Depth dependent properties of ITO thin films grown by pulsed DC sputtering

A systematically prepared set of ITO layers for solar cell applications has been analyzed by spectroscopic variable angle ellipsometry in order to trace the dependence of free carriers’ distribution along the film depth as a function of film thickness as well as its change upon annealing. Samples were deposited on silicon substrates with various thicknesses in steps of approximately 10–20 nm. This set was duplicated and these samples were annealed, so that for each thickness an as-deposited and an annealed sample is available. Conventionally measured electrical conductivity and morphological properties (AFM measurements) of the films have been compared with the optical constants’ inhomogeneity, i.e. material properties along the film thickness modelled by variable-angle spectroscopic ellipsometry. The obtained results show that the optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. For the deposition conditions used a well-determined reproducible non-uniform distribution of free carriers within the film thickness was determined. In particular it has been found that the majority of free carriers in as-deposited ultra-thin ITO films is concentrated at sample half-depth, while their distribution becomes asymmetric for the thicker films, with a maximum located at approximately 40 nm depth. The distribution of free carriers in annealed samples is qualitatively different from that of as-deposited layers.     

Annealing induced reorientation of crystallites in Sn doped ZnO films

Tin doped ZnO thin films were prepared by employing a simplified spray pyrolysis technique using a perfume atomizer and subsequently annealed under different temperatures from 350 °C to 500 °C in steps of 50 °C. The structural, optical, electrical, photoluminescence and surface morphological properties of the as-deposited films were studied and compared with that of the annealed films. The X-ray diffraction studies showed that as-deposited film exhibits preferential orientation along the (0 0 2) plane and it changes in favour of (1 0 0) plane after annealing. The increase in crystallite size due to annealing is explained on the basis of Ostwald ripening effect. It is found that the optical transmittance and band gap increases with increase in annealing temperature. A slight decrease in resistivity caused by annealing is discussed in correlation with annealing induced defect modifications and surface morphology.     

Optical characterization of vacuum evaporated CdZnTe thin films deposited by a multilayer method

CdZnTe thin films of thickness 450–1400 nm have been evaporated under vacuum onto unheated glass substrates, using a multilayer method. During film deposition, the two evaporation sources, separated by two glass cylinders, were maintained at temperatures of 720 K for Zn and at 925–1200 K for CdTe, respectively. After deposition, the samples were annealed in air up to 775 K. The structural and optical properties of both as-deposited and heat-treated samples were investigated. Depending on the preparation conditions and the annealing temperature, the value of the optical band gap, E_g, of respective films varied between 1.16 and 1.63 eV. The obtained results are discussed in correlation with the structure of the films and the role of Zn atoms in CdTe films.     

Effect of sputtering power on the electrical and optical properties of Ca-doped ZnO thin films sputtered from nanopowders compacted target

In the present work, we have deposited calcium doped zinc oxide thin films by magnetron sputtering technique using nanocrystalline particles elaborated by sol–gel method as a target material. In the first step, the nanoparticles were synthesized by sol–gel method using supercritical drying in ethyl alcohol. The structural properties studied by X-ray diffractometry indicates that Ca doped ZnO has a polycrystalline hexagonal wurzite structure with a grain size of about 30 nm. Transmission electron microscopy (TEM) measurements have shown that the synthesized CZO is a nanosized powder. Then, thin films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature. The influence of RF sputtering power on structural, morphological, electrical, and optical properties were investigated. It has been found that all the films deposited were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (0 0 2) crystallographic direction. They have a typical columnar structure and a very smooth surface. The as-deposited films show a high transmittance in the visible range over 85% and low electrical resistivity at room temperature.     

