Plasma Enhancement Research Articles

Performance Improvement of Proton Ceramic Fuel Cells through Surface Treatment of Cobalt Oxide Nanoparticles on Perovskite Oxide

This study reports on the performance improvement of a protonic ceramic fuel cell (PCFC) after a CoOx nanoparticle treatment has been applied to a PrBa0.5Sr0.5Co2-xFexO5+δ(PBSCF) cathode with a perovskite structure. CoOx nanoparticles are deposited on the sintered PBSCF surface using a plasma-enhanced (PE) atomic layer deposition (ALD) process, thereby avoiding any unwanted reactions or phase changes. The CoOx nanoparticles are successfully deposited uniformly onto the entire surface of the porous and complex cathode structure. A constant deposition rate is observed because of the self-limiting characteristics of the ALD process by a thickness difference as a function of a change in the cycle count. In our experiment, the performance of the fuel cells increases by approximately 36 % compared with the untreated cells at an operating temperature of 650 °C. In addition, all cells feature long-term stability. Impedance analysis reveals that the CoOx nanoparticle treatment results in a significant polarization and some ohmic loss improvement within all temperature regions. This is due to the synergistic effect with PBSCF and self-catalytic effects. The results imply that the proposed method enables high-performance PCFC fabrication; additionally it helps lowering the operating temperature.

High-performance proton ceramic fuel cells using a perovskite oxide cathode surface decorated with CoOx nanoparticles

This study reports on the performance improvement of a protonic ceramic fuel cell (PCFC) after a CoOx nanoparticle treatment has been applied to a PrBa0.5Sr0.5Co2-xFexO5+δ(PBSCF) cathode with a perovskite structure. CoOx nanoparticles are deposited on the sintered PBSCF surface using a plasma-enhanced (PE) atomic layer deposition (ALD) process, thereby avoiding any unwanted reactions or phase changes. The CoOx nanoparticles are successfully deposited uniformly onto the entire surface of the porous and complex cathode structure. A constant deposition rate is observed because of the self-limiting characteristics of the ALD process by a thickness difference as a function of a change in the cycle count. In our experiment, the performance of the fuel cells increases by approximately 36 % compared with the untreated cells at an operating temperature of 650 °C. In addition, all cells feature long-term stability. Impedance analysis reveals that the CoOx nanoparticle treatment results in a significant polarization and some ohmic loss improvement within all temperature regions. This is due to the synergistic effect with PBSCF and self-catalytic effects. The results imply that the proposed method enables high-performance PCFC fabrication; additionally it helps lowering the operating temperature.

Single-Crystal Diamond Needle Fabrication Using Hot-Filament Chemical Vapor Deposition.

Single-crystal diamonds in the form of micrometer-scale pyramids were produced using a combination of hot-filament (HF) chemical vapor deposition (CVD) and thermal oxidation processes. The diamond pyramids were compared here with similar ones that were manufactured using plasma-enhanced (PE) CVD. The similarities revealed in the morphology, Raman, and photoluminescent characteristics of the needles obtained using the hot-filament and plasma-enhanced CVD are discussed in connection with the diamond film growth mechanism. This work demonstrated that the HF CVD method has convincing potential for the fabrication of single-crystal diamond needles in the form of regularly shaped pyramids on a large surface area, even on non-conducting substrates. The experimental results demonstrated the ability for the mass production of the single-crystal needle-like diamonds, which is important for their practical application.

From Precursor Chemistry to Gas Sensors: Plasma-Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications.

The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2 ], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes.

Multiscale plasma and feature profile simulations of plasma-enhanced chemical vapor deposition and atomic layer deposition processes for titanium thin film fabrication

Mechanisms of titanium (Ti) thin films deposited in plasma-enhanced (PE) chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes have been elucidated via multiscale plasma and feature profile simulations. Firstly, by iterating a 2D reactor-scale plasma simulation and a feature-scale deposition profile simulation, a shared surface reaction model has been determined for PECVD processes. Ti film thicknesses and profiles consistently computed both along a wafer surface and inside a test structure agreed very well with the experimental data. Then, another multiscale simulation, with the same plasma model and the surface reaction model to which the Eley–Rideal surface kinetics is added, has been applied to a PEALD process. The simulation succeeded again in reproducing and explaining the non-conformal Ti film deposition observed in experiments, while conformal Ti films are obtained in PECVD processes. Through the simulation work, comprehensive mechanisms of Ti film deposition, which cover both PECVD and PEALD processes, have been discussed and proposed in this study.

