Shift In Optical Band Gap Research Articles

Surface wettability of titania thin films with increasing Nb content

TiO2 and TiO2/Nb amorphous thin films were grown on glass substrates by a sol-gel technique (spin coating). Films' surface composition, structure, and morphology were derived from x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy data. The investigated films showed a smooth surface (roughness values below 5 nm). A separate surface wettability investigation showed that by increasing the Nb amount in pristine titania films results in a decrease of contact angle (CA) values from 40° to nearly 0°, thus, indicating a super-hydrophilic conversion under UV illumination. This conversion rate is greatly enhanced by increasing the Nb content, the surface super-hydrophilic behavior occurring after a couple of minutes in the TiO2/Nb samples, but after 4 h in the pristine titania specimen. The current results are discussed in terms of the optical band gap shift towards higher energies, by increasing the Nb content in the films, a process explained based on small polaron hopping model.

Blue shift of optical band-gap in LiNbO3 thin films deposited by sol–gel technique

Lithium niobate (LN) thin films were deposited on quartz substrates by sol–gel technique. The measured absorption and photoluminescence spectra show that the band structure of LN thin films is direct unlike indirect band-gap in bulk LN and the optical band-gap of these LN thin films was measured to be 4.7eV which is ~1eV greater than that for stoichiometric bulk LN. The dependence of the blue shift of band-gap on several parameters like quantum confinement, composition (Li:Nb ratios of LN thin films) and strain was also investigated. The results obtained show that the large blue shift in band-gap of LN thin films is primarily due to strain in the film.

Influence of argon ambience on the structural, morphological and optical properties of pulsed laser ablated zinc sulfide thin films

Nanostructured zinc suplhide thin films are successfully deposited on quartz substrates using pulsed laser deposition (PLD) under different argon pressures (0, 5, 10, 15 and 20 Pa). The influence of argon ambience on the microstructural, optical and luminescence properties of zinc sulfide (ZnS) thin films is systematically investigated. The GIXRD data suggests rhombohedral structure for ZnS films prepared under different argon ambience. Self-assembly of grains into well-defined patterns along the y direction is observed in the AFM image of the film deposited under argon pressure 20 Pa. All the films show a blue shift in optical band gap. This can be due to the quantum confinement effect and less widening of conduction and valence band for the films with less thickness and smaller grain size. The PL spectra of the different films are recorded at excitation wavelengths 250 nm and 325 nm and the spectra are interpreted. The PL spectra of the films recorded at excitation wavelength 325 nm show intense yellow emission. The film deposited under an argon pressure of 15 Pa shows the highest PL intensity for excitation wavelength 325 nm. For the PL spectra (excitation at 250 nm), the highest PL intensity is observed for the film prepared under argon free ambience. In our study, 15 Pa is the optimum argon pressure for better crystallinity and intense yellow emission when excited at 325 nm.

Synthesis and characterization of CdS/PVA nanocomposite thin films from a complexing agent free system

CdS/PVA nanocomposite thin films have been deposited on glass substrates by in situ thermolysis of precursors dispersed in polyvinyl alcohol (PVA). The synthetic technique reported in this study is free from complexing agent and hence no need to control the pH of the solution as in the case of conventional CBD. The as-prepared films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), UV–vis spectroscopy, and photoluminescence (PL) spectra. The XRD and SAED results indicated the formation of CdS nanoparticles with hexagonal phase in the PVA matrix. The photoluminescence and UV–vis spectroscopy revealed that CdS/PVA films showed quantum confinement effect. From the shift in optical band gap, particle sizes were calculated using effective mass approximation (EMA) method and it was found to be in agreement with the results obtained from TEM observations. The SEM results indicated that as grown films were homogeneous with no visible pinholes and cracks. The film prepared at100 °C was found to be suitable for application as a window layer in solar cell.

Mechanism of Blue Shift of Optical Band Gap in Aluminum-Doped ZnO Thin Films with Blend Bond

In this paper, aluminum–doped nano-crystalline zinc oxide (ZnO:Al or AZO) thin films were deposited on fused quartz substrate by pulsed laser ablation at various temperatures. The physical phase and surface morphology were characterized by using X-ray diffraction (XRD) and atomic force microscope (AFM), respectively. The variation of the optical band gap (Egopt) of the films with the temperature was measured through transmittance in UV-VIS wavelength. The results showed that there was a blue shift of Egoptfrom 3.44 eV to 3.62 eV as the growing temperature decreased from 500°C to 200°C. The value of Egoptimmediately recurred from 3.30 to 3.35 eV after a thermal annealing of the samples at 700 °C, inclining to a normal value of bulk zinc oxide. The analyses of XRD and AFM testified that the presence of amorphous phase in the AZO films was the main reason for the blue shift of the optical band gap Egopt.