Formation of F-doped ZnO transparent conductive films by sputtering of ZnF2

Fluorine-doped ZnO transparent conductive thin films were successfully deposited on glass substrate by radio frequency magnetron sputtering of ZnF₂. The effects of rapid thermal annealing in vacuum on the optical and electrical properties of fluorine-doped ZnO thin films have been investigated. X-ray diffraction spectra indicate that no fluorine compounds, such as ZnF₂, except ZnO were observed. The specimen annealed at 500 °C has the lowest resistivity of 6.65 × 10^? 4 Ω cm, the highest carrier concentration of 1.95 × 10²¹ cm^? 3, and the highest energy band gap of 3.46 eV. The average transmittance in the visible region of the F-doped ZnO thin films as-deposited and annealed is over 90%.     

Effect of sputtering pressure and annealing temperature on the properties of indium tin oxide thin films

Indium tin oxide (ITO) thin films were deposited on glass substrates by RF sputtering system at different sputtering pressure (SP) (20–34 mTorr) and room temperature. The sputtering pressure effects on the deposition rate, electro-optical and structural properties of the as-deposited films were systematically investigated. The optimum sputtering pressure of 27 mTorr, giving a good compromise between electrical conductivity and optical transmittance was found to deposit films. The films were heat-treated in vacuum (200–450 °C) and their electro-optical and structural properties investigated with temperature. A criterion factor Q, which is the ratio between the normalized average transmission to normalized resistivity was defined. It has been observed that Q has its maximum value for heat treatment at 400 °C and the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis proves the films have preferred crystal growth towards (2 2 2) direction and average size of grains are 35–40 nm.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.     

Phase change and optical band gap behavior of Se0.8S0.2 chalcogenide glass films

Se_0.8S_0.2 chalcogenide glass films have been prepared by thermal vacuum evaporation technique with thickness 583 nm. Annealing process at T ≥ 333 K crystallizes the films and nanostructured films are formed. The crystallite size was increased to 24 nm as the annealing temperature increased to 373 K. Orthorhombic crystalline system was identified for the annealed films. SEM micrographs show that films consist of two parallel surfaces and the thickness was determined by cross section imaging. The optical transmittance is characterized by interference patterns as a result of these two parallel surfaces, besides their average value at longer wavelength decreases as a result of annealing process. The band gap, E_g is red shifted due to crystallization by annealing. As the phase of the films changes from amorphous to crystalline in the annealing temperature range 333–363 K, a non sharp change of the band gap (E_g) is observed. This change was explained by Brus’s model of the energy gap confinement behavior of the nanostructured films. The optical refractive index increases suddenly when the system starts to be crystallized by annealing.     

Highly elastic and transparent multiwalled carbon nanotube/polydimethylsiloxane bilayer films as electric heating materials

Highly elastic and transparent bilayer films composed of MWCNT and polydimethylsiloxane (PDMS) layers were fabricated by spin-coating of MWCNT aqueous solution on glass plates and following curing of PDMS applied on the MWCNT layer. Morphological feature, optical transparency, tensile property, electrical property, and electric heating behavior of the bilayer films with different MWCNT layer thicknesses of 65–185 nm were investigated. SEM images confirmed that pristine MWCNTs were uniformly deposited on glass substrates and the PDMS layer was combined well with the MWCNT layer, resulting in high structural stability of the bilayer films to high elongational or twisting deformations. With the increase of the thickness of the MWCNT layer, the sheet resistance of the bilayer films decreased substantially from ~ 10⁵ Ω/sq to ~ 10³ Ω/sq, in addition to the change of the optical transmittance from ~ 75% to ~ 40% at a 550 nm wavelength. The electric heating behavior of MWCNT/PDMS bilayer films was strongly dependent on the thickness of the MWCNT layer as well as the applied voltage. Even under high twisting by 540° or continuous stepwise voltage changes for long periods of time, the MWCNT/PDMS bilayer films retained stable electrical heating performance in aspects of temperature responsiveness, steady-state maximum temperature, and electric power efficiency.     

