As-grown Chemical Vapor Deposition Research Articles

Microscopic investigation of intrinsic defects in CVD grown MoS2 monolayers

Defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) greatly influence their electronic and optical properties by introducing localized in-gap states. Using different non-invasive techniques, we have investigated the spatial distribution of intrinsic defects in as-grown chemical vapor deposition (CVD) MoS2 monolayers and correlated the results with the growth temperature of the sample. We have shown that by increasing the CVD growth temperature the concentration of defects decreases and their spatial distribution and type change, influencing the sample’s electronic and optical properties.

Evaluating the defects in CVD diamonds: A statistical approach to spectroscopy

Laboratory-grown diamonds produced by chemical vapor deposition (CVD) can have similar sizes and colors to natural diamonds, and their color can be enhanced by post-growth treatment. Despite considerable published research on CVD diamond growth, post-growth treatment remains difficult to detect in some CVD diamonds. To improve the detection of post-growth treatment, we apply multivariate statistics to photoluminescence (PL) emission spectra and surface fluorescence images from 11,606 gem-quality CVD diamonds within the D-Z color range. Unsupervised statistics indicate as-grown CVD diamonds commonly have 3H defects, with zero-phonon line (ZPL) at 503.5 nm, and relatively intense PL peaks at 467.6 nm. These features are absent or minor in diamonds treated under high-pressure, high-temperature (HPHT) conditions. HPHT-treated CVD diamonds tend to exhibit the H3 system (with ZPL at 503.2 nm) as well as larger 491.9 nm and 566.6 nm peak areas. A subset of CVD diamonds showing the H3 system could have experienced treatment at low-pressure high-temperature (LPHT) conditions (including inside the reactor during or after CVD growth).

Mechanical origin of martensite-like structures in two-dimensional ReS2

Martensite is a needle-shaped microstructure formed by a rapid, diffusionless transformation and significantly affects the mechanical properties of materials. Here, in two-dimensional ReS2 we show that martensite-like domain structures can form via a diffusionless transformation, involving small lattice deformations. By analyzing the strain distribution and topology of the as-grown chemical vapor deposition samples, we find that cooling-induced strain at the ReS2/substrate interface is responsible for the mechanical loading and is essential for martensite-like domain formation. Meanwhile, the effect of cooling rate, flake size and substrate on the microstructures revealed the mechanical origin of the transformation. The strain-induced lattice reconstructions are rationalized and possibly lead to ferroelastic effects. In view of the strong anisotropy in electronic and optical properties in two dimensional materials like ReS2, opportunities exist for strain-correlated micro/nanostructure engineering, which has potential use in next-generation strain-tunable devices.

Evolution of defects, morphologies and fundamental growth characteristics of CVD diamond films induced by nitrogen addition

Diamond is a very promising material with exceptional properties for many high value-added applications especially due to the incorporation of impurities. As one of the most important impurities, nitrogen is widely found in natural and synthetic diamonds, and the control of nitrogen-related defects in diamond is definitely at the spotlight for its potentials in room temperature quantum applications. Since chemical vapor deposition (CVD) is one of the most promising techniques for diamond doping and realization of defect control, it is thus crucial to investigate the nitrogen impurities in as-grown CVD diamond. In this work, single-crystal diamonds were grown with different nitrogen concentration and the evolution of defects, morphologies and fundamental growth characteristics of the CVD diamond films was explored by various modern analysis and detection methods. It could be concluded that the introduction of nitrogen could result in an evident increase in supersaturation of the growth radicals at the growth interface, which would eventually distinctly promote the (100)-oriented diamond growth. In consequence, single-crystal diamonds of high crystalline quality and few carbonaceous impurities could be achieved at high growth rates with appropriate amounts of nitrogen addition. As the nitrogen concentration increased, the structure and content of the defects would evolve as well with the domination of point defects, probably leading to lattice deformation in the as-grown diamond films. The surface morphologies of the nitrogen-doped CVD diamond films could also be distinctly altered with the compromise between the development of step bunching process and the occurrence of two-dimensional growth, which would eventually lead to variations of the surface roughness and uniformity of the as-grown diamond films.

Spectroscopic and Electrical Characterizations of Low-Damage Phosphorous-Doped Graphene via Ion Implantation.

Development of n-/p-type semiconducting graphenes is a critical route to implement in graphene-based nanoelectronics and optronics. Compared to the p-type graphene, the n-type graphene is more difficult to be prepared. Recently, phosphorous doping was reported to achieve air-stable and high mobility of n-typed graphene. The phosphorous-doped graphene (P-Gra) by ion implantation is considered as an ideal method for tailoring graphene due to its IC compatible process; however, for a conventional ion implanter, the acceleration energy is in the order of kiloelectron volts (keV), thus severely destroys the sp2 bonding of graphene owing to its high energy of accelerated ions. The introduced defects, therefore, degrade the electrical performance of graphene. Here, for the first time, we report a low-damage n-typed chemical vapor deposition (CVD) graphene by an industrial-compatible ion implanter with an energy of 20 keV where the designed protection layer (thin Au film) covered on as-grown CVD graphene is employed to efficiently reduce defect formation. The additional post-annealing is found to heal the crystal defects of graphene. Moreover, this method allows transferring ultraclean and residue-free P-Gra onto versatile target substrates directly. The doping configuration, crystallinity, and electrical properties on P-Gra were comprehensively studied. The results indicate that the low-damaged P-Gra with a controllable doping concentration of up to 4.22 at % was achieved, which is the highest concentration ever recorded. The doped graphenes with tunable work functions (4.85-4.15 eV) and stable n-type doping while keeping high-carrier mobility are realized. This work contributes to the proof-of-concept for tailoring graphene or 2D materials through doping with an exceptional low defect density by the low energy ion implantation, suggesting a great potential for unconventional doping technologies for next-generation 2D-based nanoelectronics.

Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity.

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm2 ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

Study of the Substrate-Induced Strain of As-Grown Graphene on Cu(100) Using Temperature-Dependent Raman Spectroscopy: Estimating the Mode Grüneisen Parameter with Temperature

The strategy of using the thermal expansion of copper single crystal can provide an approach of homogenous strain along the basal plane to study the strain characteristic of graphene with temperature. Using an in-situ Raman measurement under a ultra-high vacuum (UHV) environment, the ability to remove contaminations allowed the direct observation of the strain property in as-grown chemical vapor deposition (CVD)-graphene with temperature on a Cu(100) substrate. In this study, the strain coefficients of G and G’ band with temperature were investigated from the in-situ temperature-dependent Raman spectra of the as-grown CVD-graphene on the single crystal of Cu(100) under UHV. By eliminating the minor contributions of the lattice expansion and anharmonic phonon scattering effects, we were able to estimate the strain coefficient of the G and G’ bands of graphene over a wide temperature from 100 K to 800 K. Based on the strain coefficients and the correlation map of the G- and G’-band frequencies, we made a re...

The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2.

We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties.

Substrate crystal recovery for homoepitaxial diamond synthesis

Abstract Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.

Single-crystal diamond microneedles shaped at growth stage

Abstract Single-crystal diamond microneedles were extracted from (001) textured polycrystalline films. The films were produced using a plasma enhanced chemical vapor deposition (CVD) from a CH 4 /H 2 gas mixture activated by a direct current discharge. The as-grown textured polycrystalline CVD films consist of pyramid-shaped micrometer size diamond crystallites embedded into a nanodiamond ballas-like material. The less ordered fraction of the CVD film material was removed selectively using thermal oxidation. A dependence of the diamond needle shape on the CVD and the oxidation process parameters was revealed via a computer simulation and experimental studies. Ability for mass production of the diamond microneedles of different shapes was demonstrated. The needles are suitable for various applications from microcutting tools to quantum information processing.

Topographic and Spectroscopic Characterization of Electronic Edge States in CVD Grown Graphene Nanoribbons

We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron-electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5-7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.

Growth selectivity of hexagonal-boron nitride layers on Ni with various crystal orientations

Layered hexagonal-boron nitride (h-BN) films were synthesized by chemical vapor deposition (CVD) on Ni foils using ammonia borane as a precursor. Confocal Raman spectroscopy and electron backscatter diffraction (EBSD) were used to probe the effect of underlying Ni crystals with various orientations on growth behaviors of h-BN layers. The growth of the h-BN layers strongly depends on the Ni crystal orientations, where the growth rate of h-BN is larger on Ni(100)-like crystal surfaces but the growth on Ni(111)-like surfaces is not detectable, suggesting that Ni (100)-like facets are likely to promote the growth of h-BN compared with Ni (111)-like surfaces. The observation is in clear contrast to the reported growth of h-BN on Ni(111) in an ultrahigh vacuum environment. The as-grown CVD h-BN films on Ni exhibit a layered structure as revealed by atomic force microscopy (AFM). Thin h-BN layers are found on the Ni domain with a low growth rate. The observation of h-BN growth on various Ni grains may provide insights for the control of thickness, size and morphology of CVD h-BN films.

Enhanced optical properties of chemical vapor deposited single crystal diamond by low-pressure/high-temperature annealing

Single crystal diamond produced by chemical vapor deposition (CVD) at very high growth rates (up to 150 μm/h) has been successfully annealed without graphitization at temperatures up to 2200 °C and pressures <300 torr. Crystals were annealed in a hydrogen environment by using microwave plasma techniques for periods of time ranging from a fraction of minute to a few hours. This low-pressure/high-temperature (LPHT) annealing enhances the optical properties of this high-growth rate CVD single crystal diamond. Significant decreases are observed in UV, visible, and infrared absorption and photoluminescence spectra. The decrease in optical absorption after the LPHT annealing arises from the changes in defect structure associated with hydrogen incorporation during CVD growth. There is a decrease in sharp line spectral features indicating a reduction in nitrogen-vacancy-hydrogen (NVH − ) defects. These measurements indicate an increase in relative concentration of nitrogen-vacancy (NV) centers in nitrogen-containing LPHT-annealed diamond as compared with as-grown CVD material. The large overall changes in optical properties and the specific types of alterations in defect structure induced by this facile LPHT processing of high-growth rate single-crystal CVD diamond will be useful in the creation of diamond for a variety of scientific and technological applications.

