The epitaxial growth of InN on Sc2O3 is promising due to the small lattice mismatch of 1.6%. In this work, InN layers were grown on Si substrates with a Sc2O3 interlayer by metal-organic vapor phase epitaxy (MOVPE). The growth temperature and pressure were fixed at 500 °C and 400 mbar, respectively. The results show that the crystalline structure, surface morphology, electrical, and optical properties strongly depend on the initial growth conditions. Prior to InN growth, the surface was treated with ammonia gas through a nitridation process. A mixture of cubic and hexagonal phases was observed in layers grown with a short nitridation time (1200 s). Increasing the nitridation time to 3600 s led to the dominance of the hexagonal phase. Room-temperature photoluminescence was observed from both the hexagonal and cubic phases. The 300 nm thick InN layers were found to be tensile strained. The charge carrier concentration and mobility were extracted from fast Fourier transform infrared (FTIR) reflectance spectrum modeling and compared with values obtained from Hall effect measurements.

Abstract The technological impact of III-nitrides in lighting and power electronics is well established. However, heteroepitaxy on rigid foreign substrates often results in high defect densities and limited mechanical compliance. This poses hurdles for the exploitation of electronic and photonic integrated circuits. Graphene interlayers offer an appealing alternative: by weakening interfacial adhesion while maintaining crystalline alignment under specific epitaxy conditions, they facilitate the lift-off of freestanding nitride membranes, allowing for substrate reuse. This review maps the experimental landscape with the theoretical background of graphene-assisted III-nitride growth, connecting macroscopic outcomes to competing interfacial regimes of van der Waals (vdW) epitaxy, quasi-vdW epitaxy, thru-hole growth, and remote epitaxy. Each process has a specific operational window under metal-organic chemical vapor deposition (MOCVD) conditions. Central challenges associated with the selection of a regime are traced to the exact root causes: suppressed nucleation on pristine graphene, uncontrolled defect formation and contamination during graphene preparation, and graphene degradation or interfacial reactions under the aggressive MOCVD environment, each of which can shift the dominant regime. Recent advances in direct graphene synthesis, controlled functionalization, buffers, and flow-modulation schemes have been assessed as practical routes to coalesced, transferable GaN- and AlN-based membranes, which constitute the main focus of this review. Future priorities for field advancement include standardized metrics for distinguishing growth modes, robust interface engineering, and considerations for novel device creation.

In this work, we present a detailed analysis of GaN layers up to 500 nm thick, directly grown on Sc2O3(111)/Si(111) templates using metal-organic vapor phase epitaxy. A range of measurement techniques, including X-ray diffraction, Raman spectroscopy, atomic force microscopy, cathodoluminescence, and scanning electron microscopy (SEM), were used to evaluate structural quality, strain/stress states, surface morphology, and dislocation densities. The micro-stripe formation was observed when the growth was conducted in a nitrogen atmosphere, with the stripes completely disappearing when the growth atmosphere was switched to hydrogen. The stripes were determined to be of a cubic GaN phase. The epitaxial relationships between the cubic GaN crystalline lattice and Sc2O3, Si, and hexagonal GaN were examined in detail. Continuous, c-axis-oriented, monocrystalline GaN layers on Sc2O3 can be achieved in both $$\hbox {N}_2$$ and $$\hbox {H}_2$$ atmospheres. Prolonged nitridation processes of up to 1200 s improved the smoothness and crystallinity of the GaN layers, significantly reducing the number of extended defects. Switching the growth atmosphere from $$\hbox {N}_2$$ to $$\hbox {H}_2$$ led to reduced dislocation densities, minimized cubic GaN formation, and improved the surface morphology of the GaN layers. Our analysis shows that due to the lattice and thermal mismatch between GaN and the Si substrate, the GaN layers experience tensile strain. To manage this strain, $$\hbox {Al}_x$$ $$\hbox {Ga}_{1-x}$$ N interlayers were inserted after 100 nm of GaN growth. This strain-engineering approach resulted in smooth, crack-free GaN epitaxial layers, demonstrating the potential for integrating GaN into silicon technology using a $$\hbox {Sc}_{{2}}$$ $$\hbox {O}_{{3}}$$ .

