Chemical Vapor Deposition Films Research Articles

Gas-Phase Synthesis of Iron Silicide Nanostructures Using a Single-Source Precursor: Comparing Direct-Write Processing and Thermal Conversion.

The investigation of precursor classes for the fabrication of nanostructures is of specific interest for maskless fabrication and direct nanoprinting. In this study, the differences in material composition depending on the employed process are illustrated for focused-ion-beam- and focused-electron-beam-induced deposition (FIBID/FEBID) and compared to the thermal decomposition in chemical vapor deposition (CVD). This article reports on specific differences in the deposit composition and microstructure when the (H3Si)2Fe(CO)4 precursor is converted into an inorganic material. Maximum metal/metalloid contents of up to 90 at. % are obtained in FIBID deposits and higher than 90 at. % in CVD films, while FEBID with the same precursor provides material containing less than 45 at. % total metal/metalloid content. Moreover, the Fe:Si ratio is retained well in FEBID and CVD processes, but FIBID using Ga+ ions liberates more than 50% of the initial Si provided by the precursor. This suggests that precursors for FIBID processes targeting binary materials should include multiple bonding such as bridging positions for nonmetals. In addition, an in situ method for investigations of supporting thermal effects of precursor fragmentation during the direct-writing processes is presented, and the applicability of the precursor for nanoscale 3D FEBID writing is demonstrated.

Nano- and micro-crystalline diamond film structuring with electron beam lithography mask

Direct current plasma enhanced chemical vapor deposition (CVD) was employed to create polycrystalline diamond films from CH4/H2 gaseous mixture at 98 mbar pressure and various substrate temperatures between 720 °C and 960 °C. The Si chips with patterns of periodic masked and open seeded zones were used as substrates. The mask free seeded areas evolved into polycrystalline diamond films after CVD process. The diamond crystallites of the films featured single crystal ordering individually with distinct cubic (100) or octahedral (111) facets on the film surfaces. Notably, specific growth conditions were determined for obtaining diamond films composed of the crystallites of nanometre and micrometre scale. These conditions are differing from those observed for non-pattern-prepared Si substrates. The nano-crystalline diamonds emerged within the 4.5–5 A current range, with growth conditions involving 3% CH4/H2 mixture at 98 mbar. The micro-crystalline diamonds (MCDs) predominantly characterized by well-developed rectangular (100) crystal faces on the film surface were successfully grown with current settings of 5.5–6 A, under 3% CH4/H2 mixture at 98 mbar. Furthermore, MCDs characterized by entirely crystalline (111) diamond faces forming CVD film surface were attained within a growth parameter range of 4.5–5.8 A, employing 3% CH4/H2 mixture for certain samples, or alternatively, utilizing 5 A with a 1.5% CH4/H2 mixture for others. Upon thorough evaluation, it was established that SiO2, TiO2, and Cr masks are well-suited materials for the planar patterning of both nano- and micro-crystalline diamond films, and the bottom-up approach can pave the way for the production of diamond planar structures through CVD, facilitated by electron beam lithography (EBL).

Plasma deposited carbon containing zirconia films as thermal barriers

Carbon containing zirconia films are deposited from chemical vapor deposition (CVD) and from plasma enhanced chemical vapor deposition (PECVD), as being used as thermal barrier coatings for many applications. Their thermal conductivity has been measured for temperatures ranging from room temperature up to 450 K using the 3 ω method. It is shown that the samples exhibit a lattice and an electronic contribution to the thermal conductivity reaching values up to 13 W/m/K for samples from CVD and up to 5 W/m/K for the samples from PECVD at 450 K. At room temperature, the thermal conductivities for PECVD films are in the range of 1 W/m/K, whereas the CVD film shows values of 8 W/m/K. The large differences are explained by the microstructure of the films, showing smaller grain sizes with poor interfaces for the PECVD films compared to the denser CVD films. This shows that plasma deposited carbon containing zirconia films may serve as excellent thermal barriers.

Systematic Studies into the Area Selectivity of Chemical Vapor Deposition Polymerization

As the current top-down microchip manufacturing processes approach their resolution limits, there is a need for alternative patterning technologies that offer high feature densities and edge fidelity with single-digit nanometer resolution. To address this challenge, bottom-up processes have been considered, but they typically require sophisticated masking and alignment schemes and/or face materials' compatibility issues. In this work, we report a systematic study into the impact of thermodynamic processes on the area selectivity of chemical vapor deposition (CVD) polymerization of functional [2.2]paracyclophanes (PCP). Adhesion mapping of preclosure CVD films by atomic force microscopy (AFM) provided a detailed understanding of the geometric features of the polymer islands that form under different deposition conditions. Our results suggest a correlation between interfacial transport processes, including adsorption, diffusion, and desorption, and thermodynamic control parameters, such as substrate temperature and working pressure. This work culminates in a kinetic model that predictes both area-selective and nonselective CVD parameters for the same polymer/substrate ensemble (PPX-C + Cu). While limited to a focused subset of CVD polymers and substrates, this work provides an improved mechanistic understanding of area-selective CVD polymerization and highlights the potential for thermodynamic control in tuning area selectivity.

