Characteristics of electron beam evaporated nanocrystalline SnO2 thin films annealed in air

Tin oxide (SnO₂) thin films (about 200 nm thick) have been deposited by electron beam evaporation followed by annealing in air at 350-550 °C for two hours. Optical, electrical and structural properties were studied as a function of annealing temperature. The as-deposited film is amorphous, while all other annealed films are crystalline (having tetragonal structure). XRD suggest that the films are composed of nanoparticles of 5-10 nm. Raman analysis and optical measurements suggest quantum confinement effects that are enhanced with annealing temperature. For instance, Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk SnO₂. Blue shift becomes more pronounced with annealing temperature. Optical band gap energy of amorphous SnO₂ film is 3.61 eV, which increases to about 4.22 eV after crystallization. Two orders of magnitude decrease in resistivity is observed after annealing at 350-400 °C due to structural ordering and crystallization. The resistivity, however, increases slightly with annealing temperature above 400 °C, possibly due to improvement in stoichiometry and associated decrease in charge carrier density.     

Influence of films thickness and structure on the photo-response of ZnO films

ZnO thin films were grown using Successive Ionic Layer Adsorption and Reaction (SILAR) method on glass substrates at room temperature. Annealing temperatures and film thickness effect on the structural, morphological, optical and electrical properties of the films were studied. For this as-deposited films were annealed at 200, 300, 400 and 500 °C for 30 min in oxygen atmosphere. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that the films are covered well with glass substrates and have good polycrystalline structure and crystalline levels. The film thickness effect on band gap values was investigated and band gap values were found to be within the range of 3.49-3.19 eV. The annealing temperature and light effect on electrical properties of the films were investigated and it was found that the current increased with increasing light intensity. The resistivity values were found as 10⁵ Ω-cm for as-deposited films from electrical measurements. The resistivity decreased decuple with annealing temperature and decreased centuple with light emission for annealed films.     

Investigation of thermal annealing effects on microstructural and optical properties of HfO2 thin films

In the present paper, we investigate the effect of thermal annealing on optical and microstructural properties of HfO₂ thin films (from 20 to 190 nm) obtained by plasma ion assisted deposition (PIAD). After deposition, the HfO₂ films were annealed in N₂ ambient for 3 h at 300, 350, 450, 500 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE), UV Raman and FTIR were used for the physical characterisation of the as-deposited and annealed HfO₂ thin films. The results indicate that as-deposited PIAD HfO₂ films are mainly amorphous and a transition to a crystalline phase occurs at a temperature higher than 450 °C depending on the layer thickness. The crystalline grains consist of cubic and monoclinic phases already classified in literature but this work provides the first evidence of amorphous-cubic phase transition at a temperature as low as 500 °C. According to SE, XRR and FTIR results, an increase in the interfacial layer thickness can be observed only for high temperature annealing. The SE results show that the amorphous phase of HfO₂ (in 20 nm thick samples) has an optical bandgap of 5.51 eV. Following its transition to a crystalline phase upon annealing at 750 °C, the optical bandgap increases to 5.85 eV.     

Effect of annealing temperature on the microstructure and optical–electrical properties of Cu–Al–O thin films

We have successfully prepared Cu–Al–O thin films on silicon (100) and quartz substrates by radio frequency (RF) magnetron sputtering method. The as-deposited Cu–Al–O film is amorphous in nature and post-annealing treatment in argon ambience results in crystallization of the films and the formation of CuAlO₂. The annealing temperature plays an important role in the surface morphology, phase constitution and preferred growth orientation of CuAlO₂ phase, thus affecting the properties of the film. The film annealed at 900 °C is mainly composed of CuAlO₂ phase and shows smooth surface morphology with well-defined grain boundaries, thus exhibiting the optimum optical–electrical properties with electrical resistivity being 79.7 Ω·cm at room temperature and optical transmittance being 80% in visible region. The direct optical band gaps of the films are found in the range of 3.3–3.8 eV depending on the annealing temperature.     

