Influence of temperature annealing on optical properties of SrTiO3/BaTiO3 multilayered films on indium tin oxide

We have prepared SrTiO₃/BaTiO₃ thin films with multilayered structures deposited on indium tin oxide (ITO) coated glass by a sol-gel deposition and heating at 300-650 °C. The optical properties were obtained by UV-vis spectroscopy. The films show a high transmittance (approximately 85%) in the visible region. The optical band gap of the films is tunable in the 3.64-4.19 eV range by varying the annealing temperature. An abrupt decrease towards the bulk band gap value is observed at annealing temperatures above 600 °C. The multilayered film annealed at 650 ° C exhibited the maximum refractive index of 2.09-1.91 in the 450-750 nm wavelength range. The XRD and AFM results indicate that the films annealed above 600 ° C are substantially more crystalline than the films prepared at lower temperatures which were used to change their optical band gap and complex refractive index to an extent that depended on the annealing temperature.     

Effects of carbonization and substrate temperature on the growth of 3C-SiC on Si(1 1 1) by SSMBE

The growth of 3C-SiC on Si(1 1 1) substrate was performed at different carbonization temperatures and substrate temperatures by solid-source molecular beam epitaxy (SSMBE). The properties of SiC film were analyzed with in situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The best carbonization temperature of 810 °C was found to be optimal for the surface carbonization. The quality of SiC film grown on Si at substrate temperature of 1000 °C is best. The worse crystalline quality for the sample grown at higher temperature was attributed to the large mismatch of thermal expansion coefficient between SiC and Si which caused more dislocation when sample was cooled down to room temperature from higher substrate temperature. Furthermore, the larger size of single pit and the total area of the pits make the quality of SiC films grown at higher temperature worse. More Si atoms for the sample grown at lower temperature were responsible for the degradation of crystalline quality for the sample grown at lower temperature.     

Amorphous to crystalline phase transition in pulsed laser deposited silicon carbide

SiC thin films were grown on Si (1 0 0) substrates by excimer laser ablation of a SiC target in vacuum. The effect of deposition temperature (up to 950 °C), post-deposition annealing and laser energy on the nanostructure, bonding and crystalline properties of the films was studied, in order to elucidate their transition from an amorphous to a crystalline phase. Infra-red spectroscopy shows that growth at temperatures greater than 600 °C produces layers with increasingly uniform environment of the Si-C bonds, while the appearance of large crystallites is detected, by X-ray diffraction, at 800 °C. Electron paramagnetic resonance confirms the presence of clustered paramagnetic centers within the sp² carbon domains. Increasing deposition temperature leads to a decrease of the spin density and to a temperature-dependent component of the EPR linewidth induced by spin hopping. For films grown below 650 °C, post-deposition annealing at 1100 °C reduces the spin density as a result of a more uniform Si-C nanostructure, though large scale crystallization is not observed. For greater deposition temperatures, annealing leads to little changes in the bonding properties, but suppresses the temperature dependent component of the EPR linewidth. These findings are explained by a relaxation of the stress in the layers, through the annealing of the bond angle disorder that inhibits spin hopping processes.     

Ion-induced secondary electron emission from ZnS thin films deposited by closed-spaced sublimation

ZnS thin films were deposited on soda lime glass and aluminum substrates by close-spaced sublimation technique. The change in composition, structural and optical properties of the films was investigated as a function of the substrate temperature. The deposited films were stoichiometric and crystalline in nature having cubic structure oriented only along (1 1 1) plane. The energy band gap of the films deposited at the substrate temperature of 150, 250 and 350 °C was 3.52, 3.58 and 3.63 eV respectively. These films were then bombarded with 2-10 keV energy pulsed Ar⁺ beam and their electron yield was determined from impinging ion and emitted electron currents. The electron yield of ZnS films was much high as compared to the metals. The electron yield of ZnS films increased with energy of the incident ion and got saturated at about 8 keV. The most important result of this study was that the electron yield of ZnS films having same composition was different. Monte Carlo simulations performed to interpret these experimental findings showed that the dissimilar electron yields of ZnS films is due to the combined effect of energy band gap, surface barrier potential and density of the films.     

Effect of nanostructured AlN coatings on the oxidation-resistant properties of optical diamond films

Diamond film is an ultra-durable optical material with high thermal conductivity and good transmission in near-infrared and far-IR (8-14 μm) wavebands. CVD diamond is subjected to oxidation at temperature higher than 780 °C bared in air for 3 min, while it can be protected from oxidation for extended exposure in air at temperature up to 900 °C by a coating of aluminum nitride. Highly oriented AlN coatings were prepared for infrared windows on diamond films by reactive sputtering method and the average surface roughness (R_a) of the coatings was about 10 nm. The deposited films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). XRD confirmed the preferential orientation nature and AFM showed nanostructures. Optical properties of diamond films coated AlN thin film was investigated using infrared spectrum (IR) compared with that for as-grown diamond films.     