Effect of oxygen deficiency on optical band gap shift in Er-doped ZnO thin films

Er-doped ZnO films were deposited by reactive magnetron sputtering technique at different oxygen flow rate. The microstructures, the chemical state of the oxygen and the optical absorption properties of ZnO:Er films were investigated. The X-ray diffraction spectroscopy (XRD) results and the X-ray photoelectron spectroscopy (XPS) analyses about the oxygen in the doped samples indicated that oxygen flow rate has great effect on the crystalline quality of ZnO:Er films. It was concluded that the decrease of the crystalline quality of the samples was caused by the oxygen deficiency. The optical absorption properties and the shift of the optical band gaps were investigated. The analysis reveals that the blue shift of the band gaps was caused due to the decreasing of O 2−ions at the intrinsic sites and the increasing of O 2-ions at the oxygen deficient regions.

Growth and characterization of electrodeposited Cu(In,Al)Se 2 thin films

Thin films of CuIn 1 − x Al xSe 2 were grown using a cathodic electrodeposition technique. The CuIn 1 − x Al xSe 2 films were electrodeposited on SnO 2 coated glass from aqueous baths containing different Al contents using deposition potentials ranging from − 650 mV to − 850 mV versus a saturated calomel electrode. The electrodeposited films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-rays, atomic force microscopy, and UV–VIS–NIR spectroscopy. The results show that single phase CuIn 1 − x Al xSe 2 films with Al content x around 0.27–0.33 having good stoichiometry can be produced in the above potential range. XRD and SEM studies show that films deposited at − 650 mV and − 750 mV have good crystallinity while those grown at − 850 mV have comparatively poorer crystallinity. SEM studies show that the particle size of the films grown at − 650 and − 750 mV is in the micron range but is around 100 nm when grown at − 850 mV. Optical studies show that the optical band gap shifts with Al content from 1.21 eV for x = 0.27 to about 1.42 eV for x = 0.33. The as-grown as well as vacuum annealed films were n-type in conductivity with resistivity in the range 3–5 × 10 −3 Ω cm.

N-doped MgZnO alloy thin film prepared by sol–gel method

In this work, nitrogen-doped MgZnO thin films were deposited on glass substrate by spin-coating technique and thermal annealing treatment. The X-ray diffraction (XRD), scanning electron microscope (SEM) and optical transmission measurement were carried out to investigate the effect of growth temperature and nitrogen doping on the physical and optical properties of the films. XRD and SEM results revealed that the crystallinity and preferred c-axis orientation was enhanced with increasing annealing temperature and certain concentration of N-doping. In addition, the films incorporated with nitrogen doping exhibit significant blue shift in optical band gap and improvement in its transparency.

Optical and electrical properties of zinc oxide/indium/zinc oxide multilayer structures

Zinc oxide/indium/zinc oxide multilayer structures have been obtained on glass substrates by magnetron sputtering. The effects of indium thickness on optical and electrical properties of the multilayer structures are investigated. Compared to a single zinc oxide layer, the carrier concentration increases from 8×1018cm−3 to 1.8×1020cm−3 and Hall mobility decreases from 10cm2/vs to 2cm2/vs for the multilayer structure at 8nm of indium thickness. With the increase of indium thickness, the transmittance decreases and optical band gap shifts to lower energy in multilayer structures. Results are understood based on Schottky theory, interface scattering mechanism and the absorption of indium layer.

Effect of oxygen partial pressure and VO2 content on hexagonal WO3 thin films synthesized by pulsed laser deposition technique

We report on the effect of oxygen partial pressure and vacuum annealing on structural and optical properties of pulsed laser-deposited nanocrystalline WO3 thin films. XRD results show the hexagonal phase of deposited WO3 thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO3 thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effects of VO2 content on structural, electrochemical, and optical properties of (WO3)1−x (VO2) x nanocomposite thin films have also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for VO2–WO3 thin film electrode with higher VO2 content (x ≥ 0.2). Increase of VO2 content in (WO3)1−x (VO2) x films leads to red shift in optical band gap.

