Electrically conductive silicon oxycarbide thin films prepared from preceramic polymers

This work focuses on silicon oxycarbide thin film preparation and characterization. The Taguchi method of experimental design was used to optimize the process of film deposition. The prepared ceramic thin films with a thickness of c. 500 nm were characterized concerning their morphology, composition, and electrical properties. The molecular structure of the preceramic polymers used for the preparation of the ceramic thin films as well as the thermomechanical properties of the resulting SiOC significantly influenced the quality of the ceramic films. Thus, an increase in the content of carbon was found beneficial for the preparation of crack-free thin films. The obtained ceramic films exhibited increased electrical conductivity as compared to monolithic SiOC of similar chemical composition. This was shown to correlate with the unique hierarchical microstructure of the SiOC films, which contain large oxygen-depleted particles, mainly consisting of highly graphitized carbon and SiC, homogeneously dispersed in an oxygen-containing amorphous matrix. The matrix was shown to also contain free carbon and to contribute to charge carrier transport between the highly conductive large particles. The ceramic thin films possess electrical conductivities in the range from 5.4 to 8.8 S/cm and may be suitable for implementation in miniaturized piezoresistive strain gauges.     

On the shrinkage during pyrolysis of thin films and bulk components: The case of a hybrid silica gel precursor for SiOC glasses

This paper compares the shrinkage during pyrolysis of a gel precursor as thin film and as bulk sample. The hybrid silica gel, precursor for SiOC glasses, contains Si-CH₃ and Si-H moieties. The shrinkage of bulk samples has been measured with conventional dilatometry. Shrinkage of thin films has been studied for the first time with in situ dilatometry allowing to measure the thickness and the refractive index during pyrolysis. Thin films shrink more compared to bulk samples and the pyrolytic transformation occurs at lower temperature (100-150 °C) compared to the bulk samples.     

High thermoelectric performance of high-mobility Ga-doped ZnO films via homogenous interface design

Ga-doped ZnO (GZO) thin films grown on sapphire substrates have been widely investigated as a promising transparent thermoelectric (TE) material. However, due to the large lattice mismatch and thermal expansion between the sapphire substrate and GZO film, strain-induced lattice distortion impedes the transport of electrons, leading to low carrier mobility. In this study, ZnO homo-buffer layers with different thicknesses were inserted between sapphire substrates and GZO films, and their effect on the TE properties was investigated. A thin ZnO interlayer (10 nm) effectively reduced the lattice mismatch of the GZO film and improved the carrier mobility, which contributed to the large enhancement in the electrical conductivity. Simultaneously, energy filtering occurred at the interface between GZO and ZnO, resulting in a relatively high density of states (DOS) effective mass and maintaining a high Seebeck coefficient compared to that of the unbuffered GZO films. Consequently, the GZO film with a 10 nm thick ZnO buffer layer possessed a high power factor value of 449 μW m⁻¹ K⁻² at 623 K. This study provides a facile and effective method for optimizing the TE performance of oxide thin films by synergistically improving their carrier mobility and enhancing their effective mass.     

High‐Temperature Creep Behavior of Dense SiOC‐Based Ceramic Nanocomposites: Microstructural and Phase Composition Effects

In this work, dense monolithic polymer‐derived ceramic nanocomposites (SiOC, SiZrOC, and SiHfOC) were synthesized via hot‐pressing techniques and were evaluated with respect to their compression creep behavior at temperatures beyond 1000°C. The creep rates, stress exponents as well as activation energies were determined. The high‐temperature creep in all materials has been shown to rely on viscous flow. In the quaternary materials (i.e., SiZrOC and SiHfOC), higher creep rates and activation energies were determined as compared to those of monolithic SiOC. The increase in the creep rates upon modification of SiOC with Zr/Hf relies on the significant decrease in the volume fraction of segregated carbon; whereas the increase of the activation energies corresponds to an increase of the size of the silica nanodomains upon Zr/Hf modification. Within this context, a model is proposed, which correlates the phase composition as well as network architecture of the investigated samples with their creep behavior and agrees well with the experimentally determined data.     

