Influence of plasma power and substrate temperature on structure of nanocrystalline germanium carbon thin films by VHF plasma CVD

The influence of plasma power and substrate temperature on the structure of nanocrystalline germanium carbon thin films was investigated. Films were deposited by the very high frequency plasma chemical vapor deposition technique using hydrogen diluted monomethylgermane (MMG). Plasma power strongly affected the decomposition of hydrogen and MMG. Crystalline volume fraction and bonding states of the atoms in the films depends on plasma power and substrate temperature. FT-IR measurements also revealed that Ge–H_n and CH_n bonds are sensitive to these factors.     

Formation kinetics and control of microcrystallite in μc-Si:H from glow discharge plasma

The formation kinetics of μc-Si:H has been investigated through the film depositions and plasma diagnoses in widely-scanned glow discharge plasma conditions; RF power density, SiH₄/H₂ ratio and substrate temperature. The roles of H and SiH_x adsorbed on the surface as well as impinging ions have been discussed in relation to volume fraction and crystallite size of μc films, and continuous control of crystallite size has been demonstrated using a triode system. Hall mobility of the deposited μc-Si:H films has also been presented as a function of the volume fraction of μc.     

Control of cluster synthesis in nano-glassy carbon films

Carbon films were prepared by pulsed laser deposition (PLD), changing buffer gas nature and pressure and laser power density. Nanometer-sized cluster assembled (CA) films, resulting from direct aggregation of carbon clusters in the ablation plume, were obtained. Visible Raman spectroscopy shows that all films are trigonally co-ordinated and structurally disordered, with a dependence of the degree of disorder on the deposition parameters. The microstructure and morphology of the films were studied in a complementary way by scanning electron microscopy (SEM) both in plane and in cross-section, and by atomic force microscopy (AFM). Different growth modes are found in the deposited CA films, depending on the interplay of laser fluence and nature-pressure of the buffer gas. Threshold fluences of increasing value separate dense columnar growth from sponge like morphology, from an open dendritic structure. AFM pictures show that our glass-like carbon films consist of agglomerates of nanometer-sized clusters. Cluster formation in the plume is modeled, allowing to estimate the average number of carbon atoms per cluster. The calculated size of the clusters depends mainly on ambient gas pressure. Cluster sizes obtained by model predictions agree with those directly observed by transmission electron microscopy (TEM) imaging and with the deduced film coherence length from Raman spectroscopy conducted herein.     

Ion bombardment effect on the growth of a-Si:H films deposited from a pure silane plasma

The growth process, optical and structural properties of a-Si:H films deposited from a pure silane multipole DC discharge are analysed by real time and spectroscopic ellipsometry and I.R. absorption spectroscopy. The relative Si flux going onto the substrate under a ionized form can vary from 20% to 80%. In each extreme case the film properties are correlated with ion bombardment energy up to 300 eV. An increase of incident ion energy is found to favour the formation of high density homogeneous and isotropic films correlated with the predominance of monohydride SiH groups.     

Study of the conductivity of nitrogen doped tetrahedral amorphous carbon films

The conductivity of nitrogen incorporated tetrahedral amorphous carbon (taC:N) films prepared by filtered cathodic vacuum arc (FCVA) system was studied. The film resistivity varied as a function of nitrogen content in a wide nitrogen content range from 1 to 23 at.%. An interesting phenomenon of the photoconductivity for the films was found, that the resistivity of those films with low nitrogen content (<5 at.%) increased due to the light irradiation. This kind of behavior of photoconductivity had not been discussed in previous papers on taC:N films.     

Doping of microcrystalline Si:H,Cl films in R.F. glow discharge

The deposition rate of n-doped μc-Si:H,Cl GD films has been found to increase with the gas phase phosphine content, while the opposite behaviour has been observed for p-doped specimens. The optical gap ranges between 1.8 ÷ 2.6 eV and its variation seems related to hydrogen and chlorine contents. The effectiveness of the doping has been tested by electrical conductivity measurements.     

