Decrease In Optical Band Gap Research Articles

Structural Origin of Optical Band Edge Shift in Se70Ge30‐xBix (0≤x≤10) Glassy Alloys

AbstractBulk glasses of Se−Ge‐Bi system have been prepared by quenching the melt. The XRD exhibits very weak peaks riding over halos. DSC scans show a single stage to a multistage crystallization with an incremental substitution of Ge by Bi, indicating evolution of structurally inhomogeneous phases at the molecular level with the increasing content of Bi. The Raman spectra (50‐500 cm‐1) reveal the formation of the corner and edge‐sharing GeSe4/2terahedral and Bi2Se3 pyramidal units. Transmission electron microscope (TEM) Images depicts the formation of submicron to nano‐sized spherical shaped structural units in a chain‐like configuration. Fragmentation of chains and refinement effect of Bi are noticed in TEM and SEM Images. The optical absorption edge found to be red‐shifted along with a sharp decrease in optical band gap (1.74‐0.82), with the increasing fraction of Bi in alloys. Attempts have been made to elucidate the results on the structural considerations (bond and band diagram) and Burstein‐Moss effect.

Effects of Cu doping on the microstructural, thermal, optical and photocatalytic properties of α-FeOOH and α-Fe2O3 1D nanoparticles

Nanostructural morphology and cation (divalent or tetravalent) doping are known to be important factors for photocatalytic and photoelectrochemical activities of iron oxides. Modifications of iron oxides in an appropriate way could significantly improve these activities. In the present work the effects of Cu doping on the microstructural, thermal, optical and photocatalytic properties of goethite (α-FeOOH) and hematite (α-Fe2O3) 1D nanoparticles were investigated. Goethite and Cu-doped goethite 1D nanoparticles were synthesized by precipitation in a highly alkaline medium. Hematite and Cu-doped hematite 1D nanoparticles were prepared by calcination of goethite samples at 500 °C. Cu2+-for-Fe3+ substitution in goethite and hematite was confirmed by determining the change in unit cell size using X-ray powder diffraction and by measuring the hyperfine magnetic field reduction using Mössbauer spectroscopy. An increased Cu2+-for-Fe3+ substitution in goethite caused a gradual elongation and narrowing of nanorods with the formation of nanoneedles at 3 or 4 mol% Cu. The shape and size of nanoparticles were not changed by thermal transformation of goethite to hematite. Thermal analysis showed a lower temperature of goethite dehydroxylation upon increased Cu doping. Cu2+-for-Fe3+ substitution in goethite and hematite caused increased absorption of visible and near-IR radiation beyond the absorption edge, as well as a slight decrease in direct and indirect optical band gaps compared with pure phases. The visible light photocatalytic activities of Cu-doped goethite and hematite nanoneedles were significantly improved compared with undoped goethite and hematite nanorods, which could be attributed to a combined effect of nanoneedle morphology and Cu doping.

Nonlinear optical and dielectric properties of TiO2 nanoparticles incorporated PEO/PVP blend matrix based multifunctional polymer nanocomposites

