Search Results (56)

Direct chemical vapor deposition (CVD) growth of hexagonal boron nitride (h-BN) on insulating substrates offers a promising pathway to circumvent transfer-induced defects and enhance device integration. This comprehensive review systematically evaluates recent advances in CVD techniques for h-BN synthesis on insulating substrates, including metal–organic CVD (MOCVD), low-pressure CVD (LPCVD), atmospheric-pressure CVD (APCVD), and plasma-enhanced CVD (PECVD). Key challenges, including precursor selection, high-temperature processing, achieving single-crystalline films, and maintaining phase purity, are critically analyzed. Special emphasis is placed on comparative performance metrics across different growth methodologies. Furthermore, crucial research directions for future development in this field are outlined. This review aims to serve as a reference for advancing h-BN synthesis toward practical applications in next-generation electronic and optoelectronic devices. Full article

Micro-size light-emitting diodes (μLEDs) are high-brightness, low-power optoelectronic devices with significant potential in display technology, lighting, and biomedical applications. AlGaInP-based red LEDs experience severe size-dependent effects when scaled to the micron level, and addressing the fabrication challenges of GaN-based red μLED arrays is crucial for achieving homogeneous integration. This study investigates the employment of KOH wet treatments to alleviate efficiency degradation caused by sidewall leakage currents. GaN-based red μLED arrays with pixel sizes ranging from 5 × 5 µm² to 20 × 20 µm² were grown using metal-organic chemical vapor deposition (MOCVD), and then fabricated via rapid thermal annealing, mesa etching, sidewall wet treatment, electrode deposition, sidewall passivation, chemical-mechanical polishing, and via processes. The arrays, with pixel densities ranging from 668 PPI (Pixel Per Inch) to 1336 PPI, consist of 10,000 to 40,000 emitting pixels, and their optoelectronic properties were systematically evaluated. The arrays with varying pixel sizes fabricated in this study were subjected to three distinct processing conditions: without KOH treatment, 3 min of KOH treatment, and 5 min of KOH treatment. Electrical characterization reveals that the 5-min KOH treatment significantly reduces leakage current, enhancing the electrical performance, as compared to the samples without KOH treatment or 3-min treatment. In terms of optical properties, while the arrays without any KOH treatment failed to emit light, the ones with 3- and 5-min KOH treatment exhibit excellent optical uniformity and negligible blue shift. Most arrays treated for 5 min demonstrate superior light output power (LOP) and optoelectronic efficiency, with the 5 µm pixel arrays exhibiting unexpectedly high performance. The results suggest that extending the KOH wet treatment time effectively mitigates sidewall defects, reduces non-radiative recombination, and enhances surface roughness, thereby minimizing optical losses. These findings provide valuable insights for optimizing the fabrication of high-performance GaN-based red μLEDs and contribute to the development of stable, high-quality small-pixel μLEDs for advanced display and lighting applications. Full article

N-polar GaN HEMTs feature a natural back-barrier and enable the formation of low-resistance Ohmic contacts, with the potential to suppress short-channel effects and current collapse effects at sub-100 nm gate lengths, rendering them particularly promising for high-frequency communication applications. In this study, N-polar GaN films were grown on C-face SiC substrates with a 4° misorientation angle via MOCVD. By employing a two-step growth process involving LT-GaN or LT-AlGaN, the surface roughness of N-polar GaN films was reduced to varying degrees, accompanied by an improvement in crystalline quality. The growth processes, including surface morphology at each growth stage, such as the AlN nucleation layer, LT-GaN, LT-AlGaN, and the initial 90 nm HT-GaN, were investigated. The results revealed that a high V/III ratio and low-temperature growth conditions for the low-temperature layers, along with the introduction of a minor amount of Al, influenced adatom migration behavior and facilitated the suppression of step bunching. Suppressing step bunching during the initial growth stages was demonstrated to be critical for improving the surface quality and crystalline quality of N-polar GaN films. An N-polar GaN HEMT epitaxial structure was successfully achieved using the optimized surface morphology with a dedicated Fe-doped buffer process. Full article

