Metalorganic Chemical Vapor Deposition Process Research Articles

Lateral heterostructures of WS2 and MoS2 monolayers for photo-synaptic transistor

Von Neumann architecture-based computing, while widely successful in personal computers and embedded systems, faces inherent challenges including the von Neumann bottleneck, particularly amidst the ongoing surge of data-intensive tasks. Neuromorphic computing, designed to integrate arithmetic, logic, and memory operations, has emerged as a promising solution for improving energy efficiency and performance. This approach requires the construction of an artificial synaptic device that can simultaneously perform signal processing, learning, and memory operations. We present a photo-synaptic device with 32 analog multi-states by exploiting field-effect transistors based on the lateral heterostructures of two-dimensional (2D) WS2 and MoS2 monolayers, formed through a two-step metal–organic chemical vapor deposition process. These lateral heterostructures offer high photoresponsivity and enhanced efficiency of charge trapping at the interface between the heterostructures and SiO2 due to the presence of the WS2 monolayer with large trap densities. As a result, it enables the photo-synaptic transistor to implement synaptic behaviors of long-term plasticity and high recognition accuracy. To confirm the feasibility of the photo-synapse, we investigated its synaptic characteristics under optical and electrical stimuli, including the retention of excitatory post-synaptic currents, potentiation, habituation, nonlinearity factor, and paired-pulse facilitation. Our findings suggest the potential of versatile 2D material-synapse with a high density of device integration.

Elastoplastic mechanical behavior of high-field magnet of REBCO high temperature superconducting composite tape under extreme environment

Rare Earth-Barium-Copper-Oxide (REBCO) high temperature superconducting (HTS) coated conductors (CCs) are one of the best candidate materials for high-field magnets. However, due to its laminated structure, each component materials of the CCs are inevitably subjected to residual thermal stress and electromagnetic force during heat treatment, processing, cooling, coil winding, and operation of the magnet structure. It causes changes in its superconducting properties, and even leads to irreversible degradation of superconducting critical characteristics, brittle layer fracture and peeling delamination within the coil. To reveal the synthesis of the tape and the stress state of the high-field coil under multi-physical fields. The residual thermal stress/strain accumulation of each component material of the tape is investigated under two thermal processing and manufacturing methods by combining analytical theory with numerical calculation. In which, the two thermal processing methods are metal-organic chemical vapor deposition (MOCVD) process and one-step cooling simplified process respectively. The results show that the residual thermal stresses accumulated by five materials, Hastelloy, Buffer, REBCO, Ag and Cu, under the step-by-step synthesis process are about 14.7%, 97.3%, 96.3%, 80% and 49.2% of the simplified model, respectively. And the residual thermal strains are about 15.1%, 96.8%, 95.2%, 85.2% and 27.5% of the simplified model. In addition, after the five materials are manufactured according to the step-by-step thermal processing process, the accumulated residual thermal stresses are about 1.88%, 86.3%, 61%, 114.3%, and 34% of their respective yield strengths, respectively. The elastoplastic mechanical behavior of the magnet under the background of high magnetic field and strong current is also studied. The results indicate that the maximum values of effective and hoop stresses in each component material of the coil are distributed in the innermost side of the magnet, and decrease gradually along the radial direction of the magnet. In the innermost coil of the magnet, the effective stress of each component material increases with the increase of the transmission current. When the transmission current Iope = 183 A, the material Cu reaches the yield strength and enters the plastic deformation stage. Whereas the material Hastelloy reaches the yield strength and enters into the plastic deformation stage when the transmission current Iope = 222 A.

Van der Waals Epitaxy, Superlubricity, and Polarization of the 2D Ferroelectric SnS.

Tin monosulfide (SnS) is a two-dimensional layered semiconductor that exhibits in-plane ferroelectric order at very small thicknesses and is of interest in highly scaled devices. Here we report the epitaxial growth of SnS on hexagonal boron nitride (hBN) using a pulsed metal-organic chemical vapor deposition process. Lattice matching is observed between the SnS(100) and hBN{11̅0} planes, with no evidence of strain. Atomic force microscopy reveals superlubricity along the commensurate direction of the SnS/hBN interface, and first-principles calculations suggest that friction is controlled by the edges of the SnS islands, rather than interface interactions. Differential phase contrast imaging detects remnant polarization in SnS islands with domains that are not dictated by step-edges in the SnS. The growth of ferroelectric SnS on high quality hBN substrates is a promising step toward electrically switchable ferroelectric semiconducting devices.

