Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm⁻¹, an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm⁻¹, the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm⁻¹ is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.     

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO₂ thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle-like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.     

Interfacial oxide and carbide phases in the deposition of diamond films on beryllium metal

We have investigated beryllium metal as a substrate for the microwave plasma chemical vapor deposition of diamond films. The dependence of oxide and carbide interfacial phase formation with temperature and their influence on diamond nucleation and growth behavior were studied by thin film X-ray diffraction. Although a native oxide (BeO with the hexagonal wurtzite structure) remains for the range of substrate temperatures studied (700–800°C), the formation of a carbide (Be₂C with the cubic antifluorite structure) is found only above a critical substrate temperature of approximately 750°C. Without the formation of Be₂C, the diamond growth rate is low and a significant amorphous carbon component is observed. Just above the critical temperature, films exhibited high growth rates with high phase-purity diamond. Thick films (>30 μm) grown above the critical temperature were observed to fracture completely within the Be₂C layer, suggesting this to be the weak structural link.     

Low temperature and large area deposition of nanocrystalline diamond films with distributed antenna array microwave-plasma reactor

Diamond films grown at low temperature (< 400 °C) on large area of different substrates can open new applications based on the thermal, electrical and mechanical properties of diamond. In this paper, we present a new distributed antenna array PECVD system, with 16 microwave plasma sources arranged in a 2D matrix, which enables the growth of 4-inch nanocrystalline diamond films (NCD) at substrate temperature in the range of 300–500 °C. The effect of substrate temperature, gas pressure and CH₄ concentration in the total gas mixture of H₂/CH₄/CO₂ on the morphology and growth rate of the NCD films is reported. The total gas pressure is found to be a critical deposition parameter for which growth rates and crystalline quality both increasing with decreasing the pressure. Under optimized conditions, the process enables deposition of uniform (~ 10%) and high purity NCD films with very low surface roughness (5–10 nm), grain size of 10 to 20 nm at growth rates close to 40 nm/h. Nanotribology tests result in the friction coefficient of the NCD films close to that obtained for the standard tetrahedral amorphous carbon coatings (ta-C) indicating the suitability of this low-temperature diamond coating for mechanical applications such as bearing or micro-tools.     

Graphitized filament plasma enhanced CVD deposition of nanocrystalline diamond

A novel approach to the deposition of polycrystalline diamond is presented. The technique is based on the hot filament chemical vapour deposition technique (HFCVD). While it is similar to a high plasma power “bias enhanced growth” HFCVD, it relies on a graphite filament rather than on a metal one. It was found that with an appropriate choice of the growth parameters, 4–9% CH₄ in H₂, filament temperature > 2200 °C, 25 mBar gas pressure, plasma power > 500 W, a long filament lifetime can be achieved, when a simultaneous deposition of graphitic carbon on the hot graphite filament and of nanocrystalline diamond on a substrate facing the filament assembly is realized. In this paper the growth of nanocrystalline diamond films and their characterization (SEM, XRD, AFM) are presented. While the technique is promising for low cost, large area deposition of nanocrystalline diamond films, also the growth of microcrystalline diamond has been observed.     

Structure and properties of hard carbon films depending on heat treatment temperatures via polymer precursor

Phenylcarbyne polymer films were coated on silicon substrates and heat treated in 1 atm pressure of argon at various temperatures. The structural changes occurring during the heat treatment process of the polymer were investigated by Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The Raman and FTIR spectra features of the polymer showed a dependence on the heat treatment temperatures. At low temperatures (below 400°C), the Raman and IR spectra of the polymer were similar to those of the original polymer. The hardness and Young's modulus of the polymer films were below 1 and 50 GPa, respectively. With increasing temperature (above 400°C), thermal decomposition of the polymer occurred, resulting in structural changes of the polymer from soft amorphous hydrocarbon (400–600°C) phases to hard carbon phases (above 600°C). The hardness and Young's modulus increased from 1.5 and 65 GPa at 600°C to 9 and 120 GPa at 1000°C, respectively. It is assumed that the hard carbon film converted from the polymer might contain sp² and sp³ carbon phases; high temperature of heat treatment resulted in increasing sp² (glassy) carbon phase in the films.     