Electrical and optical properties of Sb-doped BaSnO3 epitaxial films grown by pulsed laser deposition

In this paper we report the structural, electrical and optical properties of epitaxial Ba(SbxSn1−x)O3 (x = 0–0.30) (BSSO) films grown on SrTiO3(0 0 1) substrates by the pulsed laser deposition method. The investigation reveals that the transport and optical characteristics of BSSO films depend very sensitively on the Sb-doping content. Temperature-dependent resistivity measurements show that at low Sb contents (x = 0.03, 0.07) the metal–semiconductor transition occurs at 150 K and 80 K, respectively, and the semiconductor behaviour appears in high doped (x = 0.15, 0.30) films. The transmittance decreases significantly from about 80% to nearly zero in the visible region and the optical band gap shifts from 3.48 to 4.0 eV with increasing Sb content in the films. The lowest room-temperature resistivity of 2.43 mΩ cm with carrier density and mobility of 1.65 × 1021 cm−3 and 1.75 cm2 V−1 s−1 was obtained in the films with doping at x = 0.07. By employing them as bottom electrodes we have fabricated transparent Pb(Zr0.52Ti0.48)O3 ferroelectric capacitors showing square polarization–electric field hysteresis loops, indicating that these perovskite-type BSSO films at low doping can be potentially used in transparent devices especially based on all-perovskite heterostructures.

Nanocrystalline Mg Doped ZnO Dilute Magnetic Semiconductor Prepared by Chemical Route

This paper reports on the comparative investigation of ZnO and Mg doped ZnO (MZO) dilute magnetic semiconducting thin films prepared by chemical bath deposition. The films were characterized for their structural properties, optical and morphological studies. The films were polycrystalline in nature with hexagonal phase having (002) preferential orientation. The typical grain size were also estimated and found to be of an average of 200 nm. It was found that Mg doping into ZnO, modulated a shift in optical band gap of the MZO films in the UV region of 3.22 eV. The films exhibit ferromagnetic properties for magnesium doping concentration of 2 at%.

Effect of post-annealing temperature on nano-structure and energy band gap of indium tin oxide (ITO) nano-particles synthesized by polymerizing–complexing sol–gel method

In this paper, we report on the structural, microstructural and optical properties of nano-crystalline indium tin oxide (ITO) particle, which has been synthesized by sol–gel process using a simple starting hydro-alcoholic solution consisting of In(NO 3) 3.5H 2O, SnCl 4.5H 2O, citric acid as complexing and ethylene glycol as polymerization agents. The structural properties of indium tin oxide nano-powders annealed at different temperatures ( T=350–650 °C) have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. The XRD patterns show In 2O 3-cubic phase in nano-powders without any indication of crystalline SnO x as an additional phase. The TEM images show the nano-particles as nearly spherical shaped with size in the range of 10–45 nm as the size of grains increases by increasing the annealing temperatures. The optical direct band gap of ITO nano-particles was calculated to be about 3.39–4.02 eV in the temperature range 350–650 °C by optical absorption measurements. The optical band gap shifts to a higher energy with increasing annealing temperature is due to the improvement of the crystallinity, thereby increasing the carrier concentration as determined from the Burstein–Moss effect.

Blue shift of optical band gap in Er-doped ZnO thin films deposited by direct current reactive magnetron sputtering technique

Er-doped ZnO films were deposited by direct current co-reactive magnetron sputtering technique and the microstructure and the optical properties of the ZnO films were investigated. XRD analysis reveals that the substrate temperatures have an evident effect on the (0 0 2) preferential orientation and the Er-doped ZnO film deposited at room temperature consists of an amorphous phase. The blue shift of the optical band gap was observed in the ZnO:Er films compared with undoped ZnO film, and the Er-doped ZnO film deposited at room temperature has the largest band gap (about 3.99 eV). The main reason for the broadening of the band gap is attributed to the amorphous phase in the Er-doped ZnO film. Meanwhile, the red shift of the band gap in the ZnO:Er films deposited at high temperature was observed as the crystallinity of the fim became better.

Structural and optical characterization of pulsed laser‐ablated potassium lithium niobate thin films

AbstractThin films of potassium lithium niobate (K3Li2Nb5O15: KLN) have been prepared on glass substrate, as a function of substrate temperature, using a pulsed laser‐deposition (PLD) technique for the first time. Grazing‐incidence X‐ray diffraction (GIXRD) analysis suggests that KLN films can be grown successfully at a substrate temperature as low as 300 K. The anomalous behavior of the decline of crystalline structure with increase in substrate temperature is explained. The atomic force microscopic (AFM) and scanning electron microscopic (SEM) images show an agglomerated growth mode for the films deposited at a substrate temperature of 300 K and a decrease in grain size with increase in substrate temperature. The films deposited at higher substrate temperatures show ring‐like structures. The AFM analysis shows that the rms surface roughness of the film decrease with increase in substrate temperature. The UV–Vis transmission spectra suggest that the nature of the transition in the films is directly allowed. A blue shift in optical bandgap is observed for the films compared to bulk material. The changes in the optical bandgap with substrate temperature are also discussed.