A GaN layer with a thickness of 2 µm was grown on a sapphire substrate using atmospheric pressure metal–organic chemical vapor deposition (AP-MOCVD). Subsequently, the layer was annealed under a nitrogen atmosphere at temperatures ranging from 1000 °C to 1120 °C. High-resolution X-ray diffraction (HRXRD) analysis reveals the impact of thermal annealing on the mosaic structure of the GaN, specifically the tilt and twist variations in four planes: (00.2), (10.3), (10.2), and (10.1). Interestingly, the observed trends suggest a differential effect of annealing on screw and edge dislocation densities. The annealing process reduces the edge and screw dislocation density. Lower values (D_screw = 1.2 × 10⁸ cm⁻²; D_edge = 1.6 × 10⁹ cm⁻²) were obtained for the sample annealed at 1050 °C. Notably, both tilt and twist angles exhibited a minimum at 1050 °C (tilt = 252 arcsecs, and twist = 558 arcsecs), indicating improved crystal quality at this specific temperature. Photoluminescence (PL) spectroscopy further complemented the structural analysis. The intensity and broadening of the yellow band (YL) in the PL spectra progressively increased with the increasing annealing temperature, suggesting the presence of additional defect states. The near band edge PL emission (3.35 and 3.41 eV) variation upon thermal annealing was correlated with the mosaic structure evolution. Full article

In this paper, we report the growth of high-quality ${\mathrm{In}}_{0.59}{\mathrm{Ga}}_{0.41}\mathrm{As}/{\mathrm{In}}_{0.37}{\mathrm{Al}}_{0.63}\mathrm{As}$ strain-balanced quantum cascade lasers (QCLs) in the low-pressure MOCVD production type multi-wafer planetary reactor addressing, in particular, quality and scaled manufacturing issues. Special attention was given to achieving the sharp interfaces (IFs), by optimizing the growth interruptions time and time of exposure of InAlAs layer to oxygen contamination in the reactor, which all result in extremely narrow IFs width, below 0.5 nm. The lasers were designed for emission at $7.7\mu$m. The active region was based on diagonal two-phonon resonance design with 40 cascade stages. For epitaxial process control, the High Resolution X-Ray Diffraction (HR XRD) and Transmission Electron Microscopy (TEM) were used to characterize the structural quality of the QCL samples. The grown structures were processed into mesa Fabry-Perot lasers using dry etching RIE ICP processing technology. The basic electro-optical characterization of the lasers is provided. We also present results of Green’s function modeling of QCLs and demonstrate the capability of non-equilibrium Green’s function (NEGF) approach for sophisticated, but still computationally effective simulation of laser’s characteristics. The sharpness of the grown IFs was confirmed by direct measurements of their chemical profiles and as well as the agreement between experimental and calculated wavelength obtained for the bandstructure with ideally abrupt (non-graded) IFs. Full article

Lanthanum oxide (La₂O₃) layers are widely used in electronics, optics, and optoelectronics due to their properties. Lanthanum oxide is also used as a dopant, modifying and improving the properties of other materials in the form of layers, as well as having a large volume. In this work, lanthanum oxide layers were obtained using MOCVD (Metalorganic Chemical Vapor Deposition) on the inner walls of tubular substrates at 600–750 °C. The basic reactant was La(tmhd)₃ (tris(2,2,6,6-tetramethyl-3,5-heptanedionato)lanthanum(III)). The evaporation temperature of La(tmhd)₃ amounted to 170–200 °C. Pure argon (99.9999%) and air were used as the carrier gases. The air was also intended to remove the carbon from the synthesized layers. Tubes of quartz glass were used as the substrates. La₂O₃ layers were found to be growing on their inner surfaces. The value of the extended Gr_x/Re_x² criterion, where Gr—Grashof’s number, Re—Reynolds’ number, x—the distance from the gas inflow point, was below 0.01. The microstructure of the deposited layers of lanthanum oxide was investigated using an electron scanning microscope (SEM). Their chemical composition was analyzed via energy-dispersive X-ray (EDS) analysis. Their phase composition was tested via X-ray diffraction. The transmittance of the layers of lanthanum oxide was determined with the use of UV-Vis spectroscopy. The obtained layers of lanthanum oxide were characterized by a nanocrystalline microstructure and stable cubic structure. They also exhibited good transparency in both ultraviolet (UV) and visible (Vis) light. Full article