We demonstrate a broadband second-harmonic generator for the green spectral range based on a periodical polarity GaN waveguide structure. We grew the structure with a periodicity of 4 μ m using an MOCVD (metal-organic chemical vapor deposition) reactor. We tested the structure using femtosecond laser pulse pumping and obtained the second harmonic ( λ _SH =512 nm) conversion efficiency 0.79 · 10 ^-7 · W ^-1 for the pump peak power of 440 kW inside the waveguide. Based on the theoretical modeling of the dispersion curves of the pump and the second harmonic, we found multiple crossings, including an extended crossing. We relate this fact to the experimentally measured broadened second harmonic spectrum.

We demonstrate an MOCVD-grown quasi-symmetric GaN waveguide structure featuring modal phase matching property to generate a second harmonic in the green light spectra range with a central wavelength of 546 nm. The core of the planar waveguide structure is based on GaN polarity inversion, important for an efficient frequency conversion due to improved optical mode field overlap. The spectral characteristics of the picosecond second harmonic generation revealed high homogeneity of the fabricated waveguide structure.

BGaN epilayers with boron contents up to 5.6% were grown on SiC substrates by metal-organic chemical vapor deposition. The effects of boron incorporation on the structural and optical properties were studied by high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) spectroscopy. XRD reciprocal-space maps around the symmetric 0002 and asymmetric 1124 reflections allowed evaluation of the lattice constants and lattice mismatch with respect to the underlying substrate. XRD rocking curves and AFM measurements indicated the mosaic microstructure of the epilayer. The impact of boron content on crystallite size, tilt and twist is evaluated and the correlation with threading dislocation density is discussed. The deterioration of optical properties with increasing boron content was assessed by Raman and PL spectroscopy.

Gallium nitride is a wide-direct-bandgap semiconductor suitable for the creation of modern optoelectronic devices and radiation tolerant detectors. However, formation of dislocations is inevitable in MOCVD GaN materials. Dislocations serve as accumulators of point defects within space charge regions covering cores of dislocations. Space charge regions also may act as local volumes of enhanced non-radiative recombination, deteriorating the photoluminescence efficiency. Surface etching has appeared to be an efficient means to increase the photoluminescence yield from MOCVD GaN materials. This work aimed to improve the scintillation characteristics of MOCVD GaN by a wet etching method. An additional blue photo-luminescence (B-PL) band peaking at 2.7-2.9 eV and related to dislocations was discovered. This B-PL band intensity appeared to be dependent on wet etching exposure. The intensity of the B-PL was considerably enhanced when recorded at rather low temperatures. This finding resembles PL thermal quenching of B-PL centers. The mechanisms of scintillation intensity and spectrum variations were examined by coordinating the complementary photo-ionization and PL spectroscopy techniques. Analysis of dislocation etch pits was additionally performed by scanning techniques, such as confocal and atomic force microscopy. It was proved that this blue luminescence band, which peaked at 2.7-2.9 eV, is related to point defects those decorate dislocation cores. It was shown that the intensity of this blue PL band was increased due to enhancement of light extraction efficiency, dependent on the surface area of either single etch-pit or total etched crystal surface.

Epitaxial lateral overgrowth (ELO) of GaN epilayers on a sapphire substrate was studied by using a laser-patterned graphene interlayer. Monolayer graphene was transferred onto the sapphire substrate using a wet transfer technique, and its quality was confirmed by Raman spectroscopy. The graphene layer was ablated using a femtosecond laser, which produced well-defined patterns without damaging the underlying sapphire substrate. Different types of patterns were produced for ELO of GaN epilayers: stripe patterns were ablated along the [1¯100]sapphire and [112¯0]sapphire directions, a square island pattern was ablated additionally. The impact of the graphene pattern on GaN nucleation was analyzed by scanning electron microscopy. The structural quality of GaN epilayers was studied by cathodoluminescence. The investigation shows that the laser-ablated graphene can be integrated into the III-nitride growth process to improve crystal quality.

This study presents the results of N-face GaN layers MOCVD grown on the 20 nm thick Al2O3 layer (ALD-AlO), as a composite of a waveguiding structure. The shortening of GaN nucleation time from 195 to 60 s revealed the appearance of Ga-face inclusions in the N-face GaN, which has improved the layers' crystal quality. It also changed surfaces' morphology and roughness which decreased from 17 divided by 22 nm to 5 nm. To determine the GaN face, the samples were rinsed in a KOH solution. Time-dependent etching experiment and scanning electron microscopy inspection revealed that the etching stops at the ALD-AlO layer. After 60 min of etching, we have not detected any V-pits on the surface (on the underneath Ga-face GaN), indicating etching suppression by the ALD-AlO layer. We have performed in-plane (11 (2) over bar0) X-ray diffraction (XRD) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging, electron-beam-induced-current (EBIC) investigations.