Swift heavy ion irradiation effect on Cu-doped CdS nanocrystals embedded in PMMA

Semiconductor nanocrystals (NCs) have received much interest for their optical and electronic properties. When these NCs dispersed in polymer matrix, brightness of the light emission is enhanced due to their quantum dot size. The CdCuS NCs have been synthesized by chemical route method and then dispersed in PMMA matrix. These nanocomposite polymer films were irradiated by swift heavy ion (SHI) (100 MeV, Si+7 ions beam) at different fluences of 1 × 1010 and 1 × 1012 ions/cm2 and then compared their structural and optical properties by XRD, atomic force microscopy, photoluminescence, and UV-Vis spectroscopy before and after irradiation. The XRD spectra showed a broad hump around 2θ ≈ 11·83° due to amorphous PMMA and other peaks corresponding to hexagonal structure of CdS nanocrystals in PMMA matrix. The photoluminescence spectra shows a broad peak at 530 nm corresponding to green emission due to Cu impurities in CdS. The UV-Vis measurement showed red shift in optical absorption and bandgap changed from 4·38–3·60 eV as the irradiation fluency increased with respect to pristine CdCuS nanocomposite polymer film.

Stress-induced anomalous shift of optical band gap in ZnO:Al thin films

Thickness-dependent stress relaxation and its unreported effect on optical band gap of Al-doped ZnO thin films have been investigated. The thinnest film (∼84 nm) had a stress of −8.39×109 Nm−2, carrier concentration of 1.73×1019 cm−3 and optical band gap of 3.69 eV, a value significantly higher than the reported ones. With increase in thickness, magnitude of the stress decreased, and correspondingly a redshift of fundamental absorption band edge was observed. A linear dependence of optical band gap on stress in the films with a coefficient of 54.6 meV/GPa has been observed.

Colloidal Synthesis of Ternary Copper Indium Diselenide Quantum Dots and Their Optical Properties

Ternary direct semiconductor CuInSe2 quantum dots (QDs) with average particle sizes ranging from 1.2 to 5.6 nm have been synthesized by a colloidal route using commercially available materials. The size-dependent optical band gap and photoluminescence (PL) band shift due to the quantum size effect were successfully observed. The optical band gap evaluated from the optical absorption spectra was varied from 1.48 eV for the 5.6-nm QDs to 1.66 eV for the 1.2-nm QDs. In the PL spectra, the wavelength of the emission band varied from 918 to 838 nm corresponding to the crystal size variation from 5.6 to 1.2 nm. Because the PL band showed a large Stokes shift above 100 meV, the origin of the PL band was related to the electronic transition via defect levels. Based on the slightly In-rich composition of the QDs and the comparatively large increase in energy of the PL band with decreasing crystal size, the emission was suggested to be due to the recombination between electrons in the quantized conduction band and holes in the copper vacancy acceptor.

Temperature Dependent Optical Band Gap Measurements of III-V Films by Low Temperature Photoluminescence Spectroscopy

AbstractPhotoluminescence (PL) spectroscopy is a powerful technique for probing the structures of many types of III-V semiconductor materials. When a semiconductor material is excited at a particular wavelength, electron-hole pairs are generated. The most intense radiative transition is between the conduction band and valence band, and this measurement is used to determine the material band gap. Radiative and non-radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration, and thus predict device quality. At ambient temperatures, the PL signal is typically broad, as much as 100 nm in width. When cooled, structural details may be resolved, and a small spectral shift between 2 samples may represent a change in a structural parameter. Thus a system with high spectral resolution is required.In this paper, a modular Low Temperature Photoluminescence system (LTPL) for measuring optical band gap as a function of temperature is described. Results show that the optical band gap shifts towards higher energy as the sample temperature decreases.

Influence of reactive oxygen ambience on the structural, morphological and optical properties of pulsed laser ablated potassium lithium niobate thin films

The effect of oxygen ambience on the structure and properties of potassium lithium niobate (K 3Li 2Nb 5O 15: KLN) films prepared on glass substrates by pulsed laser ablation technique (PLD) are studied. The influence of annealing on the properties of vacuum deposited films is also investigated. The Gracing Incidence X-ray Diffraction (GIXRD) data suggests the tetragonal structure for the KLN film whose grain sizes increase on thermal annealing. The Atomic Force Microscopic (AFM) analysis reveals the four-fold symmetric nature of the grains in the films. Self assembly of grains in the form of rings and rods are observed in AFM images of the films deposited in an oxygen ambience of 2 Pa. The films deposited at higher oxygen ambience show a blue shift in optical band gap. The direct current (DC) resistance measurement on the films deposited at non-reactive ambience reveals resistivity in the range of kΩ m.