Effect of post-annealing on the properties of copper oxide thin films obtained from the oxidation of evaporated metallic copper

Thin films of copper oxide were obtained through thermal oxidation (100-450 °C) of evaporated metallic copper (Cu) films on glass substrates. The X-ray diffraction (XRD) studies confirmed the cubic Cu phase of the as-deposited films. The films annealed at 100 °C showed mixed Cu-Cu₂O phase, whereas those annealed between 200 and 300 °C showed a single cubic Cu₂O phase. A single monoclinic CuO phase was obtained from the films annealed between 350 and 450 °C. The positive sign of the Hall coefficient confirmed the p-type conductivity in the films with Cu₂O phase. However, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the as-deposited is smooth (RMS roughness of 1.47 nm) and comprised of uniformly distributed grains (AFM and SEM analysis). The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of ∼80% (800 nm) on annealing at 200 °C. The direct allowed band gap is varied between 2.03 and 3.02 eV.     

Influence of air annealing on the structural, morphological, optical and electrical properties of chemically deposited ZnSe thin films

Zinc selenide nanocrystalline thin films are grown onto amorphous glass substrate from an aqueous alkaline medium, using chemical bath deposition (CBD) method. The ZnSe thin films are annealed in air for 4 h at various temperatures and characterized by structural, morphological, optical and electrical properties. The as-deposited ZnSe film grew with nanocrystalline cubic phase alongwith some amorphous phase present in it. After annealing metastable nanocrystalline cubic phase was transformed into stable polycrystalline hexagonal phase with partial conversion of ZnSe into ZnO. The optical band gap, E_g, of as-deposited film is 2.85 eV and electrical resistivity of the order of 10⁶-10⁷ Ω cm. Depending upon annealing temperature, decrease up to 0.15 eV and 10² Ω cm were observed in the optical band gap, E_g, and electrical resistivity, respectively.     

Structural and optical properties of low temperature synthesized Nanostructured BiFeO3 thin films

Nanostructured bismuth ferrite (BiFeO₃) thin films were deposited on glass substrate by the sol-gel process. The as-fired film at 250 °C was found to be amorphous crystallizing to pure rhombohedral phase after annealing at 450 °C for 2 h in air. The XRD pattern shows that the sample is polycrystalline in nature. The average grain size of the film calculated from the XRD data was found to be 16 nm. The as-fired film show high transmittance that decreases after crystallization. The absorption edge of the films was found to be sharper and shifting towards the lower energy as the annealing temperature increases. The optical energy band gaps of the amorphous and crystalline films were found to be 2.63 and 2.31 eV, respectively. The refractive indices of the amorphous and crystalline films were 2.05 and 2.26, respectively.     

The influence of substrate temperature on the morphology, optical and electrical properties of thermal-evaporated ZnTe Thin Films

The structural, morphological, optical and electrical properties of ZnTe films deposited by evaporation were investigated as a function of substrate temperature (at −123 and 27 °C) and post-deposition annealing temperature (at 200, 300 and 400 °C). It was determined that films deposited at both substrate temperatures were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. A small Te peak was detected in XRD spectra for both substrate temperatures, indicating that as-deposited ZnTe films were slightly rich in Te. Larger grains and a tighter grain size distribution were obtained with increased substrate temperature. Scanning electron microscopy (SEM) studies showed that the microstructures of the as-deposited films agreed well with the expectations from structure zone model. Post-deposition annealing induced further grain growth and tightened the grain size distribution. Annealing at 400 °C resulted in randomization in the texture of films deposited at both substrate temperatures. Optical spectroscopy results of the films indicated that the optical band gap value increased from 2.13 to 2.16 eV with increased substrate temperature. Increasing the annealing temperature sharpened the band-edge. Resistivity measurements showed that the resistivity of films deposited at substrate temperatures of −123 and 27 °C were 32 Ω cm, and 1.0 × 10⁴ Ω cm, respectively with corresponding carrier concentrations of 8.9 × 10¹⁵ cm⁻³ and 1.5 × 10¹⁴ cm⁻³. Annealing caused opposite changes in the film resistivity between the samples prepared at substrate temperatures of −123 and 27 °C.     