All-BN distributed Bragg reflectors fabricated in a single MOCVD process

Distributed Bragg Reflectors (DBR) are well-established photonic structures that are used in many photonic applications. However, most of the DBRs are based on different materials or require post-process etching which can hinder integration with other components in the final photonic structure. Here, we demonstrate the fabrication of DBR structures consisting only of undoped boron nitride (BN) layers with high refractive index contrast by using metal–organic chemical vapor deposition (MOCVD). This has been achieved in a single process, without the need for any post-process etching. The difference in the refractive index of the component BN layers stems from different degrees of porosity of the individual BN layers, which is a direct result of a different growth temperature. The fabricated DBR structures consist of 15.5 pairs of BN layers and exhibit a reflectance of 87 ± 1% at the maximum. The wavelength of maximum reflectance can be tuned from 500 nm up to the infrared region (IR), by simply adjusting the growth periods of subsequent BN layers. We also demonstrate that the fabricated structures can be used to create an optical microcavity. The fabricated DBRs are very promising candidates for future applications, for example in combination with single-photon emitters in h-BN, which could allow the building of a cavity-based all-BN single-photon source.

Adsorption mechanism of dimeric Ga precursors in metalorganic chemical vapor deposition of gallium nitride

Gallium nitride (GaN) has attracted significant interest as a next-generation semiconductor material with various potential applications. During metalorganic chemical vapor deposition (MOCVD) of GaN using trimethyl gallium (TMG) and NH3, dimeric precursors are produced by gas-phase reactions such as adduct formation or thermal decomposition. In this work, the surface adsorption reactions of monomeric and dimeric Ga molecules including TMG, [(CH3)2Ga(NH2)]2, and [(CH3)GaNH]2 on the GaN surface are investigated using density functional theory calculations. It is found that [(CH3)2Ga(NH2)]2 is the most predominant form among the various dimeric precursors under typical GaN MOCVD process conditions. Our results indicate that the dimeric [(CH3)GaNH]2 precursor, which is generated through the thermal decomposition of [(CH3)2Ga(NH2)]2, would have higher reactivity on the GaN surface. Our work provides critical insights that can inform the optimization of GaN MOCVD processes, leading to advancements in GaN-based high-performance semiconductors.

Exploiting the Close-to-Dirac Point Shift of the Fermi Level in the Sb2Te3/Bi2Te3 Topological Insulator Heterostructure for Spin-Charge Conversion.

Properly tuning the Fermi level position in topological insulators is of vital importance to tailor their spin-polarized electronic transport and to improve the efficiency of any functional device based on them. Here, we report the full in situ metal organic chemical vapor deposition (MOCVD) and study of a highly crystalline Bi2Te3/Sb2Te3 topological insulator heterostructure on top of large area (4″) Si(111) substrates. The bottom Sb2Te3 layer serves as an ideal seed layer for the growth of highly crystalline Bi2Te3 on top, also inducing a remarkable shift of the Fermi level to place it very close to the Dirac point, as visualized by angle-resolved photoemission spectroscopy. To exploit such ideal topologically protected surface states, we fabricate the simple spin-charge converter Si(111)/Sb2Te3/Bi2Te3/Au/Co/Au and probe the spin-charge conversion (SCC) by spin pumping ferromagnetic resonance. A large SCC is measured at room temperature and is interpreted within the inverse Edelstein effect, thus resulting in a conversion efficiency of λIEEE ∼ 0.44 nm. Our results demonstrate the successful tuning of the surface Fermi level of Bi2Te3 when grown on top of Sb2Te3 with a full in situ MOCVD process, which is highly interesting in view of its future technology transfer.

Highly Tunable MOCVD Process of Vanadium Dioxide Thin Films: Relationship between Structural/Morphological Features and Electrodynamic Properties.

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.