Post-deposition annealed MoO3 film based high performance MSM UV photodetector fabricated on Si (100)

Molybdenum oxide (MoO₃) films were prepared on Si (100) at room temperature using radiofrequency (RF) magnetron sputtering technique. The films were annealed in the presence of air at different temperatures from 100 to 550 °C. The as-prepared films were amorphous as revealed by the X-ray diffraction analysis. Post-deposition annealing of MoO₃ film enhanced its crystalline structure, showing β-MoO₃ phase at 100 °C and a mixture of α-MoO₃ and β-MoO₃ phases at 300 °C. The crystallinity of α-MoO₃ improved with increasing the annealing temperature to 500 °C, however, the β-MoO₃ phase became amorphous. The film was dissolved at 550 °C as no diffraction peak of MoO₃ was detected at this temperature. The band gap of MoO₃ was evaluated through ultraviolet–visible spectroscopy. The results showed a decrease in the band gap from 3.70 to 3.39 eV with increasing the annealing temperature to 500 °C. The film with optimum crystalline quality was used to fabricate a metal-semiconductor-metal (MSM) photodetector device. The photo-detection characteristics of the film were studied after the deposition of Nickel contacts on MoO₃ using a metal mask having interdigitated electrodes. The fabricated device exhibited a high current gain and sensitivity under 365 nm UV illumination. The responsivity of the device under UV light was 0.41 A/W at 7 V. The rise and decay time of UV photodetector were 0.32 and 0.23 s respectively. These findings suggested that the MoO₃ film with dominant orthorhombic α-phase can potentially be used for the photodetector application.     

Crystallization and Photoactivity of Tio2 Films Formed on Soda Lime Glass by a Sol-Gel Dip-Coating Process

Transparent nanophase TiO₂ thin films on soda lime glass were prepared from titanium tetraisopropoxide (TTIP) by a sol-gel dip-coating method. The TiO₂ films had amorphous phase up to 400°C and anatase phase at 500°C. The amorphous TiO₂ films obtained at 300–400°C showed considerable photoactivity for the degradation of formic acid. The photoactivity of the TiO₂ films was enhanced with increasing calcination temperature from 300° to 500°C. The crystallinity of the anatase films at 500°C was improved with increasing calcination time up to 2 h and reduced with a further increase in calcination time to 4 h due to the significant formation of sodium titanate phase as a result of sodium diffusion. The four-time-dipping anatase films at 500°C exhibited the greatest photoactivity at the calcination time of 2 h. Sodium diffusion into TiO₂ films was retarded by a SiO₂ underlayer of 50 nm in thickness.     

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

Remote plasma enhanced metal organic chemical vapor deposition of TiN for diffusion barrier

TiN films were deposited with remote plasma metal organic chemical vapor deposition (MOCVD) from tetrakis-diethyl-amido-titanium (TDEAT) at substrate temperature of 250–500°C and plasma power of 20–80 W. The growth rate using N₂ plasma is slower than that with H₂ plasma and showed 9.33 kcal/mol of activation energy. In the range of 350–400°C., higher crystallinity and surface roughness were observed and resistivity was relatively low. As the temperature increased to 500°C., randomely oriented structure and smooth surface with higher resistivity were obtained. At low deposition temperature, carbon was incorporated as TiC phase, as the deposition temperature increases, carbon was found as hydrocarbon. At 40 W of plasma power, higher crystallinity and rough surface with lower resistivity were obtained and increasing the plasma power to 80 W leads to low crystallinity, smooth surface and higher resistivity. It may be due to the incorporation of hydrocarbon decomposed in the gas phase. Surface roughness was found to be related to the crystallinity of the film.     

