Direct Optical Band Gap Energy Research Articles

Effect of TiO 2 on thermal, structural and third-order nonlinear optical properties of Ca–La–B–O glass system

A series of calcium lanthanum metaborate glasses in the composition (wt%) of 23.88CaO–28.33La 2O 3–47.79B 2O 3 modified with TiO 2 up to 20 wt% are prepared by a melt quenching technique to study the influence of TiO 2 on their thermal, structural, linear and nonlinear optical properties. The differential thermal analysis (DTA) studies have demonstrated significant effects due to the presence of TiO 2 on the glass forming ability and crystallization situations. The glass with 15 wt% TiO 2 has achieved a eutectic composition and also exhibited a better glass forming ability among the glasses studied. The FT-IR spectra of these glasses show mainly vibration modes corresponding to stretching of BO 3 trigonal, BO 4 tetrahedral units and of B–O–B bending bonds. At higher concentrations of TiO 2, development of vibration band around 400 cm −1 has indicated the formation of TiO 6 structural units in the glass network. The red shift of optical absorption edge (UV cutoff) shows a monotonous decrease in direct and indirect optical band gap energies ( E opt ) with an increase of TiO 2 content in the glasses based on their absorption spectra. The optical transparency of these glasses is found to be varied from 64 to 87% within the wavelength range 450–1100 nm depending on the TiO 2 content. Besides these studies, linear refractive indices, the nonlinear optical properties of these glasses have also been evaluated.

Electrical, structural and optical properties of SnO 2:F thin films: Effect of the substrate temperature

SnO 2:F thin films have been deposited from stannic chloride solution by simple and cost effective chemical spray pyrolysis technique onto glass substrates at different substrate temperatures. The as-deposited films were characterized by X-ray diffraction (XRD), SEM, optical and electrical characterization techniques. XRD analysis showed that, at lower substrate temperature; amorphous films have been obtained, while at higher temperatures ( T > 450 °C); crystalline SnO 2:F films with preferential growth along (2 0 0) plane have been observed. The average transmission in the visible region has been found to vary from 60% to 87% depending upon the substrate temperature. In the visible region of the spectrum, the transmission is very high (high enough to observe interference). For films prepared at 475 °C, relatively higher transmittance of about 87% at 850 nm has been observed. A thickness was found to vary from 440 nm to 740 nm with substrate temperature. The direct optical band gap energy for the SnO 2:F thin film is found to be 4.15 eV. The films deposited at 475 °C substrate temperature were found to have relatively lower resistivity of 3.91 × 10 −4 Ω cm. Hall Effect studies reveal that the films exhibit n-type conductivity.

Fabrication and electrical properties of organic-on-inorganic Schottky devices

In this paper, we fabricated an Al/new fuchsin/p-Si organic–inorganic (OI) Schottkydiode structure by direct evaporation of an organic compound solution on a p-Sisemiconductor wafer. A direct optical band gap energy value of the new fuchsinorganic film on a glass substrate was obtained as 1.95 eV. Current–voltage(I–V) andcapacitance–voltage (C–V) measurements of the OI device were carried out at room temperature. From theI–V characteristics, it was seen that the Al/new fuchsin/p-Si contacts showed goodrectifying behavior. An ideality factor value of 1.47 and a barrier height (BH) value of0.75 eV for the Al/new fuchsin/p-Si contact were determined from the forward biasI–V characteristics. A barrier height value of 0.78 eV was obtained from the capacitance–voltage(C–V) characteristics. It has been seen that the BH value of 0.75 eV obtained for the Al/newfuchsin/p-Si contact is significantly larger than that of conventional Al/p-Si Schottkymetal–semiconductor (MS) diodes. Thus, modification of the interfacial potential barrierfor Al/p-Si diodes has been achieved using a thin interlayer of the new fuchsin organicsemiconductor; this has been ascribed to the fact that the new fuchsin interlayer increasesthe effective barrier height because of the interface dipole induced by passivation of theorganic layer.

Investigation of optical parameters of Ag–In–Se thin films deposited by e-beam technique

The optical properties of the Ag–In–Se (AIS) thin films deposited by e-beam technique were investigated by means of the optical transmittance measurements. The optical absorption coefficients of the films were found to vary from 10 3 to 10 5 cm −1 over the wavelength range of 300–1100 nm. The real and imaginary parts of the refractive index and dielectric constant for the as-grown and annealed films in between 100 and 400 °C were evaluated by means of both envelope method (EM) and continuous wavelet transform (CWT) method and the results were in quite good agreement with each other. The refractive index, n, dispersion over the measured wavelength range was explained by applying single-oscillator model (SOM) and the related parameters were calculated. The optical absorption process for the AIS thin films was characterized by three direct transitions from three closely spaced valence bands to a single conduction band due to the splitting of the valence band under the influence of the tetragonal crystalline field and spin–orbit interaction. The direct optical band gap energy decreases as the annealing temperature increases because of the increase in the width of band tail states near valence-band edge caused by the Se segregation. The two distinct parameters of the quasicubic model; crystal-field splitting, Δ CF , and spin–orbit splitting, Δ SO , were calculated for as-grown and annealed AIS thin films.