Poly(ethylene oxide) (PEO)/poly(vinyl pyrrolidone) (PVP) blend (50/50 wt%) and mixed-phase (anatase and rutile) titanium dioxide (TiO2) nanoparticles are used as organic host matrix and inorganic nanofiller, respectively, for the preparation of hybrid polymer nanocomposite (PNC) films (i.e. (PEO/PVP)–x wt% TiO2; x = 0, 1, 3, 5, 10, and 15) by solution casting method with deionized water as solvent. These PNC films are characterized by employing SEM, FTIR, XRD, UV–Vis, DSC, and DRS techniques. The effect of TiO2 nanofiller loading on the structural, morphological, thermal, optical, dielectric, and electrical properties of the PEO/PVP blend matrix, and also on the chain segmental dynamics of PEO in the PNC structures is investigated. It is revealed that the crystalline phase and spherulite morphology of PEO, and the polymer–polymer interactions in these PNC films have been primarily modified by the polymer–nanoparticle interactions. The optical energy band gap decreases, whereas UV absorbance enhances non-linearly with the increase of TiO2 concentration in the PNC films. The enthalpy of melting of the PEO irregularly reduces when the incorporated amount of TiO2 enhances in these films. The real part of complex permittivity and the dielectric loss tangent of these materials are found frequency independent in the range from 20 kHz to 1 MHz but these parameters increase non-linearly from ~ 1.6 to 2 and 0.006–0.008, respectively with the increase of TiO2 concentration up to 10 wt%, at 30 °C. The dominant contribution of interfacial polarization process increases the real part of complex permittivity of these PNC materials by about 1.3 times with the decrease of frequency from 20 kHz to 20 Hz which further enhances with the increment in the temperature of the film. The electrical behaviour of these materials has been examined by analyzing their ac electrical conductivity, complex impedance, and electric modulus spectra over the frequency range 20 Hz–1 MHz. The dc electrical conductivity of these PNC materials enhances nonlinearly with the increase of TiO2 loading in the PEO/PVP blend matrix. The optical and dielectric parameters of these flexible-type PNC materials confirm their multifunctional properties as UV absorber, optical energy band gap tuner, low permittivity tunable nanodielectric, electrical conductivity regulator, and novel host matrix for ion conducting materials. The results infer that these innovative technologically advanced engineered materials can be potential candidates for the next generation microelectronic devices.

Electrical and thermoelectric properties of ternary Cu-Ge-Te films

Chalcogenides are motivating members of crystalline and/or amorphous materials because of their use in many application devices as optoelectronics, detectors of thermally imaging, optical waveguides, recording materials, solar cells, bio- and chemical-sensors. Bulk samples for ternary Ge26CuxTe74−x(2.5≤x≤12.5at.%) glasses were prepared using the well-known melt quenching method. Thermal evaporation method has been used to prepare thin films. The amorphous state of the prepared films was confirmed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The dark dc-electrical conductivity (σdc) and Seebeck coefficient (S) were measured over an extensive range of temperature (300–450 K). The activation energies for electrical conduction (ΔEdc) and thermoelectric effect (ΔEs) decrease with an increase of Cu content. This reflects the observed increase in the carrier concentrations (nσ for electrical conduction and ns for thermoelectric). With the addition of Cu content, the defects as the weak Te–Te homopolar ponds (1.43 eV) increase which may tend to lower the energy of the conduction-band edge. Consequentially, both of the ΔEdc and the optical band gap (Eg) decrease with the increase in Cu content. The obtained results for the dc conductivity and the Seebeck coefficient may lead to a high value of the power factor. The current study is carried out to obtain high-efficient films for their using in the potential applications of thermoelectric phenomenon by an easy, simple, and low economic cost methods. A good correlation between the activation energy for electric conduction (ΔEdc) and the glass transition temperature (Tg) has been established.

Structural and optical studies on PVA capped SnS films grown by chemical bath deposition for solar cell application

Tin monosulphide (SnS) thin films capped by PVA have been successfully deposited on glass substrates for cost effective photovoltaic device applications by a simple and low-cost wet chemical process, chemical bath deposition (CBD) at different bath temperatures varying in the range, 50–80 °C. X–ray diffraction analysis showed that the deposited films were polycrystalline in nature, showing orthorhombic structure with an intense peak corresponding to (040) plane of SnS. These observations were further confirmed by Raman analysis. FTIR spectra showed the absorption bands which corresponds to PVA in addition to SnS. The scanning electron microscopy and atomic force microscopy studies revealed that the deposited SnS films were uniform and nanostructured with an average particle size of 4.9 to 7.6 nm. The optical investigations showed that the layers were highly absorbing with the optical absorption coefficient ~105 cm–1. A decrease in optical band gap from 1.92 to 1.55 eV with an increase of bath temperature was observed. The observed band gap values were higher than the bulk value of 1.3 eV, which might be due to quantum confinement effect. The optical band gap values were also used to calculate particle size and the results are discussed.