Influence of Ar/C2H2 ratio on the structure of hydrogenated carbon films

The amorphous hydrogenated carbon films (a-C:H) were obtained on Si (1 1 1) wafers by plasma jet chemical vapor deposition (PJCVD). a-C:H coatings have been prepared at 1000 Pa in argon/acetylene mixture. The Ar/C₂H₂ gas volume ratio varied from 1:1 to 8:1. It was demonstrated that by varying the Ar/C₂H₂ ratio the composition, growth rate of the coatings, and consequently the structure of the film, can be controlled. The growth rate and surface porosity of coatings deposited at Ar/C₂H₂ = 8:1 ratio decrease slightly with an increase in the distance between the plasma torch nozzle and substrate from 0.04 to 0.095 m. The transmittance of the coatings in the IR region of 2.5–25 μm slightly increases, while the absorption peaks at 2850–2960 cm^?1 related with sp³ CH_2–3 modes remain unchanged with an increase in the distance. The Raman spectroscopy indicated that the a-C:H coating formed at the Ar/C₂H₂ = 8:1 and 0.06 m has the highest sp³ C–C fraction. The proposed PJCVD technique allows to achieve the growth rates up to 300 nm/s.     

Deposition of mechanically hard amorphous carbon nitride films from decomposition of BrCN. Effects of substrate cooling and pulsed rf-bias voltage

Mechanically hard amorphous carbon nitride films were formed by applying a combination of radio frequency (RF) bias voltage to the substrate and the chemical vapor deposition process using the decomposition reaction of BrCN with the microwave discharge flow of Ar. Cooling water was circulated inside the substrate stage. The maximum hardness was (17 ± 1) GPa for the film prepared under the negative RF bias voltage, −V_RF, of 30 V. This hardness was nearly twice that of the film prepared without cooling, suggesting that substrate cooling was effective for suppressing the relaxation of the internal stress of the film due to the temperature rise during the application of the RF bias voltage. Under the continuous operation of the RF bias voltage, films cannot be formed for −V_RF > 40 V because of the sputtering by the bombardment of energetic Ar⁺. Then, the RF bias voltage was applied with a pulsed operation. By using this operation films were prepared in the range of −V_RF = 40-100 V. The hardness, (36 ± 10) GPa, was obtained for the film obtained under the conditions of −V_RF = 100 V, the pulse period of 1000 s, and the pulse-on time of 800 s. The observed hardness scattered largely for the different observation points within this film; a single observation point in that film showed the maximum hardness of 46 GPa. According to the IR spectra of the films, the three-dimensional C-N network structure was developed.     

ESR investigation of graphite-like amorphous carbon films revealing itinerant states as the ones responsible for the signal

The origin of paramagnetic centers in graphite-like amorphous carbon is investigated. The films were deposited by the ion beam assisted deposition (IBAD) and have a concentration of sp² sites of about 90% and zero energy band gap. The density of the film and the electrical resistivity are close to these of crystalline graphite. However, the hardness and stress of the films are similar to those of diamond-like carbon. Electron spin resonance (ESR) performed at the X-band (9.4 GHz) revealed an unexpected low density of paramagnetic centers, ascribed to conduction electrons with a g-value of about 2.003.     

Effects of carbon structure orientation on the performance of glucose sensors fabricated from electrospun carbon fibers

High-performance enzyme-based glucose sensors were prepared by electrospinning carbon fibers. The efficiency of the glucose sensor was assessed based on efficient enzyme immobilization and electrical resistance transfer by examining improved porosity and electrical properties, respectively. The porosity of the electrospun carbon fiber electrode was improved by physical activation to increase the immobilization sites of the glucose oxidase enzyme. The electrical properties were improved by a thermal treatment, which caused carbon orientation effects because of the high thermal energy. The glucose oxidase enzyme immobilization was developed based on improved specific surface area and pore volume, which were studied by pore structure and image analyzers. The glucose sensor was evaluated by amperometric measurements and cyclic voltammetry. The measured current increased with higher glucose concentrations based on the effects of the developed pore structure and the electrical properties. The enzymatic kinetics were also studied using the Lineweaver–Burk equation. The sensitivity of the glucose sensor was improved significantly with increased maximum current, whereas the GOD enzyme activity was diminished by efficient GOD immobilization. It is concluded that a high-performance glucose sensor was obtained using electrospun carbon fibers based on the effects of efficient GOD enzyme immobilization and electrical resistance transfer.     

