Low-pressure Chemical Vapor Deposition System Research Articles

Impact of alternating precursor supply and gas flow on the LPCVD growth behavior of polycrystalline 3C-SiC thin films on Si

In this paper, we will provide information on the growth mechanism of 3C-SiC using alternating supply deposition (ASD). SiH4 and C3H8 were introduced successively in a low-pressure chemical vapor deposition (LPCVD) system at 1000 °C, forming a stoichiometric polycrystalline 3C-SiC thin film on Si substrates. We demonstrate the influence of different gas flow rates on thin film properties. In detail, altering the flow rates of the precursors and the carrier gas resulted in changes of the growth rate, the surface roughness, the crystal growth mechanisms as well as the crystal quality. Hydrogen inhibition and passivation are proposed to be the main effects influencing the growth behavior of ASD thin films. Applying ASD to grow 3C-SiC thin films with precursors from low to high flow rates resulted in a transition of different growth mechanisms influencing the surface roughness and grain size. Furthermore, we could prove that ASD is a cyclic step-by-step carbon-assisted redistribution of a prior deposited silicon film. This is shown by contact angle measurements of three differently terminated surfaces during the specific phases of one ASD cycle. High resolution transmission electron microscopy (HRTEM) analysis showed a continuous and homogenous thin film and confirmed the absence of silicon- or carbon-rich layers. ASD enables 3C-SiC thin films with customized properties for tailored micro electromechanical system (MEMS) applications.

Effect of tin concentration on the morphological & crystal structure, and optical properties of chemical vapor deposited MAPb1−xSnxI3 perovskite films for 0 ≤ x < 0.5

In this article, we evaluate the impact of Sn proportion on the morphology and crystal structure, and its influence on the optical properties of MAPb1−xSnxI3 (0 ≤ x < 0.5) perovskite films. The uniform and compact Sn-Pb halide films with a variety of grain sizes were deposited using a low-pressure chemical vapor deposition (LP-CVD) system and were sequentially converted into perovskite films. Rutherford backscattering spectrometry (RBS) measurements were performed to determine the depth profile of elements and the simulated spectra suggested multilayered films. The x-ray diffraction (XRD) patterns confirmed the formation of the perovskite phase, i.e. tetragonal structure with I4cm space group. The energy bandgap (Eg) and the Urbach energy (Eu) approximation insinuate a tradeoff between the attainment of low bandgap Sn-Pb halide perovskites and defect-disorder tolerant films.

Effects of Buffer Layer on Structural Properties of Nonpolar (112¯0)-Plane GaN Film

Nonpolar (112¯0) a-plane GaN films were grown on semipolar (11¯02) r-plane sapphire substrates using various buffer layers within a low-pressure metal organic chemical vapor deposition system. The structural properties of nonpolar a-plane GaN films were intensively investigated by X-ray diffraction and Raman spectra measurements. A set of buffer layers were adopted from a GaN layer to a composite layer containing a multiple AlN layers and a gradually varied-Al-content AlGaN layer, the full width at half maximum of the X-ray rocking curves measured along the [0001] and [101¯0] directions of a-plane GaN were reduced by 35% and 37%, respectively. It was also found that the basal-plane stacking faults (BSFs) density can be effectively reduced by the heterogeneous interface introduced together with the composite buffer layer. An order of magnitude reduction in BSFs density, as low as 2.95 × 104 cm−1, and a pit-free surface morphology were achieved for the a-plane GaN film grown with the composite buffer layer, which is promising for the development of nonpolar GaN-based devices in the future.

Scalable growth of atomically thin MoS2 layers in a conventional MOCVD system using molybdenum dichloride dioxide as the molybdenum source

To bring atomically thin transition metal dichalcogenides (TMDs) to practical application, a highly reproducible process to grow them over a large scale is indispensable. Here, we develop a carbon-free reproducible route for the scalable growth of high-quality monolayer MoS2 by selecting molybdenum dichloride dioxide (MoO2Cl2) as the Mo source and integrating the precursor with a standard low-pressure cold-wall metalorganic chemical vapor deposition (MOCVD) system. Specifically, combined with H2S as the sulfur source and using catalytic Dragontrail glass (DT-glass) as the main substrate, we investigate the effect of MoO2Cl2 flux, temperature, and deposition time on the growth, confirming MoO2Cl2 with reasonably high vapor pressure is well compatible with the MOCVD system that enables precise control of sources for uniform nucleation and growth of MoS2 over a large area. We successfully demonstrate the growth of carbon-free MoS2 monolayers on DT-glass with decent crystalline, optical, and electrical properties. Additionally, the initial attempts also manifest that the proposed strategy could be generalized for growing ultrathin MoS2 on other technologically important substrates, such as SiO2/Si and quartz, exhibiting superior optical quality and wafer-scale uniformity. This work provides a new avenue for the large-scale production of high-quality MoS2 monolayers and facilitates their use in practical applications.