Influence of the molar concentration and substrate temperature on fluorine-doped zinc oxide thin films chemically sprayed

The effect of both the molar concentration of the starting solution and the substrate temperature on the electrical, morphological, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films deposited on glass substrates is analyzed in this work. All the starting solutions employed were aged for 10 days before the deposition. The results show that as the molar concentration increases, a decrease in the electrical resistivity values is obtained, reaching the minimum resistivity in films ZnO:F deposited from a 0.4 M solution at 500 °C. A further increase in the molar concentration leads to a very slight increase in the resistivity. On the other hand, as the substrate temperature is increased, the resistivity decreases and a tendency towards to minimum value is evidenced; taking the molar concentration as parameter, minimum values are reached at 500 °C. The obtaining of ZnO:F thin films, with a resistivity as low as 7.8 × 10^− 3 Ω cm (sheet resistance of 130 Ω/□ and film thickness of 600 nm) measured in as-deposited films is reported here for the first time. The concurrent effect of the high molar concentration of the starting solution, the substrate temperature values used, and the ageing of the starting solution, which might cause polymerization of the zinc ions with the fluorine species, enhance the electrical properties. The structure of the films is polycrystalline, with a (002) preferential growth. Molar concentration rules the surface morphology as at low concentration an hexagonal and porous structure is developed changing to a uniform compact and small grain size surface in the films deposited with the high molar concentrations.     

Study of the properties of CuInSe2 materials prepared from nanoparticle powder

This work studies the properties of CuInSe₂ (CIS) thin films obtained from nanoparticle powder. The samples were prepared by direct thermal evaporation of ball milled powder from a tungsten crucible onto precleaned glass substrates, using a BALZERS coating unit. The as deposited films were annealed in a vacuum at different temperatures. The composition, structure, morphology and optical properties of these samples have been investigated. X-ray diffraction (XRD) results showed that the CuInSe₂ phase may be prepared by mechanical alloying method. The crystallites of the CIS powder were found to be partially copper-rich. The as-deposited film was polycrystalline in nature having chalcopyrite structure as a mainly phase. The surface morphology is homogeneous across the surface. Expected optical properties (E_g = 1.04 eV, α ≈ 10⁴ cm^? 1) and near stoichiometric composition (Cu: 23.62 at.%, In: 24.35 at.% and Se: 52.03 at.%) were determined.     

Growth technology,X-ray and optical properties of CdSe thin films

An ammonia-free chemical-bath deposition was used to obtain CdSe thin films on glass substrate. The materials used in the chemical bath were cadmium chloride complexed with sodium citrate and sodium selenosulphate. The preparation conditions, especially the starting solution characteristics, such as concentration of dissolved materials, temperature, pH value as well as deposition time and immersion cycles were optimized to obtain homogeneous stoichiometric films with good adherence to the glass substrate. The films thickness was in the range of 400–500 nm with a growing time of 4 h. The material obtained was characterized by optical absorption, SEM with the energy dispersive X-ray analysis (EDS) and X-ray diffraction. The films obtained at bath temperatures of 70 and 80 °C had the hexagonal structure (of wurtzite type), with crystallite size of about 20 nm. Room temperature deposition results in films with the cubic structure and crystallite size of about 4 nm. From optical transmission data, an energy gap equal to 1.88 eV was found. The material is interesting for applications in hybrid systems for solar energy conversion.     

Influence of annealing on the optical properties of 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride thin films

Optical constants (refractive index, n, and absorption index, k) of the as-deposited and annealed films of 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) have been obtained in the wavelength range 190–2500 nm by using spectrophotometric measurements. The obtained optical constants were used to estimate the type of transition for the as-deposited and annealed films. We present a single oscillator model that describes the dispersion of refractive index. Drude model of free carriers absorption have been described for the analysis the dispersion of refractive index dispersion before and after annealing.     

