Shift In Optical Band Gap Research Articles

Stress-induced anomalous shift of optical band gap in Ga-doped ZnO thin films: Experimental and first-principles study

In this work, highly c-axis oriented Ga-doped ZnO thin films have been deposited on glass substrates by RF magnetron sputtering under different sputtering times. The optical band gap is observed to shift linearly with the electron concentration and in-plane stress. The failure of fitting the shift of band gap as a function of electron concentration using the available theoretical models suggests the in-plane stress, instead of the electron concentration, be regarded as the dominant cause to this anomalous redshift of the optical band gap. And the mechanism of stress-dependent optical band gap is supported by the first-principles calculation based on density functional theory.

Electrical and optical properties of ZnO:Al films with different hydrogen contents in sputtering gas

Aluminum-doped zinc oxide (ZnO:Al) films were deposited by direct current magnetron sputtering in incorporating hydrogen in sputtering gas at room temperature. The influences of hydrogen content in sputtering gas on the structural, optical, and electrical properties of ZnO:Al films were systematically investigated. It is found that hydrogen incorporated into ZnO lattice forms shallow donors in ZnO:Al films and plays an important role in the properties of ZnO:Al films. The electrical conductivity and infrared (IR) reflectance are improved due to the increase of electron carrier concentration, and the average transmittance decreases, which is ascribed to the strong scattering from the hydrogen incorporated and oxygen vacancies in ZnO:Al films. In this study, the resistivity of 5.5 × 10−4 Ω·cm is obtained, the average transmittance of the wavelength in the range of 400–900 nm is almost 86 %, and the IR reflectance reaches 75 % at 2,500 nm, which is higher than that of reported TCO films. The band gap determined by optical absorption is a result of competition between Burstein–Moss effect and many-body perturbation effect. However, the hydrogen content in sputtering gas is above 10 %, and the optical band gap shift is independent of hydrogen content in sputtering gas.

Structure and optical properties of Ni1−xCoxWO4 solid solutions

Ni1−xCoxWO4, x=0–1, polycrystalline samples were prepared by solid state synthesis. Structure, composition, and optical properties of the samples were characterized. The results show that Ni1−xCoxWO4 adopt wolframite structure in all compositions. Cell parameters of the samples systematically increase with Co content. While Co content does not have much effects on FT-IR spectra, it does indirectly change band gap energy of the compounds in the series. Gaussian fitting of diffuse reflectance spectra show that both Ni d–d transition and Co–W metal to metal charge transfer contribute to the shift of optical band gap of the samples.

Anion-controlled passivation effect of the atomic layer deposited ZnO films by F substitution to O-related defects on the electronic band structure for transparent contact layer of solar cell applications

Anion-controlled chemistry in ZnO films is essential to obtain stable charge–carrier transport and to prevent degradation in the performance of the transparent contact layer in solar cells through passivation of O-related defects by F substitution. Therefore, in this work, the passivation effect of F doping in ZnO was confirmed by analysis for the chemical state of O in ZnO films using XPS surface analysis in the O 1s region. Furthermore, we investigated the effect of F doping in a ZnO matrix on the electronic structure of the resulting films as a function of electron concentration by optical band gap shift (Burstein–Moss theory) and near-band-edge transition (photoluminescence with Stokes shift theory) measurements in ZnO films with different F doping concentrations. The band gap narrowing effect was not significant in the F-doped ZnO films due to valence band perturbation from anion doping rather than conduction band perturbation. Finally, the band structure could be approximated by the relationship between experimental analysis and the formation of valence and conduction bands in ZnO orbitals.