Comparing the effect of synthesis techniques on the semiconductor-metal transition of VO2 thin films

We compare the effect of two synthesis methods namely Chemical Vapor Deposition (CVD) and Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) on the Semiconductor-Metal Transition (SMT) characteristics of VO2 thin films. The SMT characteristics of the films are measured electrically and optically. CVD films show a lower resistance ratio and SMT strength, whereas films synthesized via UNSPACM are rougher. We ascribe the reason for the observed behaviour to morphology and off-stoichiometry arising from the deposition process.

Combined HF+MW CVD Approach for the Growth of Polycrystalline Diamond Films with Reduced Bow

A combination of two methods of chemical vapor deposition (CVD) of diamond films, microwave plasma–assisted (MW CVD) and hot filament (HF CVD), was used for the growth of 100 µm-thick polycrystalline diamond (PCD) layers on Si substrates. The bow of HF CVD and MW CVD films showed opposite convex\concave trends; thus, the combined material allowed reducing the overall bow by a factor of 2–3. Using MW CVD for the growth of the initial 25 µm-thick PCD layer allowed achieving much higher thermal conductivity of the combined 110 µm-thick film at 210 W/m·K in comparison to 130 W/m·K for the 93 µm-thick pure HF CVD film.

Boron-carbon-silicon film chemical vapor deposition by boron trichloride, dichlorosilane and monomethylsilane gases

A boron-carbon-silicon film was formed by an atmospheric pressure chemical vapor deposition method using a gas mixture of boron trichloride, dichlorosilane and monomethylsilane in ambient hydrogen at 800–1000 °C. The boron, carbon and silicon concentrations in the obtained film were about 15–30 %, 0–10 % and 50–80 %, respectively. With the increasing monomethylsilane gas flow rate, the carbon and silicon concentrations in the obtained film increased and decreased, respectively. In the obtained film, various chemical bonds between the boron, carbon and silicon existed, except for the carboncarbon bond. The following conclusions were obtained. (i) The carbon atoms existing at the surface have a chemical bond with the boron atoms, but not with the carbon and silicon atoms. (ii) The surface intermediate compound of boron chloride helps incorporating the carbon, while that of dichlorosilane does not. (iii) The etching by the hydrogen chloride gas significantly influenced the film formation process at higher than 900 °C.

Characteristics of Vertical Ga2O3 Schottky Junctions with the Interfacial Hexagonal Boron Nitride Film.

We present the device properties of a nickel (Ni)–gallium oxide (Ga2O3) Schottky junction with an interfacial hexagonal boron nitride (hBN) layer. A vertical Schottky junction with the configuration Ni/hBN/Ga2O3/In was created using a chemical vapor-deposited hBN film on a Ga2O3 substrate. The current–voltage characteristics of the Schottky junction were investigated with and without the hBN interfacial layer. We observed that the turn-on voltage for the forward current of the Schottky junction was significantly enhanced with the hBN interfacial film. Furthermore, the Schottky junction was analyzed under the illumination of deep ultraviolet light (254 nm), obtaining a photoresponsivity of 95.11 mA/W under an applied bias voltage (−7.2 V). The hBN interfacial layer for the Ga2O3-based Schottky junction can serve as a barrier layer to control the turn-on voltage and optimize the device properties for deep-UV photosensor applications. Furthermore, the demonstrated vertical heterojunction with an hBN layer has the potential to be significant for temperature management at the junction interface to develop reliable Ga2O3-based Schottky junction devices.