Processing and characterization of particles reinforced SiOC composites via pyrolysis of polysiloxane with SiC or/and Al fillers

Polysiloxane loaded with SiC as inert filler, and Al as active filler, was pyrolyzed in nitrogen to fabricate SiOC composites, and the processing and properties of the filled SiOC composites were investigated. Adding SiC fillers could reduce the linear shrinkage of filler-free cured polysiloxane in order to obtain monolithic SiC/SiOC composites. The flexural strength of SiC/SiOC composites reached 201.3 MPa at a SiC filler content of 27.6 vol.%. However, SiC/SiOC composites exhibited poor oxidation resistance, thermal shock resistance and high temperature resistance. Al fillers could react with hydrocarbon generated during polysiloxane pyrolysis at 600 °C and N₂ at 800 °C to form Al₄C₃ and AlN, respectively. The volume expansions resulting from these two reactions were in favor of the reduction in linear shrinkage and the improvement in flexural strength of SiC/SiOC composites. The flexural strength of Al-containing SiC/SiOC composites was 1.36 times that of SiC/SiOC composites without Al at an Al filler content of 20 vol.%. The addition of Al fillers remarkably improved the high temperature resistance and oxidation resistance of SiC/SiOC composites, but not thermal shock resistance.     

High-Temperature Hall Measurements on Cerium Dioxide Thin Films

Simultaneous Hall and conductivity measurements have been performed on sputtered polycrystalline thin films and on bulk ceramic specimens of nearly stoichiometric CeO₂ in the temperature range between 900° and 1040°C. The measurements have been performed in air using low-frequency alternating current. In the case of the bulk ceramic specimens, an upper limit for the carrier mobility of ≤0.2 cm²/(V·s) has been obtained, which is in accordance with data from the literature for bulk samples. The conductivity of the thin films (l/1Ω·m) at 1000°C) is in accordance with data from the literature for bulk ceramics. The carrier density derived from the Hall measurements (3 × 10¹⁶/cm³ at 1000°C) increases with increasing temperature, whereas the Hall mobility (4 cm²(V·s) at 1000°C) decreases with increasing temperature. These values differ from literature data for bulk ceramic specimens. The difference may be duelo the small grain diameters (∼200 nm) in the 1-μm-thick thin films.     

Photoacoustic effect of piezoelectric ZnO thin films

This paper describes the photoacoustic (PA) effect of piezoelectric ZnO thin films. The one-dimensional theory of the PA-effect was derived. The relationships between PA-signals and modulation frequency, thin film properties such as piezoelectric coefficients, dielectric constant, specific heat and thermal conductivity coefficient were obtained. The experimental results on the PA-effect of sputter-deposited ZnO thin films exhibited good agreement with the theoretical analysis. The piezoelectric stress coefficients, e₃₃ and e₃₁, of some ZnO films were determined from the PA-signals, typical values of which are 0.82 C/m² and -0.43 C/m², respectively.     

Effects of substrate temperature on thermal stability of Al-doped ZnO thin films capped by AlOx

Effects of substrate temperature on the thermal stability of Al-doped ZnO (AZO) films have been studied. Degradation of electrical properties of AZO films by annealing under flowing N₂ gas depends on their crystallinity controlled by the substrate temperature. A thin AlO_x capping layer was employed to passivate the thermal degradation of the AZO layer. A strong correlation between Zn desorption and reduction in carrier concentration was observed. Thermal desorption of Zn was prevented by the AlO_x layer, retaining carrier concentration. With the AlO_x capping layer, the reduction in Hall mobility was prevented in samples with good c-axis orientation, while the reduction in Hall mobility was still observed in poor c-axis oriented films. However, the reduction was smaller than that in bare AZO films. The dependence of Hall mobility evolution on the substrate temperature, and therefore, on crystallinity, strongly suggests the impact of grain boundary scattering on thermal degradation. An increase in optical mobility, which was evaluated from optical spectra using the Drude model, with annealing temperatures, supports the conclusion that an increase in grain boundary scattering by annealing caused the degradation of Hall mobility. The increase in grain boundary scattering induced by Zn desorption was prevented by the capping layer, while contributions of domain alignment and other segregation of defects to the grain boundary scattering, which depend on the substrate temperature retained, leading to different evolutions of Hall mobility.     