Influence of hydrogen on the thermomechanical properties of a-CNx:H and a-CNx films deposited by glow discharge and ion beam assisted deposition

The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN_x and a-CN_x:H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN_x:H films, was observed on the CTE, which reaches about ∼9 × 10⁻⁶ C⁻¹, close to that of graphite (∼8 × 10⁻⁶ C⁻¹) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10⁻⁶ C⁻¹ for 33 at.% nitrogen.     

Effect of N2/CH4 flow ratio on microstructure and composition of hydrogenated carbon nitride films prepared by a dual DC-RF plasma system

Hydrogenated carbon nitride (a-CN:H) films were deposited on n-type (1 0 0) silicon substrates making use of direct current radio frequency plasma enhanced chemical vapor deposition (DC-RF-PECVD), using a gas mixture of CH₄ and N₂ as the source gas in range of N₂/CH₄ flow ratio from 1/3 to 3/1 (sccm). The deposition rate, composition and bonding structure of the a-CN:H films were characterized by means of X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectrometry (FTIR). The mechanical properties of the deposited films were evaluated using nano-indentation test. It was found that the parameter for the DC-RF-PECVD process had significant effects on the growth rate, structure and properties of the deposited films. The deposition rate of the films decreased clearly, while the N/C ratio in the films increased with increasing N₂/CH₄ flow ratio. CN radicals were remarkably formed in the deposited films at different N₂/CH₄ flow ratio, and their contents are related to the nitrogen concentrations in the deposited films. Moreover, the hardness and Young’s modulus of the a-CN:H films sharply increased at first with increasing N₂/CH₄ flow ratio, then dramatically decreased with further increase of the N₂/CH₄ flow ratio, and the a-CN:H film deposited at 1/1 had the maximum hardness and Young’s modulus. In addition, the structural transformation from sp³-like to sp²-like carbon-nitrogen network in the deposited films also was revealed.     

Nitridation of (Ba,Sr)TiO3 films in an inductively coupled plasma

The effects of surface treatment, using ammonia in an inductively coupled plasma, of (Ba,Sr)TiO₃ films on the leakage and dielectric properties of the Pt/(Ba, Sr) TiO₃/Pt capacitors were investigated. To obtain data of the ammonia plasma, a Langmuir probe was used during the experiments. As a result of the exposure of (Ba,Sr)TiO₃ to the plasma, the leakage current density of the (Ba,Sr)TiO₃ capacitor was decreased by two orders of magnitude as compared to that of the non-plasma-treated sample at an applied voltage of 1.5 V. The decrease of leakage currents of (Ba,Sr)TiO₃ films is attributed to the reduction of oxygen vacancies. However, the dielectric constant may be changed by the plasma-induced space charge or ion bombardment. The X-ray photoelectron spectroscopy measurements detected the N 1s peak in the plasma-treated sample. Additional space charges were induced and result in the reduction of the dielectric constant. We assume that the competition of nitrogen incorporation into the oxygen vacancies and plasma-induced damage is the main effect of the nitridation of (Ba,Sr)TiO₃ films in ammonia plasma.     

Kinetic characteristics of the crystallization from model solutions of blood plasma

Crystallography Reports - The kinetic regularities of crystallization (growth order and constants) in a model solution of blood plasma have been investigated. The particular order and...     