Structural determination of pyrolyzed PI-2525 polyimide thin films

Polyimide was pyrolyzed in an argon atmosphere at various temperatures, and thermally converted to amorphous carbon films. The irreversible change of polyimide under progressive heat treatment is characterized by three successive structural changes: pyrolysis, carbonization, and graphitization. X-ray photoelectron spectroscopy (XPS) studies show that the polyimide starts to dissociate at pyrolysis temperatures above 500°C. At temperatures higher than 650°C most functional groups of polyimide decompose to evolve gases from the sample. The polyimide then gradually becomes more carbon rich. It is believed that at pyrolysis temperature higher than 650°C the polyimide starts to form heterocyclic structures with residual oxygen and nitrogen incorporated into the heterocyclic carbon rings. X-ray analyses indicate that the polyimide at pyrolysis temperatures less than 1000°C is amorphous carbon and no long-term periodic structure can be detected. At pyrolysis temperatures higher than 2000°C, the polyimide is converted to microcrystalline graphite.     

Self-growth of nanocrystalline structures for amorphous Sr0.925Bi0.05TiO3 thin films with high energy density

Process of self-growth nanocrystalline structure was explored to improve the dielectric properties of amorphous Sr_0.925Bi_0.05TiO₃ (SBT) thin films through controlling the annealing temperature. The crystallinity of the material was 15% when annealed at 550?°C, and the resulting nanocrystalline particles were 14?nm in size as determined by XRD analysis. Therefore, the proposed process could produce a novel film of an amorphous matrix coating nanocrystalline particles. Finite element analysis results showed that the applied electric field was focused primarily in the amorphous matrix, which could effectively decrease the probability of low voltage breakdown of the nanocrystalline particles. Simultaneously, the nanocrystalline particles supplied more polarization charges, which helped to improve the dielectric constant of the inorganic composite. Combining the merits of amorphous and crystalline phases, the ultimate energy storage density of the modified sample was as high as 21.2?J/cm³, which represents an improvement of 5.1?J/cm³ compared with that of pure amorphous SBT thin film.     

Temperature influence on the interlayer and surface morphology of diamond coating on 3D porous titanium substrates

Nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on three-dimensional porous titanium (Ti) substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The morphology variation of diamond films grown on porous three-dimensional titanium substrate was studied at four different deposition temperatures to investigate their influence on nucleation density. Scanning electron microscopy images depicted the continuous change from microcrystalline diamond grains with a random crystallographic orientation, at 500 °C and 600 °C, to a cauliflower-like structure for deposits at 700 °C and 800 °C. Visible Raman spectroscopy confirmed the good quality of diamond films and revealed that the amount of amorphous carbon increased associated to the film morphology changes from MCD to NCD. X-ray diffraction analyses, performed both through θ–2θ scans and at grazing incidence angle, allowed the investigation of the crystallographic properties and structural evolution of the different film/substrate interface phases, such as TiC(111), TiC(200) and TiH₂. The results revealed that the temperature enhanced the nucleation sites for diamond growth.     

Me-DLC films as material for highly sensitive temperature compensated strain gauges

In technical applications strain gauges are widely used. Apart from conventional polymer foil based strain gauges that are glued to the work piece surface, sputtered strain gauges are already commercially used in special applications. Those sputter strain gauges are typically made of NiCr alloy and the sensor layer is as sensitive to strain as the ones used in the glued strain gauges with a gauge factor of 2, but neglecting problems of creeping and swelling of the involved polymer materials. Diamond-like carbon (DLC) films offer significantly higher strain sensitivity, but usually they are also very sensitive to temperature effects. Using metal doped diamond-like carbon (Me-DLC), higher strain sensitivity than conventional metal based systems, in combination with thermal compensation, is possible. The influence of different process parameters on the gauge factor and temperature coefficient of resistance (TCR) of DLC and Me-DLC films produced in industrial sputtering systems was investigated. Gauge factors up to 13 in combination with a high negative TCR in the range of a few thousand ppm/K were reached with sputtered DLC films. The substrate bias voltage in particular showed a strong influence on the resulting gauge factor of the films. For Me-DLC films different deposition methods (dc and rf sputtering) and various doping metals (Ag, Ni, Ti, and W) were investigated. Using dc sputtering of the Me-DLC films only Ni-DLC showed gauge factors slightly higher than 2. Furthermore, only for Ni-DLC zero crossing of the TCR was observed by variation of the metal content. Using rf excitation especially Ni-DLC films showed gauge factors exceeding values of 15 in combination with a TCR close to zero.     