Dependence of resistivity on structure and composition of AZO films fabricated by ion beam co-sputtering deposition

The correlation between the resistivity and the structure/composition in the aluminum doped zinc oxide (AZO) films fabricated by the ion beam co-sputtering deposition at room temperature was investigated. The various compositions of AZO films were controlled by the sputtered area ratio of Al to Zn target. The structure, Al concentrations and resistivities of the as-deposited films were determined by X-ray diffractometer (XRD), energy dispersive spectrometer (EDS) and four-point probe station, respectively. The lowest resistivity of the deposited film was 5.66 × 10⁻⁴ Ω-cm at the 0.7 wt.% aluminum concentration. The most intense ZnO (0 0 2) diffraction peak, the largest grain size, the longest mean free path, and the highest free carrier concentration in the film result in the lowest resistivity of 5.66 × 10⁻⁴ Ω-cm at room temperature; simultaneously, the thermal stability of the resistivity of the AZO film as a function of the sample temperature was investigated. Below 200 °C the film's resistivity was almost kept at a fixed value and the lowest resistivity of 4.64 × 10⁻⁴ Ω-cm at 247 °C was observed.     

Boron doped ZnO thin films fabricated by RF-magnetron sputtering

By using the radio frequency-magnetron sputtering (RF-MS) method, both pure ZnO and boron doped ZnO (ZnO:B) thin films were deposited on glass substrates at ambient temperature and then annealed at 450 °C for 2 h in air. It is found that both ZnO and ZnO:B thin films have wurtzite structure of ZnO with (0 0 2) preferred orientation and high average optical transmission (≥80%). Compared with the resistivity of 6.3 × 10² Ω cm for ZnO film, both as-deposited and annealed ZnO:B films exhibit much lower resistivity of 9.2 × 10⁻³ Ω cm and 7.5 × 10⁻³ Ω cm, respectively, due to increase in the carrier concentration. Furthermore, the optical band gaps of 3.38 eV and 3.42 eV for as-deposited and annealed ZnO:B films are broader than that of 3.35 eV for ZnO film. The first-principles calculations show that in ZnO:B thin films not only the band gap becomes narrower but also the Fermi level shifts up into the conduction band with respect to the pure ZnO film. These are consistent with their lower resistivities and suggest that in the process of annealing some substituted B in the lattice change into interstitial B because of its smaller ion radius and this transformation widens the optical band gap of ZnO:B thin film.     

Microstructural and compositional studies of liquid-phase deposition derived PbTiO3 thin films on LaNiO3 substrates

PbTiO₃ thin films were successfully deposited on the LaNiO₃ (LNO) substrates by the liquid-phase deposition (LPD) method and post-annealing at various temperatures. The structure, morphology and composition of the films were investigated by some analytical techniques. The as-deposited films are amorphous and composed of densely packed spherical particles. The films with the grain size of 180 nm start to decompose and crystallize into perovskite structure at 450 °C and show a perovskite single phase with tetragonal structure after annealing at 650 °C. X-ray photoelectron spectroscopy (XPS) analysis reveals that the as-deposited film contains fluorine and carbon as major impurities. Fluorine could be completely eliminated by annealing at 650 °C in air.     

Structural and optical properties of electrodeposited molybdenum oxide thin films

Electrosynthesis of Mo(IV) oxide thin films on F-doped SnO₂ conducting glass (10-20/Ω/□) substrates were carried from aqueous alkaline solution of ammonium molybdate at room temperature. The physical characterization of as-deposited films carried by thermogravimetric/differential thermogravimetric analysis (TGA/DTA), infrared spectroscopy and X-ray diffraction (XRD) showed the formation of hydrous and amorphous MoO₂. Scanning electron microscopy (SEM) revealed a smooth but cracked surface with multi-layered growth. Annealing of these films in dry argon at 450 °C for 1 h resulted into polycrystalline MoO₂ with crystallites aligned perpendicular to the substrate. Optical absorption study indicated a direct band gap of 2.83 eV. The band gap variation consistent with Moss rule and band gap narrowing upon crystallization was observed.Structure tailoring of as-deposited thin films by thermal oxidation in ambient air to obtain electrochromic Mo(VI) oxide thin films was exploited for the first time by this novel route. The results of this study will be reported elsewhere.     

