Effect of substrate biasing and temperature on AlN thin film deposited by cathodic arc ion

Aluminum nitride (AlN) films have been grown in pure N₂ plasma using cathodic arc ion deposition process. The films were prepared at different substrate bias voltages and temperatures. The aim was to investigate their influence on the Al macro-particles, structural and optical properties of deposited films. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscope (SEM) and Rutherford backscattering spectrometry (RBS) were employed to characterize AlN thin films. XRD patterns indicated the formation of polycrystalline (hexagonal) films with preferential orientation of (002), which is suppressed at higher substrate bias voltage. FTIR and Raman spectroscopic analysis were used to assess the nature of chemical bonding and vibrational phonon modes of AlN thin films respectively. FTIR spectra depicted a dominant peak around 850 cm^?1 corresponding to the longitudinal optical (LO) mode of vibration. A shift in this LO mode peak towards higher wavenumbers was observed with the increase of substrate bias voltage and temperature, showing the upsurge of nitrogen concentration in the deposited film. Raman spectra illustrated a peak at 650 cm^?1 corresponding to E₂ (high) phonon mode depicting the c-axis oriented (perpendicular to substrate) AlN film. SEM analysis showed the AlN film deposited at higher substrate bias voltage contains fewer amounts of Al macro-particles.     

Preparation,characterization and photocatalytic activity of porous zinc oxide superstructure

A porous ZnO “timber-like” superstructures comprising nanoparticles was synthesized through the thermal decomposition of zinc oxalate precursor, prepared from zinc sulfate and oxalic acid, without additive(s) in water. The resulting ZnO superstructure was systematically characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence measurement. Using methyl orange as model, photocatalytic activity of ZnO superstructure was investigated. The heat treatment temperature and time was proved to have important influence for photocatalytic activity of ZnO superstructure. Longer heat treatment time at fixed heat treatment temperature and lower heat treatment temperature at fixed heat treatment time were favorable for the enhancement of photocatalytic activity. The optimum preparation condition for ZnO superstructure with higher photocatalytic activity was at 450 °C for 6 h.     

Effect of annealing on the structural and optical properties of laser ablated nanostructured barium tungstate thin films

High quality BaWO₄ thin films are successfully deposited on quartz substrate for a duration of 30 min using pulsed laser ablation technique and using a laser radiation of wavelength 355 nm and the effect of thermal annealing on the structural and optical properties is studied by using techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy, micro-Raman, FTIR and UV–visible spectroscopy. All the films show monoclinic crystalline structure with (2 0 2) plane as the preferred orientation of crystal growth. From the XRD analysis it is found that the optimum annealing temperature for better crystallization of the BaWO₄ film is 700 °C and there is no phase change observed with annealing temperature. The presence of the characteristic bands for the BaWO₄ in the Raman spectra of the films suggests the formation of BaWO₄ crystalline phase in all the films. SEM and AFM analyses show that as the annealing temperature increases the connectivity between individual grains increases and shows an ordered packing. The geometrical optimization and energy calculation of the title compound were done using the Gaussian 09 software package and the calculations were carried out using the CAM-B3LYP functional combined with standard Lanl2Dz basis set. The thickness of the films was calculated using lateral SEM images and also from optical transmission spectral data using PUMA software.     

Effects of high-temperature rapid thermal annealing for seed layers on the crystallographic evolution in hydrothermal ZnO nanostructures

In this work, we investigated effects of high temperature rapid thermal annealing for the zinc oxide (ZnO) seed layers on the growth morphology and crystal orientation of hydrothermal ZnO nanorods (NRs). The seed layers were prepared by sol–gel spin coating and annealed by two-step rapid thermal processes at different peak temperatures ranging from 600 to 900 °C for a short time period of 1 min. The seed layers annealed in a temperature range of 600–800 °C were all polycrystalline; however, they exhibited a highly Zn-deficient amorphous state when annealed at 900 °C as observed by X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and cross-sectional transmission electron microscopy (TEM). The vertical NRs normal to the substrate were grown along [001] direction atop the polycrystalline seeds annealed at 600–800 °C, whereas different growth morphology of flower-like NRs was observed on the seeds annealed at 900 °C with the strongest XRD peak along the [100] orientation. From our cross-sectional TEM analysis, this flower-like architecture was initiated from the pioneer crystals laterally grown along [001] direction guiding the subsequent growth of petal NRs oriented by a slight difference in growth direction.     

