Band Gap Of ZnO Films Research Articles

Influences of lead and magnesium co-doping on the nanostructural, optical properties and wettability of spin coated zinc oxide films

The quality and optical homogeneity of thin films are necessary for optoelectronic devices and semiconductor technology. The influence of Pb doping, Zn1−xPbxO (0≤x≤0.05), and Mg co-doping, Zn0.95−yPb0.05MgyO (0≤y≤0.05), on the microstructural properties, optical parameters and wettability of ZnO films were investigated. X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM) results show that the films have polycrystalline and hexagonal structure with a preferred (002) orientation combined with wrinkle network structure for the Pb doped films. The crystalline quality is slightly enhanced with the Pb doping and then deteriorated after Mg co-doping. The influences of the crystallinity and chemical composition on the film wettability are studied. All films show transparency between 85–93%. The reflectance and optical band gap of ZnO films decrease for Pb doping and then increase with Mg co-doping. Well-known Swanepoel׳s method is employed to determine the refractive index (n) and film thickness (d). The influences of Pb and Mg co-doping on the Urbach energy and optical dispersion parameters are also discussed.

Structural, optical and electronic structure studies of Al doped ZnO thin films

Structural, optical and electronic structure of Al doped ZnO thin films grown using pulsed laser deposition on glass substrate are investigated. X-ray diffraction measurements reveal that all the films are textured along the c-axis and have wurtzite structure. Al doping in ZnO films leads to increase in grain size due to relaxation in compressive stress. Enhancement in band gap of ZnO films with the Al doping is also noticed which can be ascribed to the Brustein–Moss shift. The changes in the electronic structure caused by Al in the doped thin film samples are understood through X-ray absorption measurements.

Studies on effect of oxygen flow rate in textured grain growth of ZnO thin films

ZnO thin films were deposited on glass substrate by Successive Ionic Layer Adsorption Reaction (SILAR) method. Effect of oxygen flow rate in textured grain growth, resistance and band gap of the thin films have been done. Textured grain growth of the samples were measured by comparing the peak intensities from XRD. Textured grain growth was found to be maximum when the oxygen flow rate is 2.5 litre/minute. It is found that as the oxygen flow rate increases above this limit, textured grain growth decreases and resistance the samples increases. The optical band gap of ZnO film was found to be increased with the increase of oxygen flow rate.

Low resistive gallium doped nanocrystalline zinc oxide for gas sensor application via sol–gel process

This paper reports the effect of low concentration of Ga doping on the performance of ZnO thin-film based gas sensors. The ZnO films were synthesized on the alkali-free glass substrates by employing a sol–gel process. Different concentrations of Ga were added to the sol–gel mixture for systematic comparison of doping effect. The effect of Ga doping on the microstructure, optical properties, surface morphology, electrical conductivity and gas sensing characteristics was investigated comparing undoped and Ga-doped ZnO thin films. The results revealed that the grain size decreased after the doping of Ga in ZnO. The optical transmittance and bandgap of ZnO films were not affected by the doping of Ga. The Ga doping resulted in a significant increase in electrical conductivity of ZnO thin films. The gas sensing behavior was studied under different concentrations of H2 exposure in the air at an operating temperature of 130°C. The results indicated that the sensing response for H2 was enhanced after the doping of a trace amount of Ga. It was found that 0.3at.% was the optimum Ga doping concentration for ZnO sensors in the low Ga doping concentrations, which exhibited both high response and short response time. The selectivity of 0.3at.% Ga-doped ZnO sensor was investigated for important industrial gas mixtures. It was found that the Ga-doped ZnO sensors exhibited a better detection capability of H2 than NH3 and CH4.