Investigation of p-type behavior in Ag-doped ZnO thin films by E-beam evaporation

Ag-doped ZnO (ZnO:Ag) thin films were grown on glass substrates by E-beam evaporation technique. The structural, electrical and optical properties of the films were investigated as a function of annealing temperature. The films were subjected to post annealing at different temperatures in the range of 350-650 °C in an air ambient. All the as grown and annealed films at temperature of 350 °C showed p-type conduction. The films lost p-type conduction after post annealing treatment temperature of above 350 °C, suggesting a narrow post annealing temperature window for the fabrication of p-type ZnO:Ag films. ZnO:Ag film annealed at 350 °C revealed lowest resistivity of 7.25 × 10⁻² Ω cm with hole concentration and mobility of 5.09 × 10¹⁹ cm⁻³ and 1.69 cm²/V s, respectively. Observation of a free-to-neutral-acceptor (e,A^o) and donor-acceptor-pair (DAP) emissions in the low temperature photoluminescence measurement confirms p-type conduction in the ZnO:Ag films.     

High-throughput synthesis and characterization of Mg1−xCaxO films as a lattice and valence-matched gate dielectric for ZnO based field effect transistors

Using composition-spread technique, we have grown metastable Mg_1−xCa_xO solid solution films on ZnO layers by pulsed laser deposition. All the films exhibited (1 1 1) oriented cubic phase. Despite a large miscibility gap, no phase separation took place at growth temperatures up to 700 °C, whereas an optimal growth temperature was found at 400 °C in terms of the crystallinity. The composition-spread films were characterized by X-ray diffraction mapping technique. Both lattice parameters and diffraction intensity increased with increasing the CaO composition. The present isovalent heterointerfaces realized the perfect lattice-matching by properly adjusting the CaO composition, leading to particular interest for ZnO based field effect transistors.     

Structural and optical properties of YSZ thin films grown by PLD technique

We report on the structural and optical properties of yttria stabilized zirconia (YSZ) thin films grown by pulsed laser deposition (PLD) technique and in situ crystallized at different substrate temperatures (T_s = 400 °C, 500 °C and 600 °C). Yttria-stabilized zirconia target of ∼1 in. diameter (∼95% density) was fabricated by solid state reaction method for thin film deposition by PLD. The YSZ thin films were grown on an optically polished quartz substrates and the deposition time was 30 min for all the films. XRD analysis shows cubic crystalline phase of YSZ films with preferred orientation along 〈1 1 1〉. The surface roughness was determined by AFM for the films deposited at different substrate temperatures. The nano-sized surface roughness is found to increase with the increase of deposition temperatures. For the optical analysis, a UV-vis-NIR spectrophotometer was used and the optical band gap of ∼5.7 eV was calculated from transmittance curves.     

Effect of annealing on optical and structural properties of ZnS/MnS and MnS/ZnS superlattices thin films for solar energy application

ZnS/MnS super lattice thin films were grown on glass substrates by Chemical Bath Deposition technique. Equimolar aqueous solutions of ZnCl₂:thiourea and MnCl₂·2H₂O:thiourea were taken separately. The substrates were placed vertically in the beakers containing the precursor described above, and the films are deposited at 85 °C for an hour. The as deposited films are annealed at 200 °C for about two hours. X-ray diffractometry method was used to obtain structural characterization. The UV–vis absorption spectrometry was employed to find the optical properties. The refractive-index, dielectric constant, optical conductivity, electrical conductivity and extinction coefficient were determined by various equations based on the data. The valence band and conduction band offset voltages for ZnS/MnS were determined as 0.7 eV and 0.1 eV respectively and for MnS/ZnS were 0.4 eV and 0.3 eV respectively. The band alignment of both superlattice was found to be as Type I.     

Electron transport in high quality undoped ZnO film grown by plasma-assisted molecular beam epitaxy

High quality ZnO films were grown on c-plane sapphire substrate using low temperature ZnO buffer layer by plasma-assisted molecular beam epitaxy. The film deposited at 720 °C showed the lowest value of full-width at half maximum for the symmetric (0002) diffraction peak of about 86 arcsec. The highest electron mobility in the films was about 103-105 cm²/V s. From temperature-dependent Hall effect measurements, the mobility strongly depends on the dislocation density at low temperature region and the polar optical phonon scattering at high temperature, respectively. Moreover, by obtaining the activation energy of the shallow donors, it was supposed that hydrogen was source of n-type conductivity in as-grown ZnO films.     