A leaf-like structured ITO conductive transparent thin film from visible to near-infrared region with enhanced stability

ITO thin films as the optical and electrical windows to transform photons and charges have been applied in many areas. Here, a leaf-like structured particle is composed of small particles growing along three different orientations leading to low thermal stress accompanied by well transmittance (85%) in a wide wavelength range from visible to near-infrared region and a narrowed band gap 3.07 eV. The evolution of structure and electronic performance was studied to obtain the low resistivity (12 μΩ m) and enhanced stability of the film (1000 °C). The leaf-like structure can be maintained under 600 ℃ and the electrical properties can be modified in He and N₂ atmosphere, owing to the reduced defects, increased concentration of Sn and carrier mobility. Although the structure has changed after being annealed at 1000 °C in N₂, the thin film performs excellent electrical properties (?3.44 × 10²⁰ cm^?3 and 28 cm² V^?1 s^?1).     

Hierarchical microstructure growth in a precursor-derived SiOC thin film prepared on silicon substrate

Silicon oxycarbide film deposited on a silicon substrate has shown superior electrical conductivity relative to its monolithic counterpart. In this work, the evolution of different microstructures detected on the SiOC film reveals its hierarchical microstructure. The existence of sp²-hybridized carbon domains has been unambiguously confirmed by means of Raman spectroscopy and transmission electron microscopy corroborated with electron energy loss spectroscopy. The diffusion coefficient of carbon in silica and its dependence on temperature were studied by assessing energy-dispersive X-ray spectroscopy profiles taken from the cross-sections of samples annealed at temperatures in the range from 1100°C to 1400°C. The activation energy for diffusion of carbon in silica was determined to be approximately 3.05 eV, which is significantly lower than the values related to the self-diffusion of silicon and oxygen. The microstructural evolution of precursor to SiC_nO_4-n and SiC serves as migration path of sp²-hybridized carbon to the SiO_x layer. With increasing temperature, the formation of microscale carbon-rich segregation is promoted while the SiOC film becomes thinner.     

Comparison of Ion Irradiation Effects in Silicon-Based Preceramic Thin Films

Thin films of polymers (polysiloxanes, polycarbosilanes, and polysilazanes) and alkoxide-derived siloxane gels, precursors for SiC, SiCN, SiOC, and SiOBC ceramics, were irradiated with increasing fluences of C or Au ions to study the kinetics of their conversion into ceramics. Ion beam analyses showed that the main effect of irradiation on the composition of the films is the selective release of H₂ by radiolysis. During subsequent high-temperature annealing of films converted as much as possible by irradiation, CO _x , CH _x , or silane molecules do not evolve, contrary to what is observed during the pyrolysis of unirradiated precursor films. According to Raman analyses, a large proportion of the carbon atoms segregate into clusters after irradiation and in films converted by direct pyrolysis (or combined treatments). However, carbon particles formed during irradiation are more diamond-like, affording films with 2—3 times higher hardness, as shown by nanoindentation tests. In both types of ceramics (SiC or SiOC), the optimal properties (hardness, thermal stability, and photoluminescence) associated with C segregation are obtained for a C/Si ratio of the order of 1. Boron addition is detrimental to hardening of SiOC glasses, in contrast to nitrogen.     

Interface Contributions to Localized Heating of Dielectric Thin Films

Measurements of the thermal conductivity of thin dielectric films in the last ten years have established that thin film thermal conductivity may be much lower than that of the corresponding bulk solid, by as much as two orders of magnitude, and that significant interfacial thermal resistance may be present along the film/substrate interface. We review such measurements of thin film thermal conductivity and interfacial thermal resistance, and use the heat conduction equation to determine their implications for the localized heating of thermally anisotropic thin films bonded to substrates. It is found that for surface heating an equivalent isotropic film can be established and that the presence of large interfacial thermal resistance leads to a strong dependence of film thermal conductivity on film thickness, especially for thin films. A microscopic model of the film/substrate interface is used to establish the dependence of the interfacial thermal resistance on porosity along the interface.     

