Evaluation of structural and mechanical properties of aluminum oxide thin films deposited by a sol–gel process: Comparison of microwave to conventional anneal

Thin films of Al₂O₃ have been deposited on polished silica glass substrates at room temperature by sol–gel dip coating technique followed by two different exposure methods. One set was annealed at different temperatures ranging from 200 °C to 800 °C for 10 h and a second set was exposed to microwave (2.45 MHz) radiation at different powers for 10 min. The lower temperature and shorter time with microwave irradiation might be ascribed to the activating and facilitating effect of microwaves on solid phase diffusion. Unlike other preparation methods, microwave heating is generally quite faster, and energy efficient. X-ray diffraction (XRD) and scanning electron microscopy (SEM), energy dispersive X-ray analysis techniques have been employed to characterize structural, morphological and elemental compositions of the films. Adhesion strength failure measurements on films performed by scratch test in progressive loading sequence have shown critical loads up to 25 N (partial perforation) for both annealed films and films exposed to microwave irradiation. Nanohardness indentation tests of the films exposed (800 W) to microwave have shown hardness of 8.3 GPa with elastic modulus of 120 GPa compared to the conventional annealed film (800°) of 4.5 GPa with elastic modulus of 90 GPa.     

I–V Characteristics of tris(2,4-pentanedionato)manganese(III) thin films

Amorphous thin films of tris(acetylacetonate)manganese(III) were deposited on Si(P) substrates by thermal sublimation in vacuum. The deposited films were probed with X-ray fluorescence. Their electrical properties were studied as insulators for Al/Mn(acac)₃/Si(P) metal–insulator–semiconductor devices. Those devices were characterized by the measurement of the gate-voltage dependence of their capacitance, from which the relative permittivity (RP) and density of the charges in the insulator were determined. It was found that values of the RP of tris(acetylacetonate)manganese(III) films grown on Si(P) wafers were in the range of 30–40, which can be find applications in gate related technological uses. The dc-electrical conduction in the complex film was studied at room temperature and in temperature range of (293–325 K). It was found that the data of the as-deposited films follow the trap-charge-limited space-charge-limited conductivity (TCL-SCLC) mechanism, while the data of the annealed sample in vacuum of about 10⁻³ Pa at about 100 °C for 10 min obey the Richardson–Schottky (RS) mechanism. The parameters of both mechanisms were determined. It was concluded that the density of charged defects in the insulating film is critically determined the mechanism of the current-transfer.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Incipient amorphous-to-crystalline transition in HfO2 as a function of thickness scaling and anneal temperature

A series of 1.4, 1.8, and 4.0 nm thick HfO₂ films deposited on Si(1 0 0) substrates have been measured by extended X-ray absorption fine-structure prior to anneal processing, following a standard post deposition anneal of 700 °C for 60 s in NH₃ ambient, and following an additional rapid thermal anneal cycle of 1000 °C for 10 s in N₂ ambient. Analysis of the second coordination shell gives clear evidence of increased ordering with increasing film thickness at each temperature. Similarly, increased ordering with increasing anneal temperature is evident for each film thickness. Although X-ray diffraction and high resolution transmission electron microscopy indicated the 1.4 nm HfO₂ samples to be amorphous, EXAFS has distinguished nanocrystalline from amorphous states for these films.     

Optical and electrical properties of as-prepared and annealed (Se80Te20)100 − xAgx (0 ≤ x ≤ 4) ultra-thin films

Optical and electrical properties of the (Se₈₀Te₂₀)_100 ? xAg_x (0 ≤ x ≤ 4) ultra-thin films have been studied. The ultra-thin films were prepared by thermal evaporation of the bulk samples. Thin films were annealed below glass transition temperature (328 K) and in between glass transition temperature and crystallization temperature (343 K). Thin films annealed at 343 K showed crystallization peaks for Se–Te–Ag phases in the XRD spectra. The transmission and reflection of as-prepared and annealed ultra-thin films were obtained in the 300–1100 nm spectral region. The optical band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical band gap increases, but the refractive index, extinction coefficient, real and imaginary dielectric constant decrease with increase in Ag content. The optical band gap and refractive index show the variation in their values with increase in the annealing temperature. The extinction coefficient increases with increasing annealing temperature. The surface morphology of ultra-thin films has been determined using a scanning electron microscope (SEM). The measured dc conductivity, under a vacuum of 10^? 5 mbar, showed thermally activated conduction with single activation energy in the measured temperature range (288–358 K) and it followed Meyer–Neldel rule. The dc activation energy decreases with increase in Ag content in pristine and annealed films. The results have been analyzed on the bases of thermal annealing effects in the chalcogenide thin films.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Thermal,vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate)

Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO₄) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO₄ complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in T_g towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO₄ exhibits maximum conductivity, 3.75 × 10^? 3 S cm^? 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures.     