Heteroepitaxial Growth of Germanium-on-Silicon Using Ultrahigh-Vacuum Chemical Vapor Deposition with RF Plasma Enhancement

Germanium (Ge) films have been grown on silicon (Si) substrate by ultrahigh-vacuum chemical vapor deposition with plasma enhancement (PE). Argon plasma was generated using high-power radiofrequency (50 W) to assist in germane decomposition at low temperature. The growth temperature was varied in the low range of 250°C to 450°C to make this growth process compatible with complementary metal–oxide–semiconductor technology. The material and optical properties of the grown Ge films were investigated. The material quality was determined by Raman and x-ray diffraction techniques, revealing growth of crystalline films in the temperature range of 350°C to 450°C. Photoluminescence spectra revealed improved optical quality at growth temperatures of 400°C and 450°C. Furthermore, material quality study using transmission electron microscopy revealed existence of defects in the Ge layer grown at 400°C. Based on the etch pit density, the average threading dislocation density in the Ge layer obtained at this growth temperature was measured to be 4.5 × 108 cm−2. This result was achieved without any material improvement steps such as use of graded buffer or thermal annealing. Comparison between PE and non-plasma-enhanced growth, in the same machine at otherwise the same growth conditions, indicated increased growth rate and improved material and optical qualities for PE growth.

Atomic layer deposition of AlN from AlCl3 using NH3 and Ar/NH3 plasma

The atomic layer deposition (ALD) of AlN from AlCl3 was investigated using a thermal process with NH3 and a plasma-enhanced (PE)ALD process with Ar/NH3 plasma. The growth was limited in the thermal process by the low reactivity of NH3, and impractically long pulses were required to reach saturation. Despite the plasma activation, the growth per cycle in the PEALD process was lower than that in the thermal process (0.4 Å vs 0.7 Å). However, the plasma process resulted in a lower concentration of impurities in the films compared to the thermal process. Both the thermal and plasma processes yielded crystalline films; however, the degree of crystallinity was higher in the plasma process. The films had a preferential orientation of the hexagonal AlN [002] direction normal to the silicon (100) wafer surface. With the plasma process, film stress control was possible and tensile, compressive, or zero stress films were obtained by simply adjusting the plasma time.

Vertical graphene nanosheets synthesized by thermal chemical vapor deposition and the field emission properties

In this paper, we explored synthesis of vertical graphene nanosheets (VGNs) by thermal chemical vapor deposition (CVD). Through optimizing the experimental condition, growth of well aligned VGNs with uniform morphologies on nickel-coated stainless steel (SS) was realized for the first time by thermal CVD. In the meantime, influence of growth parameters on the VGN morphology was understood based on the balancing between the concentration and kinetic energy of carbon-containing radicals. Structural characterizations demonstrate that the achieved VGNs are normally composed of several graphene layers and less corrugated compared to the ones synthesized by other approaches, e.g. plasma enhanced (PE) CVD. The field emission measurement indicates that the VGNs exhibit relatively stable field emission and a field enhancement factor of about 1470, which is comparable to the values of VGNs prepared by PECVD can be achieved.

PECVD of Hematite Nanoblades and Nanocolumns: Synthesis, Characterization, and Growth Model

Fe2O3 nanostructures are fabricated on Si(100) and SiO2 by plasma enhanced (PE)CVD from a FeIII β‐diketonate precursor. Depositions are carried out in Ar‐O2 plasmas, devoting particular attention to the influence of growth temperature (from 60 to 400 °C) on material chemico‐physical properties. Remarkably, high purity, single‐phase α‐Fe2O3 nanostructures are obtained at temperatures as low as 60 °C. Furthermore, the deposit nano‐organization can be tuned from <110>‐oriented 2D nanoblade arrays to denser nanocolumns upon increasing the deposition temperature. A possible growth model is proposed to account for the observed structural/morphological features and light absorption properties of the target materials.

