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Large-area epitaxial growth of III–V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1 × 109 cm−2 is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO2/Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm−2/(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III–V thin films on van der Waals substrates. [ABSTRACT FROM AUTHOR]

Reduction in the size, weight, and power (SWaP) consumption of an infrared (IR) detection system is one of the critical challenges lying ahead for the development of IR detector technology, especially for mid-/long-wavelength IR wave bands, which calls for high operating temperature (HOT) IR photodetectors (PDs) with good sensitivity that would ease the burden for cooling systems. Emerging as strong competitors to HgCdTe detectors, antimonide (Sb)-based IR PDs and focal plane array (FPA) imagers have gradually stepped into real-world applications after decades of development thanks to their outstanding material properties, tunability of cutoff wavelengths, feasibility of device designs, and great potential for mass production with low costs. Meanwhile, the emerging demands of versatile applications seek fast, compact, and smart IR detection systems, in which the integration of Sb-based IR PDs on a Si platform enables direct information readout and processing with Si-based microelectronics. This paper reviews recent progress in Sb-based HOT IR PDs and FPAs, including the fundamental material properties and device designs based on bulk InAsSb, InAs/GaSb, and InAs/InAsSb type-II superlattices, together with the cutting-edge performance achieved. This work also covers new trends of development in Sb-based IR PDs, such as optical engineering for signal harvesting, photonic integration techniques, as well as metal organic chemical vapor deposition growth of antimonides. Finally, challenges and possible solutions for future studies are provided from the perspectives of material growth, device design, and imaging systems. New advances in response to these existing challenges may cast light on designs and strategies for achieving HOT devices at thermoelectric cooling temperatures (yet with lower costs), and more extensive emerging applications may be found. [ABSTRACT FROM AUTHOR]

Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe2) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe2 ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1000 up to 100 000 leads to less defect-related PL at low temperatures. [ABSTRACT FROM AUTHOR]

The spinel zinc gallate ZnGa2O4 stands out among the emerging ultra-wide bandgap (∼5 eV) semiconductors as the ternary complex oxide with the widest gap where bipolar conductivity has been demonstrated. For power and energy electronics, a fundamental property of the material is its critical electric field ( E CR ) although, for ZnGa2O4, is yet unknown. In this work, highly resistive p -type ZnGa2O4 thin films on sapphire and Si substrates were grown by metal organic chemical vapor deposition to determine both, the remote acceptor concentration and vertical breakdown voltage. Hall Effect measurements confirmed a low carrier concentration at room temperature of ∼1011 cm−3. From vertical metal-semiconductor-metal structures the average E CR has been estimated to be of at least 5.3 MV cm−1, which already is significantly larger than the one of SiC and GaN. [ABSTRACT FROM AUTHOR]

Lateral epitaxial growth in vertical β-Ga2O3 nanowires was studied on sapphire substrates via metal organic chemical vapor deposition by controlling growth temperature and O2 flow rate. The lateral epitaxial growth rate of β-Ga2O3 rise up with increasing growth temperature and decreasing O2 flow rate, which are related to reaction kinetics and parasitic reaction, respectively. In addition, a growth model is proposed to explain the morphology of nanowires when growth condition changes. These results can provide important guidance on addressing slow coalescence rate of β-Ga2O3 nuclear islands on heterogeneous substrates and open new doors to β-Ga2O3 optoelectronic and electronic devices on cost-effective foreign substrates. [ABSTRACT FROM AUTHOR]

GaN-based high electron mobility transistors (HEMT) on Si (111) substrates have large potential for applications in the 5G telecommunication field. However, for this potential to be fully realized, all loss mechanisms need to be minimized. It is known that typical metal-organic chemical vapor deposition (MOCVD) processes used to grow the GaN epitaxial layers can cause considerable parasitic conductivity at the interface of the AlN nucleation layer to the high-resistivity Si substrate, leading to reduced gain and power added efficiency in amplifiers. Reducing this parasitic conductivity is hence of utmost importance to render GaN-on-Si a significant contributor to next-generation 5G power amplifier technology. In this work, we employ secondary ion mass spectroscopy, spreading resistance profiling and insertion loss measurements up to 28 GHz using coplanar waveguides fabricated on the epitaxial layer stacks to study the origin and characterize the parasitic conductivity. While a single heat-up process in an AIXTRON G5+ reactor chamber cleaned using Cl2 does not introduce any extra dopants in the Si substrate, the epitaxial growth of (Al,Ga)N-based HEMT buffer layer stacks leads to the diffusion of Al and, to a lower extent, Ga acceptors into the Si substrate. Optimization of the MOCVD process towards lower growth temperatures leads to a strong reduction of density of diffused acceptors. This reduction goes in line with a significant decrease of the insertion loss from 0.45 dB mm−1 to only 0.20 dB mm−1 at a frequency of 28 GHz. [ABSTRACT FROM AUTHOR]

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy. [ABSTRACT FROM AUTHOR]