Effect of linking topology on the properties of star-shaped derivatives of triazine and fluorene

Three star-shaped molecules, having 2,4,6-triphenyl-1,3,5-triazine core and fluorene side arms linked through different linkages, were designed and synthesized. The obtained compounds were characterized by UV and fluorescence spectroscopies, differential scanning calorimetry, thermogravimetric analysis, cyclic voltammetry, time-of-flight and CELIV techniques. All the three star-shaped compounds possess high thermal stability with the temperatures of the onsets of thermal degradation around 400°C and glass formation ability with close glass transition temperatures (56–61°C). The synthesized compounds show broadband absorption with the absorption maxima of dilute solutions in the range of 350–382nm. Dilute solutions of the fluorenyl-substituted derivatives of 2,4,6-triphenyl-1,3,5-triazine showed monomer fluorescence with fluorescence quantum yields ranging from 0.50 to 0.70. The theoretical DFT calculations showed that the geometry, optical and electrochemical properties of the synthesized star-shaped molecules depend on their linking topologies. Thus, completely flat star-shaped molecules, in which the donor and acceptor moieties are linked through the linking bridges having double and triple bonds, are characterized by smaller optical band gap and bathochromic shift compared to the derivative with twisted skeleton in which the chromophores are linked directly via single bond. The best charge-transporting properties were shown by the compound in which 2,4,6-triphenyl-1,3,5-triazine and 2-[9,9-bis(2-ethylhexyl)-9H-fluorene] moieties are linked via the linking bridges containing ethenyl linkages. Hole mobility of the amorphous layer of this compound reached 1.9×10−3cm/Vs at an electric field of 1.15×106V/cm.

Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32eV to 3.45eV with the increasing carrier density from 2.0×1020cm−3 to 3.24×1020cm−3. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices.

Microstructure and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition.

Cuprous oxide (Cu2O) thin films were prepared by using electrodeposition technique at different applied potentials (−0.1, −0.3, −0.5, −0.7, and −0.9 V) and were annealed in vacuum at a temperature of 100°C for 1 h. Microstructure and optical properties of these films have been investigated by X-ray diffractometer (XRD), field-emission scanning electron microscope (SEM), UV-visible (vis) spectrophotometer, and fluorescence spectrophotometer. The morphology of these films varies obviously at different applied potentials. Analyses from these characterizations have confirmed that these films are composed of regular, well-faceted, polyhedral crystallites. UV–vis absorption spectra measurements have shown apparent shift in optical band gap from 1.69 to 2.03 eV as the applied potential becomes more cathodic. The emission of FL spectra at 603 nm may be assigned as the near band-edge emission.

Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange

We demonstrate bandgap tuning of a single-layer MoS2 film on SiO2/Si via substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air.

The shift of optical band gap in W-doped ZnO with oxygen pressure and doping level

Tungsten-doped (W-doped) zinc oxide (ZnO) nanostructures were synthesized on quartz substrates by pulsed laser and hot filament chemical vapor co-deposition technique under different oxygen pressures and doping levels. We studied in detail the morphological, structural and optical properties of W-doped ZnO by SEM, XPS, Raman scattering, and optical transmission spectra. A close correlation among the oxygen pressure, morphology, W concentrations and the variation of band gaps were investigated. XPS and Raman measurements show that the sample grown under the oxygen pressure of 2.7Pa has the maximum tungsten concentration and best crystalline structure, which induces the redshift of the optical band gap. The effect of W concentration on the change of morphology and shift of optical band gap was also studied for the samples grown under the fixed oxygen pressure of 2.7Pa.

Infrared-optical spectroscopy of transparent conducting perovskite (La,Ba)SnO3 thin films

We have performed optical transmission, reflection, spectroscopic ellipsometry, and Hall effect measurements on the electron-doped LaxBa1–xSnO3 (x = 0.04) transparent thin films. From the infrared Drude response and plasma frequency analysis we determine the effective mass of the conducting electron m* = 0.35m0. In the visible-UV region the optical band gap shifts to high energy in (La,Ba)SnO3 by 0.18 eV compared with undoped BaSnO3 which, in the context of the Burstein-Moss analysis, is consistent with the infrared-m*. m* of BaSnO3 is compared with other existing transparent conducting oxides (TCO), and implication on search for high-mobility TCO compounds is discussed.