Wettability measurement, optical characteristics, and investigation of the quantum confinement effect of ZnS-scotch tape nanocomposite films prepared by successive ionic layer adsorption and reaction (SILAR) method

In the present work, size-tunable ZnS-scotch tape nanocomposite materials were successfully prepared using SILAR method. Scanning electron microscopy (SEM), Atomic Force Microscopy (AFM), X-ray powder diffraction (XRD) and Brunauer-Emmett-Teller (BET) porosity measuring techniques were used for morphological, structural and porosity determination of the prepared nanocomposite materials. UV–Visible absorption and Fourier-Transform infrared (FTIR) spectroscopy techniques were used for investigation of optical properties of the prepared nanocomposite. The confinement of electrons and holes in semiconductor nanostructured materials in lower cycles of SILAR depicts a blue shift in the band gap, which obtained by UV–Visible absorption spectra and described by Effective Mass Approximation (EMA) and Hyperbolic Band Model (HBM). In higher cycles of SILAR, with increasing in particles sizes, a red shift was observed at the position of the peak in the absorption spectrum and consequently optical band gap decreases. Wettability measurement of ZnS-scotch tape nanocomposite materials was investigated by contact angle measurement of water drop on the surface. BET results show a various and alternative behavior. At initial cycles of SILAR, the special surfaces are decreases by increasing cycles of SILAR until 4 cycles, then after 4 cycles of SILAR, the special surfaces are increased with increasing cycles of SILAR, until 12 cycles, then, the special surfaces have decreased with increasing cycles until 16 cycles of SILAR.

Structural, optical and thermal properties of nickel doped bismuth borate glasses

Nickel doped Bismuth Borate glasses with composition (70B2O3-(30-x)Bi2O3-xNiO) where x = 0,0.5,1.0,1.5,2.0 (wt%) have been synthesized by conventional melt quenching technique. X-Ray Diffractograms have confirmed the amorphous nature of the prepared glasses. The calculated physical parameters such as density, molar volume, average boron‑boron separation, ion concentration and inter ionic distance provided the information about the structural stability of glass samples. DTA and OPD confirmed the decrease in glass transition temperature (Tg) with increase in Ni content. The FTIR and Raman studies clearly revealed that the glass network mainly comprises of [BiO3], [BiO6], [BO3] and [BO4] units. It is also observed that the introduction of dopant (Ni) and modifier (Bi) leads to the conversion of [BO3] trigonal units into the [BO4] tetragonal units due to the increase in non-bridging oxygen atoms which results in the increase of degree of disorder in the glass network. The optical absorption measurements (UV) carried out for well polished glass samples show a decrease in optical band gap with the increase in Ni doping. This observation was further supported by the urbach energy calculations and metallization criterion of the prepared glass samples. The electronegativity and electronic polarizability values revealed the ionic character of glass samples.

Dopant Dependent Microstructural and Fluorescence Properties of Biodegradable Polymer Films for DSSC application

The effects of methyl orange (MO) dye doping on optical and fluorescence properties of Sodium Alginate (NaAlg) is studied using various experimental techniques. The FTIR results shows shift and variation in intensity of bands with dopant concentration and the appearance of new band at higher dopant concentration confirm the complex formation and modified chemical structure of polymer. UV-Visible study shows different types of electronic transition with two absorption peaks. These peaks are red shifted with dopant concentration and results in the decrease of optical band gap and increase of activation energy. The decrease of band gap is attributed to the creation of defects and results in increase of amorphous nature, is confirmed from XRD results. The Fluorescence emission spectra observed at 535nm for 236nm excitation shows that the emission peak intensity decreases with increase of dopant concentration. This deduction in peak intensity is explained on the basis of transient effect in quenching as well as charge transfer complex (CTC). These fluorescence quenched polymer composites are good candidate for the dye sensitized solar cell devices.