Effects of Annealing Temperature on the Structure and Capacitive Performance of Nanoscale Ti/IrO2–ZrO2 Electrodes

Electrodes consisting of coating of the Iridium oxide–Zirconium oxide (70%IrO₂–30%ZrO₂) binary oxide were formed on Ti substrates by thermal decomposition and annealing at 340°C–450°C. The effects of the annealing temperature on the structure, surface morphology, surface composition, and capacitive performance of the coatings were investigated using X‐ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy, X‐ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The XRD and TEM analyses showed that 360°C is greater than but very close to the crystallization temperature of the 70%IrO₂–30%ZrO₂ oxide coating. The 70%IrO₂–30%ZrO₂ oxide coatings annealed at this temperature consisted of an amorphous matrix containing a few IrO₂ nanocrystalline particles (diameter of 1–2 nm). The degree of crystallinity of the coatings was approximately 13.2%. EIS analysis showed that the electrode annealed at 360°C exhibited the highest specific capacitance, which was much higher than that of the electrode annealed at 340°C (which had a purely amorphous structure) as well as those of the electrodes annealed at 380°C and 400°C (which had higher degrees of crystallinity). On the basis of the obtained results, the following conclusion can be drawn: oxide coatings prepared at temperatures slightly higher than the crystallization temperature of the oxide and containing conductive nanocrystalline particles exhibit the best capacitive performance. We suggest that this phenomenon can be explained by the fact that the electronic conductivity of the coating is greatly improved by the presence of the homogeneously distributed conductive nanocrystalline particles in the amorphous matrix. Furthermore, the protonic conductivity and loose atomic configuration of the amorphous structure of the electrode are not adversely affected by the annealing treatment.     

Effect of Substrate Temperatures on Structural and Morphological Properties of Nano-Crystalline Silver Thin Films Grown on Silicon Substrates

The silver (Ag) thin films were deposited on silicon substrates by DC magnetron sputtering method under different substrate temperatures of 100–500?°C. Then the as-deposited films were subjected to annealing treatment. The XRD results revealed that the Ag thin films have a good nanocrystalline structure and a considerable increase in the crystallinity of Ag (111) peak was observed at substrate temperature of 200?°C. The average crystalline size of Ag films varied between 18 and 44 nm which confirms the presence of nanocrystal’s in the films. The AFM and SEM images demonstrated that the grain size and surface roughness of the films are sensitive to substrate temperature during deposition of the films and annealing treatment. The SEM results is in good agreement with the results of XRD and AFM analysis.     

Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique

Tetrahedral amorphous carbon (ta-C) is a potential low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. The thermal stability of tetrahedral amorphous carbon (ta-C) films has been investigated by heat treatment of the films at temperatures from room temperature to 450°C in high vacuum, low vacuum and oxygen ambient. It was found that heat treatment in oxygen ambient leads to a much more prominent variation in film thickness, stress and hardness than in both low and high vacuum. Raman studies also show an increase of the G-band frequency to higher values, an increase of the integrated intensity ratio and a narrowing of the G bands for films annealed in oxygen ambient with increasing temperature. By contrast, ta-C films exhibit a high resistance to degradation during treatment in low and high vacuum. They sustain their structure, thickness, stress and hardness for temperatures up to 400°C.     

Thermal stability of lead sulfide nanocrystalline films

Thermally stable nanocrystalline films of lead sulfide PbS were prepared using the chemical deposition method. The thickness of the films measured by interferometry was approximately equal to 100 nm, and the average particle size determined from the broadening of the X-ray diffraction reflections was approximately 80 nm. X-ray diffraction analysis revealed that annealing of the film at a temperature of 350°C in air results in the formation of a protective oxide sulfate phase of the composition PbO · PbSO₄ on the surface of the film. It was established that the oxide sulfate phase prevents a further oxidation of the film and serves as an inhibitor of the growth of PbS nanoparticles upon heating up to a temperature of 500°C. The factors responsible for the high thermal stability of the sizes of nanoparticles and the optical properties of the PbS nanocrystalline film were discussed.     

Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200°C to 1800°C

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature.     

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC₄ from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.