Effect of solvent ratio on the properties of highly oriented sprayed fluorine-doped tin oxide thin films

Transparent conducting thin films of F:SnO₂ have been deposited onto preheated glass substrates by a spray pyrolysis technique using pentahydrate stannic chloride (SnCl₄·5H₂O) and ammonium fluoride (NH₄F) as precursors and mixture of water and propane-2-ol as solvent. The concentration of SnCl₄·5H₂O and NH₄F is kept fixed and the ratio of water and propane-2-ol solvent in the spraying solution is varied. A fine spray of the source solution using air as a carrier gas has grown films of thickness up to 995 nm. Optical absorption, X-ray diffraction, Van der Pauw technique for measurement of a sheet resistance and Hall effect measurements at room temperature for determination of carrier density and conductivity have been used. The as-deposited films are of polycrystalline SnO₂ with a tetragonal crystal structure and are preferentially having orientation along the (200) direction with texture coefficient as high as 6.16. The average grain size for the as-deposited sample is found to be of the order of 44 nm. The films have moderate optical transmission (up to 70–85% at 550 nm). The figure of merit (ϕ) values vary from 1.95 · 10^− 3 to 35.68 · 10^− 3 Ω^− 1. The films are heavily doped, degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance (R_s) for the optimized sample is 5.1 Ω. The films have a resistivity of 5.43 · 10^− 4 Ω cm and mobility around 7.38 cm² V^− 1 s^− 1.     

Structural and optical properties of thermally evaporated zinc phthalocyanine thin films

Thermally evaporated zinc phthalocyanine (ZnPc) films in the as deposited condition were identified to be as-amorphous. It undergoes structural transformation upon annealing up to 613 K. The optical properties and spectral behavior of as deposited and annealed thin films of ZnPc were studied using spectrophotometric measurements of the transmissivity and reflectivity at normal incidence of light in the wavelength range 200–2500 nm. The refractive index, n, and absorption index, k, were calculated and it was found that they are independent of film thickness in the thickness range 205–530 nm. Annealing at 613 K increases absorbance of films by 5–6 times in comparison with absorbance of as deposited ones and shifts peak positions of all bands towards low energy side of spectra except the peak position of N-band is shifted towards high energy side of spectra. The absorption spectra in the UV–VIS. region has been analyzed in terms of both molecular orbital and band theories. Indirect allowed transitions near the onset and fundamental absorption edges were observed. The energy at the onset was obtained and equals to 1.45 and 1.51 eV for as deposited and annealed films, respectively. The fundamental energy gap was obtained and equals to 2.94 and 2.88 eV for as deposited and annealed films, respectively. The absorption spectra shows four absorption bands. The oscillator strength, f, the electric dipole strength, q², the molar extinction coefficient, ζ_molar, were calculated for as deposited and annealed ZnPc thin films.     

Thickness and temperature effects on electrical resistivity of (Bi0.5Sb0.5)2Te3 thin films

Thin films of (Bi_0.5Sb_0.5)₂Te₃ of different thickness were deposited on glass substrate by the flash evaporation method in a vacuum of 1 × 10^− 5 Torr. X-ray diffraction and transmission electron microscope analysis indicates that these films are polycrystalline even in the as-deposited state and the post-deposition annealing leads to grain growth. Electrical resistivity studies were carried out on these films as a function of temperature (300– 450 K) and film thickness (450–2000 Å). Temperature dependence of electrical resistivity shows that (Bi_0.5Sb_0.5)₂Te₃ films are semiconducting. It is found that electrical conduction activation energy decreases with increase of film thickness and this observation is explained based on the Slater model. Thickness dependence of electrical resistivity is analyzed using the effective mean free path model of size effect with perfect diffuse scattering. This analysis leads to the evaluation of the important physical parameters i.e., mean free path and bulk resistivity of hypothetical bulk.     