Modification of Microstructural, Optical and Electrical Properties of PVA by Zinc Ferrite Nano Fillers

Zinc ferrite nano fillers are synthesized by using chemical precipitation method. The size of the synthesized nanoparticles is determined from XRD result. The pure and nano sized zinc ferrite doped PVA composite films were prepared by solvent casting method. The prepared polymer nanocomposite was characterized by XRD, UV-Vis and Electrical conductivity measurements. From XRD results it is found that the semicrystalline polymer get turns in to crystalline form upon doping. Using the observed UV-Vis study Molar extinction coefficient, dipole moment, dipole strength, dipole length and oscillator strength were estimated by adopting standard method and variation of these parameters with doping level are attributed to modifications in the orientation of the crystallites within the composite. The optical energy band gap and activation energy were also calculated using absorption spectra and it is observed that the optical band gap decreases and activation energy increases with filler concentration. The decrease in the band gap and creation of new dipoles enhances the electrical conductivity of the composite films. The maximum conductivity of 6.52571 × 10−5 S/cm is achieved for 2 wt% dopant concentration.

Enhancement of electrical and magnetic properties in acceptor-doped BaTiO3 ferroelectric ceramics

BaTi1−xMnxO3 (x = 0.00, 0.25, 0.50, 0.75 mol%) (BTMO) ceramics were synthesized through sol–gel combustion method. The structural studies suggested that Mn-doped BaTiO3 (BTO) ceramics exhibit a tetragonal structure with P4mm space group via Rietveld refinement analysis. Also, the phonon mode at 308 cm−1 through Raman spectral analysis confirms the local structure of tetragonal symmetries. The redshift observed in UV-absorption spectra indicates a decrease of optical band gap from 3.13 to 2.71 eV with increasing Mn2+ doping. The observed decrease in the intensity of PL emission spectra was due to an increase of Mn2+ concentration. This indicates that a decrease in oxygen vacancies and a reduction in the number of electrons attributed to the Burstein–Moss shift. The carrier hopping process between Mn2+ and Mn3+ is responsible for dielectric as well as magnetization behavior. The ferroelectric double hysteresis loops are related to a ferroelectric and anti-ferroelectric order and it increases with the increase of Mn ion concentration in BTO. Through P–E measurements, the value of remnant electric polarization and coercive field found to be increased with Mn concentration in BTO samples. For BTO sample, two EPR signals with g = 1.969 and g = 2.000 singlets can be assigned with ionized Ba and Ti-vacancy defects. In addition, the EPR signal for BTMO shows a good correlation with Ti vacancies as compensating for lattice defects. Further, the Mn doping induced a weak ferromagnetic to ferromagnetic state due to free carrier concentrations.

Impact of Heat Treatment on The Structural and Optical Properties of Ge4Se60Te30In6 Phase Change Thin Film

Abstract Bulk Ge4Se60Te30In6 chalcogenide glass was synthesized by melt quenching process. Thin films of prepared glassy alloy were deposited onto highly cleaned glass substrate using thermal evaporation technique. In the present work the effect of annealing temperature on the structural and optical behavior of Ge4Se60Te30In6 thin film is investigated by DSC, XRD and UV-Vis spectroscopy. Optical transmittance with wavelength was recorded, for as-prepared and annealed films by a double beam spectrophotometer in the wavelength span 300 to 1000 nm. The optical constants like α, k and Eg were calculated as a function of annealing temperature. The optical transition was found to be allowed direct transition. The decrease of optical band gap with increasing the annealing temperature from 373K to 413 K was explained by Mott and Davis model. The decrease of optical bandgap with increase of annealing temperature were explained by structural relaxation as well as change in defect states and density of localized states due to amorphous-crystalline transformation.

