Multiple glass transition temperatures of polymer thin films as probed by multi-wavelength ellipsometry

Ellipsometry was used as a technique to determine glass temperature transitions of homopolymer thin films of polystyrene and poly(methyl methacrylate) ranging from 5 to 200 nm. The technique operated systematically at various energies reveals the presence of multiple transitions in these films under certain conditions of preparation. It was observed that in the case of strong polymer-substrate interactions, several Tg were measured and ascribed to a layered structure in the film. In the absence of strong interactions, a more uniform film presenting a single Tg was observed no matter the thickness. The results confirm the recent assumption of thin films organised in multilayers and emphasize the importance of using a highly sensitive multi-wavelength technique in probing such properties.     

Vanadium dioxide thin film with low phase transition temperature deposited on borosilicate glass substrate

A nanostructured vanadium dioxide (VO₂) thin film showing a low metal-insulator transition temperature of 30 °C has been fabricated through reactive ion beam sputtering followed by thermal annealing. The thin film was grown on borosilicate glass substrate at the temperature of 280 °C with a Si₃N₄ buffer layer. Both scanning electron microscopy and atomic force microscopy images have been taken to investigate the configuration of VO₂ thin film. The average height of the crystallite is 20 nm and the grain size ranges from 40 nm to 100 nm. The transmittance measured from low to high temperatures also reveals that the film possesses excellent switching property in infrared light at critical transition temperature, with switching efficiency of 52% at 2600 nm. This experiment paves the way of VO₂ thin film's application in smart windows.     

Lithium borosulphate glasses: an analysis of glass transition temperature results

Lithium borosulphate glasses have been prepared in three different series: (a) (42·5 −x)Li₂O:57·5 B₂O₃:xLi₂SO₄; (b) 42·5Li₂O: (57·5 −x)B₂O₃:xLi₂SO₄ and (c) 42·5Li₂O:57·5B₂O₃:xLi₂SO₄. The glass transition temperature (T _g) of these glasses has been analysed on the basis of the fraction of four coordinated boron which governs the glass structure. The analysis reveals that the addition of Li₂SO₄ in series (a) and (b) gives rise to increased value of N₄ whereas, in series (c) it increases the number of non-bridging oxygens.     

Electrical conductivity and transition temperature of NbN thin films

Niobium nitride thin films are grown using reactive RF sputtering technique for four different partial nitrogen pressures in argon atmosphere. The superconducting transition temperature of the films has been measured. The films exhibit a negative temperature coefficient of resistivity. The electrical characterization of the films has been carried out and the conductivity measured between room temperature and liquid nitrogen temperature. This is fitted usingT ^P law.     

Abrasion of thin films deposited onto glass by the Taber test

The mechanism of abrasion of sputtered metal nitride thin films on glass by the Taber abrasion test and the effects of mechanical and tribological properties of thin films on Taber abrasion were investigated. The abrasion occurred in a peeling mode: the film was peeled off the substrate and the peeled area increased with increasing number of Taber abrading cycles. The peel rate per Taber cycle, a quantitative measure to represent the abrasion resistance of the thin films, was small in the film with greater adhesion strength, higher hardness and lower frictional resistance of the surface. The effect of these properties on the peel rate was represented with a mathematical equation of exponential form. The peel rate was found to vary most sensitively with the hardness.     

Processing and characterization of functionally graded materials through mechanical properties and glass transition temperature

Functionally graded materials (FGMs) were prepared by construction based layering method through dispersion of carbon nanoparticles in the styrene butadiene rubber matrix. The gradation of material property i.e., ‘glass transition temperature’ was brought in the nanocomposite by varying the concentration of process oil. Glass transition temperature (T_g) of FGMs was varied from − 56 to − 80 °C along the span of 3 mm thick sheet. The gradation of oil in FGMs also changes other properties like tensile strength, elongation at break, modulus, etc. Tensile strength and modulus at 100% drops down while elongation at break continuously increases while moving from one end to other end along the sheet thickness. Thermal analysis of FGMs verifies the compositional changes as well as the change in T_g along the thickness.     

Anomalous behavior in ZnMgO thin films deposited by sol-gel method

The magnesium doped zinc oxide is a promising optical material to enhance the luminescence for possible application in solid state lighting. Magnesium doped zinc oxide thin films (Zn_0.85Mg_0.15O) were deposited by sol-gel route on p-type silicon and annealed at different temperatures in oxygen environment for an hour. The doping of magnesium in zinc oxide was confirmed by X-Ray diffraction and the samples were found to have wurtzite crystal structure with (002) preferred orientation. The films were characterized by Hall-effect, atomic force microscopy, UV-VIS spectroscopy, photoluminescence (PL) and work function measurements. The different studies exhibited an anomalous behavior for the film annealed at 900 °C. The Hall effect, work function measurements and UV-VIS spectroscopy indicated that the resistivity, work function and optical band gap increased as a function of annealing temperature (from 300 °C to 700 °C) however these parameters were found to decrease for the films annealed above 700 °C. The particle size increased with the annealing but for the samples annealed at 900 °C, the shape of the grains changed and became elongated like fibers as observed by the atomic force microscopy. The PL measurements displayed the existence of oxygen vacancies defects for the samples annealed at and above 600 °C. The possible mechanism for this anomaly has been discussed in this work.     