Investigation on the effect of Zr doping in ZnO thin films by spray pyrolysis

Zirconium doped zinc oxide thin films with enhanced optical transparency were prepared on Corning 1737 glass substrates at the substrate temperature of 400 °C by spray pyrolysis method for various doping concentrations of zirconium (IV) chloride in the spray solution. The X-ray diffraction studies reveal that the films exhibit hexagonal crystal structure with polycrystalline grains oriented along (0 0 2) direction. The crystalline quality of the films is found to be deteriorating with the increase of doping concentration and acquires amorphous state for higher concentration of 8 at.% in precursor solution. The average transmittance for 5 at.% (solution) zirconium doped ZnO film is significantly increased to ∼92% in the visible region of 500-800 nm. The room temperature photoluminescence (PL) spectra of films show a band edge between 3.41 and 3.2 eV and strong blue emission at 2.8 eV irrespective of doping concentration and however intensity increases consistently with doping levels. The vacuum annealing at 400 °C reduced the resistivity of the films significantly due to the coalescence of grains and the lowest resistivity of 2 × 10⁻³ Ω cm is observed for 3 at.% (solution) Zr doped ZnO films which envisages that it is a good candidate for stable TCO material.     

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.     

Electrical properties of vacuum-annealed titanium-doped indium oxide films

Titanium-doped indium oxide (ITiO) films were deposited on Corning glass 2000 substrates at room temperature by radio frequency magnetron sputtering followed by vacuum post-annealing. With increasing deposition power, the as-deposited films showed an increasingly crystalline nature. As-deposited amorphous ITiO films obtained at 20 W began to crystallize at the annealing temperature of 155 °C. Although there was no significant change in the crystalline structure of the films, electron mobility improved gradually with further increase in the annealing temperature. After post-annealing at 580 °C, the highest electron mobility of 50 cm² V⁻¹ s⁻¹ was obtained. Compared with the amorphous ITiO films, the ITiO films with a certain degree of crystallinity obtained at high deposition power were less affected by the vacuum annealing. Their electron mobility also improved due to post-annealing, but the increase was insignificant. After post-annealing, the optical transmission of the 325 nm-thick ITiO films showed approximately 80% at wavelengths ranging from 530 to 1100 nm, while the sheet resistance decreased to 10 Ω/sq. This makes them suitable for use as transparent conductive oxide layers of low bandgap solar cells.     

Effect of annealing on electrical resistivity of rf-magnetron sputtered nanostructured SnO2 thin films

Tin oxide (SnO₂) thin films were deposited by radio frequency (RF) magnetron sputtering on clean corning glass substrates. These films were then annealed for 15 min at various temperatures in the range of 100-500°C. The films were investigated by studying their structural and electrical properties. X-ray diffraction (XRD) results suggested that the deposited SnO₂ films were formed by nanoparticles with average particle size in the range of 23-28 nm. XRD patterns of annealed films showed the formation of small amount of SnO phase in the matrix of SnO₂. The initial surface RMS roughness measured with atomic force microscopy (AFM) was 25.76 nm which reduces to 17.72 nm with annealing. Electrical resistivity was measured as a function of annealing temperature and found to lie between 1.25 and 1.38 mΩ cm. RMS roughness and resistivity show almost opposite trend with annealing.     

Thermal stability of Ag films in air prepared by thermal evaporation

The thermal stability of silver films in air has been studied. Pure Ag films, 250 nm in thickness, were prepared on glass substrates by thermal evaporation process, and subsequently annealed in air for 1 h at temperatures between 200 and 400 °C. The structure and morphology of the samples were investigated by X-ray diffraction, Raman spectra and atomic force microscopy. It is found that the crystallization enhances for the annealed films, and film surface becomes oxidized when annealing temperature is higher than 350 °C. The electrical and optical properties of the films were studied by van der Pauw method and spectrophotometer, respectively. Reflectance drops sharply as Ag films are annealed at temperatures above 250 °C. Film annealed at 250 °C has the maximum surface roughness and the minimum reflectance at 600 nm optical wavelength. Film annealed at 200 °C has the minimum resistivity, and resistivity increases with the increasing of the annealing temperature when temperature is above 200 °C. The results show that both oxidization on film surface and agglomeration of silver film result in infinite of electrical resistivity as the annealing temperature is above 350 °C.     