AlGaN/AlN/SiC Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors with Al2O3 Gate-Oxide and Step-Graded AlGaN Channel

Al0.75Ga0.25N/n-AlxGa1-xN/Al0.75Ga0.25N/AlN metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs), grown on a SiC substrate, with step-graded Si-doped (n = 3 × 1018 cm-3) widegap AlxGa1-xN channel and Al2O3 gate-dielectric are investigated. Increased Al-compositions with x = 0.25, 0.5, and 0.75 towards the buffer were devised in the composite widegap channel. The high-k Al2O3 dielectric/passivation layer was grown by using a non-vacuum ultrasonic spray pyrolysis deposition (USPD) method. Experimental comparisons were made with respect to a conventional Schottky-gate HFET.The epitaxial structure of the studied Al2O3-dielectric Al0.75Ga0.25N/n-AlxGa1-xN/Al0.75Ga0.25N/AlN MOS-HFET (sample A) and Schottky-gate HFET (sample B). Both devices have the identical epitaxial structure grown on a SiC substrate by using a low-pressure metal-organic chemical vapor deposition (LP-MOCVD) system. The layer structure includes an undoped AlN buffer, a 20-nm undoped Al0.75Ga0.25N barrier, a 150-nm step-graded Si-doped (n = 3 × 1018 cm-3) AlxGa1-xN channel (x = 0.25, 0.5, and 0.75), and a 20-nm undoped Al0.75Ga0.25N barrier. Standard photolithography and lift-off techniques were used for device processing. For sample B, mesa etching was performed with respect to a 100-nm thick Ni barrier by using an inductively coupled-plasma reactive ion etcher (ICP-RIE). Flow rates were tuned and set at 10 sccm and 20 sccm for the mixed etching gases of BCl3 and Cl2, respectively. The 20-nm undoped Al0.75Ga0.25N barrier was etched away before deposition of source/drain electrodes. Ti (10 nm)/Al (50 nm)/Ni (10 nm)/Au (50 nm) were evaporated. The source/drain ohmic contacts were formed by annealing the sample for 25 seconds at 900°C by using a rapid thermal annealing (RTA) system (ULVAC MILA-5000). Then, 30-nm thick Al2O3 layer was deposited on the Al0.75Ga0.25N barrier by using the USPD technique. Finally, gate electrode of Ni (100 nm)/Au (50 nm) was evaporated after gate photolithography. As for sample A, the gate electrode was formed directly on the surface of Al0.75Ga0.25N barrier without oxide deposition.Improved device performance of the present MOS-HFET design have been obtained, including maximum drain-source current density (IDS, max ) of 130.1 A/mm at VGS = 10 V and VDS = 20 V, IDS at VGS = 0 V (IDSS0 ) of 83.1 mA/mm, on/off-current ratio (Ion /Ioff ) of 1.4 × 107, maximum extrinsic transconductance (gm, max ) of 11.8 mS/mm, two-terminal off-state gate-drain breakdown voltage (BVGD ) of -404 V, and three-terminal on-state drain-source breakdown voltage (BVDS ) of 364 V at 300 K. The present MOS-HFET design has also shown high spectral responsivity (SR) of 737 A/W under 250-nm deep-UV radiation at 300 K.

Piezoresistive strain sensor based on monolayer molybdenum disulfide continuous film deposited by chemical vapor deposition

In this paper, a centimeter-scale monolayer molybdenum disulfide (MoS2) film deposition method has been developed through a simple low-pressure chemical vapor deposition (LPCVD) growth system. The growth pressure dependence on film quality is investigated in this LPCVD system. The layer nature, electrical characteristic of the as-grown MoS2 films indicate that high quality films have been achieved. In addition, a hydrofluoric acid treated SiO2/Si substrate is used to improve the quality of the MoS2 films. Piezoresistive strain sensor based on the monolayer MoS2 film elements is fabricated by directly patterning metal contact pads on MoS2 films through a silicon stencil mask. A gauge factor of 104 ± 26 under compressive strain is obtained by using a four-point bending method, which may inspire new possibilities for two-dimensional (2D) material-based microsystems and electronics.