Influence of low energy (keV) negative Li ion implantation on properties of electrochemically induced scaffold-based growth of PbSe nanowires

In the present work, PbSe nanowires were synthesized using restrictive template-based electrodeposition technique and implanted under vacuum with 200 keV negative Li ion at different fluences. The morphology of the nanowires was characterized by field emission scanning electron microscopy (FESEM). X-ray diffraction patterns confirmed the same crystal structure for pristine and ion implanted samples besides change in intensity of diffraction peaks were observed. Crystallite size was evaluated using modified Scherrer method and Williamson–Hall methods (UDM and USDM). Stress, strain, stacking fault, dislocation density and lattice parameters were evaluated for the pristine and ion implanted samples. UV–Vis spectroscopy results showed decrease in optical bandgap with increasing ion fluence. Photoluminescence spectra manifested decrease in emission peak with increasing ion fluence. The I–V graphs of the nanowires depicted space charge limiting current (SCLC) characteristics at higher voltage. An increase in current was observed with increase in ion fluence due to increase in free charge carriers.

Characterization and Optical Properties of MnO Doped CuO-Containing Phosphate Glass as Absorption Filters

In order to demonstrate the intrinsic and extrinsic changes induced by MnO in the phosphate glass network, a glass system of composition 44P2O5-38ZnO-2CuO-(16-x)Na2O-xMnO (where, x = 1, 2, 3, 4, and 5 mol%) was prepared using the conventional melt quenching technique. XRD patterns confirmed the non-crystalline nature of the prepared samples. The density and molar volume as a function of MnO content were determined to characterize structure of the obtained glasses. The optical spectroscopic studies of the prepared samples were carried out over (190-1000 nm) spectral range. The prepared samples were found to behave as bandpass filters in the visible region of spectrum. The UV cutoff wavelength was shifted from 308 nm to 352 nm by increasing content of MnO which is very useful in the optical technology requirements such as ultraviolet preventing applications and the protection from UV-LASER. The optical band gap was decreased by increasing content of MnO whilst, the Urbach energy was increased. The decrease in optical band gap was attributed to the progressive increase in non-bridging oxygens (NBOs) concentration with the addition of MnO. The refractive index was found to be enhanced with the addition of MnO by the effect of increase in overall polarizability. However, the optical polarizability was found to be correlated to the chemical composition of the glass. The results reveal the role of MnO as a network modifier in the phosphate glass matrix.

Investigation of the optical and electrical parameters of As47.5Se47.5Ag5 thin films with different thicknesses for optoelectronic applications

The impact of film thickness on both optical and electrical parameters of amorphous As47.5Se47.5Ag5 thin films at different thickness (174–1383 nm) was studied. In this study the envelope method was employed to determine the optical parameters of the studied films using the experimentally measured data of the optical transmittance Tλ. The optical absorption data of the investigated films at all different thicknesses was successfully described by Tauc’s relation and it was found that the optical absorption mechanism follows the allowed indirect transition rule. Analyzing the results showed that the optical band gap decreases and localized states width increases with increasing the film thickness. Furthermore, the refractive index as well as dispersion parameters of As47.5Se47.5Ag5 thin films were discussed using the single oscillator model suggested by Wemple-DiDomenico. The D.C electrical conductivity of the investigated films in the temperature region (300–588 K) was investigated. In the higher temperature region (400–588 K) the conduction mechanism refer to the conduction of the carriers excited into the extended states, while at lower temperature region (300–400 K) the conduction mechanism is due to carriers transport in the localized states near the valence and conduction bands. The Mott’s parameters were calculated as a function of the film thickness near the Fermi level of the studied films. The results revealed that the variation of the film thickness strongly affects the optical as well as the electrical parameters of As47.5Se47.5Ag5 thin films.