Spray pyrolysed bismuth oxide thin films and their characterization

Uniform, adherent and reproducible bismuth oxide thin films have been deposited on glass substrates from aqueous Bi(NO₃)₃ solution, using the solution spray technique. Their structural, surface morphological, optical, and electrical properties were investigated by XRD, AFM, optical absorption, electrical resistivity and thermo-emf measurements. The structural analysis from XRD pattern showed the formation of mixed phases of monoclinic Bi₂O₃ (predominant), tetragonal β-Bi₂O₃ and nonstiochiometric Bi₂O_2.33. The surface morphological studies on atomic force micrographs revealed round grain morphology of bismuth oxide crystallites. The optical studies showed a direct band gap of 2.90 eV for as-prepared bismuth oxide films. The electrical resistivity measurements of bismuth oxide films indicated a semiconducting behavior with the room temperature electrical resistivity of the order of 10⁷ Ω cm. From thermo-emf measurements, the electrical conductivity was found to be of n-type.     

Crystallographic parameters and electro-optical constants in ITO thin films

Indium tin oxide thin films were deposited onto soda lime glass substrates using an e-beam evaporation system. In order to improve the structural, electrical and optical properties, the films were annealed at 450 °C and 500 °C in vacuum for 1 h. X-ray diffractions of samples were analyzed by Rietveld refinement and Warren–Averbach methods. By application of Levenberg–Marquardt least square method, the experimental transmittance data were fitted completely with the transmittance data calculated via a combination of modified Drude and Forouhi–Bloomer models. Focusing on the results, it was shown that the samples had a nanosize crystallite and enhancement of the annealing temperature resulted in an increase in the conductivity, carrier concentration, lattice parameter, crystallite size and micro-strain. However, it was found that the defects were preferentially accumulated along grain boundaries in sample annealed at lower temperature. Moreover, the finding revealed that conductivity of the samples was dominated by intra-grain in indium tin oxide films. Furthermore, increasing annealing temperature resulted in the orientation in < 111> crystal texture and also brought in an additional Burstein–Moss shift.     

Structural,optical and electrical characteristics of transparent bismuth vanadate films deposited on indium tin oxide coated glass substrates

Bismuth vanadate (BVO) thin films were fabricated on indium tin oxide (ITO) coated glass substrates using pulsed laser ablation technique and investigated their structural, optical and electrical properties. The use of the indium tin oxide coated glass substrate resulted in reducing the leakage current characteristics of crystalline BVO thin films. The X-ray diffraction (XRD) studies confirmed the monophasic nature of the post annealed (500 °C/1 h) films. The atomic force microscopy indicated the homogeneous distribution of crystallites in the as-deposited films. The as-deposited and the post annealed films were almost 90% transparent (380–900 nm) as confirmed by optical transmission studies. Dielectric constant of around 52 was attained accompanied by the low dielectric loss of 0.002 at 10 kHz for post annealed films. The leakage current of the post annealed BVO films on ITO coated glass substrates measured at room temperature was 8.1 × 10⁻⁸ A at an applied electric field of 33 kV/cm, which was lower than that of the films with platinum and SrRuO₃ as the bottom electrodes.     

Room temperature deposition of IZTO transparent anode films for organic light-emitting diodes

This study was to investigate anodic electrode IZTO films deposited by pulsed DC magnetron sputter at room temperature with various oxygen partial pressures onto glass substrate and to analyze the structural, electrical, and optical properties, as well as the relationship between the chemical binding state of the surface and the characteristics of IZTO films. In addition, the prepared IZTO films were used to fabricate the organic light emitting diodes (OLEDs) as an anode layer to study the device performances. The IZTO film deposited at optimal oxygen partial pressure of 2.0% in sputtering process showed the best properties, such as a low electrical resistivity and high optical transmittance of <5.1 × 10^?4 Ω cm and >80% in the visible wavelength of 400–800 nm, respectively. The OLED characteristics with the optimum condition showed good brightness and the lowest turn-on voltage of >10,000 cd/m² and 4.67 V. These results indicate that IZTO films can be a promising candidate as an alternative TCO electrode material for flexible and OLED devices.