Plasma boriding of a cobalt–chromium alloy as an interlayer for nanostructured diamond growth

Chemical vapor deposited (CVD) diamond coatings can potentially improve the wear resistance of cobalt–chromium medical implant surfaces, but the high cobalt content in these alloys acts as a catalyst to form graphitic carbon. Boriding by high temperature liquid baths and powder packing has been shown to improve CVD diamond compatibility with cobalt alloys. We use the microwave plasma-enhanced (PE) CVD process to deposit interlayers composed primarily of the borides of cobalt and chromium. The use of diborane (B2H6) in the plasma feedgas allows for the formation of a robust boride interlayer for suppressing graphitic carbon during subsequent CVD of nano-structured diamond (NSD). This metal–boride interlayer is shown to be an effective diffusion barrier against elemental cobalt for improving nucleation and adhesion of NSD coatings on a CoCrMo alloy. Migration of elemental cobalt to the surface of the interlayer is significantly reduced and undetectable on the surface of the subsequently-grown NSD coating. The effects of PECVD boriding are compared for a range of substrate temperatures and deposition times and are evaluated using glancing-angle X-ray diffraction (XRD), cross-sectional scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and micro-Raman spectroscopy. Boriding of CoCrMo results in adhered nanostructured diamond coatings with low surface roughness.

Recent Advances in Atmospheric Vapor‐Phase Deposition of Transparent and Conductive Zinc Oxide

The industrial need for high‐throughput and low‐cost ZnO deposition processes has triggered the development of atmospheric vapor‐phase deposition techniques which can be easily applied to continuous, in‐line manufacturing. While atmospheric CVD is a mature technology, new processes for the growth of transparent conductive oxides on thermally sensitive materials or flexible substrates are being developed, such as atmospheric plasma‐enhanced (PE)‐CVD and atmospheric spatial atomic layer deposition (ALD). In this article, the challenges and recent results on the growth of ZnO under atmospheric pressure by CVD, PE‐CVD, and spatial ALD are reviewed and the use of these films as transparent electrodes in thin film solar cells are presented.

Substrate‐thickness Dependence of Hydrogenated Microcrystalline Silicon Nucleation Rate on Amorphous Silicon Layer

AbstractThe nucleation rate of hydrogenated microcrystalline silicon (µc‐Si:H) films deposited by plasma‐enhanced (PE)CVD on hydrogenated amorphous silicon (a‐Si:H) substrates is investigated through structural and electrical characterization, with special attention paid to the initial growth stage of µc‐Si:H films. It is found that the nucleation rate of µc‐Si is dependent on the thickness of the a‐Si:H substrate. The µc‐Si:H film exhibits a rapid nucleation on a thin a‐Si:H layer, leaving a thin incubation layer at the µc‐Si/substrate interface. This substrate‐thickness dependence of the nucleation rate is proposed to be correlated with the stress inside the a‐Si:H layer. The high interfacial stress existing in the thin a‐Si:H layer facilitates the formation of high concentration, strained Si‐Si bonds, which are responsible for the rapid µc‐Si nucleation. The thick a‐Si:H layer relaxes the interfacial stress through the formation of islands in the Stranski‐Krastanow (S‐K) growth mode, while the intrinsic stress is still low, resulting in a long nucleation process allowing for the intrinsic compressive stress to be accumulated that is necessary for the µc‐Si deposited on it.