InOx semiconductor films deposited on glass substrates for transparent electronics

Transparent undoped semiconductor indium oxide films were deposited by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium at low substrate temperature. It was experimentally verified that the variation of rf power density has a strong influence on the electrical and structural properties of the films. The thickness of the InO_x films is of about 100 nm. Results show that InO_x films show an average visible transmittance of about 85% and energy gap of about 2.6 eV. Structural and electrical conductivity measurements show that films are polycrystalline and there exists a linear variation of conductivity logarithm vs reciprocal of temperature. Electrical conductivity variation of 17.6 to 5.8 × 10⁻³ (Ω cm)⁻¹ for films produced at rf power densities ranging from 3.9 to 78.1 mW cm⁻³ was obtained. This controllable semiconductor behavior can therefore satisfy the requirement of a particular application for these type of films.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film

Li⁺ ion conducting Li–Al–Ti–P–O thin films were fabricated on ITO-glass substrates at various temperatures from 25 to 400 °C by RF magnetron sputtering method. When the substrate temperature is higher than 300 °C, severe destruction of ITO films were confirmed by XRD (X-ray diffraction) and the abrupt transformation of one semi-circle into two semi-circles on the impedance spectra. These as-deposited Li–Al–Ti–P–O solid state electrolyte thin films have an amorphous structure confirmed by XRD and a single semicircle on the impedance spectra. Good transmission higher than 80% in the visible light range of these electrolyte thin films can fulfill the demand of electro-chromic devices. Field emission scanning electron microscopy and atomic force microscopy showed the denser, smoother and more uniform film structure with the enhanced substrate temperature. Measurements of impedance spectra indicate that the gradual increased conductivity of these Li–Al–Ti–P–O thin films with the elevation of substrate temperature from room temperature to 300 °C is originated from the increase of the pre-exponential factor (σ₀). The largest Li-ion conductivity can come to 2.46 × 10^? 5 S cm^? 1. This inorganic solid lithium ion conductor film will have a potential application as an electrolyte layer in the field such as lithium batteries or all-solid-state EC devices.     

Crystallization of bismuth titanate and bismuth silicate grown as thin films by atomic layer deposition

Bismuth silicate and bismuth titanate thin films were deposited by atomic layer deposition (ALD). A novel approach with pulsing of two Bi-precursors was studied to control the Si/Bi atomic ratio in bismuth silicate thin films. The crystallization of compounds formed in the Bi₂O₃–SiO₂ and Bi₂O₃–TiO₂ systems was investigated. Control of the stoichiometry of Bi–Si–O thin films was studied when deposited on Si(1 0 0) and crystallization was studied for films on sapphire and MgO-, ZrO₂- and YSZ-buffered Si(1 0 0). The Bi–Ti–O thin films were deposited on Si(1 0 0) substrate. Both Bi–Si–O and Bi–Ti–O thin films were amorphous after deposition. Highly a-axis oriented Bi₂SiO₅ thin films were obtained when the Bi–Si–O thin films deposited on MgO-buffered Si(1 0 0) were annealed at 800 °C in nitrogen. The full-width half-maximum values for 200 peak were also studied. An excess of bismuth was found to improve the crystallization of Bi–Ti–O thin films and the best crystallinity was observed with Ti/Bi atomic ratio of 0.28 for films annealed at nitrogen at 1000 °C. Roughness of the thin films as well as the concentration depth distribution were also examined.     

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe₂O₃–60P₂O₅ (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe₂O₃ content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe₂O₃ content from 10 to 30 mol%.     

