Optical Band Gap Of Films Research Articles

Residual stress induced crystalline to amorphous phase transformation in Nb2O5 quantum dots

Nb2O5 quantum dots (QDs) were grown using a simple technique of vacuum thermal evaporation. QDs were found to be crystalline in nature by selected area electron diffraction (SAED) in TEM. Samples with thickness up to 20 nm did not show any significant residual strain. Residual stress effect on band gap of crystalline Nb2O5 was studied for films thicker than 20 nm. Residual strain was determined using SAED of the films with reference to powder X-ray diffraction (XRD). Films thicker than 45 nm become amorphous as analyzed by both SAED and XRD. The optical absorption of films in the range 25–60 nm indicates significantly varying optical band gap of films. The varying band gap with film thickness scales linearly very well with the variation of residual stress with film thickness. The residual stress dependence of band gap of crystalline films yields stress free band gap as 3.37 eV with pressure coefficient of band gap (∂Eg/∂P)T = −29.3 meV/GPa. From this study, the crystalline to amorphous transformation in tetragonal form of M-Nb2O5 has been determined to be at about 14 GPa. Both pressure coefficient of band gap and crystalline to amorphous transition for tetragonal M-Nb2O5 have been determined for the first time in the literature.

Room-Temperature Deposition of ZnO Thin Films by Using DC Magnetron Sputtering

ZnO thin films have successfully been deposited using DC magnetron sputtering at room temperature by means of plasma power variation. XRD results show that films were grown at a plasma power of 30 W and 40 W are polycrystalline, while at 20 W is considered as amorphous. The optical bandgap of films are shrinkage by increasing the plasma power. The broadest transmittance range is belongs to ZnO film growth at plasma power of 40 W. The electrical conductivity of ZnO films increase from 4.02x10-7 (Ωcm)-1 to 8.92x10-7 (Ωcm)-1 once the plasma power is increased. Based on the electrical and optical properties of the films it clearly be seen that ZnO film grown at plasma power of 40 W has highest transmittance and lower electrical resistivity therefore it appropriate for transparent conductive oxide (TCO).

Optical properties of Al-CdO nano-clusters thin films

The aluminum doped cadmium oxide (CdO:Al) thin films were grown onto glass substrates by sol–gel spin-coating method. The structural properties of undoped and Al-doped CdO thin films were studied by atomic force microscopy. AFM results reveal that the studied CdO films were formed from the nano-clusters. The optical transmittance of undoped and Al-doped CdO is decreased with increasing Al contents. The optical band gaps of the CdO films were varied from 2.54eV to 2.32eV with increasing Al dopants. The width of localized states in the optical band gap of the films is increased with increasing Al content. The improvement of the optical constant of Al-doped CdO has potential applications as transparent conducting oxide for different optoelectronic device applications.

Structural and Optical Properties of Al–Zn–Cu Thin Films Prepared by DC-Magnetron Sputtering at Different Sputtering Powers

Al–Zn–Cu thin films were deposited using DC-magnetron sputtering on glass substrates at room temperature in pure Ar gas environment. The influence of variation power sputtering from 30–120 W on the structural and optical properties has been studied. The X-ray diffraction (XRD) patterns show amorphous structure for all films. The atomic force microscopy (AFM) images revealed a homogeneous Al–Zn–Cu dispersion within the layer in low power sputtering. The optical transmittance spectra showed that the optical transmittance in the visible was decreased with increased power sputtering. By increasing thickness of films, optical band gap in films decreased, and intensity of PL is increased.

Electronic structure and optical band gap of silver photo-diffused Ge2Sb2Te5 thin film

Thin films of amorphous chalcogenide with composition of Ge22Sb22Te56 (thickness d=250nm) and silver film (thickness d=50nm) on top of chalcogenide film were deposited by thermal evaporation technique. Photo-diffusion of Ag into the amorphous Ge2Sb2Te5 thin films has been carried out by illuminating the prepared Ge-Sb-Te:Ag bilayer with halogen lamp. The photo-diffused silver depth profile was traced by means of Time of Flight Secondary Ion Mass Spectroscopy. The films remain amorphous after Ag photo-diffusion into the amorphous Ge2Sb2Te5 films. The composition of Ge2Sb2Te5 thin films and the amount of Ag photo-diffused has been gathered from Electron Probe Micro-analyzer having a Wavelength Dispersive Spectrometer. The composition of the films was found to be very close to the bulk used to deposit films and the amount of Ag photo-diffused was ∼5.20at.%. Changes in the electronic structures of Ge2Sb2Te5 film on Ag photo-diffusion were studied using X-ray photoelectron spectroscopy. The incorporation of silver also increases the optical band gap of the film due to the decrease in the density of defect states on Ag photo-diffusion.