EFFECT OF Mn DOPING ON MICROSTRUCTURE AND OPTICAL PROPERTIES OF NANOCRYSTALLINE ZnO

Mn -doped ZnO has emerged as the most studied system for prototype applications in spintronics devices because of its interesting magnetic properties. In this report, nanocrystalline ZnO doped with various concentration of Mn have been synthesized from different precursors using modified ceramic route. Samples are characterized by using X-ray diffractometer (XRD), high resolution transmission electron microscopy (HRTEM) and UV-Vis spectrophotometer. XRD and HRTEM studies confirm the growth of single phase, well crystallized Mn -doped ZnO nanoparticles. Particle size estimated from Rietveld analysis as well as TEM images show a decreasing tendency with the increase in Mn concentration. We observe both band bowing and red shift in optical bandgap by varying the concentration of Mn and precursors. This study demonstrates that Mn concentration is not the only factor but precursors have a definite role on the variation of optical bandgap.

Temperature-Dependent Photoluminescence of ZnCuInS/ZnSe/ZnS Quantum Dots

Colloidal ZnCuInS/ZnSe/ZnS core/shell/shell quantum dots (QDs) with average particle sizes of 2.3, 2.7, and 3.3 nm were prepared in a noncoordinating solvent. The size-dependent optical band gap and photoluminescence (PL) band shift due to the quantum confinement effect were observed. Because the PL band showed a large Stokes shifts over 400 meV, the origin of the PL band was related to the electronic transition via defect levels. A time-resolved PL measurement indicated that the PL lifetime of the QDs was a characteristic feature of three dominating transitions from the conduction band to surface defect level, from the conduction band to an acceptor level, and from the donor level to an acceptor level. It was investigated as a function of temperature in the range from 50 to 373 K to understand the radiative and nonradiative relaxation processes and fitted with two empirical expressions, from which the Huang–Rhys factor and the phonon energy were calculated. According to the fitting data, the size-dependent parameters were analyzed including the Huang–Rhys factor, the average phonon energy, and the excitonic-acoustic phonon coupling coefficient. The temperature coefficient was about −2.32 × 10–4 eV/K. The results showed that, in the temperature range from 50 to 373 K, the variations of the energy band gap and the photoluminescence line broadening were predominantly due to an optical transition from the band edge to the defect-related level and the coupling of the carriers to acoustic phonon, respectively.

Structure and red shift of optical band gap in CdO–ZnO nanocomposite synthesized by the sol gel method

Abstract The structure and the optical band gap of CdO–ZnO nanocomposites were studied. Characterization using X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS) analysis confirms that CdO phase is present in the nanocomposites. TEM analysis confirms the formation of spheroidal nanoparticles and nanorods. The particle size was calculated from Debey–Sherrer′s formula and corroborated by TEM images. FTIR spectroscopy shows residual organic materials (aromatic/Olefinic carbon) from nanocomposites surface. CdO content was modified in the nanocomposites in function of polyvinylalcohol (PVA) added. The optical band gap is found to be red shift from 3.21 eV to 3.11 eV with the increase of CdO content. Photoluminescence (PL) measurements reveal the existence of defects in the synthesized CdO–ZnO nanocomposites.

Synthesis of Al-doping ZnO nanoparticles via mechanochemical method and investigation of their structural and optical properties

In this study, pure and Al-doped ZnO nanoparticles were successfully synthesized by the mechanochemical method. The effect of doping on structural and optical properties was studied. The structural investigations of products confirmed the hexagonal wurtzite structure for all products and also it was observed that doping caused a little change in lattice parameters. The results obtained by SEM revealed that the average particle size of products increases by doping and nanoparticles have almost spherical morphology. UV–visible absorption and PL spectra showed that the maximum ultraviolet absorption wavelength of products increases and the optical band gap shifts from 3.15eV to 3.11eV, with increasing the Al concentration.

Blue shift in optical band gap of sol–gel derived Sn1−xZnxO2 polycrystalline thin films

Undoped and Zn doped SnO2 thin films are deposited by sol–gel spin coating on glass substrate. XRD spectra with prominent peaks along (110) and (101) planes shows the polycrystalline nature of thin films. The particle size lies between 9.30 and 42.09 nm as estimated by Debye–Scherer method. SEM micrographs of the films contain pebble like structures spread throughout whose diameter decreases with increase in dopant concentration. Surface topology of the films is studied by atomic force microscopy. Transmission spectra show that all the films are highly transparent in the visible and IR spectral region (80–90 %) and a sharp absorption occurs near 300 nm. Approximately a change of 4 % is observed in the optical band gap by Zn doping. The optical band gap is tunable between 3.55 and 3.68 eV for 0 ≤ x ≤ 0.15 in nanocrystalline Sn1−xZnxO2. Broad UV emission at 395 nm is observed in photoluminescence spectra of the films along with a blue emission. Emission intensity decreases as amount of Zn incorporated into SnO2 increases.