Numerical investigation of SiO<sub>2</sub> film deposition enhanced by capacitively coupled discharge plasma

Higher requirements for the performances of thin films need to be fulfilled in the rapid development of integrated circuit technology, due to the more complicate structure and smaller size of chips. In plasma-enhanced chemical vapor deposition , high-density and high-performance thin films can be deposited at low temperature, compared with traditional chemical vapor deposition. In this work, a two-dimensional fluid/MC model coupled with the deposition module is used to describe the capacitively coupled SiH<sub>4</sub>/N<sub>2</sub>O/Ar discharges as well as the deposition processes, focusing on the influences of the radial position, gas ratio and gas pressure on the deposition of silicon oxide films. The results show that the edge effect which leads the plasma density to rise near the electrode edges gives rise to the non-uniform deposition rate along the radial direction. It is also found that the more N<sub>2</sub>O and less Ar content in the gas mixture, as well as an increased gas pressure will improve this uniformity. However, an excessive deposition rate will lead to a series of undesirable phenomena, such as “key hole structure”, vacancies and excessive impurities in films. These problems are also troublesome in the microelectronics manufacture processes. More detailed investigation into the deposition mechanism can be expected in the future .

Toward the commercialization of chemical vapor deposition graphene films

Since its first successful isolation over a decade ago, academic and industrial interest has triggered the steady progress of the commercialization of graphene, as evidenced by a wealth of graphene-related patents, products, institutes, and startups. Among currently available graphene materials, graphene films derived from chemical vapor deposition (CVD) techniques, with fine controllability and uniformity, have been proven to be a promising candidate for various applications, with exciting demonstrations in electronics, optoelectronics, sensors, and filtering membrane. In this review, recent progress toward the commercialization of CVD films is summarized, covering the state-of-the-art methods for controllable synthesis, up-scale technologies for mass production, and demonstrations in potential commercial applications, which will propel the successful commercialization of graphene films by transforming the laboratory-scale advances. Moreover, a brief summary of the current market of CVD graphene films is provided with regarding to the commercial graphene products and production equipment. Finally, a perspective on the critical challenges and future direction of CVD graphene films will be presented.

Effect of Low-Pressure Plasma Nitriding with Hollow Cathode Discharge on the Surface Microstructure of WC-Co Cermet

WC-Co cermet was plasma-nitrided with the assistance of a hollow cathode ion source at 400 °C under a vacuum of 3–8 Pa. Hot film chemical vapor deposition (HFCVD) of a diamond coating was carried out on the nitrided specimen, without chemical etching. Scanning electronic microscopy, electron probing microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the surface microstructure of the nitride specimens and the coatings. A thin surface conversion layer with a specific structure was formed, in which the primary Co binder was transformed into Co-rich particles. The Co-rich particles consisted of a γ-Co core and a Co4N outer layer. This specific surface conversion layer significantly suppresses the out-diffusion and catalytic graphitization of Co during HFCVD. The existent phase, morphology, and density distribution of Co compounds can be tuned by varying the nitriding parameters, such as gas media, ionization ratio, bombardment energy flux, and nitriding duration.

Two-dimensional ReS2 : Solution to the unresolved queries on its structure and inter-layer coupling leading to potential optical applications

Over the last few years, ReS2 has generated a myriad of unattended queries regarding its structure, the concomitant thickness dependent electronic properties and apparently contrasting experimental optical response. In this work, with elaborate first-principles investigations, using density functional theory (DFT) and time-dependent DFT (TDDFT), we identify the structure of ReS2, which is capable of reproducing and analyzing the layer-dependent optical response. The theoretical results are further validated by an in-depth structural, chemical, optical and optoelectronic analysis of the large-area ReS2 thin films, grown by chemical vapor deposition (CVD) process. Micro-Raman (MR), X-ray photoelectron spectroscopy (XPS), cross-sectional transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDAX) have enabled the optimization of the uniform growth of the CVD films. The correlation between the layer-dependent optical and electronic properties of the excited states was established by static photoluminescence (PL) and transient absorption (TA) measurements. Sulfur vacancy-induced localized mid-gap states render a significantly long life-time of the excitons in these films. The ionic gel top-gated photo-detectors, fabricated from the as-prepared CVD films, exhibit a large photo-response of ~ 5 A/W and a remarkable detectivity of ~ 1011 Jones. The outcome of the present work will be useful to promote the application of vertically grown large-area films in the field of optics and opto-electronics.

Investigation of novel optical and waveguide characteristics for an air–graphene–LiNbO3 system

The optical characteristics of a planar thin film waveguide system composed of air–graphene–LiNbO3 have been investigated. Monolayer or bilayer graphene of high quality are characterized by Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The refractivity and reflectivity of the air–graphene–LiNbO3 system are measured experimentally and compared with those of a LiNbO3 waveguide by the prism coupling method. The reflectivity shows an overall decrease due to the lower transmittance for graphene on the LiNbO3 substrate. The refractivity increases significantly at the wavelength of 1540 nm, which may be attributed to the generation of graphene surface plasmons excited by infrared radiation. A shaped air–graphene–LiNbO3 waveguide is designed and simulated by Mode Solutions. The distribution of an optical field is performed and analyzed. The preparation of the proposed air–graphene–LiNbO3 structure incorporates the commonly used chemical vapor deposition and thin film transfer techniques, and is compatible with existing optoelectronic integration processes, which can be employed for building various optical integrated devices.