Investigation on structural, optical, morphological and electrical properties of thermally deposited lead selenide (PbSe) nanocrystalline thin films

In this paper, we report the substrate temperature induced changes in physical properties of thermal evaporated lead selenide (PbSe) thin films from the chemically synthesized nanocrystalline PbSe powders. As the first step, nanocrystalline lead selenide was synthesized by simple chemical method at 80°C using lead nitrate [Pb(NO3)2] and sodium selenosulphate [Na2SeSO3] in the aqueous alkaline media. Ethylene Diamine Tetra acetic acid (0.1M) was used as a complexing agent to form stable complexes with metal ions. Later on, the lead selenide thin films were deposited on the degreased glass substrates under a vacuum of 10−5Torr at various substrate temperatures by thermal evaporation technique using the pre-synthesized nanocrystalline PbSe powders. X-ray diffraction results show the synthesized powders and the deposited PbSe films belong to cubic structure. A gradual reduction in optical bandgap of films was observed with increasing substrate temperatures, which revealed the crystallization of the films. These observations are corroborated by photoluminescence spectroscopy study. Changes in surface morphology of the films with respect to substrate temperature were studied by high resolution scanning electron microscopy and atomic force microscopy. Electrical study infers the deposited films are of p-type semiconducting nature.

Synthesis and Optical Properties of Sprayed ZnO and ZnO:Ga Thin Films

Characteristics and optical constants of pure and Ga-doped ZnO thin films have been studied. Pure and Ga-doped zinc oxide thin films were deposited onto glass substrates using the spray pyrolysis technique. Optical absorption studies in the wavelength range of 300–900 nm showed a peak near 450 nm for different doped films (in addition to the peak for un-doped ZnO). Increasing the doping concentration of Ga to 7% induced an increase in the optical constants of films. This increase is attributed to the formation of chargetransfer complexes. ZnO thin films doped with Ga have improved the optical transmittance in the visible region. The addition of Ga also induced an obvious increase in the optical band gap of these films; the optical band gap of Ga-doped films was slightly higher than that of undoped samples (3.1 eV). This study also found that the highest band gap (Eg=3.4 eV) occurred for the film deposited with a doping concentration of 5% gallium.

Determination of Optical and Physical Properties of ZrO2 Films by Spectroscopic Ellipsometry

Film characterization based on spectroscopic ellipsometry (SE) is desirable in order to understand physical and optical characteristics of films. In this work, ZrO2 films were deposited on Si substrates by d.c. reactive magnetron sputtering with deposition times from 4 to 7 hr. The film thickness determined by SE technique was compared to field emission scanning electron microscopy (FE-SEM) results. From SE modelling process, the double-layer physical model and the Tauc-Lorentz for two oscillator dispersion models offered the best result. The refractive index at photon energy of 2.25eV of the film was decreased from 2.14 to 2.08 with time of 4 to 7 hr, respectively. The optical band gap of films was found to be in the range of 5.25-5.3eV. Finally, the film thickness was determined by SE technique which the result showed the thickness values of all films closely to the film thickness measured by FESEM technique.

Effects of Controlling the AZO Thin Film's Optical Band Gap on AZO/MEH-PPV Devices with Buffer Layer

Organic/inorganic hybrid solar cells were fabricated incorporating aluminum-doped zinc oxide (AZO) thin films of varying optical band gap in AZO/poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) structures. The band gaps were controlled by varying the flow rates of Ar andO2used to deposit the AZO. Devices with CdS buffer layer were also fabricated for improved efficiency. The effects of AZO optical band gap were assessed by testing theI–Vcharacteristics of devices with structures of glass/ITO/AZO/MEH-PPV/Ag under AM1.5 illumination (100 mW/cm2). Efficiency was improved about 30 times by decreasing the AZO optical band gap, except in devices deposited without oxygen. A power conversion efficiency of 0.102% was obtained with the incorporation of a CdS buffer layer.

The Properties of Amorphous Silicon Thin Film Solar Cell Fabricated by Intrinsic Layer's Structure and Materials

The hydrogenated amorphous silicon and hydrogenated amorphous silicon-gemanium alloys have been prepared by the PECVD method. Hydrogenated amorphous thin film solar cells consisted of P-a-Si:H/I-a-Si:H(a-Si:H, a-SiGe:H, a-Si:H/a-SiGe:H superlattice) / N-a-Si:H structure. Solar cells were fabricated with thickness 200 nm of intrinsic layer, while the thicknesses of p- and n-layer were fixed to 30 nm and 60 nm respectively. UV-Vis Spectrometer analysis of a-Si:H and a-SiGe:H thin films optical bandgap were 1.77, 1.12–1.71 eV. Conversion efficiencies were a-Si:H (1.36%), a-SiGe:H (3.11%) and a-Si:H/a-SiGe:H superlattice (4.52%).

