eV For Films Research Articles

Deposition and characterization of BiVO4 thin films and evaluation as photoanodes for methylene blue degradation

Thin films of bismuth vanadate (BiVO4) are deposited through the solution combustion synthesis technique coupled with the dip-coating process. Thermal gravimetric analyis shows a total mass loss of 71 % besides the formation of the monoclinic phase, about 300 °C, which is also revealed by X-ray diffraction. UV–Vis optical absorption spectra show direct bandgap transition about 2.5 eV for films, in good agreement with semiconducting monoclinic phase. Scanning electron microscopic images reveal that thermal annealing time at 500 °C is a very important parameter to control the thickness and shape of the particles and yields an average thickness of about 800 nm for 10 dip-coated deposited layers, with round-shaped nanometric-sized particles, homogeneously distributed on the film surface. Photoelectrochemical degradation of methylene blue by a bismuth vanadate film deposited on fluor-doped tin oxide substrate shows up as a very efficient process. The first-order rate constant for the photoinduced process is about five times the rate constant for degradation in the dark, showing the capacity of the BiVO4/fluorine-doped tin oxide film for electrochemical degradation, mainly in the presence of light.

Preparation and Properties of N-ZnS Film/P-Si Heterojunction

ZnS films were prepared on silicon (Si) and glass substrates by radio-frequency (RF) magnetron sputtering method. The effect of annealing treatment on the structural and optical properties of ZnS films was studied. The results showed that annealing treatment was helpful in improving the crystalline quality of the ZnS films, and the bandgap was about 3.61eV and 3.49eV for films with and without annealing, respectively. A ZnS/ Si heterojunction diode was fabricated successfully by depositing ZnS films on p-type single-crystalline Si substrates. The electrical and optical property of the device was reported.

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175°C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N2O), hydrogen (H2), silane (SiH4), and diborane (B2H6) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71eV to 2.0eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7×10−5S/cm to 5.1×10−7S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (Voc)=800mV, short circuit current density (Jsc)=16.3mA/cm2, fill-factor (FF)=72%, and photovoltaic conversion efficiency (η)=9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. Jsc, FF and Voc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.

Structural and optical properties of ZnO thin films prepared by sol–gel method

In this Letter thin films of zinc oxide (ZnO) are prepared by sol–gel method and deposited on glass substrate to evaluate structural and optical properties of the film. The films were grown at different thicknesses in the range of 100–180 nm. The properties of layers were characterised using X-ray diffractometer (XRD), atomic force microscope (AFM) and optical spectrophotometer to evaluate the quality of the thin film for photovoltaic applications. The results from XRD pattern revealed that the grown films exhibit wurtzite structure with (002) preferential plane. The results from AFM data revealed that the films had nano-sized grains with a grain size from∼25 to 60 nm depending on the film thickness. The optical spectrophotometer studies exhibited direct allowed transition with an energy bandgap of 3.12–3.02 eV for films with thicknesses of 100–180 nm, respectively. The refractive index as well as extinction coefficient of films was also measured.

Zn1‐xMgxO thin films deposited by spray pyrolysis

AbstractThis work reports on an analysis of the chemical composition, structural, and optical properties of Mg‐doped zinc oxide (ZMO) thin films with Mg concentration up to 54 at.% and prepared by the spray pyrolysis technique from acetate precursors. Chemical analysis show that the [Mg]/[Zn] ratio incorporated into the films is higher than that present in the starting solution. X‐ray diffraction, Raman scattering, and transmission measurements show that all samples exhibit a single phase wurtzite structure with a transmittance above 70% in the visible range. Increasing Mg content leads to an increase of the band gap energy from 3.23 in pure ZnO to 3.57 eV in films with 54 at% of Mg content. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Poly(3-dodedyl-2,5-thienylenevinylene)s from the Stille coupling and the Horner–Emmons reaction

Regioregular (RR) conjugated polymers are critical for electronic and optoelectronic properties of polymer based semiconducting devices. Monosubstituted polythienylvinylene (PTV), a relatively low band-gap conjugated polymer, has been reportedly synthesized using the Stille coupling reaction between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene. Although a small fraction of the product shows good (∼90%) regioregularity, the whole product (S-C12-PTV) consists of mostly unidentified structures. To gain better control of the polymer structure and regioregularity, a difunctionalized C12-thiophene monomer, 3-dodecyl-5-formyl-thiophen-2-ylmethyl)-phosphonic acid diethyl ester, has been polymerized in near quantitative yield via the Horner–Emmons reaction. The full (100%) regioregularity of the resulting polymer (HE-C12-PTV) has been confirmed by its 1H and 13C NMR spectra. The full regioregularity is also reflected in its strong tendency to crystallize and practically no solubility in boiling hexane, in sharp contrast to S-C12-PTV. UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltammetry, thermal analysis (DSC & TGA) and X-ray diffraction have been used to characterize both polymers. UV-vis absorption spectra of the HE-C12-PTV chloroform solutions of different concentrations show well-resolved vibronic structures with an absorption maximum at 577 nm and a prominent shoulder at 614 nm. The optical bandgaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO energy levels of the HE-C12-PTV film were found to be at −4.98 eV and −2.88 eV, respectively. The electrochemical bandgap of the polymer in the film is estimated to be 2.1 eV. The DSC curve shows a pronounced melting peak at 205 °C. XRD study shows that a decent crystalline structure is formed without any annealing of the as-cast films.