The effects of oxygen, nitrogen, and hydrogen annealing on Mg acceptors in GaN as monitored by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to study the unpassivated Mg-related acceptor in GaN films. As expected, the trends observed before and after O2, N2, or forming-gas anneals at temperatures 850°C in O2 or N2 permanently removes the signal, contrary to the results of conductivity measurements. Approximately 1019 cm−3 Mg acceptors were detected in some GaN films grown by chemical vapor deposition (CVD) before acceptor activation, suggesting that it is possible to have electrically active Mg in as-grown CVD material.

Photoemission properties and surface structures of homoepitaxially grown CVD diamond(1 0 0) surfaces

In the present study, the surface properties of homoepitaxially grown chemical vapor deposition (CVD) diamond have been investigated using total photoyield measurements, scanning tunneling microscope (STM) observations and scanning tunneling spectroscopy (STS). The STM results show that as-grown CVD diamond(100) surfaces are characterized mainly by two types of regions with 2×1/1×2 structures and much less structures. These well-ordered surface structures can be significantly clearer after a low-temperature annealing at 150–200°C in ultra high vacuum. On the other hand, the STS results indicate the presence of band gap states, which agrees well with the fact that the observed photoemission threshold energy is located at 5eV, well below the band gap energy of 5.5eV, for the as-grown CVD diamond surface. The absolute value measurements of the photoemission total yields using a calibrated Si photodetector demonstrate substantially low photoemission efficiencies even for negative electron affinity (NEA) diamond surfaces, being consistent with the STM/STS data.

UHV μ-electron beam evaluation of the CVD diamond particles grown on Si(001)

Abstract The electronic and structural properties of diamond particles chemical vapor deposition (CVD)-grown on Si(001) surfaces were studied using a scanning probe μ-electron beam with SEM, RHEED, secondary-electron spectroscopy (SES) and Auger-electron spectroscopy in ultra-high-vacuum (UHV) conditions. The CVD grown diamond particles studied were made with a source gas of either CO/H 2 (sample A) or CH 4 /H 2 (sample B) on Si(001). The UHV-SEM images showed that sample A was composed of diamond particles with well-determined low-index surfaces while those for sample B with poorly determined surfaces. μ-RHEED patterns from diamond single-crystallites for sample A showed well-developed Kikuchi-bands together with surface spots indicating a good order of diamond framework near the surface. The diamond crystallites of sample A showed varieties of SES spectra and only a small fraction of them had negative electron affinity (NEA) although no differences are found in RHEED. The SES spectra for sample B showed that essentially all the diamond particles are NEA. It is found that the Fermi-level is located at ∼1.0 eV above the valence band maximum for NEA diamond particles and ∼1.5 eV for PEA ones. The change in electron affinity and the origin of surface conductive layers present on as-grown CVD diamonds are discussed in terms of the Fermi-level positions found presently.

Oxygen in silicon carbide: shallow donors and deep acceptors

The electrical and optical properties of oxygen (O)-implanted 6H-silicon carbide (SiC) chemical vapor deposition (CVD) epilayers are investigated by Hall effect, admittance spectroscopy, deep level transient spectroscopy (DLTS) and low temperature photoluminescence (LTPL). Two types of O-related centers are found: shallow donors in the energy range of (129–360) meV below the conduction band edge as well as deep acceptor-like defects at E C−480 meV, E C−560 meV and E C−610 meV. For the shallow donors, a certain sensitivity to heat treatments is demonstrated in terms of a decrease of their ionization energies when exposing the O +-implanted epilayers to temperatures at 1650–1800°C. In addition, evidence has been found for the incorporation of O in as-grown 4H-SiC CVD epilayers indicated by the observation of deep O-related defect centers in DLTS spectra.

Imaging micro-twin distributions in as-grown CVD diamond films with TEM

Fractured edges of diamond films grown by chemical vapor deposition (CVD) have been examined directly in a conventional transmission electron microscope (TEM) without thinning, An important advantage of the fracture specimen preparation technique is that the microstructures in the diamond grains at the growth face can be characterized directly by bright-field (BF) and dark-field (DF) TEM imaging and diffraction. Additionally, the topography of the same region can be directly determined from secondary electron (SE) images available in the same TEM.

A Novel TEM Technique for Characterizing As-Grown CVD Diamond Films

Microstructural studies by transmission electron microscopy (TEM) of diamond films grown by chemical vapor deposition (CVD) usually involve tedious specimen preparation. This process has been avoided with a technique that is described in this paper. For the first time, thick as-grown diamond films have been examined directly in a conventional TEM without thinning. With this technique, the important microstructures near the growth surface have been characterized. An as-grown diamond film was fractured on a plane containing the growth direction. It took about 5 min to prepare a sample. For TEM examination, the film was tilted about 30-45° (see Fig. 1). Microstructures of the diamond grains on the top edge of the growth face can be characterized directly by transmitted electron bright-field (BF) and dark-field (DF) images and diffraction patterns.