Carbon-assisted chemical vapor deposition of hexagonal boron nitride

We show that in a low-pressure chemical vapor deposition (CVD) system, the residual oxygen and/or air play a crucial role in the mechanism of the growth of hexagonal boron nitride (h-BN) films on Ni foil ‘enclosures’. Hexagonal-BN films grow on the Ni foil surface via the formation of an intermediate boric-oxide (BOx) phase followed by a thermal reduction of the BOx by a carbon source (either amorphous carbon powder or methane), leading to the formation of single- and bi-layer h-BN. Low energy electron microscopy (LEEM) and diffraction (LEED) were used to map the number of layers over large areas; Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structure and physical quality of the ultra-thin h-BN film. The growth procedure reported here leads to a better understanding and control of the synthesis of ultra-thin h-BN films.

Graphene growth with ‘no’ feedstock

Synthesis of graphene by chemical vapor deposition (CVD) from hydrocarbons on Cu foil substrates can yield high quality and large area graphene films. In a typical CVD process, a hydrocarbon in the gas phase is introduced for graphene growth and hydrogen is usually required to achieve high quality graphene. We have found that in a low pressure CVD system equipped with an oil mechanical vacuum pump located downstream, graphene can be grown without deliberate introduction of a carbon feedstock but with only trace amounts of C present in the system, the origin of which we attribute to the vapor of the pump oil. This finding may help to rationalize the differences in graphene growth reported by different research groups. It should also help to gain an in-depth understanding of graphene growth mechanisms with the aim to improve the reproducibility and structure control in graphene synthesis, e.g. the formation of large area single crystal graphene and uniform bilayer graphene.

Influence of copper foil polycrystalline structure on graphene anisotropic etching

Anisotropic etching of graphene and other two dimensional materials is an important tool to understand the growth process as well as enabling fabrication of various well-defined structures. Here, we reveal the influence of copper foil polycrystalline structure on anisotropic etching process of as-synthesized graphene. Graphene crystals were synthesized on the polycrystalline Cu foil by a low-pressure chemical vapor deposition (LPCVD) system. Microscopic analysis shows difference in shape, size and stripes alignment of graphene crystals with dissimilar nucleation within closure vicinity of neighboring Cu grains. Post-growth etching of such graphene crystals also significantly affected by the crystallographic nature of Cu grains as observed by the field emission scanning electron microscope (FE-SEM) and electron back scattered diffraction (EBSD) analysis. Hexagonal hole formation with anisotropic etching is observed to be independent of the stripes and wrinkles in the synthesized graphene. We also observed variation in etched pattern of the graphene depending on the base Cu grain orientations, attributing to difference in nucleation and growth process. The findings can facilitate to understand the nature of microscopic etched pattern depending on metal catalyst crystallographic structure.

The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

Theoretical calculation and experimental measurement have shown that twin grain boundary (GB) of molybdenum disulphide (MoS2) exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD) system, and we investigated how the twin GB affects the electrical transport properties of MoS2 by temperature-dependent transport studies. At low temperature, the twin GB can increase the in-plane electrical conductivity of MoS2 and the transport exhibits variable-range hopping (VRH), while at high temperature, the twin GB impedes the electrical transport of MoS2 and the transport exhibits nearest-neighbor hopping (NNH). Our results elucidate carrier transport mechanism of twin GB and give an important indication of twin GB in tailoring the electronic properties of MoS2 for its applications in next-generation electronics and optoelectronic devices.

Fabrication of GaN-Based Heterostructures with an InAlGaN/AlGaN Composite Barrier**Supported by the National Science and Technology Major Project under Grant No 2013ZX02308-002, and the National Natural Science Foundation of China under Grant Nos 11435010, 61474086 and 61334002.