Preparation, characterisation and photocurrent study of sol-gel-derived Al, Mg-doped ZnO transparent thin films

In this work, sol-gel spin-coating was employed to prepare ZnO, Al-doped ZnO (AZO) and Mg-doped ZnO (MZO) thin films onto glass substrates. Structural properties and surface morphologies of as-prepared films were characterised by X-ray diffraction (XRD) and scanning electron microscope (SEM) while their optical properties were scrutinised from their transmission spectra measured by UV-VIS spectrophotometer. Their optical responses and sensitivity of the films were investigated by photocurrent spectroscopy. The XRD and SEM results show that the crystallinity and physical structure of as-prepared films are highly affected by Al and Mg doping. UV-VIS spectrophotometer results suggest that Mg and Al dopants could not only improve their optical transparency but also significantly cause the blue-shift in optical band gap of ZnO films. In addition, the enhancement in photosensitivity of ZnO films can be attained by specific Al and Mg doping content.

First-principles study and electronic structures of Mn-doped ultrathin ZnO nanofilms

The first-principles density functional calculation is used to investigate the electronic structures and magnetic properties of Mn-doped and N-co-doped ZnO nanofilms. The band structure calculation shows that the band gaps of ZnO films with 2, 4, and 6 layers are larger than the band gap of the bulk with wurtzite structure and decrease with the increase of film thickness. However, the four-layer ZnO nanofilms exhibit ferromagnetic phases for Mn concentrations less than 24% and 12% for Mn-doping performed in the whole layers and two layers of the film respectively, while they exhibit spin glass phases for higher Mn concentrations. It is also found, on the one hand, that the spin glass phase turns into the ferromagnetic one, with the substitution of nitrogen atoms for oxygen atoms, for nitrogen concentrations higher than 16% and 5% for Mn-doping performed in the whole layers and two layers of the film respectively. On the other hand, the spin-glass state is more stable for ZnO bulk containing 5% of Mn impurities, while the ferromagnetic phase is stable by introducing the p-type carriers into the bulk system. Moreover, it is shown that using the effective field theory for ferromagnetic system, the Curie temperature is close to the room temperature for the undamped Ruderman—Kittel—Kasuya—Yoshida (RKKY) interaction.

Characterization of ZnO threads obtained using dip coating method at room temperature

In this work ZnO thin films were prepared using the sol–gel method; these films have been deposited on glass substrates using the dip coating technique. The surface characteristics of the films were analyzed with atomic force microscope and could see that the morphology of these films was in thread forms. These threads are formed at the time of deposit of the sol–gel solution with the deep coating. After the heat treatment, these threads keep their shape. The structural characteristics of the films were analyzed using an X-ray diffractometer using the X-ray beam technique, we can found the wurtzite (ZnO) phase in our films and the crystallite sizes were calculated using Scherrer equation. The optical properties were studied by a UV–Visible spectrophotometer where we find that the transmittance percentage is over 50%. The band gap of ZnO films was found in 3.02 for the film with 1 layer and conform we increased the number of layers the optical band gap decrease.

Direct observation of Al-doping-induced electronic states in the valence band and band gap of ZnO films

We present a synchrotron radiation hard x-ray photoemission spectroscopy study of the electronic structure of Al-doped ZnO films. Doping-induced states appear between the Zn3$d$ and O2$p$ levels and in the band gap just below the conduction band minimum (CBM). Ab initio calculations confirm the Al impurity origin of these induced states. The drop in the film resistivity with Al doping is not due to the progressive shifting of the Fermi level above the CBM, but rather to the filling of the Al impurity band state, which pins the Fermi level just below the CBM.