Effect of O2/Ar Mixture on the Structural and Optical Properties of ZnO Thin Films Fabricated by DC Cylindrical Magnetron Sputtering

Direct Current Cylindrical Magnetron Sputtering Setup was used to deposit ZnO thin films on BK7 substrates. The effects of changing O2/Ar reactive gas mixtures on the structural and optical properties of films were studied. Crystallinity and structure of films were obtained by X-ray diffraction (XRD). Preferential crystalline growth orientation of ZnO films detected by XRD was always along the (002) orientation. The thickness of films was measured by surface profilometer which showed thickness increasing from 68.7 to 80.8 nm for 3–6% O2 amount respectively. The morphology and roughness of the films were investigated by Atomic Force Microscopy (AFM). As oxygen gas amount was increased, the roughness and the grain size were decreased and the deposition rate was increased. The optical transmittance of ZnO films are obviously affected by the changing of O2/Ar reactive gas mixtures. All films exhibit a transmittance higher than 70% in the visible region. The optical band gap of films was measured by Tauc’s method. The results show that by increasing the amount of O2 in reactive gas mixture, the optical band gap of deposited films increases.

Influence of annealing on the structure formation and optical absorption parameters of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl) acetonitrile thin films

The effect of annealing temperatures (343 and 425) K on structure formation and optical absorption of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl)acetonitrile (DOPNA) thin films has been reported. The structural properties are investigated by X-ray diffraction and transmission electron microscope techniques. The electronic transitions of DOPNA thin films are investigated in terms of ultravioletvisible (UV-vis) spectroscopy. The absorption coefficient of films has been calculated and analyzed in order to determine the type of inter-band electronic transition as well as the value of optical band gap for films. The films have been found to have indirect allowed optical band gap: which was found to decrease by increasing annealing temperatures.

The effect of substrate bias voltage on PbTe films deposited by magnetron sputtering

The PbTe films were deposited onto ITO glass substrate by radio frequency magnetron sputtering. Effect of external direct current electrical field applied between substrate and target on the quality of films was investigated. Stylus surface profile, X-ray diffraction (XRD), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the films. The film thickness was measured by a conventional stylus surface profile. The crystal structure and lattice parameters of films were determined by using XRD. The surface morphology of the films was measured by AFM. The absorption coefficients and optical band gaps of films were found from FTIR. The sheet resistance of the samples was measured with a four-point probe and the resistivity of the film was calculated. All the obtained films were highly textured with a strong (2 0 0) orientation. With increasing bias voltage to −30 V, the property of crystal structure, surface morphology and absorption coefficients and resistivity were improved. However, further increase of substrate bias leads to transformation of the property.

Structural and optical properties of Ag2SeTe nano thin films prepared by thermal evaporation

Semiconducting Ag2SeTe thin films were prepared with different thicknesses onto glass substrates at room temperature using thermal evaporation technique. The structural properties were determined as a function of thickness by XRD exhibiting no preferential orientation along any plane, however the films are found to have peaks corresponding to mixed phase. The XRD studies were used to calculate the crystallite size and microstrain of the Ag2SeTe films. The calculated microstructure parameters reveal that the crystallite size increases and micro strain decreases with increasing film thickness. The refractive index, dielectric constants and thereby the optical bandgap of the films were calculated from transmittance spectral data recorded in the range 400–1200 nm by UV–VIS-Spectrometer. The direct optical bandgap of the Ag2SeTe thin films deposited on glass substrates with different thicknesses 50–230 nm were found to be in the range 1.48–1.59 eV. The carrier density value is estimated to be around 9.8 × 1021 cm−1 for the film thickness of 150 nm. The compositions estimated from the optical band gap studies reveal a value of 0.75 for Tellurium concentration. These structural and optical parameters are found to be very sensitive to the thin film thickness.