Broad-band, High-efficiency Optical Absorbers Derived From Carbon Nanomaterials

ABSTRACTOptical absorption efficiency, an important metric for sensing, radiometric and energy harvesting applications, has been studied theoretically and experimentally in porous, ordered nanostructures, including multi-walled- (MW) carbon nanotubes (CNTs) and single-walled- (SW) CNTs. We have characterized the absorption efficiencies in the 350 nm -7000 nm wavelength range of vertically aligned MWCNT arrays with high site densities synthesized directly on metallic substrates using a plasma-enhanced (PE)- chemical vapor deposition (CVD) process. Our ultra-thin absorbers exhibit a reflectance as low as ∼ 0.02 % (100 X lower than the benchmark). Such high efficiency absorbers are particularly attractive for radiometry, as well as energy harnessing applications. This work increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures arising from a plasma process. Optical modeling calculations were conducted that enabled a determination of the extinction coefficient in the films.

In-line deposition of silicon-based films by hot-wire chemical vapor deposition

Silicon-based films such as hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and hydrogenated amorphous silicon nitride (a-SiNx:H) can be deposited by hot-wire chemical vapor deposition (HW-CVD). The HW-CVD technology differs from conventional plasma-enhanced (PE)-CVD in a number of technological aspects, such as soft activation, high growth rates, and low system costs. To evaluate the HW-CVD technology for thin film deposition in solar industry an in-line hot-wire CVD system was used to deposit a-Si:H films for passivation of crystalline silicon solar cells as well as for the fabrication of thin film silicon solar cells. The HW-CVD system consists of seven vacuum chambers including three hot-wire systems with maximum deposition areas of 500mm by 600mm for each hot-wire activation source. The deposition processes were investigated by applying design of experiment to identify the effects and interactions of the process parameters on the deposition characteristics and film properties. The process parameters investigated were silane flow, deposition pressure, substrate temperature, film thickness, as well as temperature, diameter and number of wires, respectively. Growth rates up to 2.5nm/s were achieved for a-Si:H films. Intrinsic a-Si:H films for passivation of different crystalline solar cell types yielded carrier lifetimes of more than 1000μs for film thickness values below 20nm. For n-doped a-Si:H films prepared with PH3 as dopant gas, electrical resistivity is in the range of 102Ωcm. P-doped a-Si:H films prepared with B2H6 as dopant gas show electrical resistivity of about 105Ωcm. Crystalline silicon heterojunction solar cells with intrinsic thin layer (HIT cells) exhibit energy conversion efficiencies of more than 17% when fabricated with intrinsic HW-CVD amorphous silicon films as passivation layers.

Remote Plasma‐Assisted CVD Growth of Carbon Nanotubes in an Optimised Rapid Thermal Reactor

AbstractThe advances in the use of a remote plasma in combination with a rapid radiative reactor in plasma‐enhanced (PE) CVD is presented here. The characteristics and parameters of this system are fully analyzed and compared with conventional CVD growth methods. Growth of multi‐ and single‐walled carbon nanotubes (CNTs) at low temperatures with high quality and reproducibility has been achieved in this way. Further, a new catalytic system, which gives dense multi‐wall CNTs on metal substrates, is presented.

Resistive switching properties of plasma enhanced-ALD La2O3 for novel nonvolatile memory application

Plasma enhanced (PE) atomic layer deposition (ALD) of lanthanum oxide films on silicon and platinum substrates were examined using lanthanum-2,2,6,6-tetramethyl-3,5-heptanedione, and O2 plasma as precursors. The effect of pulse time and deposition temperature on the growth rate was investigated. Resistive switching behaviors of La2O3 prepared by PE-ALD were investigated as a promising candidate for next generation nonvolatile memory technology. The crystalline structure of the obtained film was found to be polycrystalline by transmission electron microscope and the chemical composition of the film was estimated to be stoichiometric La2O3 by x ray photoelectron spectroscopy. The low resistance ON state and high resistance OFF state can be reversibly altered under a low voltage about 1.5 and −0.6 V. More than 1000 reproducible switching cycles by dc voltage sweep were observed with a resistance ratio above 100, which was large enough to read out without obvious degradation. Moreover, the estimated working characteristics such as set and reset voltages distribution were sufficiently stable to fulfill requirement for memory application. Considering the excellent memory switching behavior, resistance switch device composed of a promising ALD high-k La2O3 dielectric film is a possible candidate to be integrated into future memory processes.