Effect of annealing on structural and optical properties of zinc oxide films

Nanocrystalline ZnO thin films were prepared using modified liquid flow deposition method (LF method). The effects of annealing in temperature range of 350–550 °C on the structural and optical properties of the ZnO films have been studied. The structure, surface morphology, chemical composition and optical properties of the films were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–vis–NIR and photoluminescence (PL) spectroscopy. X-ray diffraction measurements showed that the annealed ZnO films were polycrystalline with (1 0 0), (0 0 2) and (1 0 1) oriented crystallites of hexagonal wurtzite structure and full width half maximum (FWHM) of (0 0 2) XRD peak is observed to decrease as annealing temperature increases above 350 °C. The film annealed at 550 °C possesses c-axis oriented hexagonal wurtzite ZnO nanoparticles. The crystallinity as well as grain size of the films was found to increase on post-annealing. SEM micrographs of the films reveal the presence of hexagonal shaped nanoparticles and EDAX measurements confirm the removal of impurities after annealing treatment. The optical band gap of ZnO films initially blue shifted (3.39–3.41 eV) when annealed at 350 °C and a red shift (3.41–3.32 eV) was observed above the annealing temperature 350–550 °C. Room temperature photoluminescence (PL) measurement of the films showed UV and defect related deep level emissions including blue, green and yellow emissions in the visible region. The possible mechanisms of these emissions were studied in detail.

Preparation and characterization of supported ZnO photocatalyst by zincate method

Supported ZnO film photocatalysts are prepared on glass and ceramic substrates by zincate method. Prepared ZnO films are seen rather strongly durable. They resist to chemical dissolution in a broad range of pH from 3 to 10. Various physical tests are conducted to measure the relevant characteristics of the films. Band gap of ZnO film is calculated as 3.24 eV. SEM analysis shows that the ZnO film has granular morphology with uniform particle size about 300-400 nm. The film thickness is calculated as 1.41 microm after twenty coating cycles and the thickness of the thin film per cycle was approximately 70 nm. Photochemical activity tests are performed by measuring photodecolorization rate of methyl orange solution. First order rate constants are correlated to principal process parameters. The results show that ceramic is the preferable supporting material with high activity. According to simulation study, the photocatalytic activity of film coated on ceramic ring of 8mm diameter is nearly equal to the activity of powder ZnO slurry of 120 mg/dm(3) density. It carried out about decolorization 65% in 3h. The calculations show that it is possible to prepare ZnO film on ceramic ring with similar activity as powder ZnO.

Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering

In this paper we report the effect of deposition temperature on the structural and optical properties of ZnO thin films prepared by rf magnetron sputtering. The films grown at lower deposition temperatures were in a state of large compressive stress, whereas the films grown at higher temperature (450 °C) were almost stress free. In the absorption spectra, the ZnO excitonic and the Zn surface plasmon resonance (SPR) peaks have been observed. A redshift in the optical band gap of ZnO films has also been observed with the increase in the deposition temperature. The shift in the band gap calculated from the size effect did not match with the observed shift values and the observed shift has been attributed to the compressive stress present in the films.

Structural and optical properties of La-doped ZnO films prepared by magnetron sputtering

La-doped ZnO films were prepared by RF magnetron sputtering using different composition powder compacted targets (0, 1, 2, 3 and 5 at.%). All films show a preferred c-axis growth orientation. Furthermore, the (002) diffraction peak shifts to a small angle and the full-width at half-maximum augments with increasing La concentration up to 2 at.%, which indicate that a small quantity of La atoms are incorporated into the ZnO lattice. The average transmittance in the visible range is over 80%, and a blue shift of the absorption edge is observed. With increasing La concentration, the band gap of ZnO films evaluated by the linear fitting linearly increases from 3.270 to 3.326 eV. In the photoluminescence spectra, a strong violet emission peak and a weak green emission band can be observed. The former is due to the electron transition between the defect energy levels, associated with the interfacial traps existing at the ZnO grain boundaries, and valence band. The latter could be ascribed to crystal defects related to oxygen vacancies.

Structure and UV emission of nanocrystal ZnO films by thermal oxidation of ZnS films

ZnO films prepared by the thermal oxidation of the ZnS films through thermal evaporation are reported. The as-deposited ZnS films have transformed to ZnO films completely at 400 °C. The 400–700 °C annealed films with a preferential c-axis (0 0 2) orientation have a hexagonal wurtzite structure. The band gap of ZnO films shifts towards longer wavelength with the increase of the annealing temperature. The relationship between the band gap energy of ZnO films and the grain size is discussed. The shift of the band gap energy can be ascribed to the quantum confinement effect in nanocrystal ZnO films. The photoluminescence spectra of ZnO films show a dominant ultraviolet emission and no deep level or trap state defect emission in the green region. It confirms the absence of interstitial zinc or oxygen vacancies in ZnO films. These results indicate that ZnO film prepared by this simple thermal oxidation method is a promising candidate for optoelectronic devices and UV laser.

