZTSSe solar cells with LDS layers showed increased ultraviolet responses of up to 25% because of wavelength conversion by the QDs. In addition, the impact of the capping layer, which was formed to protect the QDs from oxygen and moisture, on the solar cell performance was analyzed. Thus, a maximal conversion efficiency of 7.3% was achieved with the 1.0 mL QD condition; furthermore, to the best of our knowledge, this is the first time that LDS layers have been experimentally demonstrated for CZTSSe solar cells.

In this study, we integrated RGB inorganic LEDs onto a sapphire substrate. Blue and green subpixels were monolithically integrated using a selective area growth method, while red subpixels were formed by adhesive bonding and epitaxial lift-off. The fabricated RGB subpixels could be independently controlled to realize the full color spectrum.