Low-temperature growth of polycrystalline Ge thin film on glass by in situ deposition and ex situ solid-phase crystallization for photovoltaic applications

Poly-crystalline germanium (poly-Ge) thin films have potential for lowering the manufacturing cost of photovoltaic devices especially in tandem solar cells, but high crystalline quality would be required. This work investigates the crystallinity of sputtered Ge thin films on glass prepared by in situ growth and ex situ solid-phase crystallization (SPC). Structural properties of the films were characterized by Raman, X-ray diffraction and ultraviolet-visible reflectance measurements. The results show the transition temperature from amorphous to polycrystalline is between 255 °C and 280 °C for in situ grown poly-Ge films, whereas the transition temperature is between 400 °C and 500 °C for films produced by SPC for a 20 h annealing time. The in situ growth in situ crystallized poly-Ge films at 450 °C exhibit significantly better crystalline quality than those formed by solid-phase crystallization at 600 °C. High crystalline quality at low substrate temperature obtained in this work suggests the poly-Ge films could be promising for use in thin film solar cells on glass.     

Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach

Zn-Sn-O (ZTO) films with continuous compositional gradient of Sn 16-89 at.% were prepared by co-sputtering of two targets of ZnO and SnO₂ in a combinatorial method. The resistivities of the ZTO films were severely dependent on oxygen content in sputtering gas and Zn/Sn ratio. Except for the films with Sn 16 at.%, all the as-prepared films were amorphous and maintaining the stable amorphous states up to the annealing temperature of 450 °C. Annealing at 650 °C resulted in crystallization for all the composition, in which ZnO, Zn₂SnO₄, ZnSnO₃, and SnO₂ peaks were appeared successively with increasing Sn content. Above Sn 54 at.%, the ZTO films were deduced to have a local structure mixed with ZnSnO₃ and SnO₂ phases which were more conductive and stable in thermal oxidation than ZnO and Zn₂SnO₄ phases. The lowest resistivity of 1.9 × 10⁻³ Ω cm was obtained for the films with Sn 89 at.% when annealed at 450 °C in a vacuum. The carrier concentrations of the amorphous ZTO films that contained Sn contents higher than 36 at.% and annealed at 450 °C in a vacuum were proportional to the Sn contents, while the Hall mobilities were insensitive to Sn contents and leveling in the range of 23-26 cm²/V s.     

Effects of annealing temperature and method on structural and optical properties of TiO2 films prepared by RF magnetron sputtering at room temperature

TiO₂ thin film was deposited on non-heated Si(1 0 0) substrate by RF magnetron sputtering. The as-deposited films were annealed by a conventional thermal annealing (CTA) and rapid thermal annealing (RTA) at 700 and 800 °C, and the effects of annealing temperature and method on optical properties of studied films were investigated by measuring the optical band gaps and FT-IR spectra. And we also compared the XRD patterns of the studied samples. The as-deposited film showed a mixed structure of anatase and brookite. Only rutile structures were found in samples annealed above 800 °C by CTA, while there are no special peaks except the weak brookite B(2 3 2) peak for the sample annealed at (or above) 800 °C by RTA. FT-IR spectra show the broad peaks due to Ti-O vibration mode in the range of 590-620 cm⁻¹ for the as-deposited film as well as samples annealed by both annealing methods at 700 °C. The studied samples all had the peaks from Si-O vibration mode, which seemed to be due to the reaction between TiO₂ and Si substrate, and the intensities of these peaks increased with increasing of annealing temperature. The optical band gap of the as-deposited film was 3.29 eV but it varied from 3.39 to 3.43 eV as the annealing temperature increased from 700 to 800 °C in the samples annealed by CTA. However, it varied from 3.38 to 3.32 eV as the annealing temperature increased from 700 to 800 °C by RTA.