Localized Probing of Dielectric Breakdown in Multilayer Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) has emerged as a promising 2D/layered dielectric owing to its successful integration with graphene and other 2D materials, although a coherent picture of the overall dielectric breakdown mechanism in h-BN is yet to emerge. Here, we have carried out a systematic study using conduction atomic force microscopy to provide insights into the process of defect generation and dielectric degradation in the progressive breakdown (PBD) and hard breakdown (HBD) stages in 2-5 nm thick chemical vapor deposition (CVD)-grown multilayer h-BN films. The PBD and HBD regimes show different behaviors. Under electrical stress in the PBD stage, defects are generated progressively in the h-BN, leading to a gradual reduction of the effective barrier resistance and continuous soft breakdowns (SBDs) of the dielectric material. Random telegraph noise nano-spectroscopy shows that low frequency noise becomes dominant after an SBD event due to the creation of additional defects around the percolation path. We also observe a wide variation in the current-voltage (I-V) breakdown plots in the PBD stage, giving rise to non-Weibull statistical distribution of the breakdown voltage. We attribute this observation to the significant thickness inhomogeneity in the CVD films. At HBD, h-BN materials are always physically removed from the film, leading to the formation of pits at the breakdown location. Interestingly, pit formation is also occasionally observed in the PBD stage under very low current compliances, suggesting that breakdown may proceed by a mixture of defect generation and material removal in h-BN CVD films.

Effects of Substrate Bias on the Diamond Nanostructure by Reactive Ion Etching

Dense and uniform diamond nanocone arrays on titanium substrate were constructed by using bias-assisted reactive ion etching (RIE) of diamond films in a microwave plasma chemical vapor deposition (MPCVD) system. A hydrogen/argon mixture was employed as work gas with ratio of 3/1. The formation of nanocone structure was generated the lengthways physical bombardment/sputtering by argon ions, and selective chemical reaction of graphite and disordered carbons by hydrogen atoms and ions. The structure, size, and density depended on the substrate bias. The surface morphology of diamond film and nanostructures were characterized by field emission scanning electron microscope (FESME), and the composition of diamond film and nanostructures were characterized by Raman spectra. The Stopping and Range of Trons in Matter (SRIM) software was used to simulate the bombardment effect of the diamond film on different Ar+ ion incident angles during the etching process. The experimental results showed substrate bias at −180 V and −210 V was beneficial to the formation of high density, small size and sharp nanocones, meanwhile larger bias contributed to the formation of large size nanocones. Besides, as-prepared nanocones still maintained significant diamond phase.

Rapid and Large-Area Visualization of Grain Boundaries in MoS2 on SiO2 Using Vapor Hydrofluoric Acid.

Grain boundaries in two-dimensional (2D) material layers have an impact on their electrical, optoelectronic, and mechanical properties. Therefore, the availability of simple large-area characterization approaches that can directly visualize grains and grain boundaries in 2D materials such as molybdenum disulfide (MoS2) is critical. Previous approaches for visualizing grains and grain boundaries in MoS2 are typically based on atomic resolution microscopy or optical imaging techniques (i.e., Raman spectroscopy or photoluminescence), which are complex or limited to the characterization of small, micrometer-sized areas. Here, we show a simple approach for an efficient large-area visualization of the grain boundaries in continuous chemical vapor-deposited films and domains of MoS2 that are grown on a silicon dioxide (SiO2) substrate. In our approach, the MoS2 layer on a SiO2/Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO2 at the MoS2 grain boundaries and SiO2 underneath the MoS2 grains as a result of VHF diffusing through the defects in the MoS2 layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO2 pattern using optical microscopy, scanning electron microscopy, or Raman spectroscopy. This method allows for a simple and rapid evaluation of grain sizes in 2D material films over large areas, thereby potentially facilitating the optimization of synthesis processes and advancing applications of 2D materials in science and technology.