Anisotropic crystallite size analysis of textured nanocrystalline silicon thin films probed by X-ray diffraction

A newly developed X-ray technique is used, which is able to quantitatively combine texture, structure, anisotropic crystallite shape and film thickness analyses of nanocrystalline silicon films. The films are grown by reactive magnetron sputtering in a plasma mixture of H₂ and Ar onto amorphous SiO₂ and single-crystal (100)-Si substrates. Whatever the used substrate, preferred orientations are observed with texture strengths around 2-3 times a random distribution, with a tendency to achieve lower strengths for films grown on SiO₂ substrates. As a global trend, anisotropic shapes and textures are correlated with longest crystallite sizes along the 〈111〉 direction but absence of 〈111〉 oriented crystallites. Cell parameters are systematically observed larger than the value for bulk silicon, by approximately 0.005-0.015 Å.     

Fabrication of single crystal PZT thin films on glass substrates

 We have successfully transferred heteroepitaxial Pb(Zr,Ti)O₃ (PZT) thin films from MgO substrates on to glass substrates. The transferred PZT thin films exhibit single crystal structure with ferroelectric properties similar to the as-grown epitaxial films. The transferring process comprises coating of Cr-metallized surface of epitaxial PZT thin films, pressing and cementing the Cr-metallized surface on to the glass substrates by silicone rubber, and removing the MgO substrates by chemical etching. This process realizes a fabrication of high-temperature processed PZT thin films onto the glass at room temperature. The process is also available for the transformation of PZT thin films on organic film sheet. The present transfer process reduces the effects of the inevitable strain and/or constraint to rigid substrates for heteroepitaxial growth and has a potential for integration of single crystal piezoelectric PZT devices onto a wide variety of MEMS.     

Physical aging in amorphous polymers far below the glass transition temperature

Constitutive equations are derived for the viscoelastic response and enthalpy recovery in amorphous polymers quenched far below the glass transition temperature. The model is based on the concept of cooperative relaxation, which treats a polymer as an ensemble of independent relaxing regions. Any flow unit is trapped in a cage, where it randomly hops in the potential well being thermally activated. Rearrangement occurs, when a relaxing region reaches in a hop some liquid-like state. Structural relaxation in a disordered medium after quenching is thought of as an increase in roughness of the energy landscape. Using plausible hypotheses about the rearrangement process, stress–strain relations are derived which account for the effect of physical aging on the viscoelastic behavior, and a formula is developed for the increment of the specific enthalpy. These relationships are validated using experimental data in mechanical and calorimetric tests on polycarbonate and polystyrene. Fair agreement is demonstrated between observations and results of numerical simulation.     

Reactivity of mixed rhodium/aluminium thin films deposited on gamma-alumina

Rh and Al mixed thin films deposited on the gamma-alumina surface were studied. Auger electron spectroscopy as well as thermal desorption spectroscopy (TDS) results showed significant alloying of Rh and Al after the first annealing of the system. CO thermal desorption results were strongly influenced by the previous CO adsorption-desorption cycles, showing strong CO adsorbate-induced reversible changes on the surface. TDS and molecular beam reactive scattering (CO oxidation) results depended on Rh:Al atomic ratio significantly. Shapes of CO oxidation curves were similar to the results obtained on clean Rh foil previously and their intensity also corresponded to Rh surface concentration.     

Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates

We have studied the dependence of dielectric properties on the deposition temperature of BiFeO₃ thin films grown by the pulsed laser deposition technique. Thin films have been grown onto amorphous silica glass substrates with pre-patterned Au in-plane capacitor structures. It is shown that on the amorphous glass substrate, BiFeO₃ films with a near-bulk permittivity of 26 and coercive field of 80 kV/cm may be grown at a deposition temperature of about 600 °C and 1 Pa oxygen pressure. Low permittivity and higher coercive field of the films grown at the temperatures below and above 600 °C are associated with an increased amount of secondary phases. It is also shown that the deposition of BiFeO₃ at low temperature (i.e. 500 °C) and post deposition ex-situ annealing at elevated temperature (700 °C) increases the permittivity of a film. The applied bias and time dependence of capacitance of the films deposited at 700 °C and ex-situ annealed films are explained by the de-pinning of the ferroelectric domain-walls.     

Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N₂) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. Fourier transform infrared and UV–visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T_s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B–O containing disordered tissue at higher T_s. The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E₀₄ gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm⁻¹ (O–B–O/B–N) infrared peak area ratio plays an indicator role to reveal the stability of BN films.     

Influence of heating rate on the hysteresis in the phase transition in silver selenide thin films

The electrical resistance of vacuum-deposited silver selenide thin films, of thickness 70 nm was measured in the temperature range from 300 to 430 K, at different heating rates and at a pressure of 2×10⁻⁵ mbar. The films were annealed at 430 K at a pressure of 2×10⁻⁵ mbar for an hour. It is found that annealed silver selenide films undergo a structural phase transition exhibiting hysteresis. The structural phase transition with hysteresis in silver selenide thin films is influenced by the heating rates of the films. The effect of the heating rate on the phase transition temperature and hysteresis are discussed taking into account the presence of potential barrier and the defects. Our studies reveal that hysteresis width decreases with decreasing heating rates.     

Mechanical and deformation behaviour of titanium diboride thin films deposited by magnetron sputtering

Nanoindentation studies have been carried out for TiB₂ films deposited on Si, glass and steel by sputtering for studying the influence of the substrates. It was observed that the modulus of the film was influenced by the substrates from 30 nm onwards. Plastic energy analysis has shown that as load increases more energy is absorbed by the substrate. Quantitative indentation depth limits for obtaining film only hardness, using a combination of log-log plot of load vs displacement and load vs (displacement)² functions, have shown the dependence on the threshold load for crack formation. Comparison of the hardness data with composite hardness models has been performed. Fracture toughness of the coatings was also evaluated using two methods which resulted in comparable results.     

The growth of indium-tin-oxide thin films on glass substrates using DC reactive magnetron sputtering

The growth of indium-tin-oxide thin films as a function of thickness using DC reactive magnetron sputtering was investigated. As the film thickness grew, the crystallinity increased showing both (2 2 2) and (4 0 0) planes. However, the peak intensity ratio of I₂₂₂/I₄₀₀ in the X-ray diffraction pattern decreased with the thickness, implying a preferred orientation along the (4 0 0) planes at the higher thickness. The grain sizes and domain boundaries grew clearly and the specific resistivities decreased with the film thickness. Two components of the specific resistivities, carrier mobility and carrier concentration, showed opposite behaviour: (i) increasing carrier concentration; (ii) decreasing carrier mobility with increase in the film thickness. Furthermore, the graded growth of the ITO thin film could also be shown from the optical properties and morphological properties by UV/Vis/NIR spectroscopy and scanning electron microscopy.     

Kinetics of the glass transition in As22S78 chalcogenide glass: Activation energy and fragility index

Differential scanning calorimetry (DSC) has been used to probe the dynamics of the glass transition in As₂₂S₇₈ chalcogenide glass. Non-isothermal measurements were performed at different heating rates (5–35 K min⁻¹). The experimental result of this kinetic glass transition phenomenon was analyzed on the basis of the relaxation process occurring in the transition temperature range. The activation energy of the glass transition was determined from the heating rate dependence of the glass transition temperature. The fragility index m of the glass was estimated from the measurements of the activation energy of the relaxation process, which characterizes the glass transition. Different kinetic methods as well as isoconversional methods were used. Isoconversional analysis of the experimental data shows that the activation energy of glass transition process is varying with the degree of transformation (and hence with temperature) from the glassy to the supercooled phase.     

Properties of atomic layer deposited HfO2 thin films

The growth, composition and morphology of HfO₂ films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H₂O at 250° and ii) tetrakis dimethyl amino hafnium and H₂O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO₂ peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO₂ films. The index of refraction for all films is very close to that for bulk crystalline HfO₂.     

Spatial spectral evolution in pulsed laser deposited lead-germanate thin films by micro-infrared spectroscopy

Lead-germanate thin films were developed on silicon substrates by pulsed laser deposition from bulk glassy targets of composition 0.4PbO-0.6GeO₂, and micro-infrared transmittance measurements were performed to assess the state of the grown films. Measurements across the radius of films revealed surprisingly large spectral changes, reminiscent of lead-oxide variations in corresponding bulk glasses. To search for the origin of this effect, the infrared spectra were simulated by employing the rigorous expression for the transmittance of a bilayer system to take into full account multiple internal reflections in both thin film and substrate. The results showed that the profiles of the experimental spectra can be accurately described by using as input the complex refractive index of the target glassy material and by considering film thickness variations from the center to the edges of the film. This work demonstrates the strong influence of optical effects on the infrared spectra of thin films, and manifests also the effectiveness of infrared spectroscopy when coupled with rigorous calculations to characterize the structure of thin films.