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N₂ or H₂ as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N₂ carrier gas show two additional broad peaks, which are ascribed to carbon sp² clusters related modes, spreading in the frequency range 1300–1600 cm^?1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ～520 cm^?1, considered as some defects related local vibrations, appears in the samples grown with N₂ as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp² clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H₂ as carrier gas, the bands related to carbon sp² clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm^?1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N₂ carrier gas, were only weakened.     

Photoluminescence from c-axis oriented ZnO films synthesized by sol-gel with diethanolamine as chelating agent

ZnO films were synthesized on SiO₂/Si substrates through the sol-gel technique using diethanolamine as chelating agent and annealed in Ar+O₂ atmospheres with different O₂ flow-rates in the 10–100 sccm range. Samples were studied by scanning electron microscopy and X-ray diffraction, evidencing a nanostructured morphology with a preferential orientation along the (0 0 2) direction (c-axis orientation), which is uncommon when diethanolamine is used as the chelating agent. The room temperature photoluminescence spectra show strong UV emissions at around 375 and 384 nm from near band-edge transitions and phonon replica, and a broad defect-related band extending from the visible to near infrared (∼500–800 nm). The analysis of the defect-related emission band and its various components as a function of the O₂ flow-rate is discussed in terms of contributions from specific luminescent point defect centers established during annealing.     

Post-annealing modification in structural properties of ZnO thin films on p-type Si substrate deposited by evaporation

Zinc oxide (ZnO) films of thickness ∼380 nm were deposited on p-type Si (1 1 1) substrate maintained at 300 °C under 3×10⁻⁶ Torr by a radio frequency (RF) heating source. Transmission Fourier transform infrared (FTIR) spectrum exhibited a clear Zn–O bond excitation frequency of ∼408 cm⁻¹. X-ray diffraction spectrum demonstrated four peaks (P₁–P₄) at 2θ (deg) ∼36±0.06, 40±0.09, 82±0.17 and 86±0.2, which originated from (1 0 0), (0 0 2), (2 0 1) and (0 0 4) hexagonal planes, respectively. P₂ being the highest intensity peak indicated that the growth of ZnO predominantly occurred along the c-axis i.e. (0 0 2) plane. Micrographs of the samples obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) identically displayed scattered nanocrystallites, which grew bigger with the increase of sample annealing temperature (°C) in the range of 400–1000. AFM pictures, in particular, exposed the hexagonal structure of the deposited films along with voids. However, ZnO composition ∼6:1 (Zn:O) as calculated from the energy dispersive spectrum (EDS) revealed that the formation of ZnO was not stoichiometric, rather of Zincsuboxide structure ZnO_x (x<1). Arrhenius plot of the resistivity data yielded a donor level (zinc interstitial and/or Zn–on–O site) with ionization energy E_c–1.26 eV, thereby it supports our measured results, in general.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

Preparation and characterization of TiO2 thin film by thermal oxidation of sputtered Ti film

Titanium dioxide (TiO₂) thin films were successfully prepared on quartz substrate by thermal oxidation of sputtered titanium film in air. The structure, composition, morphology and optical properties of oxidized TiO₂ films were characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and UV-visible spectroscopy. Meanwhile, the photocatalytic activity of the films was evaluated on the basis of the degradation of methyl orange solution under UV irradiation. Ti films after oxidation present mainly in TiO₂ form with a larger amount of adsorbed O₂, and oxidation temperature has a strong impact on the crystal structure and properties of the films. A phase transformation of anatase to rutile for oxidized TiO₂ films occurred in the temperature range of 700–800 °C. The energy band gap of oxidized TiO₂ films decreased first and then increased with annealing temperature. Furthermore, TiO₂ film oxidized at 600 °C exhibited the best photocatalytic activity due to suitable crystal phase and size. These results might contribute to the synthesis of metal oxide thin films with expectant structural morphology and properties by thermal oxidation methods.     

Structural,optical and antibacterial properties of nanocrystalline Zn1–xLaxO compound semiconductor

The Zn_1–xLa_xO powders were synthesized by the planetary ball milling method. An accumulated milling time of 15 h with a milling speed of 400 rpm were found to be the optimum milling conditions. The crystal structure, morphology, selected area electron diffraction, and elemental analysis were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The solubility limit of La that could substitute at the Zn sites was only 5 mol% or x=0.05. The dependence of the crystallite size on the La doping content can be explained by the Zener pinning effect. The particle size of the milled ZnO powder was about 34.03 nm and the particle size was reduced to 30.90 nm when doped with 10 mol% La. The particles of all milled samples could agglomerate as a cluster. The largest E_g value of 3.14 eV was obtained from the Zn_0.97La_0.03O powder due to this sample having the smallest crystallite size. The Zn_1–xLa_xO powders can inhibit Staphylococcus aureus better than Escherichia coli due to the presence of an extra lipopolysaccharide layer on the outer surface of the latter.     