Metal–Organic Chemical Vapor Deposition‐Grown AlScN for Microelectromechanical‐Based Acoustic Filter Applications

AlScN films are produced in a commercial metal–organic chemical vapor deposition (MOCVD) system modified to low‐vapor‐pressure Sc metal–organic precursors. Growth conditions are optimized for surface morphology, film stress, and piezoelectric coefficient across a range of compositions. Epitaxial structures are designed to eliminate internal tensile stresses that develop during the film growth as well as prevent surface adatom interactions unique to the MOCVD process. From these films, wide‐bandwidth resonators and filters are manufactured using a novel microelectromechanical‐based bulk acoustic wave (BAW) transfer process. When tested on‐wafer, resonators exhibit a of 10.5%, and a value of 1400. Ladder‐type RF filters using these resonators are fabricated at 6.2 GHz and show improved performance over filters fabricated using physical vapor deposition‐deposited AlScN.

Stabilization of the Nano-Sized 1T Phase through Rhenium Doping in the Metal-Organic CVD MoS2 Films.

Heterogeneous nanostructures composed of metastable tetragonal 1T-MoS2 and stable hexagonal 2H-MoS2 phases are highly promising for a wide range of applications, including catalysis and ion batteries, due to the high electrical conductivity and catalytic activity of the 1T phase. However, a controllable synthesis of stabilized 1T-MoS2 films over the wafer-scale area is challenging. In this work, a metal-organic chemical vapor deposition process allowing us to obtain ultrathin MoS2 films containing both 1T and 2H phases and control their ratio through rhenium doping was suggested. As a result, Mo1-xRexS2 films with a 1T-MoS2 fraction up to ≈30% were obtained, which were relatively stable under normal conditions for a long time. X-ray photoelectron spectroscopy and Raman spectroscopy also indicated that the 1T-MoS2 phase fraction increased with rhenium concentration increase saturating at Re concentrations above 5 at. %. Also, its concentration was found to significantly affect the film resistivity. Thus, the resistivity of the film containing approximately 30% of the 1T phase was about 130 times lower than that of the film without the 1T phase.

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9 × 10 12 cm−2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.

Selective-area growth of β-Ga2O3 nanowire films on nano-patterned Si(111) substrate by metal-organic chemical vapor deposition

β-Ga2O3 nanowire films were prepared on Si(111) substrate by selective area growth (SAG) using metal-organic chemical vapor deposition (MOCVD). A large-area periodic nano-hole array TiN mask was fabricated. The effects of MOCVD process parameters on SAG were discussed by calculating Ga supersaturation. It was found that the SAG is closely related to the selective area nucleation and inhibition of lateral growth. Transmission electron microscopy confirmed that β-Ga2O3 nanowires have a preferential orientation of the (002) crystal plane. The sample after in-situ O2 annealing showed an x-ray diffraction intensity increase of (400), (002), and (-111) crystal planes. On this basis, P–Si/β‐Ga2O3 nanowire heterojunctions were fabricated and presented high resistance of the films. At a bias voltage of 20 V, the current decreased from 6.14 × 10−6 to 2.25 × 10−6 A. The MOCVD-based SAG paves a novel way to fabricate β-Ga2O3 nanowire films with high surface-to-volume ratio and homogeneity.

Manufacturing of Gallium Nitride Thin Films in a Multi-Wafer MOCVD Reactor

Abstract Gallium nitride (GaN) thin films have attracted considerable attention for manufacturing optical and electronic devices. They have wide bandgap and superb performance in these applications. The reliability and durability of optoelectronic devices depend on the quality of the GaN thin films. The metal-organic chemical vapor deposition (MOCVD) process is a common manufacturing technique for fabricating high-quality thin films. By manipulating the operating conditions and the reactor design, one can control the deposition rate and the uniformity of the thin film. In this paper, the manufacturing process for GaN thin films in a multi-wafer MOCVD reactor is simulated based on the three-dimensional computational model of an experimental system which provides data for validation as well as realistic design parameters. The reactor pressure and the flow rate of the precursor, trimethyl-gallium (TMG), significantly affect the deposition rate and film uniformity. The incursion of impurities in the deposition can be reduced by increasing the volumetric ratio of NH3 to TMG (V/III) and reducing the reactor pressure. The deposition rate and quality of the thin film are enhanced using an appropriate mixture of H2 and N2 as the carrier gas. The design of the inlet can also be varied to improve the utilization of metal-organic precursors and increase the deposition rate. This paper presents and discusses results on these aspects for this important manufacturing process. Thus, it leads to a better understanding of the basic mechanisms involved and provides guidelines for obtaining high deposition rates with high film quality in practical chemical vapor deposition reactors.