The remote epitaxy of GaN epilayers on GaN/sapphire templates was studied by using different graphene interlayer types. Monolayer, bilayer, double-stack of monolayer, and triple-stack of monolayer graphenes were transferred onto GaN/sapphire templates using a wet transfer technique. The quality of the graphene interlayers was examined by Raman spectroscopy. The impact of the interlayer type on GaN nucleation was analyzed by scanning electron microscopy. The graphene interface and structural quality of GaN epilayers were studied by transmission electron microscopy and X-ray diffraction, respectively. The influence of the graphene interlayer type is discussed in terms of the differences between remote epitaxy and van der Waals epitaxy. The successful exfoliation of GaN membrane is demonstrated.

Remote epitaxy via graphene has recently attracted significant attention, since it provides the possibility to lift-off the grown epitaxial layer, reuse the substrate, and produce flexible devices. However, extensive research is still necessary to fully understand the III-nitride formation on the van der Waals surface of a two-dimensional material and utilize remote epitaxy to its full potential. In this work, the growth of a GaN epilayer using a GaN/sapphire template covered with monolayer graphene is presented. Metalorganic vapor phase epitaxy is chosen to fabricate both the template and the nitride epilayer on top as a cost-effective approach toward GaN homoepitaxy. One-step and multi-step growth temperature protocols are demonstrated while paying particular attention to the graphene interface. GaN seed formation on graphene is analyzed to identify remote epitaxy. Crystalline quality improvement of the epilayer by adjusting the growth parameters is further discussed to provide useful insights into GaN growth on a GaN/sapphire template via monolayer graphene.

A systematic study of the GaN epitaxial lateral overgrowth (ELO) of the focused ion beam (FIB) patterned sapphire substrate is presented. The FIB technology with its atom at a time removal principle is a flexible and high-resolution maskless processing technique, unique to fabricate trenches of the highest quality without surface damage, debris, and cracks, therefore, best suited to investigate the improvement of the maskless GaN ELO on the patterned sapphire. Arrays of the square (size varying from 1 × 1 μm2 to 5 × 5 μm2) and rectangular (size varying from 20 × 1 μm2 to 20 × 5 μm2) trenches with the inter-trench distance varying from 1 to 5 μm, aligned parallel/perpendicular to sapphire $$\langle $$ 11 $$\overline{2}$$ 0 $$\rangle $$ direction were overgrown and investigated structurally and electrically using Raman scattering, atomic force microscopy (AFM), and electron beam induced current (EBIC). To find out the optimized ELO of the array, the threading dislocation density and the remaining strain in the ELO epilayer was analyzed. The epilayer improvement was demonstrated in a correlation between the trench size, the array geometry (the trench shape and inter-trench distance), and the epilayer thickness. A homogeneous overgrowth of the entire array was observed for the arrays of the largest trenches and the reduced inter-trench distance.

Indium nitride epilayers were grown by metalorganic chemical vapour deposition (MOCVD) on graphene/SiC substrates with different terrace widths. Photoluminescence (PL) properties in the epilayers of different thickness and along the epilayer growth direction were studied with spatial resolution using confocal microscopy. The PL properties have been linked with the structure of the epilayers studied by X-ray diffraction, transmission electron microscopy, atomic force microscopy and Raman scattering. It is revealed that InN on graphene/SiC layers with a thickness of up to similar to 2 mu m consists of chaotically oriented nanocrystals of 20 nm in diameter but tends to transform into a more homogeneous multicrystalline layer at longer deposition durations. The PL band in the top part of thick layers is peaked at 0.687 eV, closer to the band gap than in any InN epilayer grown by MOCVD before. This evidences a low density of equilibrium electrons usually deteriorating performance of InN epilayers. The PL band in thin samples or in the lower part of thick samples is shifted by 60 meV predominantly to quantum confinement in the nanocrystals.

Pulsed growth, i.e. the MOCVD growth method, when the metal precursors are delivered with interruptions, allows increasing the internal quantum efficiency (IQE) of InGaN multiple quantum well (MQW) structures. On the other hand, similar results are achieved by increasing the growth temperature. In this work, we analyze the physical mechanisms behind the IQE improvement in differently grown QWs at low and high carrier densities. At low excitations, the In0.1Ga0.9N/GaN MQW structures grown at 800 and 815 degrees C exhibited the PL efficiency of 4% and 6%, respectively. At high excitations, the benefit of higher growth temperature disappeared: peak IQE at the droop onset was similar to 34% in all structures. Moreover, the structure grown at 815 degrees C had lower than intended indium content (9% instead of 10%) and QW width (3.5 nm instead of 4.0 nm). The pulse-grown structures, while very sensitive to interruption duration, provided peak IQE up to 40% without decline in structural quality. From the provided photoluminescence, carrier lifetime and diffusion coefficient data we speculate that higher growth temperature leads to lower point defect density, which increases IQE at low excitations, but becomes saturated at high excitations. Meanwhile, the pulsed growth mainly affects the localizing potential, which governs carrier recombination pathways.