Effect of metal-ion doping on the optical properties of nanocrystalline ZnO thin films

Optical properties of metal (Al, Ag, Sb, and Sn)-ion-implanted ZnO films have been studied by ultraviolet-visible spectroscopy and spectroscopic ellipsometric techniques. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n), and extinction coefficient (k) of nanocrystalline ZnO films have been studied for the similar implantation dose of all the metal ions. The ellipsometric spectra of the ion-implanted samples could be well described by considering an air/roughness/ZnO–M (layer 1)/ZnO (layer 2)/glass model. The band gap of ZnO films increases with Al ion doping and decreases with doping of Ag, Sb, and Sn ions. The refractive index of ZnO films in the visible spectral region increases substantially on Sb and Sn ion doping, while it decreases to some extent with Al ion doping.

Electrochemical deposition and characterization of wide band semiconductor ZnO thin film

ZnO thin film was electrodeposited from an aqueous solution of Zn(NO3)2 at 65 °C on indium tin oxide (ITO)-covered glass substrates. Atomic force microscope and X-ray diffraction studies indicated that the obtained ZnO films were polycrystalline with hexagonal wurtzite-type structure. Various operating conditions were controlled, including the applied voltages, the deposition time and annealing treatment. According to applied conditions, ZnO films presented different morphologies, grain size ranging approximately from 180 to 320 nm. When depositions were carried out at −0.9 and −1.0 V, ZnO films were compact and homogeneous, and their transmittance was close to 95% at the wavelength of 500 nm. The variations in the band gap of ZnO films deposited under different conditions are also discussed.

Growth of high quality ZnO thin films at low temperature on Si(100) substrates by plasma enhanced chemical vapor deposition

High quality ZnO thin films with a c-axis-orientated wurtize structure have been grown on a Si(100) substrate by plasma enhanced chemical vapor deposition using a zinc organic source [Zn(C2H5)2] and carbon dioxide (CO2) gas mixture at low temperature. The dependence of ZnO thin-film quality on the gas flow rate ratio of Zn(C2H5)2 to CO2 (GFRRZC) is studied by x-ray diffraction (XRD), optical transmission spectra (OTS), and photoluminescence spectra (PL). The optical properties show that the GFRRZC has an obvious effect on the band gap of ZnO films. The relationship between the quality of the ZnO thin films and the substrate temperatures is also studied by XRD and PL spectra. The XRD spectra show that the full width at half maximum (FWHM) of the diffraction peak at 34.42° of (0002) oriented ZnO is 0.25° at the optimized condition, indicating the formation of high quality ZnO films. The PL spectra show a strong excitonic emission at about 3.26 eV without the emission around 2.5 eV related to deep-level defects, implying the formation of the stoichiometric ZnO thin films. The smallest FWHM of the PL spectrum of a ZnO thin film is 111 meV at room temperature.

Anomalous behavior of the optical band gap of nanocrystalline zinc oxide thin films

The optical band gap of ZnO films on fused silica in the carrier concentration regime of 1018−1020/cm3 is reported. Contrary to theoretical predictions there is an anomalous increase in the band gap of ZnO films at a carrier concentration of 5 × 1018/cm3, followed by an abrupt decrease at a critical concentration of 3−4 × 1019/cm3 before the optical band gap rises again. Similar observations have been made before, but an explanation of these observations was lacking. We propose a model based on the existence of potential barriers at the grain boundaries, causing quantum confinement of the electrons in the small grains realized in these films. Quantum confinement leads to the initial rise in the optical band gap. On increasing the carrier concentration to the critical value, the potentials at the grain boundaries collapse, leading to the abrupt decrease in the optical band gap. Above this carrier concentration the films behave according to existing many-body theories.