Carrier films behavior during thermoforming process studied using dynamic mechanical thermal analysis (DMTA)

Dynamic mechanical thermal analysis was used to investigate the thermal transitions and modulus/temperature behavior of thermoformable carrier films, and to relate the information obtained to carrier film behavior during the thermoforming process. In this study the glass transition temperatures (T _g) and the temperatures at which crystallization occurred during heating (T _c) of four thermoformable carrier films were measured by using a dynamic mechanical thermal analyzer (DMTA). These films are good candidates for the automotive process, which uses painted carrier films as moldable automotive coatings (MAC). The modulus/temperature behavior of the films was also observed over a wide temperature range, which included thermoforming temperatures. Although films of PETG and PCTG 5445, co-polyesters based on poly(1,4-cyclohexylene dimethylene terephthalate), are thermoformable, their T _g values, 92 and 99 °C, respectively, are not high enough to allow current paint systems (with bake temperature of 100–110 °C) to cure on the films without causing severe film deformation.     

Thermal,optical, and electrical characterization of thin film coated RTV 655 bilayer system

Adhesion of thin films to hydrophobic elastomeric substrates is of particular interest in the area of flexible electronics and nano‐sensor technology. Here, nanometer‐thick Au films were deposited directly onto hydrophobic RTV 655 substrates by means of sputtering, thermal evaporation, and electroless techniques without an adhesion‐promoting layer. The bilayer system was exposed to repeat thermal cycling and changes to the surface morphology of the thin film were monitored electrically and optically. Buckle formation in the as‐deposited film was attributed to stress in the film and substrate stiffness rather than thermal coefficient mismatch between films. The Au‐RTV 655 interface was water tight and maintained a strong adhesion despite repeated thermal cycles. Sputtered and thermally evaporated carbon‐coated RTV 655 substrates were also studied in parallel for comparison. Periodic arrays of buckles formed in pre‐strained RTV 655 samples showed reproducibility in their optical properties demonstrating good adhesion between the two layers without an interfacial layer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41396.     

Solution-processed semiconducting aluminum-zinc-tin-oxide thin films and their thin-film transistor applications

We prepared aluminum-zinc-tin-oxide (AZTO) thin films by the solution spin-coating method and investigated their physical and electrical properties according to different incorporated amounts of Al. AZTO films annealed at 400 °C were amorphous. Though SnO₂ crystallites were detected in films annealed at temperatures higher than 500 °C, the number of crystallites decreased as the Al content increased. Thin films had a smooth and uniform surface morphology with an optical transmittance value higher than 92% in the visible range. Electrical conductivity and its temperature dependence varied markedly according to the amount of Al incorporated in the film. We therefore systematically investigated activation energies for carrier transport for each film composition. Thin-film transistors (TFTs) were fabricated using solution-processed AZTO as an active channel layer. The effects of the amount of Al incorporated in the thin film on TFT characteristics were also evaluated. The best device performance was observed for a TFT with a 5 mol%-Al-incorporated AZTO channel. Field effect mobility, subthreshold swing, and on/off ratio were approximately 0.24 cm² V⁻¹ s⁻¹, 0.69 V/dec, and 1.03×10⁶, respectively.     

PbTiO3–PbO–SiO2 Glass-Ceramic Thin Film by a Sol–Gel Process

PbTiO₃(PT)-PbO-SiO₂ glass-ceramic thin films were pro-duced by a sol-gel process. The crystallization of PT oc-curred at ∼700°C and was higher than that in PT-PbO-B₂ O₃ sol-gel glass-ceramics. A pinhole-free thin film was obtained by a rapid thermal annealing process when the designed glass-forming phase content in the thin film was >24 vol%. The measured dielectric constants of the films fairly agreed with the predicted values, based on a parallel mixing model. The dielectric constant was 219 and the di-electric loss was 0.04 in the 0.6PT-0.4(PbO-SiO₂) film that was fired at 700°C.     