Optical and ionic transport mechanism γ-phase stabilized nanostructured Bi-Ce-O ionic conductors: a structure-property correlation study

This work reports the body-centered cubic (BCC) γ-phase stabilization of Bi2O3 nanostructures by means of varying Ce doping concentration and its effect on the optical, ionic transport and dielectric properties. The Rietveld analysis of X-ray diffraction pattern confirmed the formation of nanocrystalline Ce-doped Bi2O3 composite nanostructure having BCC as main phase in addition with small amount of monoclinic phase at room temperature for 30% doping concentration in the pure Bi2O3 matrix. The results of TEM, EDAX and XRF analysis are consistent with the Rietveld analysis. Lattice fringe and SAED pattern obtained from HRTEM analysis confirm the body-centered cubic γ-phase as dominant phase. The UV-Vis spectroscopy study shows decrease in optical band gap with doping concentration. The presence of different molecular bonds corresponding to γ-phase has been confirmed using FT-IR spectroscopy. The ionic conductivity of the samples has been found to increase with measuring temperature as well as doping concentration. The dielectric and complex modulus properties have been analyzed using Havriliak-Negami formalism and found to be non-Debye type. The conductivity data has been found to obey Variable Range Hopping phenomenon which is corroborated with the structural property of the prepared compositions.

Optical and structural properties of Se80Te15Bi5/PVA nanocomposite films

New nanocomposite films of polyvinyl alcohol (PVA) doped with different wt% (0.6, 1.6, 2.4) of Se80Te15Bi5 glass have been prepared by solution casting method for flexible optical device applications. X-ray diffractogram (XRD) of films exhibit polycrystalline nature. Scanning electron microscope (SEM) images confirmed the formation of nanoclusters having a mean diameter of 60 nm. Elemental composition of the samples was confirmed by energy dispersive X-ray analysis (EDAX) spectra. The optical constants of films were calculated based on the spectral data obtained using UV–Visible spectrophotometer in the wavelength range 200–800 nm. The possible optical transition in these films was found to be allowed indirect transition with energy gap values which decreases from 4.0 eV for pure PVA to 1.6 eV for the nanocomposite with 2.4 wt% of the glass. The calculated refractive index exhibits a normal dispersion and increases from 1.42 for pure PVA to 3.17 for the nanocomposite with 2.4 wt% of the glass. The optical dispersion parameters were determined according to Wemple and DiDomenico (WDD) model. Decrease in optical band gap while the increase in other optical parameters may be attributed to the formation of clusters during the dissolution of glass.

Composition dependent microstructure and optical properties of boron carbide (BxC) thin films deposited by radio frequency-plasma enhanced chemical vapour deposition technique

BxC thin films were deposited on silicon (100) and sodalime glass substrates using Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) technique, at different substrate self-bias varying from -100V to -250V. Optical property, structure and stoichiometry of the films were determined by spectrophotometry, X-ray diffraction and proton elastic backscattering spectrometric measurements respectively to investigate the effect of film composition on microstructure and optical properties. Film refractive index at a typical wavelength of 400 nm increased from 1.87 to 1.97 on account of increasing packing density of the films with substrate self- bias. Decrease in direct and indirect optical band gap with increasing substrate self-bias, has been explained on the basis of compositional variation i.e. boron/carbon stoichiometric ratio. Finally, soft X-ray reflectivity in the wavelength range of 40 Å–360 Å has been measured to explore its potential for application as optical material in this region of electromagnetic spectrum.

Detailed optical analysis of 100 MeV Ni7+ ion irradiated WO3 thin films using Surface Plasmon Resonance