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of Al_xGa_1−xN films with high Al fractions (60–87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal–organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E₂(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) Al_xGa_1−xN films possess large energy gaps E_g in the range of 5.0–5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20–300 K of Al_0.87Ga_0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1–2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field. Full article

Beta-phase gallium oxide (β-Ga₂O₃) is a cutting-edge ultrawide bandgap (UWBG) semiconductor, featuring a bandgap energy of around 4.8 eV and a highly critical electric field strength of about 8 MV/cm. These properties make it highly suitable for next-generation power electronics and deep ultraviolet optoelectronics. Key advantages of β-Ga₂O₃ include the availability of large-size single-crystal bulk native substrates produced from melt and the precise control of n-type doping during both bulk growth and thin-film epitaxy. A comprehensive understanding of the fundamental growth processes, control parameters, and underlying mechanisms is essential to enable scalable manufacturing of high-performance epitaxial structures. This review highlights recent advancements in the epitaxial growth of β-Ga₂O₃ through various techniques, including Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), Mist Chemical Vapor Deposition (Mist CVD), Pulsed Laser Deposition (PLD), and Low-Pressure Chemical Vapor Deposition (LPCVD). This review concentrates on the progress of Ga₂O₃ growth in achieving high growth rates, low defect densities, excellent crystalline quality, and high carrier mobilities through different approaches. It aims to advance the development of device-grade epitaxial Ga₂O₃ thin films and serves as a crucial resource for researchers and engineers focused on UWBG semiconductors and the future of power electronics. Full article

HgCdTe is a well-known material for state-of-the-art infrared photodetectors. The interd-iffused multilayer process (IMP) is used for Metal–Organic Chemical Vapor Deposition (MOCVD) of HgCdTe heterostructures, enabling precise control of composition. In this method, alternating HgTe and CdTe layers are deposited, and they homogenize during growth due to interdiffusion, resulting in a near-uniform material. However, the relatively low (350 °C) IMP MOCVD growth temperature may result in significant residual compositional inhomogeneities. In this work, we have investigated the residual inhomogeneities in the IMP-grown HgCdTe layers and their influence on material properties. Significant IMP growth-related oscillations of composition have been revealed in as-grown epilayers with the use of a high-resolution Secondary Ion Mass Spectroscopy (SIMS). The oscillations can be minimized with post-growth annealing of the layers at a temperature exceeding that of growth. The electric and photoelectric characterizations showed a significant reduction in the background doping and an increase in the recombination time, which resulted in dramatic improvement of the spectral responsivity of photoconductors. Full article

A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm². The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics. Full article

InGaN-based red micro-light-emitting diodes (µLEDs) of different sizes were prepared in this work. The red GaN epilayers were grown on 4-inch sapphire substrates through metal-organic chemical vapor deposition (MOCVD). Etching, sidewall treatment, and p- and n-contact deposition were involved in the fabrication process. Initially, the etching process would cause undesirable damage to the GaN sidewalls, which leads to an increase in leakage current. Hence, we employed KOH wet treatment to rectify the defects on the sidewalls and conducted a comparative and systematic analysis of electrical as well as optical properties. We observed that the µLEDs with a size of 5 µm exhibited a substantial leakage current, which was effectively mitigated by the application of KOH wet treatment. In terms of optical performance, the arrays with KOH demonstrated improved light output power (LOP). Additionally, while photoelectric performance exhibited a decline with increased current density, the devices treated with KOH consistently outperformed their counterparts in terms of optoelectronic efficiency. It is noteworthy that the optimized devices displayed enhanced photoelectric characteristics without significantly altering their original peak wavelength and FWHM. Our findings point to the elimination of surface non-radiative recombination by KOH wet treatment, thereby enhancing the performance of small-sized red µLEDs, which has significant potential in realizing full-color micro-displays in near-eye projection applications. Full article