Microstructural and surface property variations due to the amorphous region formed by thermal annealing in Al-doped ZnO thin films grown on n-Si (1 0 0) substrates

X-ray diffraction (XRD) patterns revealed that the as-grown and annealed Al-doped ZnO (AZO) films grown on the n-Si (1 0 0) substrates were polycrystalline. Transmission electron microscopy (TEM) images showed that bright-contrast regions existed in the grain boundary, and high-resolution TEM (HRTEM) images showed that the bright-contrast regions with an amorphous phase were embedded in the ZnO grains. While the surface roughness of the AZO film annealed at 800 °C became smoother, those of the AZO films annealed at 900 and 1000 °C became rougher. XRD patterns, TEM images, selected-area electron diffraction patterns, HRTEM images, and atomic force microscopy (AFM) images showed that the crystallinity in the AZO thin films grown on the n-Si (1 0 0) substrates was enhanced resulting from the release in the strain energy for the AZO thin films due to thermal annealing at 800 °C. XRD patterns and AFM images show that the crystallinity of the AZO thin films annealed at 1000 °C deteriorated due to the formation of the amorphous phase in the ZnO thin films.     

Luminescence and structure of ZnO-ZnS thin films prepared by oxidation of ZnS films in air and water vapor

Effect of water vapor quantity at oxidation of undoped ZnS films on structural and luminescent properties of the obtained films was investigated. The films were deposited onto glass substrates by electron beam evaporation. ZnO-ZnS layers were prepared by thermal oxidization of ZnS films at 600 °C in dry or wet atmospheres. The films were characterized by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. As-deposited ZnS films were sphalerite crystal structure. The “dry annealing” led to the ZnS phase transition from sphalerite to wurtzite structure and from ZnS to ZnO for a small fraction of the film. After the “wet annealing” the amount of ZnO phase with wurtzite structure growing along the 〈0 0 0 2〉 direction varied from 25% to 95% in dependence on the water vapor quantity. Photoluminescent spectrum at room temperature exhibits green emission with maximum at 2.4 eV. A strong influence of the water vapor on shape and intensity of the emission was observed. Photoluminescent spectra at 22 K consisted of two bands—high-energy band at 2.1-2.4 eV and low energy band at 1.7-1.8 eV. Location and intensity ratio depended on the preparation conditions.     

Investigation of structural and optical properties of UHV deposited titanium thin films under thermal oxidation

Titanium dioxide thin layers were prepared by annealing method, on glass substrate at different temperatures, 150, 250 and 350 °C, in presence of 5 cm³/s uniform oxygen flow. The structural investigations were performed by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. Roughness of the films changed due to annealing process. The optical constants of the layers were obtained by Kramers–Kronig analysis of the reflectivity curves. There was a good agreement between structural and optical properties of the layers. Annealing temperature can play an important role in nanostructures of the films.     

Effects of the substrate temperature on the properties of CuIn5S8 thin films

Structural, optical and electrical properties of CuIn₅S₈ thin films grown by thermal evaporation have been studied relating the effects of substrate heating conditions of these properties. The CuIn₅S₈ thin films were carried out at substrate temperatures in the temperature range 100-300 °C. The effects of heated substrate on their physico-chemical properties were investigated using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), optical transmission and hot probe method. X-ray diffraction revealed that the films are strong preferred orientation along the (3 1 1) plane upon substrate temperature 200 °C and amorphous for the substrate temperatures below 200 °C. No secondary phases are observed for all the films. The composition is greatly affected by heated substrate. From the optical transmission and reflection, an important absorption coefficient exceeds 10⁵ cm⁻¹ at 800 nm was found. As increasing the substrate temperature, the optical energy band gap decreases from 1.70 eV for the unheated films to 1.25 eV for the deposited films at 300 °C. It was found that CuIn₅S₈ thin film is an n-type semiconductor at 250° C.     