Electrical and optical properties of Cr2−xTixO3 thin films

Cr2−xTixO3 thin films with 0 ⩽ x ⩽ 1.82 were prepared on sapphire substrates by aerosol assisted chemical vapour deposition. The films in the 0 ⩽ x ⩽ 1.6 composition range grow preserving the α-chromite structure with a lattice constant that changes as a function of the titanium concentration [Ti]. However, films with x > 1.6 start to develop a crystalline phase that can be identified as Cr-doped TiO2. For the Cr2−xTixO3 films a Ti3+ → Cr3+ substitution is deduced from the crystalline structure and confirmed by XPS experiments. The increment in the lattice parameter agrees with the reduction in the optical band gap which goes from 3.50 eV to 2.70 eV for films with x = 0 and x ∼ 1.6, respectively. At room temperature the electrical conductivity decreases as a function of the [Ti]. The thermal stimulated conductivity indicates a trap level at ∼0.55 eV inside the gap in all the samples and the behaviour of the σ–T curves at higher temperatures indicates that the transport properties undergo a transition from a variable range hopping process to a more complicated process as the [Ti] increases.

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO 3) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm 2 C −1 (pure WO 3) to 27 cm 2 C −1 for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO 3 films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.

Physical Aging of Arsenic Trisulfide Thick Films and Bulk Materials

Differential scanning calorimetry (DSC) measurements were performed at different heating rates under nonisothermal conditions to probe the glass transition kinetics of As2S3 bulk glass and thick films. We found that, for films, the glass transition temperature (Tg) increases with increasing annealing temperature, and sharp crystalline‐like melting endothermic peaks appear in samples annealed at 160°C. Such sharp endothermic peaks could also be found for samples annealed at 140°C for 300 h, indicating a very slow crystallization process. The activation energy for the molecular motions and rearrangements around Tg could be estimated as 2.12 eV for films and 2.3 eV for the bulk, respectively.

Transport Properties Of Cu2SnS3 Thin Films

A single source vacuum evaporation process has been employed to prepare thin films of Cu 2 S n S 3 . Electrical Hall measurements using Van der Pauw technique have revealed that film resistivity was a strong function of the substrate temperature for film deposition. The activation energy for the resistivity was 0.268 eV for films deposited at 300 K, 0.162 eV for films deposited at 423 K and 0.059 eV for films deposited at 573 K. The Hall mobility was observed to decrease with an increase in temperature due to grain boundary scattering. Room temperature mobility was 110 cm 2 V -1 s -1 , reducing to 28 cm 2 V -1 s -1 at 593 K. The barrier height within the grain was determined to be 0.051 eV. Key words: Thin film, transport properties, vacuum evaporation, activation energy, Hall mobility Nig. J. of Pure & Appl. Physics Vol.3 2004: 75-78

Analysis of Cu(InGa)Se2Alloy Film Optical Properties and the Effect of Cu Off-Stoichiometry

AbstractVariable angle spectroscopic ellipsometry has been used to characterize Cu(InGa)Se2 thin films as a function of relative Ga content and to study the effects of Cu off-stoichiometry. Uniform Cu(InGa)Se2films were deposited on Mo-coated soda lime glass substrates by elemental evaporation with a wide range of relative Cu and Ga concentrations. Optical constants of Cu(InGa)Se2were determined over the energy range of 0.75–C4.6 eV for films with 0 ≤ Ga/(In+Ga) ≤ 1 and used to determine electronic transition energies. Further, the changes in the optical constants and electronic transitions as a function of Cu off-stoichiometry were determined in Cu-In-Ga-Se films with Cu atomic concentration varying from 10 to 25 % and Ga/(In+Ga) = 0.3. Films with Cu in the range 16–24 % are expected to contain 2 phases so an effective medium approximation is used to model the data. This enables the relative volume fractions of the two phases, and hence composition, to be determined. Two distinctive features are observed in the optical spectra as the Cu concentration decreases. First, the fundamental bandgaps are shifted to higher energies. Second, the critical point features at higher energies become broader suggesting degradation of the crystalline quality of the material.