Vertically aligned two-dimensional (2D) molybdenum disulfide nanoflowers (MoS2 NFs) have drawn considerable attention as a novel functional material with potential for next-generation applications owing to their inherently distinctive structure and extraordinary properties. We report a simple metal organic chemical vapor deposition (MOCVD) method that can grow high crystal quality, large-scale and highly homogeneous MoS2 NFs through precisely controlling the partial pressure ratio of H2S reaction gas, PSR, to Mo(CO)6 precursor, PMoP, at a substrate temperature of 250 °C. We investigate microscopically and spectroscopically that the S/Mo ratio, optical properties and orientation of the grown MoS2 NFs can be controlled by adjusting the partial pressure ratio, PSR/PMoP. It is also shown that the low temperature MOCVD (LT-MOCVD) growth method can regulate the petal size of MoS2 NFs through the growth time, thereby controlling photoluminescence intensity. More importantly, the MoS2 NFs/GaAs heterojunction flexible solar cell exhibiting a power conversion efficiency of ∼1.3% under air mass 1.5 G illumination demonstrates the utility of the LT-MOCVD method that enables the direct growth of MoS2 NFs on the flexible devices. Our work can pave the way for practical, easy-to-fabricate 2D materials integrated flexible devices in optical and photonic applications. [ABSTRACT FROM AUTHOR]

Planar UV-C light emitting diodes still suffer from low efficiency, mainly due to substrate crystalline quality, p doped conductivity and extraction efficiency. One possible way to overcome partly these issues is to realize the whole UV structure on AlGaN pyramids by selective area growth in order to benefit from the advantages of such structures, i.e. the dislocation filtering and the semi polar planes. We present here a detailed study about the epitaxy of AlGaN nano-sized pyramids by metal organic vapor phase epitaxy on patterned templates presenting different holes apertures and pitches as 1.5 μm and 4 μm or 100 nm and 250 nm respectively. While increasing the Al content, their height decreases while the thickness of the deposition on the mask increases whatever the design of the mask. Those changes of the pyramid shapes and deposition are directly linked to the properties of Al adatoms, i.e. low Al diffusion length. Using the conventional growth mode for the epitaxy of those pyramids did not permit the incorporation of Al from the base of the pyramids to their truncated apex. Its presence was concentrated on the edges and top of the pyramids. On the contrary, a pulsed growth mode, coupled with a strongly reduced pitch, allowed an incorporation of Al since the base of the nanopyramid, and a decrease of the deposition height on the mask. These results can be explained by the desorption of Ga species, due to the presence of H2 in the reactor chamber during the step without the metal precursors, leading to a higher Al/Ga ratio. It is even enhanced inside the holes by the reduced pitch. [ABSTRACT FROM AUTHOR]

Measurements of minority carrier lifetime have been carried out on nonintentionally doped HgCdTe(100) layers grown by metal organic chemical vapor deposition on GaAs substrates. The studies have been carried out in order to investigate the possibility of applications of HgCdTe(100) epilayers to fully depleted P–i–N photodiodes operated at near-room temperatures and the background-radiation limit. Post-growth annealed layers show n-type conductivity with a residual donor concentration of about 1015 cm−3. The cadmium molar composition (0.262 < x < 0.336) was chosen for mid-wave infrared devices operating at 230 K to 300 K. Results show that at high temperatures, the samples show clearly Auger-limited lifetimes determined by the background doping. The measured lifetimes at 300 K range from 5 to 11 µs, and from 0.2 to 1 µs, for the cut-offs of 4 µm and 5.4 µm, respectively. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (Vf) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the Vf is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low Vf penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs. [ABSTRACT FROM AUTHOR]

In this work, metal organic vapor phase epitaxy (MOVPE) is employed for selective-area growth (SAG) of undoped and lightly n-doped GaN micropillars on masked GaN-on-sapphire templates. In micropillar geometry, the limits of GaN drift layer thickness in hetereoepitaxial Schottky diodes are expected to be significantly pushed upwards. This is an important step towards the realization of GaN-based quasi-vertical power devices with high breakdown voltage (Vbr) on low-cost foreign substrates. Micropillar growth evolution and the impact of growth rate and V/III ratio on micropillar morphology and parasitic GaN deposition on AlOx hard masks are investigated. By using a combination of low growth rate and high V/III ratio, 12–22 µm high micropillars with planar pillar tops are grown in circular mask openings with radii of 35–100 µm. The threading dislocation density of the highest pillars is (1.4 ± 0.3) × 107 cm−2. Schottky diodes on micropillars with lowest net doping concentration (ND − NA) of 1.3 × 1016 cm−3 exhibit excellent forward characteristics with ideality factors , on/off -ratios up to 1010, and on -resistances R on 1 m cm−2. [ABSTRACT FROM AUTHOR]

We report on a feasibility study of using Raman micro-spectroscopy as a characterization method for quality control of (REBCO, rare-earth barium copper oxide) coated conductors (CC). Fragments of long REBCO tape deposited using advanced metal organic chemical vapor deposition with dropouts in critical current (Ic) were analyzed implementing Raman spectroscopy in area scan (map) and linear scan modes, in order to evaluate the level of information obtainable with the technique. The areal maps have been found to provide detailed information on the spatial distribution of the sample quality, providing maps of both intensity and wavelength shift characteristic of Raman-active peaks. The maps readily identify defective regions ranging from completely misoriented a-axis domains (REBCO a-axis || sample normal), as well as regions with well-textured c-axis orientation (REBCO c-axis || sample normal) but with significant variation in cation disorder. The results are complemented with linear scans carried out by two-dimensional x-ray diffraction, which do identify the a-axis grain regions but do not detect any apparent anomaly in the cation-disordered region detected by Raman micro-spectroscopy. The results indicate that scanning Raman spectroscopy has the potential to be utilized as a fast, insightful, and reliable characterization, quality, and process control technique for both CC optimization and increase of manufacturing yield. [ABSTRACT FROM AUTHOR]