Room-temperature luminescence properties of a dense electron-hole plasma were studied in a set of GaN epilayers with thickness varying from 2 to 25 mu m. The stimulated emission threshold was measured by photoluminescence and light-induced transient grating techniques under short-pulse excitation. Both techniques revealed the stimulated emission threshold increase with layer thickness despite the reduction of defect density. Numerical modeling of photoexcited carrier dynamics showed the different roles of carrier and photon populations. The stimulated emission threshold is mainly determined by the photon dynamics with weak influence of nonradiative carrier recombination. Increasing layer thickness results in the decreasing overlap of the gain layer with the optical mode, which reduces the transfer of energy from carriers to photons.

InGaN/GaN multiple quantum wells (MQWs) with the same indium content and QW widths were grown by metalorganic chemical vapor deposition using metal precursor flow interruptions of different length. The influence of the growth interruption length on photoluminescence (PL) properties was studied by combining temperature-dependent and spatially-resolved PL spectroscopy. The PL band variations are attributed to the transformation of localizing potential: growth interruptions change the density of indium clusters, which modifies the density of localized states. The optimized localizing potential in the structure grown using 9 s-long growth interruptions facilitated an increase in PL intensity by a factor of up to 2.

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al _2 O _3 layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al _2 O _3 ALD layer, the interface quality of the waveguide structure.

Carrier dynamics were studied in InN, which is a promising material for radiation-resistant optoelectronic devices. InN epilayers with different background electron densities were prepared by metalorganic chemical vapor deposition and irradiated with protons. Application of the light-induced transient grating technique in the sub-picosecond domain allowed to study the carrier lifetime and diffusion coefficient as a function of the total carrier density (intrinsic, photoexcited, and irradiation-induced). The dominance of the direct Auger process in carrier recombination was revealed. The irradiation of InN by protons, such as those encountered in space applications, decreases the carrier lifetime while simultaneously increasing the diffusion coefficient (up to 190 cm(2)/s at the highest total carrier density studied). As a result, the average carrier diffusion length does not decrease below similar to 140 nm, and exceeds the light absorption depth of InN. The lifetime damage factor was determined, and the lifetime dependence on the displacement damage dose was measured. The radiation resistance of InN surpasses that of GaAs, InGaAs, and InGaN. InN-based photovoltaic devices may work for long space missions. (C) 2019 Elsevier B.V. All rights reserved.

Spectral dependence of terahertz emission is a sensitive tool to analyze the structure of conduction band of semiconductors. In this work, we investigate the excitation spectra of THz pulses emitted from MOCVD-grown InN and InGaN epitaxial layers with indium content of 16%, 68%, and 80%. In InN and indium-rich InGaN layers we observe a gradual saturation of THz emission efficiency with increasing photon energy. This is in stark contrast to other III-V semiconductors where an abrupt drop of THz efficiency occurs at certain photon energy due to inter-valley electron scattering. From these results, we set a lower limit of the intervalley energy separation in the conduction band of InN as 2.4 eV. In terms of THz emission efficiency, the largest optical-to-THz energy conversion rate was obtained in 75 nm thick In0.16Ga0.84N layer, while lower THz emission efficiency was observed from InN and indium-rich InGaN layers due to the screening of built-in field by a high-density electron gas in these materials.

Thin films of InN were grown on sapphire by metalorganic chemical vapor deposition using multi-step growth technique (thermal cleaning and nitridation of sapphire substrate, deposition and annealing of nucleation layer, and growth of the active InN layer). The effects of different growth steps on the structural quality and luminescence properties of the samples grown for the study were analyzed by X-ray structural analysis (ω-2Θ scans, measurement of rocking curves) and photoluminescence spectroscopy with sub-micrometer spatial resolution enabled by applying confocal microscopy. The influence of nitridation duration, variation of thickness and annealing conditions of nucleation layer, and the temperature of deposition of the active InN layer were studied and discussed.