Microstructural and optical investigations of sol–gel derived ferroelectric BaTiO 3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO 3 nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO 2/Si substrates by a modified sol–gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5–5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO 3/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi–Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n ( 0 ) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262 ± 2 and 268 ± 1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.

Effect of deposition conditions on the InP thin films prepared by spray pyrolysis method

InP thin films were prepared by spray pyrolysis technique using aqueous solutions of InCl3 and Na2HPO4, which were atomized with compressed air as carrier gas. The InP thin films were obtained on glass substrates. Thin layers of InP have been grown at various substrate temperatures in the range of 450–525°C. The structural properties have been determined by using X-ray diffraction (XRD). The changes observed in the structural phases during the film formation in dependence of growth temperatures are reported and discussed. Optical properties, such as transmission and the band gap have been analyzed. An analysis of the deduced spectral absorption of the deposited films revealed an optical direct band gap energy of 1.34–1.52 eV for InP thin films. The InP films produced at a substrate temperature 500°C showed a low electrical resistivity of 8.12 × 103 Ω cm, a carrier concentration of 11.2 × 1021 cm−3, and a carrier mobility of 51.55 cm2/Vs at room temperature.

Optimization of annealing conditions of In 2S 3 thin films deposited by vacuum thermal evaporation

In 2S 3 thin films were grown on glass substrates by means of the vacuum thermal evaporation technique and subsequently thermally annealed in nitrogen and free air atmosphere from 250 to 350 °C for different durations. Experimental parameters have been adjusted in order to optimize the annealing conditions, and to obtain high band gap energy at low deposition temperature, as required for photovoltaic applications. In order to improve our understanding of the influence of the deposition and annealing parameters on device performance, we have investigated our indium sulfide material by X-ray diffraction, energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM) and spectrophotometry. The optical and structural properties of the films were studied as a function of the annealing temperature and durations. X-ray diffraction analysis shows the initial amorphous nature of deposited In–S thin films and the phase transition into crystalline In 2S 3 upon thermal annealing. Films show a good homogeneity and optical direct band gap energy about 2.2 eV. An annealing temperature of 350 °C during 60 min in air atmosphere were the optimal conditions.

Fabrication and characterization of In 2S 3 thin films deposited by thermal evaporation technique

Indium sulphide, In 2S 3, thin films present an alternative to conventional CdS films as buffer layer for CIS-based thin film solar cells. The objective is to eliminate toxic cadmium for environmental reasons. Indium sulphide is synthesized and deposited by single source vacuum thermal evaporation method on glass substrates. The films are analyzed by X-ray diffraction (XRD) and spectrophotometric measurements. They have a good crystallinity, homogeneity and adhesion. The X-ray diffraction analysis confirmed the initial amorphous nature of the deposited InS film and phase transition into crystalline In 2S 3 formed upon annealing at free air for 250 °C substrate temperature for 2 h. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range of 300–1800 nm. We note that the films annealed at 250 °C for 2 h show a good homogeneity with 80% transmission. An analysis of the optical absorption data of the deposited films revealed an optical direct band gap energy in the range of 2.0–2.2 eV.

A Study on Structural and Optical Properties of Pb1-xCdxI2 Single Crystals

<TEX>$Pb_{1-x}$</TEX> <TEX>$Cd_{x}$</TEX> <TEX>$I_2$</TEX> (x=0.0, 0.2, 0.5, 0.7, 0.9, 1.0) single crystals were grown by using Bridgman method and their structural and optical properties were investigated from the measurement of X-ray diffraction, optical absorption and photoluminescence. As-grown single crystals have hexagonal closed packed layered structure. The values of lattice constant c decrease with increasing composition x. Direct and indirect transition optical energy band gaps are calculated from optical absorption spectra measured at room temperature. They increase exponentially from 2.3eV to 3.2 eV with increasing composition x. The energies of photoluminescence peak due to donor bound exciton measured at 6K increase with increasing composition . However, the peak energies of donor-acceptor pair (DAP) are independent of the optical energy band gaps of <TEX>$Pb_{1-x}$</TEX>/<TEX>$Cd_{x}$</TEX> <TEX>$I_2$</TEX> single crystals.

Optical properties of β-In2S3 andβ-In2S3:Co2+ single crystals

β-In2S3 and β-In2S3:Co2+ singlecrystals were grown by the chemical transport reaction method usingIn2S3, S, and ZnS as starting materials and (ZnCl2 + I2) asa transport agent. The single crystals crystallized into a tetragonalstructure. The indirect optical energy band gaps of the single crystals at 298 Kwere found to be 2.240 eV and 1.814 eV for β-In2S3 andβ-In2S3:Co2+, respectively. The direct opticalenergy band gaps were found to be 2.639 eV and 2.175 eV for β-In2S3and β-In2S3:Co2+, respectively. Impurity optical absorptionpeaks were observed for the β-In2S3:Co2+ single crystal.These impurity absorption peaks were assigned, based on the crystal fieldtheory, to the electron transitions between the energy levels of the Co2+ion sited in Td symmetry.