The electric and magnetic properties of microbridges made from 440 nm thick Y–Ba–Cu–O films prepared on Al2O3 substrates with CeO2 sublayers by the pulsed injection metal organic chemical vapor deposition (MOCVD) method were studied in temperatures ranging from 20 K to 300 K in order to investigate the possibilities of using these microbridges as fast fault current limiters. The application of an external magnetic field () causes the flux flow induced magneto-resistive effect, which is proportional to and to at low and high temperatures, respectively. The experimentally obtained S-shaped I–V characteristic and the change of the microbridge resistance over time when affected by a step-like magnetic field can be well explained using a thermo-electrical model based on Joule heating and the flux flow nature of the resistive state (RS). The transition from the superconducting to the RS, studied using ns duration rectangular waveform electrical pulses able to create up to ≈2106 A cm−2 current densities without rises of microbridge temperatures, demonstrated that the I–V characteristics of the RSs consisting of an assembly of straight lines corresponding to the different pinning centers. At temperatures close to the critical temperature of superconductivity, the number of these lines increases, and the I–V characteristics can be described with high accuracy by a power law. It was concluded that microbridges made from these films can be used as protectors against complicated waveform electromagnetic pulses having short rise times, high voltage peak amplitudes and long low-value over-current 'tails'. [ABSTRACT FROM AUTHOR]

Using the mechanism of self-reactive etching between Ga and Ga2O3, Ga2O3 nanopore films were fabricated. The self-reactive etching effects based on as-grown and annealed Ga2O3 films by metal organic chemical vapor deposition were compared. It was found that the nanopore film based on as-grown Ga2O3 film has a uniform size, high density and a small diameter. Ultraviolet-visible light reflection spectra and transmission spectra show that the nanopore film could effectively reduce the reflectivity of light and enhance the light absorption. Based on the as-grown Ga2O3 film and its nanopore film, metal-semiconductor-metal structure solar blind ultraviolet photodetectors (PD) were fabricated. Under 5 V bias, the light-dark current ratio of the nanopore film PD is about 2.5 × 102 times that of the film PD, the peak responsivity of the nanopore film PD is about 49 times that of the film PD. The rejection ratio is 4.6 × 103, about 1.15 × 102 times that of the film PD. The nanopore structure effectively increases the surface-volume ratio of film. The photoelectric detection performance and response performance of the nanopore film PD could be significantly enhanced. [ABSTRACT FROM AUTHOR]

A series of InSb thin films were grown on GaAs substrates by molecular beam epitaxy (MBE). GaSb/AlInSb is used as a compound buffer layer to release the strain caused by the lattice mismatch between the substrate and the epitaxial layer, so as to reduce the system defects. At the same time, the influence of different interface structures of AlInSb on the surface morphology of buffer layer is explored. The propagation mechanism of defects with the growth of buffer layer is compared and analyzed. The relationship between the quality of InSb thin films and the structure of buffer layer is summarized. Finally, the growth of high quality InSb thin films is realized. [ABSTRACT FROM AUTHOR]

The ultra-wide bandgap semiconductor gallium oxide (Ga2O3) offers substantial promise to significantly advance power electronic devices as a result of its high breakdown electric field and maturing substrate technology. A key remaining challenge is the ability to grow electronic-grade epitaxial layers at rates consistent with 20–40 μm thick drift regions needed for 20 kV and above technologies. This work reports on extensive characterization of epitaxial layers grown in a novel metalorganic chemical vapor deposition tool that permits growth rates of 1.0–4.0 μm h−1. Specifically, optical, structural and electrical properties of epilayers grown at ∼1 μm h−1 are reported, including employment in an operating MOSFET. The films demonstrate relatively smooth surfaces with a high degree of structural order, limited point defectivity (Nd − Na ≈ 5 × 1015 cm−3) and an optical bandgap of 4.50 eV. Further, when employed in a MOSFET test structure with an n+ doped channel, a record high mobility for a transistor structure with a doped channel of 170 cm2 V−1 s−1 was measured via the Hall technique at room temperature. This work reports for the first time a β-Ga2O3 MOSFET grown using Agnitron Technology's high growth rate MOCVD homoepitaxial process. These results clearly establish a significant improvement in epilayer quality at growth rates that can support future high voltage power device technologies. [ABSTRACT FROM AUTHOR]

We propose a metal organic vapor phase epitaxy (MOVPE) method of pre-introducing TMIn during the growth of u-GaN to improve the subsequent growth of InGaN and discuss the impact of this method in detail. Monitoring the MOVPE by the interference curve generated by the laser incident on the film surface, we found that this method avoided the problem of the excessive InGaN growth rate. Further x-ray diffraction (XRD), photoluminescence (PL), and atomic force microscope (AFM) tests showed that the quality of InGaN is improved. It is inferred that by introducing TMIn in advance, the indium atom can replace the gallium atom in the reactor walls, delivery pipes, and other corners. Hence the auto-incorporation of gallium can be reduced when InGaN is grown, so as to improve the material quality. [ABSTRACT FROM AUTHOR]