Structural and Raman studies of Ga2O3 obtained on GaAs substrate

Ga₂O₃ were synthesized by controlled thermal oxidation of GaAs substrates at atmospheric pressure. The crystalline structure and vibrational modes were studied as a function of growth temperature within a range of 750–950 °C. Samples grown in the range of 750–850 °C present nanostructured surface and the samples obtained at higher temperature are oriented to the (004) β-phase. Crystalline structure was confirmed by X-ray diffraction, and Raman scattering studies. The evolution of the surface morphology was analyzed by atomic force microscopy, and scanning electron microscopy.     

Crack free ZnO thick film on sapphire substrate without GaN template grown by MVPE

A crack-free ZnO film with thickness of ∼65 μm is obtained on sapphire substrate without GaN template by metal vapor phase epitaxy. The residual stress is investigated in terms of thermal and non-thermal stress. Comparing the calculated values of the thermal stress with the experimental data of the non-thermal stress, it is shown that the non-thermal stress is much larger than the thermal stress. Annealing was used to reduce the non-thermal stress. A re-growth process was employed, using annealed ZnO sample as pseudo-substrate. The result indicates that the residual stress in annealed ZnO sample was reduced to half, and the critical thickness is improved to ∼70 μm in the ZnO film grown on annealed ZnO pseudo-substrate, compared with ∼15 μm for ZnO grown directly on sapphire substrate. In addition, crystal quality is greatly improved, determined by double-crystal X-ray diffraction.     

Structural and optical properties of ZnO films produced by a nonvacuum chemical technique

Zinc-oxide films are grown by a new nonvacuum chemical method: the pyrolysis of zinc acetylacetonate at a temperature of 280–300°C. The structural, phonon, and emission properties of the ZnO films are studied by X-ray diffraction analysis, scanning electron microscopy, Raman measurements, and photoluminescence spectroscopy. The high-intensity (0002) peak recorded in the X-ray diffraction spectra indicate the predominant orientation of crystallites in the (0001) direction in the ZnO films. From analysis of the E ₂ ^high mode in the Raman spectrum of the ZnO films, the elastic strains ? _zz (～3.2 × 10^?3) and the quality of the crystal structure are determined. The characteristics of the pyrolytic ZnO films are compared with the corresponding characteristics of ZnO films grown by molecular-beam epitaxy. As a result, the possibility of growing polycrystalline ZnO films of rather high quality by a practically feasible low-temperature technique is demonstrated.     

Effect of the substrate temperature on the electrical and structural properties of spray-deposited SnO2:F thin films

SnO₂:F thin films were prepared by the spray pyrolysis (SP) technique at substrate temperature in the range 360–480 °C. The effect of varying the substrate temperature on the electrical and structural properties of the films was investigated by studying the I–V characteristics, the X-ray diffraction patterns (XRD), and the scanning electron microscope images (SEM). The I–V characteristics of the films were improved by increasing the substrate temperature, i.e. the resistivity of the films had decreased from 98 to 0.22 Ω cm. The X-ray diffraction patterns taken at 400 and 480 °C showed that the films are polycrystalline and two directions of crystal growth appeared in the difractogram of the film deposited at the lower substrate temperature, which correspond to the reflections from the (1 1 0) and (2 0 0) planes. With the increase in the substrate temperature a new direction of crystal growth appeared, which corresponds to the reflection from the (1 0 1) plane. Also the (1 1 0) and (2 0 0) lines were slightly grown at the higher substrate temperature, which means the crystal growth was enhanced and the grain size had increased. The SEM images confirmed these results and showed larger grains and more crystallization for the higher substrate temperature too.     

Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst

A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be grown. Also, experiments indicate that ZnO nanowire arrays with different diameter along their length have been formed under various growth conditions in the same distance from source material. It is found that supersaturation is a crucial parameter determining the growth behavior of ZnO nanowire arrays. The growth mechanism of ZnO nanowires is discussed. The room temperature photoluminescence spectrums of ZnO nanowire array show two emission bands. One is the exciton emission band (centered at 380 nm) and the other is a broad visible emission band centered at around 490 nm. As the substrate temperature decreases, the intensity of UV emission increases while the intensity of visible emission peak decreases.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Vibrational and morphological properties of Sn-doped CdS films deposited by ultrasonic spray pyrolysis method

Cd_1?xSn_xS films (x=0.0, 0.1 and 0.2) were prepared by the ultrasonic spray pyrolysis (USP) method on the glass substrate at 300 °C. Effect of Sn doping on the vibrational and morphological properties of CdS films has been investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), FT-IR and Raman spectroscopy. The SEM and AFM measurements showed that the surface morphology of the films was affected by the tin incorporation. The Raman (200–700 cm^?1) and FT-IR (400–4000 cm^?1) spectra of Cd_1?xSn_xS were recorded. The Raman spectrum for Cd_1?xSn_xS films is dominated by an intense band at 300 cm^?1, assigned to the first-order longitudinal optic phonon and the second-order phonon peak 599 cm^?1. The absorption peaks in the FT-IR spectra of Cd_1?xSn_xS located at 540–700 and 1004–1045 cm^?1 can be assigned to the Cd–S and C–O stretching frequencies, respectively. Raman and FT-IR spectra shows decrease in the peak intensity with increasing Sn concentration.     

AlxGa1−xN/GaN/AlN heterostructures grown on Si(1 1 1) substrates by MBE for MSM UV photodetector applications

Al_xGa_1−xN/GaN/AlN heterostructures on silicon (Si) substrate was developed by nitrogen plasma-assisted molecular beam epitaxy (MBE) and their properties were investigated by scanning electron microscopy (SEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), high resolution X-ray diffraction (XRD), Raman spectroscopy and Hall effect measurements. High purity gallium (7N) and aluminum (6N5) were used in the Knudsen cells. High purity nitrogen with 7N purity was supplied to radio frequency (RF) source to generate reactive nitrogen species. The nitrogen pressure and a discharge power were kept at 1.5×10⁻⁵ Torr and 300 W, respectively. From SEM measurements, the surface morphology of samples presented 2- and 3-dimensional growth modes. The EDX measurements showed that there were no foreign elements in the grown samples. The HR-XRD measurement has confirmed that the Al_xGa_1−xN/GaN/AlN heterostructures samples were epitaxially grown on Si substrate. All the dominant E₂ phonon modes were found in Raman spectra results. Lastly, Al_xGa_1−xN/GaN/AlN heterostructures based metal–semiconductor–metal (MSM) UV photodetectors were fabricated and the electrical characteristics of the devices were investigated by using current–voltage (I–V) and photo-conductivity measurements. The devices presented good I–V and photoconductivity characteristics.     

Photocatalytic degradation of methyl orange and gas-sensing performance of nanosized ZnO

ZnO nanoparticles were synthesized by calcining composites of zinc nitrate and poly(vinyl pyrrolidone) (PVP, molecular weight 30 000) at a mass ratio of 1:2 at 500 °C for 2 h. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to characterize the as-synthesized ZnO nanoparticles. The particles ranged in size from 30 to 50 nm. Infrared spectra of PVP and the PVP+Zn(NO₃)₂·6H₂O composite revealed coordination between the carbonyl (C=O) of PVP and Zn²⁺ of zinc nitrate, which led to a uniform nanoparticle morphology. The gas-sensing properties and photocatalytic performance of the final product were systematically investigated. The results show that the ZnO nanoparticles exhibit both a high response for ethanol detection and excellent photocatalytic activity for degradation of methyl orange under UV irradiation for 30 min.     

Enhanced photocatalytic activity of zinc oxide synthesized by calcination of zinc sulfide precursor

ZnO nanoparticles were synthesized by calcination of ZnS precursor in an air atmosphere, in which ZnS had been firstly synthesized through precipitation with sodium sulfide (Na₂S) as the precipitator. Detailed structure and morphology of the samples were characterized by X-ray diffraction, energy dispersive spectroscopy, scanning electron microscopy, and transmission electronic microscopy. Optical properties were examined by UV–vis absorption spectroscopy. Photocatalytic activities of the samples were evaluated by degradation of Reactive Blue 14 (KGL). The results indicate that ZnS precursor converted into pure ZnO stepwise via calcination at a temperature range of 400–800 °C, and pure ZnO can be achieved above 700 °C. ZnO obtained by calcination at 700 °C had an average crystalline size around 45 nm and exhibited the highest photocatalytic activity, degrading KGL by almost 97.1% after 60 min under ultraviolet irradiation, which was superior to that of the directly synthesized and commercial ZnO. The inherent correlation between different samples and their photocatalytic activities was discussed. The phase, crystalline size, specific surface area and oxygen vacancy defects of the samples were proposed to affect their photocatalytic activity.