GaN-based heterostructures with an InAlGaN/AlGaN composite barrier on sapphire (0001) substrates are grown by a low-pressure metal organic chemical vapor deposition system. Compositions of the InAlGaN layer are determined by x-ray photoelectron spectroscopy, structure and crystal quality of the heterostructures are identified by high resolution x-ray diffraction, surface morphology of the samples are examined by an atomic force microscope, and Hall effect and capacitance-voltage measurements are performed at room temperature to evaluate the electrical properties of heterostructures. The Al/In ratio of the InAlGaN layer is 4.43, which indicates that the InAlGaN quaternary layer is nearly lattice-matched to the GaN channel. Capacitance–voltage results show that there is no parasitic channel formed between the InAlGaN layer and the AlGaN layer. Compared with the InAlGaN/GaN heterostructure, the electrical properties of the InAlGaN/AlGaN/GaN heterostructure are improved obviously. Influences of the thickness of the AlGaN layer on the electrical properties of the heterostructures are studied. With the optimal thickness of the AlGaN layer to be 5 nm, the 2DEG mobility, sheet density and the sheet resistance of the sample is 1889.61 cm2/V · s, 1.44 × 1013 cm−2 and as low as 201.1 ω/sq, respectively.

Substrate effect on the growth of monolayer dendritic MoS2 on LaAlO3 (100) and its electrocatalytic applications

In accommodating the rapid development of two-dimensional (2D) nanomaterials, chemical vapor deposition (CVD) has become a powerful tool for their batch production with desirable characteristics, such as high crystal quality, large domain size, and tunable domain shape. The crystallinity and morphology of the growth substrates usually play a crucial role in the CVD synthesis of high-quality monolayer MoS2, a kind of 2D layered material which has ignited huge interest in nanoelectronics, optoelectronics and energy harvesting, etc. Herein, by utilizing a low-pressure chemical vapor deposition (LPCVD) system, we demonstrate a regioselective synthesis of monolayer MoS2 on the corrugated single-crystal LaAlO3 (100) with twin crystal domains induced by the second-order phase transition. Unique dendritic morphologies with tunable nucleation densities were obtained in different regions of the undulated substrate, presenting a strong substrate modulation effect. Interestingly, the exposure of abundant active edge sites along with the rather high nucleation density makes the monolayer dendritic MoS2 a good electrocatalyst for hydrogen evolution reaction (HER), particularly featured by a rather high exchange current density (70.4 μA cm−2). Furthermore, uniform monolayer MoS2 films can also be obtained and transferred to arbitrary substrates. We believe that this work provides a new growth system for the controllable synthesis of 2D layered materials with unique dendritic morphologies, as well as its great application potential in energy conversion and harvesting.

Growth of Large-Area Graphene Single Crystals in Confined Reaction Space with Diffusion-Driven Chemical Vapor Deposition

To synthesize large-area graphene single crystals, we specifically designed a low-pressure chemical vapor deposition (LPCVD) reactor with confined reaction space (L 22 mm × W 13 mm × H 50 μm). Within the confined reaction space, a uniform distribution of reactant concentrations, reduced substrate roughness, and the shift of growth kinetics toward a diffusion-limited regime can be achieved, favoring the preparation of large-area, high-quality graphene single crystals. The gas flow field and mass transport pattern of reactants in the LPCVD system simulated with a finite element method support the advantages of using this confined reaction room for graphene growth. Using this space-confined reactor together with the optimized synthesis parameters, we obtained monolayer, highly uniform, and defect-free graphene single crystals of up to ∼0.8 mm in diameter with the field-effect mobility of μEF ∼ 4800 cm2 V–1 s–1 at room temperature. In addition, structural design of the confined reaction space by adjusting the...

Synthesis and characterization of graphene layers prepared by low-pressure chemical vapor deposition using triphenylphosphine as precursor

The synthesis of a single-layer graphene using a low-pressure Chemical Vapor Deposition (CVD) system with triphenylphosphine as precursor is reported. The amount of triphenylphosphine used as precursor was in the range of 10–40 mg. Raman spectroscopy was employed to analyze samples prepared with 10 mg of the precursor, and these spectra were found typical of graphene. The Raman measurements indicate that the progressive degradation of graphene occurs as the amount of triphenylphosphine increases. X-ray photoelectron spectroscopy measurements were performed to investigate the different chemical environments involving carbon and phosphorous atoms. Scanning electron microscopy and transmission electron microscopy were also employed and the results reveal the formation of dispersed nanostructures on top of the graphene layer, In addition, the number of these nanostructures is directly related to the amount of precursor used for sample growth.