Nanocrystalline anatase TiO 2 thin films: preparation and crystallite size-dependent properties

Nanocrystalline anatase TiO 2 films with crystallites of either of three different sizes (nominally 6, 12, and 20 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These suspensions were painted onto the respective substrates, dried in air and then calcined at 720 K in air for 1 h; the resulting films have a whitish appearance and are several μm thick. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. X-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) data demonstrated that two of the films are solely composed of the anatase phase whereas in the third also the brookite phase of TiO 2 might be present. The average crystallite size was derived from the width of the XRD diffraction peaks and was found to agree within ∼5% with the nominal size. The elemental composition and the amount of impurities in the different films was investigated by secondary-ion mass spectrometry. Whereas the Ti/O concentration is constant throughout all films, the presence of contaminants is substrate-dependent: while in films deposited on silicon or sapphire the atomic fraction of impurities (most prominently Na and K) is in the range of ∼10 −4, for glass substrates an about 100 times higher level of those species is observed, probably the result of outdiffusion during the calcination step or during the reduction of the samples. A detailed study of the influence of the sample temperature T and the ambient oxygen pressure p(O 2) on the DC electrical conductivity σ of the films was performed. For all films, a power-law dependence of σ on p(O 2), σ∝ p(O 2) − n , was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=470 K, n amounts to 1.31, 1.15 and 0.56 for the 6-, 12-, and 20-nm films, respectively. For reduced films, an exponential dependence of σ on T −1 was determined, yielding activation energies E A with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes. Impedance spectroscopy was utilized to determine the frequency-dependent complex resistance of the various films. In all cases, a pure RC behavior was found: whereas the magnitude of R strongly depends on the crystallite size, the sample temperature and the ambient oxygen pressure, C exhibits little variations and falls in the range ∼18 to ∼25 pF. Using thermal desorption mass spectrometry, distinct desorption peaks for O 2 species were observed which shift to higher temperature values with increasing heating rate. An activation energy for that desorption process of ∼0.9 eV was obtained thereof for a 6-nm nanocrystalline film.

Electrical properties of nanocrystalline anatase TiO 2 thin films with different crystallite size

Nanocrystalline anatase TiO 2 films with crystallites of three different sizes (approximately 6, 12, and 18 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. A detailed study of the influence of sample temperature T and the ambient oxygen pressure p(O 2) on the electrical conductivity σ of the films was performed. For all films a power-law dependence of σ on p(O 2), σ∝ p(O 2) n , was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=200 °C, n amounts to −1.31, −1.15 and −0.56 for the 6-, 12-, and 18-nm films, respectively. For reduced films, an exponential dependence of σ on T −1 was determined, yielding activation energies E A with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes.

Thin Films of Titanium Dioxide Prepared by Chemical Routes using Novel Precursors

ABSTRACTNovel, volatile, stable, oxo-β-ketoesterate complexes of titanium, whose synthesis requires only an inert atmosphere, as opposed to a glove box, have been developed. Using one of the complexes as the precursor, thin films of TiO2 have been deposited on glass substrates by metalorganic chemical vapor deposition (MOCVD) at temperatures ranging from 400°C to 525°C and characterized by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. All the films grown in this temperature range are very smooth; those grown above 480°C consist of nearly monodisperse, nanocrystals of the anatase phase. Optical studies show the bandgaps in the range 3.4–3.7 eV for films grown at different temperatures. Thin films of anatase TiO2 have also been grown by spin-coating technique using another ketoesterate complex of titanium, demonstrating that the newly developed complexes can be successfully used for thin film growth by various chemical routes.

Investigation of MgO and Mg1-xTixO thin Films by Electrostatic Spray Deposition Method for a Protective Layer of AC-Plasma Display Panel

AbstractMgO and Mg1-xTixO thin films were deposited by the electrostatic spray deposition method using Mg(tmhd)2 and Ti(OiPr)2(tmhd)2 as source materials and tetrahydrofuran and 1-octyl alcohol as solvents. The refractive index of the films shows a strong change from 1.73 for MgO to 2.39 for TiO2. The optical band gap energy values are 3.83 eV for TiO2 film and higher than 5.1 eV for films of Mg/(Mg+Ti) > 0.67. The discharge voltage and current increased with a content of Ti in Mg1-xTixO films. All these changes are closely associated with different Mg/(Mg+Ti) ratios of the films.