Synthesis and X-ray structural characterization of NiO nanoparticles obtained through gelatin

Nanoparticles of fcc-NiO phase were obtained by heating the dried resin resultant of a mixture of gelatin and NiCl₂ · 6H₂O in aqueous solution. The average particle size and microstrain were calculated from the line broadening of X-ray powder diffraction peaks, and these values were between 15 nm and 78 nm, and 0.056% and 0.172%, respectively. The Rietveld refinement method was applied to all diffraction patterns. The particle size, obtained from this procedure, changes as a function of temperature, heating time and the remarkable reduction due to the addition of NaOH to the solution, which can be attributed to the presence of NaCl crystals and carbon encapsulating NiO nanoparticles during the heating. The heating temperature was in the range of 350–700 °C. Thermo-gravimetric analysis showed that the majority of organic fraction starts to disappear after 300 °C.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Electrical properties of A2.6+xTi1.4−xCd(PO4)3.4−x (A = Li,K; x = 0.0–1.0) phosphate glasses

Electrical properties of A_2.6+xTi_1.4−xCd(PO₄)_3.4−x (A = Li, K; x = 0.0–1.0) phosphate glasses are investigated over a frequency range from 42 Hz to 1 MHz at different temperatures. Impedance spectroscopy is used to separate the bulk conductivity from electrode effect of electrical conductivity data. The bulk dc conductivity is Arrhenius activated, with activation energies and pre-exponential factors following the Meyer–Neldel rule. The real part of ac conductivity shows universal power law feature. The variation of dielectric constant with frequency is attributed to ion diffusion and polarization occurring in the phosphate glasses. The frequency dependent imaginary part of electric modulus M″(ω) plot shows non-Debye feature in conductivity relaxation. The Kohlrausch–Williams–Watts stretched exponential function was used to describe the modulus spectra and the stretching exponent β is found to be temperature independent. Scaling in M″(ω) shows that the electrical relaxation mechanisms are independent of temperature for given composition at different temperatures.     

Electrical conductivity and dielectric relaxation in non-crystalline films of tungsten trioxide

Amorphous tungsten trioxide (a-WO₃) thin films were prepared by thermal evaporation technique. The electrical conductivity and dielectric properties of the prepared films have been investigated in the frequency range from 100 Hz to 100 kHz and in the temperature range 293–393 K. In spite of the absence of the dielectric loss peaks, application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The frequency dependence of σ(ω) follows the Jonscher’s universal dynamic law with the relation σ(ω) = σ_dc + Aω^s, where s is the frequency exponent. The conductivity in the direct regime, σ_dc, is described by the small polaron model. The electrical conductivity and dielectric properties show that Hunt’s model is well adapted to a-WO₃ films.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

Preparation of indium tin oxide thin films without external heating for application in solar cells

Indium tin oxide (ITO) films were grown without external heating in an ambient of pure argon by RF-magnetron sputtering method. The influence of argon ambient pressure on the electro-optical properties of as-deposited ITO films was investigated. The morphology, structural and optical properties of ITO films were examined and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and UV–VIS transmission spectroscopy. The deposited ITO films with a thickness of 300 nm show a high transparency between 80% and 90% in the visible spectrum and 14–120 Ω/□ sheet resistance under different conditions. The ITO films deposited in the optimum argon ambient pressure were used as transparent electrical contacts for thin film Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS solar cells with efficiencies of the order of 7.0% were produced without antireflective films. The results have demonstrated that the developed ITO deposition technology has potential applications in thin film solar cells.     

Electronic transport properties of mixed transition metal ions doped borophosphate glasses

A series of borophosphate glasses in the composition (B₂O₃)_0.10–(P₂O₅)_0.40–(CuO)_0.50?x–(MoO₃)_x; 0.05 ? x ? 0.50 have been investigated for room temperature density and dc conductivity over the temperature range from 350 to 650 K. The density decreased with increase in MoO₃ over the composition range studied except a slight increase around 0.35 mole fraction. The observed initial decrease in conductivity with the addition of MoO₃ has been attributed to the hindrance offered by the Mo⁺ ions to the electronic motions. The observed peak-like behavior in conductivity in the composition range 0.20 – 0.50 mol% of MoO₃ is ascribed to the mixed transition metal ion effect (MTE). Mott’s small polaron hopping model has been used to analyze the high temperature conductivity data and the activation energy for conduction has been determined. The low temperature conductivity has been analyzed in view of Mott’s and Greaves variable range hopping models. It is for the first time that conduction mechanisms have been explored and MTE detected in mixed transition metal ions doped borophosphate glasses.