The Development of Polyethylene Naphthalate Films by Low-pressure High-frequency Plasma Chemical Vapor Deposition System with Advance Oxidations Process

AbstractThe low-pressure high-frequency plasma chemical vapor deposition (CVD) system was developed with non-thermal plasma process to study the Polyethylene naphthalate (PEN) surface characteristics. Plasma surface treatment by oxygen can improve the adhesive properties. A mixture of Ar and O

Influence of Cu dopant on the structure and optical properties of ZnO thin films prepared by MOCVD

ZnO:Cu thin films were prepared on the sapphire substrate using low-pressure metal organic chemical vapor deposition system. The influence of Cu dopant on the structure and morphology, electrical and optical properties of ZnO was investigated. Scanning electron microscope results showed that the smooth surface morphology was made up of evenly diameter nanorod, with complete hexagonal structure at some doping point. Low temperature photoluminescence results indicated that Cu dopant introduced more than one acceptor energy level in the energy gap and the energy levels were multiple and complicated. The presence of donor and acceptor pairs suggested that acceptors were really incorporated into ZnO, and it was proposed to be Cu1+. These results confirmed that moderate Cu-dopant in ZnO could improve the optical properties and it extended a rich picture of the bound excitons emission.

Effects of temperature and ammonia flow rate on the chemical vapour deposition growth of nitrogen-doped graphene.

We doped graphene in situ during synthesis from methane and ammonia on copper in a low-pressure chemical vapour deposition system, and investigated the effect of the synthesis temperature and ammonia concentration on the growth. Raman and X-ray photoelectron spectroscopy was used to investigate the quality and nitrogen content of the graphene and demonstrated that decreasing the synthesis temperature and increasing the ammonia flow rate results in an increase in the concentration of nitrogen dopants up to ca. 2.1% overall. However, concurrent scanning electron microscopy studies demonstrate that decreasing both the growth temperature from 1000 to 900 °C and increasing the N/C precursor ratio from 1/50 to 1/10 significantly decreased the growth rate by a factor of six overall. Using scanning tunnelling microscopy we show that the nitrogen was incorporated mainly in substitutional configuration, while current imaging tunnelling spectroscopy showed that the effect of the nitrogen on the density of states was visible only over a few atom distances.

The growth mechanisms of graphene directly on sapphire substrates by using the chemical vapor deposition

Uniform and large-area graphene films grown directly on sapphire substrates by using a low-pressure chemical vapor deposition system are demonstrated in this paper. The evolution process and the similar Raman spectra of the samples with different growth durations have confirmed that the continuous graphene film is formed by graphene flakes with similar sizes. The layer-by-layer growth mechanism of this approach is attributed to the preferential graphene deposition on sapphire surfaces. The etching effect of H2 gas is demonstrated to be advantageous for the larger graphene grain formation. The smooth surface of substrates is also proved to be a key parameter for continuous graphene film formation with better crystalline quality.

Influences of Au nanoparticle density on the performance of GaAs solar cells

The influence of the Au nanoparticle density on the performance of single-junction GaAs solar cells has been investigated. The single-junction GaAs solar cells are grown on (100) GaAs substrates by using a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Au nanoparticles are dispersed on the surfaces of the cells by micro-pipetted casting and the density of the dispersed nanoparticles varies from 4.7 × 109 to 4.3 × 1010 cm−2. The conversion efficiencies of the GaAs solar cells are observed to depend strongly on the Au nanoparticle densities. For Au nanoparticle densities lower than 2.8 × 1010 cm−2, the conversion efficiency improvement of the GaAs solar cells mainly originates from the antireflection coating effect. In contrast, for the sample with a Au nanoparticle density of 2.8 × 1010 cm−2, the antireflection effect decreases due to the shadow effect and the local field enhancement effect due to the Au nanoparticles starts to be effective. The GaAs solar cell with a Au nanoparticle density of 1.9 × 1010 cm−2 shows a maximum conversion enhancement compared to that of the reference sample.

Synthesis of Boron‐Doped Graphene Monolayers Using the Sole Solid Feedstock by Chemical Vapor Deposition

Substitutionally boron-doped monolayer graphene film is grown on a large scale by using a sole phenylboronic acid as the source in a low-pressure chemical vapor deposition system. The B-doped graphene film is a homogeneous monolayer with high crystalline quality, which exhibits a stable p-type doping behavior with a considerably high room-temperature carrier mobility of about 800 cm(2) V(-1) s(-1) .