Doping Effects on CdO Thin Films

ABSTRACTThis paper reports the properties of undoped CdO films and CdO films doped with the group VII element F and the group IV element Sn. The CdO films are made by low-pressure chemical-vapor deposition. We observe that undoped CdO films can achieve a carrier concentration of 1021 cm−3, apparently by controlling the intrinsic defect. However, the electron mobility of these films is only around 2 cm2 V−1 s−1. With fluorine doping, an electron mobility of ∼260 cm2 V−1 s−1 has been achieved. However, low carrier concentration results because of the low solubility of F in CdO film. CdO films doped with both Sn and F demonstrate carrier concentrations of 1021 cm−3 and reasonable electron mobilities of around 20 cm2 V−1 s−1. Due to the small effective electron mass of CdO, a large Burstein-Moss shift is observed for films with high carrier concentration. The shift enables the fundamental absorption edges of undoped CdO films to reach 3.0 eV and 3.3 eV for films doped with both Sn and F.

Polarons in Ladder-Type Polymer Films; Recombination Channels and Electron−Phonon Coupling

We applied the photomodulation spectroscopy for probe-photon energies from 0.05 to 2.5 eV in films of ladder-type poly(p-phenylene) and identified a photoinduced absorption (PA) band related to triplet excitons and two correlated PA bands due to charged polarons. The low energy polaron band peaks at a relatively low photon energy of ∼0.1 eV, showing small polaronic relaxation energy. We employed photoinduced absorption-detected magnetic resonance spectroscopy to study the polaron recombination channels and found that spin 1/2 magnetic resonance enhances the bimolecular recombination of polarons to singlet byproducts (singlet excitons and bipolarons). However, it reduces polaron recombination to triplet byproducts, such as triplet excitons. The low-energy polaron PA shows antiresonances due to its overlap with Raman- and IR-active vibrations. The antiresonances spectrum is compared to the IR and Raman spectra and found to also contain modes that are silent in the latter spectra.

Deep Levels In High Resistivity AlGaN Films Grown By MOCVD

ABSTRACTDeep levels in high resistivity layers of AlGaN grown by MOCVD on sapphire were studied by means of dark current and photocurrent versus temperature measurements and by photoinduced current relaxation spectroscopy (PICTS). Strong temperature quenching of photocurrent was observed and explained by the presence of hole traps with energies 0.2 eV, 0.3 eV and 0.35 eV for films with correspondingly 5%, 15% and 25% of Al. Two hole traps with activation energies of 0.27–0.35 eV and 0.85–1.05 eV depending on composition were detected in PICTS spectra. It is suggested that the former traps are the same as observed in temperature quenching of photocurrent while the latter traps are related to the yellow luminescence band in AlGaN. The photocurrent decay is shown to have a complex form with a rapid decrease followed by a long tail associated with detrapping of holes. It is argued that, at room temperature, the ∼1 eV hole traps are mainly responsible for the amplitude of the tail.

Electrical properties in arsenic-ion-implanted GaAs

A model of effective surface potential energy is proposed to explain unknown behavior of carrier transport in film and leakage currents in Schottky diodes. X-ray photoelectron spectroscopy is employed to investigate a series of band diagrams of arsenic-ion-implanted GaAs films. It is shown that the effective surface potential energies, affected by the shift of core levels, are due to different microstructures and atomic environments in these films at different annealing temperatures. Moreover, the effective barrier heights on the surfaces of these films are changed by the different effective surface potential energies which increase from 0.49 to 0.75 eV in films annealed from 200 to 600 °C, but slightly decrease to 0.7 eV in films annealed up to 800 °C.

Mechanisms of Thin Film Ti And Co Silicide Phase Formation on Deep-Sub-Micron Geometries and Their Implications and Applications To 0.18 °m CMOS and Beyond

AbstractWe studied the kinetics of the high resistivity to low resistivity phase transformation for Ti and Co silicides on deep-sub-micron geometries. Fundamental differences between the growth characteristics for Co and Ti result in different scaling behavior. While the the TiSi2 C54 phase grows by a random nucleation and growth process from the polymorphic TiSi2 C49 phase, CoSi 2 grows by a diffusion limited process from CoSi. A strong linewidth and C49 grain size dependence for the TiSi2 C49 to C54 transformation can be related to low nucleation density. A Johnson-Mehl-Avrami analysis was performed for the TiSi2 C49 to C54 transformation obtaining the distribution of Avrami exponents and transformation times for individual 0.26 μm lines, showing that all lines transformed with similar one-dimensional growth characteristics. We found a strong dependence of the half-transformation times on film thickness and linewidth. While activation energies depend strongly on film thickness, ranging from 4.2 to 5 eV for films 64 to 28 nm thick, they are insensitive to linewidth indicating a different nature for the effect of linewidth and thickness. We also found a reduction of agglomeration temperatures with decreasing linewidth or film thickness resulting on a narrower process window for deep-submicron devices. A small linewidth dependence was measured for CoSi to CoSi2 transformation rates, with activation energies of 2 to 2.2 eV for 0.27 μm and wider lines. Both Co and Ti selfaligned silicide processes were implemented successfully into a 0.18 μm technology.