High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunnel junctions (InGaN TJs) were grown by metalorganic chemical vapor deposition (MOCVD). The InGaN TJs µLEDs show a significant reduction of forward voltage (Vf) by ∼0.6 V compared to the common TJs µLEDs. The Vf at 20 A cm−2 is very low varied from 3.15 V to 3.19 V in small InGaN TJ µLEDs with a size less than 40 × 40 µm2, and then significantly increases in large LEDs. Selective area growth (SAG) of TJs can overcome such size limitation by vertical out diffusion of hydrogen through the apertures on top of p-GaN. The InGaN TJ µLEDs overgrown by SAG show a size-independent low Vf ranged from 3.08 V to 3.25 V. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with indium tin oxide (ITO) contact. This work solves the key challenges of MOCVD-grown TJs. [ABSTRACT FROM AUTHOR]

Micro light emitting diodes have been grown by metal organic vapor phase epitaxy on standard GaN and partly relaxed InGaNOS substrates with the purpose of incorporating higher concentrations of indium for identical growth conditions. Green emission has been demonstrated at wavelengths of 500 nm for the GaN template and 525 and 549 nm for the InGaNOS substrates, respectively. The structure, deformation, indium concentration and piezoelectric potentials have been measured with nm-scale spatial resolution in the same specimens by transmission electron microscopy. We show by off-axis electron holography that the piezoelectric potential and information about the indium concentration from the mean inner potential are obtained simultaneously. By separating the components using a model, we show that for higher concentrations of indium in the quantum wells (QWs) grown on InGaNOS substrates, the piezoelectric potentials are reduced. The measurements of the indium concentrations by electron holography have been verified by combining energy dispersive x-ray spectrometry, x-ray diffraction and from the tensile deformation made by precession electron diffraction. A discussion of the limitations of these advanced aberration-corrected transmission electron microscopy techniques when applied to nm-scale QW structures is given. [ABSTRACT FROM AUTHOR]

We report on the fabrication and characterization of solar-blind photodetectors based on metal organic chemical vapor deposition grown polycrystalline monoclinic indium gallium oxide alloys on sapphire using N2O for oxidation. The effects of growth conditions on indium incorporation efficiency and oxygen vacancies of the alloy, photo-to-dark current ratio (PDR), gain and responsivity of the fabricated photodetectors were investigated. The optical bandgap of the films was found to decrease due to the indium incorporation (x = 20.3%, 17.7%, 10.6% for samples A, B, and C, respectively) into the lattice of gallium oxide. By increasing the indium content incorporated into the lattice of , we demonstrated solar-blind photodetectors whose peak responsivity increased from 0.79 A/W () to 319.1 A/W, 66.1 A/W and 27.7 A/W for samples A, B and C, respectively at 5 V applied bias with the cut off wavelength below 280 nm. Increasing in content resulted in a higher concentration of oxygen vacancies in as-grown films. Increased oxygen vacancies as a result of the change in growth conditions lead to higher photoconductive gain, higher responsivities, and lower PDR, demonstrating a trade-off between responsivity and the PDR. To the best of our knowledge, the peak responsivity value reported in this work is the highest for based solar-blind photodetectors. Fast rise and fall times in the order of 100 ms have been measured for the photodetectors. [ABSTRACT FROM AUTHOR]

The ε-Ga2O3 p–n heterojunctions (HJ) have been demonstrated using typical p-type oxide semiconductors (NiO or SnO). The ε-Ga2O3 thin film was heteroepitaxial grown by metal organic chemical vapor deposition (MOCVD) with three-step growth method. The polycrystalline SnO and NiO thin films were deposited on the ε-Ga2O3 thin film by electron-beam evaporation and thermal oxidation, respectively. The valence band offsets (VBO) were determined by x-ray photoelectron spectroscopy (XPS) to be 2.17 eV at SnO/ε-Ga2O3 and 1.7 eV at NiO/ε-Ga2O3. Considering the bandgaps determined by ultraviolet-visible spectroscopy, the conduction band offsets (CBO) of 0.11 eV at SnO/ε-Ga2O3 and 0.44 eV at NiO/ε-Ga2O3 were obtained. The type-II band diagrams have been drawn for both p–n HJs. The results are useful to understand the electronic structures at the ε-Ga2O3 p–n HJ interface, and design optoelectronic devices based on ε-Ga2O3 with novel functionality and improved performance. [ABSTRACT FROM AUTHOR]

High quality 0.02 mol%, 0.05 mol%, and 0.08 mol% Fe: β-Ga2O3 single crystals were grown by the floating zone method. The crystal structure, optical, electrical, and thermal properties were measured and discussed. Fe: β-Ga2O3 single crystals showed transmittance of higher than 80% in the near infrared region. With the increase of the Fe doping concentration, the optical bandgaps reduced and room temperature resistivity increased. The resistivity of 0.08 mol% Fe: β-Ga2O3 crystal reached to 3.63 × 1011 Ω ⋅cm. The high resistivity Fe: β-Ga2O3 single crystals could be applied as the substrate for the high-power field effect transistors (FETs). [ABSTRACT FROM AUTHOR]