Passivation is commonly used to suppress current collapse in AlGaN/GaN HEMTs. However, the conventional PECV-fabricated SiN_x passivation layer is incompatible with the latest process, like the “passivation-prior-to-ohmic” method. Research attention has therefore turned to high-temperature passivation schemes. In this paper, we systematically investigated the differences between the SiN_x/GaN interface of two high-temperature passivation schemes, MOCVD-SiN_x and LPCVD-SiN_x, and investigated their effects on the ohmic contact mechanism. By characterizing the device interface using TEM, we reveal that during the process of MOCVD-SiN_x, etching damage and Si diffuses into the semiconductor to form a leakage path and reduce the breakdown voltage of the AlGaN/GaN HEMTs. Moreover, N enrichment at the edge of the ohmic region of the LPCVD-SiN_x device indicates that the device is more favorable for TiN formation, thus reducing the ohmic contact resistance, which is beneficial to improving the PAE of the device. Through the CW load-pull test with drain voltage V_DS = 20V, LPCVD-SiN_x devices obtain a high PAE of 66.35%, which is about 6% higher than MOCVD-SiN_x devices. This excellent result indicates that the prospect of LPCVD-SiN_x passivation devices used in 5G small terminals will be attractive. Full article

Extensive research into two-dimensional transition metal dichalcogenides (2D-TMDCs) over the past decade has paved the way for the development of (opto)electronic devices with enhanced performance and novel capabilities. To realize devices based on 2D-TMDC layers, compatible and optimized technologies such as layer transfer and photolithography are required. Challenges arise due to the ultrathin, surface-only nature of 2D layers with weak van der Waals adhesion to their substrate. This might potentially compromise their integrity during transfer and photolithography processes, in which prolonged exposure at usually high temperature to reactive chemicals and strong solvents are conventionally used. In this paper, we show that employing a dry-transfer technique based on thermal release tape (TRT) as an alternative to wet processes based on KOH solution better preserves layer quality. In the succeeding device fabrication process, an optimized photolithography as a cost-effective and widely available method for device patterning is utilized. The introduced photolithography protocol presents a near-perfect yield and reproducibility. To validate our optimized techniques, we fabricated field-effect transistors (FETs) using 2D-MoS₂ layers from metal–organic chemical vapor deposition (MOCVD), wet- and dry-transferred onto SiO₂/Si substrates. Our findings mark a significant stride towards the efficient and industry-compatible utilization of 2D van der Waals materials in device fabrication. Full article

Metal–organic chemical vapor deposition (MOCVD) is a key method for scalable synthesis of two-dimensional transition metal dichalcogenide (2D-TMDC) layers. However, it faces several challenges, such as the unintentional co-deposition of carbon impurities resulting from the pyrolysis of metal–organic precursors. This study investigates the chemical features of carbon and its impact on the photoluminescence property and air stability of 2D-MoS₂. Using X-ray photoemission spectroscopy (XPS), it was found that the carbon impurities show characteristics similar to those of sp²-bonded graphitic carbon. Upon prolonged (20–40 weeks) exposure to the atmosphere, the incorporated carbon appears to react with 2D-MoS_2, forming a MoS_2−xC_x solid solution. At the same time, a gradual decrease in the S/Mo ratio implies the formation of sulfur vacancies was also observed. These two processes lead to crystal degradation over time, as evidenced by the gradual quenching of the Raman and photoluminescence (PL) peaks. More detailed PL analyses suggest a charge transfer mechanism between sp²-carbon/2D-MoS₂ and 2D-MoS₂/air-adsorbates, which, in the short term, could alter PL emissions and appear to further intensify the degradation of 2D-MoS₂ in the long-term. The findings highlight the strong impact of unintentionally co-deposited carbon on the optical properties and air stability of MOCVD 2D-MoS₂ layers. Full article

The monoclinic structures of vanadium dioxide are widely studied as appealing systems due to a plethora of functional properties in several technological fields. In particular, the possibility to obtain the VO₂ material in the form of thin film with a high control of structure and morphology represents a key issue for their use in THz devices and sensors. Herein, a fine control of the crystal habit has been addressed through an in-depth study of the metal organic chemical vapor deposition (MOCVD) synthetic approach. The focus is devoted to the key operative parameters such as deposition temperature inside the reactor in order to stabilize the P2₁/c or the C2/m monoclinic VO₂ structures. Furthermore, the compositional purity, the morphology and the thickness of the VO₂ films have been assessed through energy dispersive X-ray (EDX) analyses and field-emission scanning electron microscopy (FE-SEM), respectively. THz time domain spectroscopy is used to validate at very high frequency the functional properties of the as-prepared VO₂ films. Full article