Theoretical adjustment of metalorganic chemical vapor deposition process parameters for high-quality gallium nitride epitaxial films

Semiconductor thin films for electronic devices are usually produced through processes such as chemical vapor deposition, which requires careful control of the gas flow, heat distribution, and concentration distribution over the substrate to ensure a uniform deposition rate and thickness. Herein, a systematic method is proposed for the theoretical adjustment of metalorganic chemical vapor deposition (MOCVD) process parameters. To this end, a GaN-MOCVD reactor with a vertical injection structure was simulated based on computational fluid dynamics to analyze the stable flow under a fixed flow rate. The orthogonal experimental design was used to analyze the influence of process conditions on film quality. A neural network and genetic algorithm were used to optimize the inlet flow under the stable flow state to render the coefficient of variation <3%. Under these conditions, the flow field in the reactor was stabilized to ensure a uniform thickness for the deposited film. This study provides not only an effective solution for high-quality epitaxial growth but also a theoretical basis for subsequent experiments and equipment improvement.

Impact of synthesis temperature and precursor ratio on the crystal quality of MOCVD WSe2 monolayers

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.

MOCVD of Hierarchical C‐MoS2 Nanobranches for ppt‐Level NO2 Detection

In the past decades, toxic gas emissions have increased significantly owing to the rapid growth of industry and road transportation. Therefore, monitoring major pollutants, such as NO2, is crucial to protecting human health. The 2D materials that contain numerous adsorption sites and exhibit ultrahigh chemical reactivity can be used as sensor materials to detect these toxic gases. Herein, highly uniform, large‐area carbon‐incorporating hierarchical MoS2 nanobranches are synthesized by metal–organic chemical vapor deposition (MOCVD). An in situ carbon‐incorporation method that uses the carbon impurity present in the precursor as the seed during the MOCVD process is employed to form a hierarchical structure containing abundant adsorption sites. A gas sensor based on the resulting C‐MoS2 nanobranches contains many edge sites exhibits high adsorption energy, and consequently, has high NO2 gas sensitivity. Hence, this hierarchical C‐MoS2 gas sensor shows excellent sensing properties, exhibiting a device response of 1.67 at an extremely low NO2 concentration (≈5 ppb). The limit of detection of the gas sensor for NO2 is calculated to be low (≈1.58 ppt), further confirming its exceptional performance. Thus, the hierarchical C‐MoS2 nanobranches deposited herein provide novel insights regarding the properties of 2D materials and are highly suited for fabricating high‐performance NO2 sensors.

Periodical Ripening for MOCVD Growth of Large 2D Transition Metal Dichalcogenide Domains

Abstract2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever‐growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer‐scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying‐temperature ripening (PVTR) process is demonstrated to grow wafer‐scale high crystalline TMDC films by MOCVD. It is found that the high‐temperature ripening significantly reduces the nucleation density and therefore enables single‐crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom‐resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high‐quality 2D semiconducting films for applications in electronics and optoelectronics.

Migration-Enhanced Metal–Organic Chemical Vapor Deposition of Wafer-Scale Fully Coalesced WS2 and WSe2 Monolayers