Mid-wavelength infrared planar photodiodes were demonstrated, in which both the epitaxy growth of InAs/GaSb superlattices and the thermal diffusion of p-type dopant were performed in production-scale metal–organic chemical vapor deposition reactors. The formation of a planar homojunction was confirmed by secondary ion mass spectroscopy and its I–V characteristics. A cut-off wavelength around 5 μm was determined in 77 K optical characterization, and photo-current as high as 600 nA was collected from a reverse-biased planar diode of 640 μm diameter. These preliminary results were obtained despite the structural degradation revealed by x-ray diffraction, and we attribute the degradation to the concert of thermal annealing and high Zn concentration behind the diffusion front. [ABSTRACT FROM AUTHOR]

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition. As the layer growth temperature increases, the cluster density decreases but their size indicted by the number of atoms increases. Moreover, APT reveals that threading dislocations are decorated with Mg atoms. Off-axis electron holography provides complementary information about the electrical activity of the Mg doping. As only a small fraction of dopant atoms are ionised at room temperature, this fraction is increased by annealing the specimen to 400 °C in situ in a transmission electron microscope (TEM). A strong reduction of the dopant electrical activity is observed for increases in the layer growth temperature. The correlation of APT with TEM-based techniques was shown to be a unique approach in order to investigate how the growth temperature affects both the chemical distribution and electrical activity of Mg dopant atoms. [ABSTRACT FROM AUTHOR]

High quality 0.02 mol%, 0.05 mol%, and 0.08 mol% Fe: β-Ga2O3 single crystals were grown by the floating zone method. The crystal structure, optical, electrical, and thermal properties were measured and discussed. Fe: β-Ga2O3 single crystals showed transmittance of higher than 80% in the near infrared region. With the increase of the Fe doping concentration, the optical bandgaps reduced and room temperature resistivity increased. The resistivity of 0.08 mol% Fe: β-Ga2O3 crystal reached to 3.63 × 1011 Ω ⋅cm. The high resistivity Fe: β-Ga2O3 single crystals could be applied as the substrate for the high-power field effect transistors (FETs). [ABSTRACT FROM AUTHOR]

Critical current (Ic) values of 1384 A/12 mm, corresponding to a critical current density of 12.47 MA cm−2 and a pinning force of 374 GN m−3, have been achieved at 30 K, 3 T in the orientation of field parallel to the c axis (B ∥ c) in (Gd, Y)BaCuO tapes with 15 mol% Zr addition made by metal organic chemical vapor deposition (MOCVD). These tapes show pinning force levels as high as 453 GN m−3 at 30 K. An analysis of the properties of 24–28 (Gd, Y)BaCuO tapes with 15 mol% Zr addition showed a lack of correlation between their critical currents at 30 K, 3 T (B ∥ c) and Ic values both at 77 K, zero field and at 77 K, 1 T (B ∥ c). However, a strong correlation was found between the critical currents at 30 K, 3 T and at 77 K, 3 T (B ∥ c). It has also been discovered that the minimum critical current (Ic, min) value at 77 K, 3 T has no influence on the Ic, min value at 30 K, 3 T, and it in turn depends on the ratio of the Ic values in the orientations of field parallel and perpendicular to the c axis at 77 K, 3 T. [ABSTRACT FROM AUTHOR]

High-performance low-leakage-current AlGaN/GaN high electron mobility transistors (HEMTs) on silicon (111) substrates grown by metal organic chemical vapor deposition (MOCVD) with a novel partially Magnesium (Mg)-doped GaN buffer scheme have been fabricated successfully. The growth and DC results were compared between Mg-doped GaN buffer layer and a unintentionally one. A 1-μm gate-length transistor with Mg-doped buffer layer exhibited an OFF-state drain leakage current of 8.3 × 10−8 A/mm, to our best knowledge, which is the lowest value reported for MOCVD-grown AlGaN/GaN HEMTs on Si featuring the same dimension and structure. The RF characteristics of 0.25-μm gate length T-shaped gate HEMTs were also investigated. [ABSTRACT FROM AUTHOR]

InGaN quantum dots (QDs) formed via spinodal and binodal decomposition are used as an optically active region for light-emitting diodes (LEDs). It is shown that the emission wavelength of the electroluminescence (EL) can be shifted from blue to green by adjusting the deposition time of the InGaN QD layer. A first approach towards a monolithic multicolour emitting diode is presented. Strong difference of the EL on the InGaN QD stacking sequence is observed and is attributed to both low hole concentration and mobility. [ABSTRACT FROM AUTHOR]

Near-ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) are grown by low-pressure metal-organic chemical vapor deposition. The scanning electronic microscope image shows that the p-GaN micro-rods are formed above the interface of p-AlGaN/p-GaN due to the rapid growth rate of p-GaN in the vertical direction. The p-GaN micro-rods greatly increase the escape probability of photons inside the LED structure. Electroluminescence intensities of the 372 nm UV LED lamps with p-GaN micro rods are 88% higher than those of the flat surface LED samples. [ABSTRACT FROM AUTHOR]

The high power GaN-based blue light emitting diode (LED) on an 80-μm-thick GaN template is proposed and even realized by several technical methods like metal organic chemical vapor deposition (MOCVD), hydride vapor-phase epitaxial (HVPE), and laser lift-off (LLO). Its advantages are demonstrated from material quality and chip processing. It is investigated by high resolution X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), Rutherford back-scattering (RBS), photoluminescence, current-voltage and light output-current measurements. The width of (0002) reflection in XRD rocking curve, which reaches 173″ for the thick GaN template LED, is less than that for the conventional one, which reaches 258″. The HRTEM images show that the multiple quantum wells (MQWs) in 80-μm-thick GaN template LED have a generally higher crystal quality. The light output at 350 mA from the thick GaN template LED is doubled compared to traditional LEDs and the forward bias is also substantially reduced. The high performance of 80-μm-thick GaN template LED depends on the high crystal quality. However, although the intensity of MQWs emission in PL spectra is doubled, both the wavelength and the width of the emission from thick GaN template LED are increased. This is due to the strain relaxation on the surface of 80-μm-thick GaN template, which changes the strain in InGaN QWs and leads to InGaN phase separation. [ABSTRACT FROM AUTHOR]