Electrochemically deposited thermoelectric n-type Bi2Te3 thin films

Electrochemically deposited n-type BiTe alloy thin films were grown from nitric acid baths on sputtered Bi_xTe_y/SiO/Si substrates. The film compositions, which varied from 57 to 63 at.% Te were strongly dependent on the deposition conditions. Surface morphologies varied from needle-like to granular structures depending on deposited Te content. Electrical and thermoelectric properties of these electrodeposited Bi_xTe_y thin films were measured before and after annealing and compared to those of bulk Bi₂Te₃. Annealing at 250 °C in reducing H₂ atmosphere enhanced thermoelectric properties by reducing film defects. In-plane electrical resistivity was highly dependent on composition and microstructure. In-plane Hall mobility decreased with increasing carrier concentration, while the magnitude of the Seebeck coefficient increased with increasing electrical conductivity to a maximum of −188.5 μV/K. Overall, the thermoelectric properties of electrodeposited n-type BiTe thin films after annealing were comparable to those of bulk BiTe films.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.     

Investigation of the coefficient of thermal expansion in nanocrystalline diamond films

Nanocrystalline diamond films are a promising class of nanomaterials with tunable properties. An especially appealing field of application for NCD are nitrogen doped semiconducting films.The residual stress in the films is an important film property directly influencing the adhesion of the film on the substrate and thus the film performance. The residual stress consists of two components: a thermal part due to the different coefficients of thermal expansion of film and substrate and an intrinsic part. The residual stress in most films that are deposited at high temperatures is dominated by an effect arising from the difference in coefficients of thermal expansion in film and substrate. By measuring the residual stress in the film at different temperatures it is possible to calculate the coefficient of thermal expansion of the films. For this purpose an ex-situ optical device was used to measure the residual stress of the film.Nanocrystalline diamond films were deposited by microwave-plasma CVD at a pressure of 200 mbar from an Ar/H₂/CH₄ plasma while the hydrogen fraction in the process gas and the substrate temperature were varied between 3 to 6% to 600 to 800 °C respectively.To investigate the influence of the nitrogen admixture in the plasma on the thermal expansion coefficient more films were deposited at a pressure of 200 mbar with admixtures of nitrogen of 2.5% and 7.5%.It is shown that by controlling the process parameters the coefficient of thermal expansion in the NCD films can be matched with the silicon substrate for insulating as well as for conductive films and therefore the thermal stress component be minimized. The results are important for the development of MEMS where silicon as a substrate is widely used.     

Enhanced electrical properties of In-Ga-Sn-O thin films at low-temperature annealing

Electrical and optical properties of In-Ga-Sn-O (IGTO) thin films deposited by radio-frequency magnetron sputtering were investigated according to annealing temperatures. While IGTO films remained an amorphous phase even after a heat treatment at temperature up to 500 °C, Hall measurements showed that annealing temperature had a significant impact on electrical properties of IGTO thin films. After investigating a wide range of annealing temperatures for samples from as-deposited state to 500 °C, IGTO film annealed at 200 °C exhibited the best electrical performance with a conductivity of 229.31 Ω^?1cm^?1, a Hall mobility of 36.89 cm²V^?1s^?1, and a carrier concentration of 3.85 × 10¹⁹ cm^?3. Changes in proportions of oxygen-related defects and percentages of Sn²⁺ and Sn⁴⁺ ions within IGTO films according to annealing temperatures were analyzed with X-ray photoelectron spectroscopy to determine the cause of the superb performance of IGTO at a low temperature. In IGTO films annealed at 200 °C, Sn⁴⁺ ions acting as donor defects accounted for a high percentage, whereas hydroxyl groups working as electron traps showed a significantly reduced percentage compared to the as-deposited film. Optical band gaps of IGTO films obtained from UV–visible spectrum were 3.38–3.47 eV. The largest band gap value of 3.47 eV for the IGTO film annealed at 200 °C could be attributed to an increase in Fermi-level due to an increase of carrier concentration in the conduction band. These spectroscopic results well matched with electrical properties of IGTO films according to annealing temperatures. Excellent electrical properties of IGTO thin films annealed at 200 °C could be largely due to Sn donors besides oxygen vacancies, resulting in a significant increase in free carriers despite a low annealing. temperature.