Synthesis and characterization of 2D platinum diselenide

Two-dimensional (2D) materials have been intensively studied due to their outstanding chemical, physical, electronic and optical properties potentially offering a breakthrough in the search of a direct integration of these materials with the silicon (Si) technology. Among them, platinum diselenide (PtSe2) is of significant interest for applications in electronics, spintronics, sensors, catalysis, etc. In this paper, we report a large-scale synthesis of PtSe2-layers by thermally-assisted selenization of pre-deposited platinum films in a horizontal quartz-tube chemical vapor deposition (CVD) reactor. The overall structural information and quality of the PtSe2-films is analyzed by Raman spectroscopy and Raman active modes identification. The electrical characterization via V-I characteristics reveals sheet resistance values in the range of ~103 Ω.

Moisture diffusion in plasma-enhanced chemical vapor deposition dielectrics characterized with three techniques under clean room conditions

Absorption of moisture by thin dielectric materials alters their properties and can cause several reliability issues. Even at standard room temperature and low humidity level, some dielectric materials are sensitive to moisture. In this study, moisture diffusion in two plasma-enhanced chemical vapor deposition (PECVD) films is investigated with three measurement methods to determine diffusion coefficients and saturated moisture concentrations: mass measurements, bending radius of curvature measurements and infrared spectroscopy. The two PECVD silicon dioxides are deposited at 200 °C and 400 °C. They were exposed to moisture in clean room environment (21 °C and 40% relative humidity) for about 800 h. The present results confirm that mass measurements, bending radius of curvature measurements and infrared spectroscopy can be used to monitor thin dielectric films in these environmental conditions. They lead to similar values for the diffusion coefficient. These values are in the range of [1.5–4.2] × 10−15 cm² s−1 for the 200 °C film and [2.3–3.6] × 10−15 cm² s−1 for the 400 °C one. Saturated moisture concentrations confirm that the two dielectrics are sensitive to moisture even at 21 °C, 40% relative humidity. Besides, the results show that standard fickean behavior does not provide the best fit to model water diffusion for some dielectric films. A dual stage model that appears to be more adapted is finally introduced.

Nucleation promoted synthesis of large-area ReS2 film for high-speed photodetectors

Rhenium disulfide (ReS2) is a transition metal dichalcogenide with a layer-independent direct bandgap. Notably, the weak interlayer coupling owing to its T-phase structure enables multi-layer ReS2 to behave similarly to decoupled monolayers. This inherent characteristic makes continuous multilayer ReS2 film a unique platform for large-area electronic applications. To date, the bulk of work on ReS2 has been conducted using mechanically exfoliated samples or small size flakes (<1 mm2) with no potential for large-scale electronics. A chemical vapor deposition (CVD) synthesis of a large area, continuous ReS2 film directly on a SiO2 substrate is also known to be more challenging compared with that of other 2D materials, such as MoS2 and WS2. This is partly due to its tendency to grow into discrete dendritic structures. In this study, a large-area (>1 cm2), continuous multilayer ReS2 film is directly synthesized on a SiO2 substrate without any transfer process. The polycrystalline ReS2 film synthesized by this method exhibits one of the fastest photoresponse speeds (0.03 s rise time and 0.025 s decay time) among the reported CVD films. The photoresponsivity Rλ was also the highest among large-area CVD films. The synthesis method for a continuous multilayer ReS2 film is amenable to large-scale integration and will pave the way for practical optoelectronic applications based on 2D layered materials.

Contrasting the electrochemical behaviors of graphene film with hexagonal boron nitride film before and after modification with stearic acid on Cu in 3.5 wt% NaCl solution

Graphene and hexagonal boron nitride (h-BN) films were grown on the copper foil using chemical vapor deposition (CVD) method. Stearic acid was used to further repair the defects existing on both of the graphene and h-BN films. Raman spectra, Scanning probe microscope (SPM, Dimension 3100) and scanning electron microscope (SEM, Quanta 250) images confirmed the existence, thickness, and distribution of graphene and h-BN film. The morphologies of graphene and h-BN modified with stearic acid were demonstrated by SEM and SPM images. Contact angles were used to show the improvement of wettability of CVD films after modification. Electrochemical results manifested that the corrosion rate of copper foil under graphene film is 1.55 times higher than that of copper foil under h-BN film, and the corrosion rate of copper foil under graphene film is 2.87 times higher than that of copper foil under modified graphene film. SEM, SPM, laser scanning confocal microscopy (LSM, Zeiss LSM700), Raman spectra, and x-ray diffraction (XRD, D8 ADVANCE) results supported the better anti-corrosion performance of h-BN film comparing with graphene film and effective auxiliary role of stearic acid after the samples immersed in 3.5 wt% NaCl solution for 30 days. The excellent anti-corrosion ability of h-BN film was attributed to the similar laminated structure as graphene and the electrically insulating nature.