Metal–organic chemical vapor deposition (MOCVD) is widely employed for the wafer-scale synthesis of transition metal dichalcogenide (TMDC) monolayers (MLs). Despite large efforts devoted to understanding the intricate nucleation and lateral growth mechanisms of TMDCs, little attention has been paid to the migration of adatoms on the top of an ML and its influence on parasitic/premature bilayer (BL) nucleation. In this work, using a commercial multi-wafer MOCVD platform, a novel two-stage migration-enhanced MOCVD process is introduced to realize the deposition of wafer-scale fully coalesced tungsten disulfide (WS2) and tungsten diselenide (WSe2) MLs with only sparse BL nucleation in a reasonable deposition time. With the WS2 ML coverage exceeding 99% on 2 in. sapphire substrates within 3 h, BL coverage is suppressed to ∼15%. Following the same migration enhancement approach, WSe2 MLs are synthesized in 90 min with <20% BL coverage. The migration of W adatoms on the already formed stable WS2 (or WSe2) ML domains is promoted by ramping down the delivery of the tungsten precursor. From the qualitative analysis of the nanomorphology, the migration length of W adatoms is estimated to be ≤100 nm. This approach can be seen as a reliable and solid basis for the development of future large-scale TMDC ML deposition techniques.

Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface.

Conceptual 2D group III nitrides and oxides (e.g., 2D InN and 2D InO) in heterostructures with graphene have been realized by metal-organic chemical vapor deposition (MOCVD). MOCVD is expected to bring forth the same impact in the advancement of 2D semiconductor materials as in the fabrication of established semiconductor materials and device heterostructures. MOCVD employs metal-organic precursors such as trimethyl-indium, -gallium, and -aluminum, with (strong) metal-carbon bonds. Mechanisms that regulate MOCVD processes at the atomic scale are largely unknown. Here, we employ density-functional molecular dynamics - accounting for van der Waals interactions - to identify the reaction pathways responsible for dissociation of the trimethylindium (TMIn) precursor in the gas phase as well as on top-layer and zero-layer graphene. The simulations reveal how collisions with hydrogen molecules, intramolecular or surface-mediated proton transfer, and direct TMIn/graphene reactions assist TMIn transformations, which ultimately enables delivery of In monomers or InH and CH3In admolecules, on graphene. This work provides knowledge for understanding the nucleation and intercalation mechanisms at the atomic scale and for carrying out epitaxial growth of 2D materials and graphene heterostructures.

Efficient preparation of the inclined La1-xCaxMnO3 films with excellent light-induced transverse thermoelectric performance based on MOCVD process

The efficient preparation of inclined La1-xCaxMnO3 (LCMO) films with excellent light-induced transverse thermoelectric (LITT) performance on c-axis miscut 12° SrTiO3 single crystal substrates is achieved based on the home-designed metal-organic chemical vapor deposition (MOCVD) system. The LCMO films with the thickness of 300 nm are prepared in 1 min, and the preparation efficiency reaches 300 nm/min. Under the radiation of 248 nm pulsed laser with the duration time of 28 ns, the voltage signal of LCMO films shows high sensitivity of 3.13 V/mJ and fast response time of 28 ns, superior to the LITT performance of the reported LCMO under the irradiation of pulsed laser with similar wavelength and duration time. The improvement of LITT performance originates from good electrical conductivity and biaxial texture. Additionally, under the radiation of 900 nm continuous laser, the obvious response voltages can be detected in the LCMO films, and the voltage amplitudes increase linearly with increasing laser power densities. These results demonstrate the application potential of the light detectors composed of LCMO films based on the efficient preparation of MOCVD, and may provide a good prospect for developing low-cost and high-performance light detectors based on the LITT effect.

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD part-I: Growth optimization

The present work is part-I of a study that demonstrates the influence of metalorganic chemical vapor deposition process parameters on the evolution of gallium nitride (GaN) deposited in a two-step growth process on c-plane sapphire substrates with a particular emphasis on nitrogen-polar GaN (N-polar GaN). First, the temperature domains of sapphire nitridation that result in Ga-polar and N-polar GaN epilayers were identified. Then, the structural and morphological evolution of low-temperature GaN nucleation layers were examined at the observed temperature domains of nitridation. Subsequently, a series of experiments were performed to study the influence of Ga and N precursor flow rates and epilayer growth temperature on the surface evolution of N-polar GaN. The observed results were analyzed in terms of thermodynamic and kinetic parameters of GaN growth. This work reports high-surface quality N-polar GaN epilayers without surface hillocks, exhibiting surface roughness about 1.5 nm over 2 × 2 μm2 area and with good crystalline quality on nominally on-axis c-plane sapphire substrates at low epilayer growth temperatures.