Electrical, optical, and structural characteristics of ITO thin films by krypton and oxygen dual ion-beam assisted evaporation at room temperature

Transparent conducting tin-doped indium oxide (ITO) thin films on polycarbonate and glass substrates were deposited without substrate heating and post-deposition annealing using a dual ion-beam assisted evaporation technique, where the bombardment of the growing film surfaces during electron beam evaporation was done using krypton (varied ion flux, J Kr + , and grid acceleration voltage, V a, of the krypton ion source) and oxygen (fixed ion flux and grid acceleration voltage of the oxygen ion source) ion beams. The electrical, optical, and structural effects of krypton ion-beam bombardment of the growing ITO thin films were investigated using Hall-effect measurements, X-ray photoelectron spectroscopy (XPS), UV-visible spectrometry, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The total film thickness and the deposition rate were 100 nm and 0.06 nm/s, respectively. All ITO films grown with J Kr + =1.92–3.76×10 14 cm −2 s −1 and V a=100–500 V showed an amorphous structure and no other crystalline phases. As J Kr + increased, the electrical conductivity and the optical transmittance of the grown films were improved compared with those of the ITO films deposited using the oxygen ion-beam only. Also, an increase of the bombardment energy by increasing V a of the krypton ion source caused the deterioration of ITO film properties. The conductivity and the optical transmittance of ITO films deposited on polycarbonate substrates were a little lower than those of films on glass substrates. At room-temperature, using optimal growth conditions, the electrical resistivity was as low as 6.4×10 −4 Ω cm with an electron carrier concentration n e=4.3×10 20 cm −3 and a Hall mobility μ H=26.7 cm 2 V −1 s −1, the visible transmittance (at λ=550 nm) was 90%, and optical direct band gap energy 3.8 eV.

Properties of amorphous tin-doped indium oxide thin films deposited by O 2/Ar mixture ion beam-assisted system at room temperature

Highly transparent and conductive thin films of tin-doped indium oxide (ITO) on glass substrates were grown by the ion beam-assisted deposition (IBAD) technique without any substrate heating. X-Ray diffraction investigations indicated that all films have an amorphous structure and no other crystalline phases. The addition of Ar to O 2 flow and the increased energy of incident ions were found to reduce the resistivity of the grown films. Observed decrease in the resistivity was attributed to the increase in the carrier concentration. In the optimal growth conditions at room temperature, we obtained the electrical resistivity of 4.6×10 −4 Ω-cm, visible transmittance (at λ=550 nm) ≥90%, and optical direct band gap energy of ≅3.75 eV.

Optical and electrical properties of flash-evaporated amorphous CuInSe 2 films

Amorphous films of CuInSe 2 were deposited on glass substrate by flash evaporation of source materials. The films were found to be p-type semiconductors. The direct optical band-gap energy was obtained to be 1.21–1.41 eV. The film DC conductivity ranged from 1.2–5.7 S cm −1 at 285 K for different film thickness with corresponding activation energy of 55.5–301 meV. From temperature dependence of conductivity, the carrier transport was interpreted to be due to band conduction above 270 K.

Photoelectrochemical behaviour of Zn 3In 2S 6 single crystals in aqueous solutions

The basic photoelectrochemical behaviour and some physical quantities of the ternary semiconductor n-Zn 3In 2S 6 have been investigated for the first time. From the photocurrent actions spectrum in 1M Na 2SO 4 a direct optical band gap energy ( E g) equal to 2.81 eV has been determined. From capacity measurements a flat band potential ( V FB 0 − 1.29 V SCE) has been calculated, as well as the donor density, the conduction and the valence band edge. With pH increasing, a shift of the photocurrent onset potential ( V on) to more cathodic values has been observed. The presence of redox systems such as Na 2S/NaOH, Na 2SO 3, K 3[Fe(CN) 6]/ K 4[Fe(CN) 6], Ce 3+/Ce 4+ improves the stability of the electrodes and produces reasonable photovoltages.

The optical properties of MoTe2 and WSe2

The dielectric constants of (2H) MoTe2 und (2H) WSe2 have been measured over the photon energy range 0.5 to 3.8 eV and the derived direct optical band gap energies compared with the measured activation energies of intrinsic electrical conduction. A systematic excitonic description of the MoSe2, MoTe2, and WSe2 spectra is given, together with an outline development of the band structures of these compounds. Die Dielektrizitätskonstante von (2H) MoTe2 und (2H) WSe2 wurde im Bereich der Photonenenergien von 0,5 bis 3,8 eV gemessen. Die so ermittelten Energien der direkten optischen Bandlücke werden mit den gemessenen Aktivierungsenergien der Eigenleitung verglichen. Es wird eine systematische exzitonische Beschreibung der MoSe2-, MoTe2- und WSe2-Spektren angegeben, sowie eine Entwicklung der Bandstruktur dieser Verbindungen skizziert.