Investigation into the structure of Langmuir-Blodgett films prepared from salts of acetylenic acids

Langmuir-Blodgett films are prepared from lead, cadmium, and copper salts of carboxylic acids, namely, 23-tetracosynoic acid HC≡C(CH₂)₂₁COOH (with the terminal triple bond) and 2-docosynoic acid CH₃(CH₂)₁₈C≡CCOOH (with the internal triple bond). The structural transformations in the films during polymerization under exposure to UV radiation are investigated using X-ray reflectomertry. It is found that the X-ray reflectograms of the initial films prepared from salts of both acids exhibit no less than four or five pronounced intense Bragg reflections. This suggests that the initial films have good layer ordering. The bilayer periods for all the films are determined, and the influence of the salt type on the bilayer period is analyzed. It is established that, under exposure to UV radiation, the structural transformations in the films prepared from the salts of 23-tetracosynoic acid occur without substantial disturbances and defects, as can be judged from the absence of significant changes in the location and intensity of the Bragg reflections. An increase in the time of irradiation of the films prepared from the salts of 2-docosynoic acid leads to an increase in the bilayer period. This effect is especially pronounced for the lead and copper salts. However, upon long-term exposure to UV radiation, the salts undergo photolysis, the bilayer period decreases, and the structure of the film begins to break down.     

Effects of vicinal angles from (0 0 1) surface on the Boron-doping features of high-quality homoepitaxial diamond films grown by the high-power microwave plasma chemical-vapor-deposition method

Vicinal surface effects on homoepitaxial growth and boron-doping processes have been studied in case of single-crystalline diamond (0 0 1) surfaces grown using the high-power microwave plasma chemical-vapor-deposition (MWPCVD) method. The off-angles inclined from the on-orientation (0 0 1) surfaces ranged to 5° along the [1 1 0] or [1 0 0] direction, while the concentration of doping B(CH₃)₃ gas was kept constant with a B/C ratio of 50 ppm. Although a number of square-like growth hillocks often appeared, depending substantially on the crystalline quality of the high-pressure/high-temperature-synthesized (HPHT) Ib diamond substrates employed, the number and shape of the hillocks changed significantly with the increasing off-angle. For the vicinal surfaces with off-angles of ≈3° inclined along the [1 1 0] direction, macroscopically flat surfaces were obtained, compared with the other off-angle cases examined. Furthermore, the growth rate and acceptor density of substitutional boron atoms in the homoepitaxial layers were found to substantially increase with the increasing off-angle. These indicate that the step density can play important roles not only in the homoepitaxial growth but also in the boron-incorporation process during the high-power MWPCVD growth.     

Growth instabilities in the deposition of amorphous films

It is proposed that during the atom-by-atom deposition of a film, the attractive forces between the oncoming atoms and those deposited become important in determining the growth of the film when surface diffusion and many atom rearrangement processes are suppressed (precisely the conditions that lead to the formation of amorphous films). The trajectories of the oncoming atoms are distorted in such a way that the formation and growth of surface irregularities are favored. Since the deflection depends inversely upon the incident kinetic energy of the atoms, sputtered films should be smoother than evaporated films for equal substrate temperatures. It is also argued that this mechanism can lead to the formation of voids.     

Optical properties and chemical bonding characteristics of amorphous SiNX:H thin films grown by the plasma enhanced chemical vapor deposition method

Amorphous silicon nitride (SiN_X:H) thin films grown by the plasma enhanced chemical vapor deposition (PECVD) method are presently the most important antireflection coatings for crystalline silicon solar cells. In this work, we investigated the optical properties and chemical bonding characteristics of the amorphous SiN_X:H thin films deposited by PECVD. Silane (SiH₄) and ammonia (NH₃) were used as the reactive precursors. The dependence of the growth rate and refractive index of the SiN_X:H thin films on the SiH₄/NH₃ gas flow ratio was studied. The chemical bonding characteristics and the surface morphologies of the SiN_X:H thin films were studied using the Fourier transform infrared spectroscopy and atomic force microscopy, respectively. We also investigated the effect of rapid thermal processing on the optical properties and surface morphologies of the SiN_X:H thin films. It was found that the rapid thermal processing resulted in a decrease in the thickness, increase in the refractive index, and coarser surfaces for the SiN_X:H thin films.