Indium phosphide nanowires were grown by metalorganic molecular beam epitaxy using the selective-area vapor–liquid–solid method. We show experimentally and theoretically that the size of the annular opening around the nanowire has a major impact on nanowire growth rate. In addition, we observed a considerable reduction of the growth rate in dense two-dimensional arrays, in agreement with a calculation of the shadowing of the scattered precursors. Due to the impact of these effects on growth, they should be considered during selective-area vapor–liquid–solid nanowire epitaxy. [ABSTRACT FROM AUTHOR]

To explain different doping effects in a buffer layer, thermally annealed interface, and upper epilayers of GaAs/Si films grown by Metalorganic Chemical Vapor Deposition (MOCVD), the behaviors of unintentional doping in GaAs/Si films are investigated in detail. A third doping mechanism of arsine impurity incorporation during the growth process of GaAs/Si films, apart from conventional mechanisms of gas phase reaction and diffusion from the silicon substrate, is proposed. The experimental results reveal that the doping behavior in the buffer layer studied is determined by the three types of doping mechanisms together. However in the thermally annealed interface and upper epilayers, the third doping mechanism is dominant. According to the third mechanism, the background carrier concentration in GaAs/Si films grown by MOCVD could be properly controlled through the arsine flow rate. [ABSTRACT FROM AUTHOR]

The authors report on advanced metalorganic chemical vapor deposition (MOCVD) grown HgCdTe device structure for an ambient temperature long wavelength infrared radiation (LWIR) detector application. MOCVD technology with a wide range of composition and donor/acceptor doping and without post grown annealing seems to be an excellent tool for HgCdTe heterostructure epitaxial growth structure. The N+ /G/π/G/P+ /G/n+ (where G denotes graded interface region) HgCdTe photovoltaic device concept of a specific barrier bandgap architecture integrated with Auger-suppression is a good solution for high operating temperature (HOT) infrared detectors. Selected problems of photodiode designing are indicated. Theoretical modelling using APSYS platform supports design and better understanding of the carrier transport mechanism in the photodiode structures. SIMS profiles allowed comparing projected and obtained structures and revealed diffusion processes in the structures. The negative differential resistance, clearly visible on current-voltage characteristics, evidences for Auger-suppression due to exclusion and extraction phenomena. It is shown that the thickness and arsenic doping of the active region influence the reverse bias dark current minimum and the response time of LWIR HOT HgCdTe photodiodes. Reverse bias highly (up to 50 times) increases responsivity that could reach 6 A/W at 300 K. [ABSTRACT FROM AUTHOR]

By using x-ray diffraction analysis, we investigate the major structural parameters such as strain state and crystal quality of non-polar a-plane InxGa1−xN thin films grown on r-sapphire substrates by metalorganic chemical vapour deposition. The results of the inplane grazing incidence diffraction technique are analyzed and compared with a complementary out-of-plane high resolution x-ray diffraction technique. When the indium composition is low, the a-plane InxGa1−xN layer is tensile strain in the growth direction (a-axis) and compressive strain in the two in-plane directions (m-axis and c-axis). The strain status becomes contrary when the indium composition is high. The stress in the m-axis direction σyy is larger than that in the c-axis direction σzz. Furthermore, strain in the two in-plane directions decrease and the crystal quality becomes better with the growing of the InxGa1−xN film. [ABSTRACT FROM AUTHOR]

Electronic transport properties of InAs nanowires are studied systematically. The nanowires are grown by molecular beam epitaxy on a SiOx-covered GaAs wafer, without using foreign catalyst particles. Room-temperature measurements revealed relatively high resistivity and low carrier concentration values, which correlate with the low background doping obtained by our growth method. Transport parameters, such as resistivity, mobility, and carrier concentration, show a relatively large spread that is attributed to variations in surface conditions. For some nanowires the conductivity has a metal-type dependence on temperature, i.e. decreasing with decreasing temperature, while other nanowires show the opposite temperature behavior, i.e. temperature-activated characteristics. An applied gate voltage in a field-effect transistor configuration can switch between the two types of behavior. The effect is explained by the presence of barriers formed by potential fluctuations. [ABSTRACT FROM AUTHOR]

The N content of a series of GaNSb samples grown by molecular-beam epitaxy is investigated using high-resolution x-ray diffraction (HRXRD) and secondary-ion mass spectrometry. The N contents determined by the two methods agree well at lower N compositions (x < 0.01), and deviate significantly at larger N contents (x > 0.02). Analysis of the HRXRD-determined lattice constant using Vegard's law, underestimates the N content for high N compositions. The underestimation is found to be up to 24% for x = 0.03. The variation of the lattice parameter with N content is modelled by considering the influence of the density of the interstitial N-Sb complex rising with increasing N content. [ABSTRACT FROM AUTHOR]