Optical and structural properties of silicon nitride thin films prepared by ion-assisted deposition

Silicon nitride (SiN x ) films were prepared by ion-assisted deposition process. The films were analyzed by measurement of scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectrometry, spectrophotometer, and ellipsometer measurements. The effects of N-ion current density ( J) on the structural and optical properties of SiN x thin films were investigated. The SEM studies revealed that films deposited at a higher value of J had highly condensed structure. FTIR analysis indicated an extremely low hydrogen content in the IAD films. It was found that the N/Si ratio of films increased when J increased . The optical constants and the optical band gap of films were determined. The stoichiometric Si 3N 4 thin film was obtained with a high refractive index of 2.073 (at a wavelength of 400 nm), an extinction coefficient less than 6 × 10 −4, and an optical band gap of 4.57 eV.

Preparation and characterization of spray-deposited Cu 2ZnSnS 4 thin films

Thin films of Cu 2ZnSnS 4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643–683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10 4 cm −1.

High temperature dependence of the cubic phase content and optical properties of BN thin films

The temperature dependence of the cubic phase content and optical properties of Boron nitrogen (BN) thin films was studied in this paper. The BN thin films were deposited on fused silica and Si substrates by radio frequency bias magnetron sputtering. The BN film properties before and after annealing were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and UV-visible transmittance and reflection spectra. The results indicate that annealing temperature has a significant effect on the optical absorption edge, optical absorption coefficient α, refractive index n, and optical conductivity σ of BN films. The optical absorption edge shift to the high energy with annealing temperature increase indicates that the film optical band gap E g becomes large. The change in the optical absorption properties results from the stress relaxation and phase transformation owing to the high temperature annealings. The dependence of α on the photon energy is fitted by the Urbach tail model in order to determine the Urbach energy E 0. In addition, it is found that refractive index n exists clearly different dependences on temperature in visible and ultraviolet regions, and the optical conductivity σ threshold moves to high energy with increasing annealing temperatures in the threshold region.

ZnO thin films prepared by a modified water-based Pechini method

ZnO films were prepared by a modified water-based Pechini method where ethanol was added to avoid some disadvantages of solely water-based precursor solution. The ratio of ethanol to water (E/W) largely influences the quality of films, and the appropriate ratio is about 1.2. FT-IR spectra show that the addition of ethanol largely promotes the esterification reaction in the dried gel. XRD results show that all films are of wurtzite structure while the annealing temperature is not higher than 700 °C. The secondary phase Zn 2SiO 4 was found in the films annealed at 800 °C because of the reaction between SiO 2 substrate and ZnO film. SEM photographs show that all films consist of nanocrystalline grains with sizes in the range of 25–82 nm, and the addition of ethanol improves the growth of grains. Transmittance spectra reveal that all films have high average transmittance of about 90% in the range of 450–800 nm except those annealed at 800 °C. The average optical band gap of films is about 3.29 eV.

Optical and electrical properties of co-sputtered amorphous transparent conducting zinc indium tin oxide thin films

Highly conductive and transparent thin films of amorphous zinc indium tin oxide are prepared at room temperature by co-sputtering of zinc oxide and indium tin oxide. Cationic contents in the films are varied by adjusting the power to the sputtering targets. Optical transmission study of films showed an average transmission greater than 85% across the visible region. Maximum conductivity of 6 × 10 2 S cm − 1 is obtained for Zn/In/Sn atomic ratio 0.4/0.4/0.2 in the film. Hall mobility strongly depends on carrier concentration and maximum mobility obtained is 18 cm 2 V − 1 s − 1 at a carrier concentration of 2.1 × 10 20 cm − 3 . Optical band gap of films varied from 3.44 eV to 3 eV with the increase of zinc content in the film while the refractive index of the films at 600 nm is about 2.0.

Real time spectroscopic ellipsometry on ultrathin (<50Å) hydrogenated amorphous silicon films on Si(100) and GaAs(100)

Real time spectroscopic ellipsometry was used to determine the time evolution of the dielectric function, bulk thickness, and surface roughness during hot-wire chemical vapor deposition of hydrogenated amorphous silicon (a-Si:H). The amorphous silicon films were deposited on native-oxide-covered c-Si(100) and GaAs(100) substrates at temperatures in the range from 70to350°C. Data analysis by a three layer optical model, consisting of substrate, bulk, and surface roughness layer, revealed that the dielectric function of the a-Si:H film changes in the initial growth regime (d<50Å), which can be attributed to a higher optical band gap for films with a smaller thickness. It is argued that the origin of this higher band gap lies most likely in quantum confinement effects of the electron wave function in the ultrathin film, with possibly a small contribution of a higher hydrogen content in the ultrathin film. In addition, we show that the trends in surface roughness and bulk thickness are only marginally affected, regardless of whether the change in dielectric function with film thickness is incorporated in the data analysis.