Structural and optical properties of spin coated Zn1−xCrxO nanostructures

Zn1−xCrxO thin films were grown on glass substrates by the sol–gel spin-coating technique. XRD results indicate that a ZnO single phase with a hexagonal wurtzite structure is formed. The films exhibit a preferential orientation along (002) direction in comparing with the other directions. AFM images of the films indicate that the Cr-doped ZnO films are consisted of a wrinkled network structure. The surface roughness of the films is increased with increasing Cr content. The optical band gap of Zn1−xCrxO thin films is decreased from 3.3eV to 3.2eV with the increase of Cr dopant ratio from x=0% to x=9.8% (in molar ratio). The observed red shift in optical band gap is due to s–d and p–d exchange interactions. Urbach energy, EU, values are changed inversely with optical band gaps of the films. The refractive index and optical conductivity of the films were changed with Cr content. The obtained results suggest that the structural and optical properties of ZnO films can be controlled by Cr doping.

Unusual near-band-edge photoluminescence at room temperature in heavily-doped ZnO:Al thin films prepared by pulsed laser deposition

Room temperature photoluminescence (PL) properties of heavily-doped ZnO:Al thin films (with carrier concentration n in the range of 5–20 × 1020 cm−3) prepared by pulsed laser deposition have been investigated. Despite their high carrier concentration, the films exhibited strong room temperature near-band-edge bound excitons at ∼3.34 eV and an unusual peak at ∼3.16 eV, and negligible deep-level emission even for the films deposited at a temperature as low as 25 °C. The radiative efficiency of the films increased with growth temperature as a result of increased n and improved crystallinity. A large blue shift of optical band gap was observed, which is consistent with the n-dependent Burstein–Moss and band gap-renormalization effects. Comparison of the results of the PL and optical measurements revealed a large Stokes shift that increased with increase in n. It has been explained by a model based on local potential fluctuations caused by randomly-distributed doping impurities.

Red shift in absorption edge of Cd1−x Ni x S dilute magnetic semiconductor nanofilms

Nickel-doped cadmium sulphide dilute semiconductor nanofilms (Cd1−x Ni x S, 0 ≤ x ≤ 0.09) have been investigated using atomic force microscopy, scanning electron microscopy, Fourier infrared transform and UV–Vis spectroscopy. With increasing Ni content a decrease in average crystallite size (26–10 nm) and surface roughness has been found. A red shift in optical band gap has been observed with decrease in average crystallite size, which is against quantum confinement.

Analysis of optical band-gap shift in impurity doped ZnO thin films by using nonparabolic conduction band parameters

Polycrystalline ZnO thin films both undoped and doped with various types of impurities, which covered the wide carrier concentration range of 1016–1021cm−3, were prepared by magnetron sputtering, and their optical-band gaps were investigated. The experimentally measured optical band-gap shifts were analyzed by taking into account the carrier density dependent effective mass determined by the first-order nonparabolicity approximation. It was shown that the measured shifts in optical band-gaps in ZnO films doped with cationic dopants, which mainly perturb the conduction band, could be well represented by theoretical estimation in which the band-gap widening due to the band-filling effect and the band-gap renormalization due to the many-body effect derived for a weakly interacting electron-gas model were combined and the carrier density dependent effective mass was incorporated.

Investigation of Annealing on the Structural and Optical Properties of Chemical Bath Deposited ZnSe Quantum Dot Thin Films

ZnSe thin films were obtained through chemical bath deposition method. Structural and optical properties of as deposited and annealed samples were investigated by X-ray Diffraction and spectrophotometer. The as deposited thin films were in nanocrystalline, with lots of strain and a blue shift of optical band gap. After annealing, the crystal grain gained, the strain eased and optical band gap enlarged. And it suggested that annealing can ease the quantum effect of chemical bath deposited ZnSe thin films.