Spray pyrolysis deposition of ZnO thin films on FTO coated substrates from zinc acetate and zinc chloride precursor solution at different growth temperatures

ZnO thin films were fabricated using zinc chloride and zinc acetate precursors by the spray pyrolysis technique on FTO coated glass substrates. The ZnO films were grown in different deposition temperature ranges varying from 400 to 550 °C. Influences of substrate temperature and zinc precursors on crystal structure, morphology and optical property of the ZnO thin films were investigated. XRD patterns of the films deposited using chloride precursor indicate that (1 0 1) is dominant at low temperatures, while those deposited using acetate precursor show that (1 0 1) is dominant at high temperatures. SEM images show that deposition temperature and type of precursor have a strong effect on the surface morphology. Optical measurements show that ZnO films are obviously influenced by the substrate temperatures and different types of precursor solutions. It is observed that as temperature increases, transmittance decreases for ZnO films obtained using zinc chloride precursor, but the optical transmittance of ZnO films obtained using zinc acetate precursor increases as temperature increases.     

Effect of temperature on pulsed laser deposition of HgCdTe films

HgCdTe thin films have been deposited on Si(1 1 1) substrates at different substrate temperatures by pulsed laser deposition (PLD). An Nd:YAG pulsed laser with a wavelength of 1064 nm was used as laser source. The influences of the substrate temperature on the crystalline quality, surface morphology and composition of HgCdTe thin films were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). The results show that in our experimental conditions, the HgCdTe thin films deposited at 200 °C have the best quality. When the substrate temperature is over 250 °C, the HgCdTe film becomes thermodynamically unstable and the quality of the film is degraded.     

Effects of deposition temperature and thickness on the structural properties of thermal evaporated bismuth thin films

Bismuth (Bi) thin films of different thicknesses were deposited onto Si(1 0 0) substrate at various substrate temperatures by thermal evaporation technique. Influences of thickness and deposition temperature on the film morphologies, microstructure, and topographies were investigated. A columnar growth of hexahedron-like grains with bimodal particle size distribution was observed at high deposition temperature. The columnar growth and the presence of large grains induce the Bi films to have large surface roughness as evidenced by atomic force microscopy (AFM). The dependence of the crystalline orientation on the substrate temperature was analyzed by X-ray diffraction (XRD), which shows that the Bi films have completely randomly oriented polycrystalline structure with a rhombohedral phase at high deposition temperature (200 °C) and were strongly textured with preferred orientation at low deposition temperatures (30 and 100 °C).     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

Influence of deposition temperature on the structural and morphological properties of Be3N2 thin films grown by reactive laser ablation

Be₃N₂ thin films have been grown on Si(1 1 1) substrates using the pulsed laser deposition method at different substrate temperatures: room temperature (RT), 200 °C, 400 °C, 600 °C and 700 °C. Additionally, two samples were deposited at RT and were annealed after deposition in situ at 600 °C and 700 °C. In order to obtain the stoichiometry of the samples, they have been characterized in situ by X-ray photoelectron (XPS) and reflection electron energy loss spectroscopy (REELS). The influence of the substrate temperature on the morphological and structural properties of the films was investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The results show that all prepared films presented the Be₃N₂ stoichiometry. Formation of whiskers with diameters of 100-200 nm appears at the surface of the films prepared with a substrate temperature of 600 °C or 700 °C. However, the samples grown at RT and annealed at 600 °C or 700 °C do not show whiskers on the surface. The average root mean square (RMS) roughness and the average grain size of the samples grown with respect the substrate temperature is presented. The films grown with a substrate temperature between the room temperature to 400 °C, and the sample annealed in situ at 600 °C were amorphous; while the αBe₃N₂ phase was presented on the samples with a substrate temperature of 600 °C, 700 °C and that deposited with the substrate at RT and annealed in situ at 700 °C.     

Transmission electron microscopy studies of HfO2 thin films grown by chloride-based atomic layer deposition

Detailed transmission electron microscopy characterization of HfO₂ films deposited on Si(1 0 0) using atomic layer deposition has been carried out. The influence of deposition temperature has been investigated. At 226 °C, a predominantly quasi-amorphous film containing large grains of cubic HfO₂ (a₀ = 5.08 Å) was formed. Grain morphology enabled the nucleation sites to be determined. Hot stage microscopy showed that both the cubic phase and the quasi-amorphous phase were very resistant to thermal modification up to 500 °C. These observations suggest that nucleation sites for the growth of the crystalline cubic phase form at the growing surface of the film, rather homogeneously within the film. The films grown at higher temperatures (300-750 °C) are crystalline and monoclinic. The principal effects of deposition temperature were on: grain size, which coarsens at the highest temperature; roughness with increases at the higher temperatures due to the prismatic faceting, and texture, with texturing being strongest at intermediate temperatures. Detailed interfacial characterization shows that interfacial layers of SiO₂ form at low and high temperatures. However, at intermediate temperatures, interfaces devoid of SiO₂ were formed.