Polymer Electrolyte Fuel Cell Electrodes Grown by Vapor Deposition Techniques

AbstractPolymer fuel cell electrode growth using vapor deposition techniques is reviewed. The supports, the nanocatalyst sizes and morphologies, and the resulting electrodes are examined as a function of the vapor deposition process; sputtering, CVD, plasma‐enhanced (PE)CVD, and metal‐organic (MO)CVD. In each case, up‐to‐date fuel cell performances are highlighted. Vapor depositions are valuable techniques for designing fuel cell electrodes of various kinds with good fuel cell performances, e.g., power density and catalyst activity

Organosilicon Polymers Deposition by PECVD and RPECVD on Micropatterned Substrates

AbstractThin films of organosilicon materials produced by plasma‐assisted deposition are frequently used because of their multifunctional character, but few comparative studies into their growth on structured surfaces are available. Two types of CVD processes, plasma‐enhanced (PE)CVD and remote plasma‐enhanced (RPE)CVD are taken as typical operating conditions. Polymer films of thicknesses ranging from 0.25 to 1.2 µm are obtained by both processes from the tetramethylsiloxane (TMDSO) precursor, on silicon substrates microstructured with a set of patterns (trenches, holes, and columns) with various spacings, and with vertical dimensions of 1.3 or 1.45 µm. Analysis by scanning electron microscopy (SEM) of the samples is carried out after sample cleavage. The effects of pattern size and shape, defined by the aspect ratio parameter, on the local growth rate are studied more specifically for trenches for both PECVD and RPECVD processes

Advanced Roll‐to‐Roll Plasma‐Enhanced CVD Silicon Carbide Barrier Technology for Protection from Detrimental Gases

AbstractThe demand for low‐cost transparent gas and moisture barrier materials initiates many activities in the area of roll‐to‐roll (R2R), CVD, thin film technology. The present article is an overview on the elaboration of plasma‐enhanced (PE)CVD technology for R2R manufacturing of amorphous silicon carbide (SiC) barrier coatings applied on polymers. A prototype R2R PECVD reactor is designed and built for uniform coatings deposition on large area web‐substrates. A key feature is the incorporation of the Penning discharge plasma source (PDPS) to provide magnetically confined, high‐density plasma (HDP) with no excessive heat. Coatings are grown on a variety of polymer substrates using trimethylsilane ((CH3)3SiH) as the main organic precursor for delivery of active SiC species in the gas phase during deposition. A variety of analytical techniques are utilized to correlate process operating factors and plasma chemistry with coating composition and properties, in particular with the barrier performance and durability of barrier coated films (BCFs). The developed R2R PECVD technology demonstrates significant achievements in barrier performance with water vapor transmission rate (WVTR) of less than 5 × 10−4 g m−2 day−1 and durability of BCFs for more than 500 h under damp‐heat conditions of 85 °C and 85% relative humidity (RH). Manufactured BCFs can be competitively implemented for protection against detrimental gasses in various applications, such as drug and food packaging, and flexible electronic devices, such as thin film batteries and solar cells, liquid‐crystal (LCD), and light‐emitting diode (LED) displays.

Measurement of strain and strain relaxation in free-standing Si membranes by convergent beam electron diffraction and finite element method

Bridge-shaped free-standing Si membranes (FSSM), strained by low-pressure (LP) Si x N y , plasma-enhanced (PE) Si x N y and Si x Ge 1− x stressors, were measured by convergent beam electron diffraction (CBED) and the finite element method (FEM). The results of CBED show that, while the strain along the length of the FSSM is compressive in an LPSi x N y /Si sample, those along the length of the FSSM are tensile in PESi x N y /Si and Si x Ge 1− x /Si samples. The average absolute values of strains are different in FSSM with LPSi x N y , PESi x N y and Si x Ge 1− x as stressors. The FEM was used to compensate the results of CBED taking into account the strain relaxation in transmission electron microscopy (TEM) sample preparation. The FEM results give the strain properties in three dimensions, and are in good agreement with the results of CBED. There is approximately no strain relaxation along the length of FSSM, and the elastic strains along the other two axes in FSSM are partially relaxed by thinning down for the preparation of TEM samples.