InGaN/GaN multiple quantum well (MQW) solar cells with stepped-thickness quantum wells (SQW) are designed and grown by metal-organic chemical vapor deposition. The stepped-thickness quantum wells structure, in which the well thickness becomes smaller and smaller along the growth direction, reveals better crystalline quality and better spectral overlap with the solar spectrum. Consequently, the short-circuit current density (Jsc) and conversion efficiency of the solar cell are enhanced by 27.12% and 56.41% compared with the conventional structure under illumination of AM1.5G (100 mW/cm²). In addition, approaches to further promote the performance of InGaN/GaN multiple quantum well solar cells are discussed and presented. [ABSTRACT FROM AUTHOR]

YBa2Cu3O7-x (YBCO) films with embedded BaZrO3 and BaTiO3 nanoparticles were fabricated by metalorganic deposition using trifluoroacetates (TFA-MOD). Both X-ray diffraction and transmission electron microscopy revealed that these BaZrO3 and BaTiO3 nanoparticles had random orientations and were distributed stochastically in the YBCO matrix. The unique combined microstructure enhances the critical current density (Jc) of the BaZrO3/BaTiO3 doped-YBCO films, while keeping the critical transition temperature (Tc) close to that in the pure YBCO films. These results indicate that BaZrO3 and BaTiO3 nanoparticles provide strong flux pinning in YBCO films. [ABSTRACT FROM AUTHOR]

III-nitride (III-N) heterojunction bipolar transistors (HBTs) are a less-explored electronic device technology due to the myriad research issues in material growth, device design and fabrication associated with these devices. For III-N HBTs, npn-GaN/InGaN heterostructures provide the benefits of mitigating the poor base electrical conductivity of p-type GaN and the problematic magnesium incorporation issues. Consequently, InGaN-base III-N HBTs are promising for next-generation high-power RF III-N systems. This paper will describe the current development status of npn GaN/InGaN HBTs grown either on sapphire or free-standing (FS) GaN substrates using optimized metalorganic chemical vapor deposition (MOCVD) and refined HBT processing techniques. Recombination current paths in GaN/InGaN HBTs are studied and small-signal equivalent circuits are developed. The extracted device model indicates that, with further device fabrication technique development, Johnson's figure of merit (JFOM) of GaN/InGaN HBTs can be as high as 5 THz V. [ABSTRACT FROM AUTHOR]

Four types of AlGaN/GaN high electron mobility transistor (HEMT) structures have been epitaxially grown on Si substrates by metalorganic chemical vapor deposition (MOCVD) and fabricated into devices. To achieve crack-free device structures, various stress-engineering methods have been employed including the use of AlGaN/AlGaN-graded layers, insertion of low-temperature AlN layers and ion implantation of the AlN/Si substrate. To improve material quality, pulsed MOCVD is used to enhance adatom diffusion length during (Al) GaN epitaxy of various layers in the HEMT structure. A comparison between structural and morphological characteristics of the HEMTs shows improvement in the (0 0 0 2) symmetric rocking curve value to 837.9 s-1 and the surface roughness of 0.21 nm for HEMT structures grown using pulsed epitaxy. An OFF-state breakdown voltage of 217 V at a drain current of 1 mA mm-1 at Vg=-8 V was measured for the structure with enhanced material quality. [ABSTRACT FROM AUTHOR]

We report on the Au-assisted vapour--liquid--solid (VLS) growth of GaAs/InxGa1-xAs/GaAs (0.2 ≤ x ≤ 1) axial double-heterostructure nanowires on GaAs (111) B substrates via the metal--organic chemical vapor deposition (MOCVD) technique. The influence of the indium (In) content in an Au particle on the morphology of nanowires is investigated systematically. A short period of pre-introduced In precursor before the growth of InxGa1-x As segment, coupled with a group III precursor interruption, is conducive to obtaining symmetrical heterointerfaces as well as the desired In/Ga ratio in the InxGa1-x As section. The nanowire morphology, such as kinking and tapering, are thought to be related to the In composition in the catalyst alloy as well as the VLS growth mechanism. [ABSTRACT FROM AUTHOR]

The growth mechanism of catalyst-free self-organized GaN nuclei and three-dimensional columns on sapphire by metal organic vapour phase epitaxy (MOVPE) is investigated. Temperature- and time-dependent growth is performed. The growth behaviour can be characterized by two different kinetic regimes: mass-transport-limited growth and thermodynamically limited growth. The sum of activation energies for thermodynamic barrier of nucleation and for surface diffusion/mass-transport limitation, i.e. Whet + Ed, is 0.57 eV in the 'low'-temperature region and 2.43 eV in the 'high'-temperature region. GaN columns grown under the same conditions have very comparable height, which is not dependent on their diameter or the distance to other columns. Therefore, the growth rate is presumably limited by the incorporation rate on the top surface of columns. The height and diameter at the top of the GaN columns increase linearly with time and no height limit is observed. The GaN columns can reach more than 40 µm in height. Moreover, the investigated GaN columns are Ga-polar. [ABSTRACT FROM AUTHOR]

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm. [ABSTRACT FROM AUTHOR]