Modifications in the structural, optical properties and surface morphology of RF sputtered WO3 thin films after irradiation with 100 MeV Ni7+ ions have been reported in the present article. The WO3 thin films were deposited at different sputtering pressures (10–50 mTorr). Despite the grain growth due to irradiation, no structural phase transformation has been observed for the WO3 thin films deposited up to 30 mTorr growth pressure. However, complete amorphization post irradiation was detected for the orthorhombic WO3 thin film grown at 50 mTorr sputtering pressure. AFM images indicated that the pristine WO3 thin film consists of uniform grains having smooth surface morphology. Upon irradiation, grain size increased consistently along with increase in surface roughness (Rq) at all sputtering pressures. The root mean square roughness (Rq) estimated from AFM analysis increased from 1.76 nm in pristine film to 9.89 nm in response to irradiation at a fluence of 1 × 1012 ions cm−2. A systematic decrease in optical band gap has been observed with irradiation, which can be correlated with Ni7+ ion induced mid-gap and near band edge defect states. Surface Plasmon Resonance (SPR) technique has been recognized as an appropriate tool to study the optical properties of pristine and irradiated WO3 thin films. SPR reflectance curves were recorded in angular interrogation mode at 633 nm excitation wavelength for pristine and Ni7+ ions irradiated WO3 films deposited at varying deposition pressure.

Investigation of the effects of acidic pH and annealing on the properties of nanostructured TiO2 thin films for waveguiding applications

In this work, titanium dioxide (TiO2) thin films are synthesized by the simple sol-gel process using the dip-coating technique at various pH values (pH 6, 4 and 2). The influence of acidic pH and annealing at 500 °C in air for one hour on the microstructure, morphology, optical and waveguide properties of the prepared thin films are investigated carefully using various characterization techniques. X-ray diffraction (XRD) and Raman spectroscopy (RS) analysis show that all the annealed films are polycrystalline and possess only anatase crystal structure with a preferential orientation along the (101) plane. The intensity of the diffraction and Raman peaks as well as crystallite size were found to increase with increasing sol acidity which indicates an improvement in the crystallinity of the films. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed that morphology, grain size and surface roughness of the films are influenced by heat treatment and pH of the sol. According to UV–Visible measurements, all the TiO2 thin films are highly transparent with an average transmittance of more than 74% in the visible region. It is also observed that the optical transmission and energy band gap decrease with sol acidity and annealing. M-lines spectroscopy (MLS) measurements at a 632.8 nm wavelength put into evidence that TiO2 planar waveguides support single confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. Moreover, both TE and TM refractive indexes and the birefringence of the anatase films are found to increase with acidity of the sol.

Determination of valence and conduction band offsets in Zn0.98Fe0.02O/ZnO hetero-junction thin films grown in oxygen environment by pulsed laser deposition technique: A study of efficient UV photodetectors

Here we present ZnO based thin films deposited in oxygen environment by pulsed laser deposition technique for UV photodetection applications. The valence and conduction band alignments are the key parameter required in understanding the type of charge carrier participating in conduction process. The valence and conduction band offset measurements have been carried out using the X-ray photoelectron spectroscopy (XPS) on the pristine ZnO, Fe (2 wt%) -doped ZnO (Zn0.98Fe0.02O) and the hetero-junction of Zn0.98Fe0.02O with ZnO (Zn0.98Fe0.02O/ZnO). XPS studies revealed that the valence band is upward shifted with 0.37 eV, whereas, there is a downward shifting of −0.393 eV in the conduction band. Structural analysis of the deposited films has been done using X-ray diffraction studies, which revealed the polycrystalline nature of all the films having wurtzite crystal phase. AFM studies suggest that the size of the grains decreases with Fe-doping in ZnO network due to different ionic radii of Zn and Fe. UV–Vis studies suggest the decrease in optical band gap with Fe-doping and hetero-junction formation. The optical band gap energy has also been obtained from the fitting of experimental data of absorption spectra with Mott -Davis's model and Hydrogenic excitonic model. Both the models are well corroborated with each other. The photoconduction behavior of the deposited films has been analyzed under UV light. The pristine ZnO film has a photoresponse of about 25% which is found to increase in Zn0.98Fe0.02O (∼36%) film. The responsivity values are found to be 1.56, 2.17 and 1.86 mA/W for the pristine ZnO, Zn0.98Fe0.02O and Zn0.98Fe0.02O/ZnO thin films, respectively.