Superconducting FeSe thin films have been deposited onto (001)-oriented single-crystalline MgO substrates by conventional radio-frequency magnetron co-sputtering of a FeSe target and a pure Se target. The composition of the films is within the homogeneity range of the tetragonal β phase close to stoichiometry. Films with a minimum residual resistivity and a maximum superconducting transition temperature can be prepared within a narrow substrate temperature range from 480 to 510 °C. In contrast to the equilibrium phase diagram the β phase surprisingly exists in thin films deposited at substrate temperatures as high as 680 °C. The polycrystalline tetragonal films grow with a preferred c-axis texture and a tiny admixture of misaligned grains. The temperature-dependent resistivity reveals a low-temperature linear part and a high-temperature part with a pronounced negative curvature. The origin of the non-Fermi liquid behaviour and the underlying electron scattering mechanisms are still unknown so far. [ABSTRACT FROM AUTHOR]

Trifluoroacetate metal–organic deposition (TFA-MOD) is a promising technique to fabricate YBa2Cu3O7-δ (YBCO) superconducting films. However, its slow pyrolysis process, which usually takes more than 10 h, constitutes a barrier for industrial production. In this study, polyethylene glycol (PEG) was utilized to reduce the stress generation inside the coated films when the strong pyrolysis reactions happen. With the addition of 30 wt% PEG2000 to the precursor solution, a smooth film surface could be obtained through a rapid pyrolysis process of 15 min. After the optimizations of the crystallization and oxygenation processes, mass percentage and molecular weight of PEG additive, YBCO thin films with Jc of about 4:5 MA cm-2 (77 K, self-field) could be routinely fabricated using (20-30) wt% PEG(1000-2000) additive with a total treatment time of about 2 h including the 15 min pyrolysis process time. The effects of PEG additive were discussed using one of the mechanisms of buckling formation. The reduction of compressive stress by PEG additive was suggested to be the reason for preventing buckling. [ABSTRACT FROM AUTHOR]

The intrinsic strain effect on critical current, Ic in Cu-stabilized GdBCO coated conductor (CC) tapes under bending and uniaxial tension has been investigated. The bending deformation tolerance of Ic in GdBCO CC tapes, fabricated by reactive co-evaporation by deposition and reaction (RCE-DR) with substrate materials of Hastelloy and stainless steel and fabricated by the metal organic chemical vapour deposition (MOCVD) process was evaluated. The intrinsic strain response of Ic under bending was found to be independent of the fabrication process, the substrate material and the geometry of the sample. For samples with a Hastelloy substrate, the intrinsic strain response of Ic/Icmax under bending was well correlated with those under uniaxial tension. However, for samples with a stainless steel substrate, these had a large strain sensitivity for Ic under uniaxial tension even though this showed a much higher irreversible strain limit up to 1.05%. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) nanostructured materials have attracted a great deal of interest owing to their potential applications in various industries. Due to the limitations and cost associated with conventional low-pressure plasma systems, atmospheric-pressure plasma techniques such as dielectric barrier discharges (DBDs) are investigated as an alternative approach for inducing specific chemical reactions. RuO2 nanomaterials are widely used as supercapacitor electrodes, in field-emission devices and for catalytic applications. In such applications, size and shape dependent properties of nanomaterials play critical roles in improving the performance. In this paper, an attempt is made to prepare 1D RuO2 nanostructured materials using a DBD plasma. It is reported here that the composition of feed gas is an important factor in determining the final morphology. For example, an Ar + H2 plasma yields aggregated RuO2 nanostructures, whereas 'nanopillar' and 'nanorod' morphologies are obtained when using Ar + O2 and Ar, respectively. Possible mechanisms behind the morphological differences are elucidated on the basis of the temperature variations inside the plasma reactor and the chemistry of the gaseous reactive species. The application of a DBD plasma to the synthesis of RuO2 nanorods is reported for the first time in this paper. [ABSTRACT FROM AUTHOR]

An exponential-doping structure is successfully applied to the preparation of AlGaAs/GaAs photocathodes through the metalorganic chemical vapor deposition (MOCVD) technique. The experimental results show that the quantum efficiency in the entire waveband region for the exponential-doping photocathodes grown by MOCVD is remarkably enhanced as compared to those grown by molecular beam epitaxy. As a result of the improved built-in electric fields and cathode performance parameters, the photoemission characteristics for the MOCVD-grown transmission-mode and reflection-mode AlGaAs/GaAs photocathodes are different over the wavelength region of interest. [ABSTRACT FROM AUTHOR]

We report single crystal phase and non-tapered wurtzite (WZ) and zincblende twinning superlattice (ZB TSL) InP nanowires (NWs). The NWs are grown in a metalorganic vapor phase epitaxy (MOVPE) reactor using the vapor-liquid-solid (VLS) mechanism and in situ etching with HCl at a high growth temperature. Our stacking fault-free WZ and ZB TSL NWs allow access to the fundamental properties of both NW crystal structures, whose optical and electronic behaviors are often screened by polytypism or incorporated impurities. The WZ NWs show no acceptor-related emission, implying that the VLS-grown NW is almost free of impurities due to sidewall removal by HCl. They only emit light at the free exciton (1.491 eV) and the donor bound exciton transition (1.4855 eV). The ZB NWs exhibit a photoluminescence spectrum being unaffected by the twinning planes. Surprisingly, the acceptor-related emission in the ZB NWs can be almost completely removed by etching away the impurity-contaminated sidewall grown via a vapor-solid mechanism. [ABSTRACT FROM AUTHOR]