Structural, electrical and optical properties of ZnS films deposited by close-spaced evaporation

ZnS films have been deposited on glass substrates by close-spaced evaporation (CSE) technique. The films were grown at different temperatures in the range, 200-350 °C. The layers have been characterized with X-ray diffractometer (XRD), atomic force microscope (AFM), energy dispersive analysis of X-rays (EDAX) and optical spectrophotometer to evaluate the quality of the layers for photovoltaic applications. The studies showed that the optimum substrate temperature for the growth of ZnS layers was 300 °C. The films grown at these temperatures exhibited cubic structure with nearly stoichiometric composition. The AFM data revealed that the films had nano-sized grains with a grain size of ∼40 nm. The optical studies exhibited direct allowed transition with an energy band gap of 3.61 eV. The other structural and optical parameters such as lattice stress, dislocation density, refractive index and extinction coefficient were also evaluated. The temperature-dependent conductivity measured in the range, 303-523 K showed a change in the conduction mechanism at 120 °C. The activation energy values evaluated using the temperature dependence of electrical conductivity are 7 and 29 meV at low and high temperature regions, respectively.     

Chemical bath-deposited ZnS thin films: Preparation and characterization

Chemical bath deposition of ZnS thin films from NH₃/SC(NH₂)₂/ZnSO₄ solutions has been studied. The effect of various process parameters on the growth and the film quality are presented. The influence on the growth rate of solution composition and the structural, optical properties of the ZnS thin films deposited by this method have been studied. The XRF analysis confirmed that volume of oxygen of the as-deposited film is very high. The XRD analysis of as-deposited films shows that the films are cubic ZnS structure. The XRD analysis of annealed films shows the annealed films are cubic ZnS and ZnO mixture structure. Those results confirmed that the as-deposited films have amorphous Zn(OH)₂. SEM studies of the ZnS thin films grown on various growth phases show that ZnS film formed in the none-film phase is discontinuous. ZnS film formed in quasi-linear phase shows a compact and a granular structure with the grain size about 100 nm. There are adsorbed particles on films formed in the saturation phase. Transmission measurement shows that an optical transmittance is about 90% when the wavelength over 500 nm. The band gap (E_g) value of the deposited film is about 3.51 eV.     

ZnSe sintered films: Growth and characterization

The II-VI compound semiconductor, ZnSe having wide band gap between 2.58 and 2.82 eV is a promising material for use in photovoltaic devices, blue light emitting diodes and laser diodes. Several methods have been used to prepare ZnSe thin films. We have deposited ZnSe films on ultra-clean glass substrate by sintering technique. The optical, structural and electrical properties of ZnSe thin films have been examined. The optical band gap of these films is studied using reflection spectra in wavelength range 325-600 nm and structure of these films is studied using XRD. The DC conductivity of the films was measured in vacuum by two-probe technique.Sintering is a very simple and viable method compared to other intensive methods. The results of the present investigation will be useful in characterizing the material ZnSe for its applications in photovoltaics.     

Synthesis of silicon carbide films by combined implantation with sputtering techniques

Silicon carbide (SiC) films were synthesized by combined metal vapor vacuum arc (MEVVA) ion implantation with ion beam assisted deposition (IBAD) techniques. Carbon ions with 40 keV energy were implanted into Si(1 0 0) substrates at ion fluence of 5 × 10¹⁶ ions/cm². Then silicon and carbon atoms were co-sputtered on the Si(1 0 0) substrate surface, at the same time the samples underwent assistant Ar-ion irradiation at 20 keV energy. A group of samples with substrate temperatures ranging from 400 to 600 °C were used to analyze the effect of temperature on formation of the SiC film. Influence of the assistant Ar-ion irradiation was also investigated. The structure, morphology and mechanical properties of the deposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nanoindentation, respectively. The bond configurations were obtained from IR absorption and Raman spectroscopy. The experimental results indicate that microcrystalline SiC films were synthesized at 600 °C. The substrate temperature and assistant Ar-ion irradiation played a key role in the process. The assistant Ar-ion irradiation also helps increasing the nanohardness and bulk modulus of the SiC films. The best values of nanohardness and bulk modulus were 24.1 and 282.6 GPa, respectively.     

Fabrication of nanostructured CuInS2 thin films by ion layer gas reaction method

CuInS₂ thin films were prepared by a two-stage ion layer gas reaction (ILGAR) process in which the Cu and In precursors were deposited on glass substrate by using a simple and low-cost dip coating technique and annealed in H₂S atmosphere at different temperatures. The influence of the annealing temperature (250-450 °C) on the particle size, crystal structure and optical properties of the CuInS₂ thin films was studied. Transmission electron microscopy revealed that the particle radii varied in the range 6-21 nm with annealing. XRD and SAED patterns indicated polycrystalline nature of the nanoparticles. The optical band gap (E_g) varied from 1.48 to 1.56 eV with variation of particle size. The variation of Urbach tail with temperature indicated higher density of the defects for the films annealed at lower temperature. From the Raman study, it was observed that the FWHM of the A₁ mode at ∼292 cm⁻¹ corresponding to the chalcopyrite phase of CuInS₂ decreased with increasing annealing temperature.     

Effect of ambient hydrogen sulfide on the physical properties of vacuum evaporated thin films of zinc sulfide

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H₂S ∼10⁻⁴ Torr). The H₂S atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH₂)₂] inside the vacuum chamber. It has been observed that at elevated substrates temperature of about 200 °C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation. The zinc ions promptly recombine with H₂S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.     

Wide band gap Cd0.83Mg0.15Al0.02O thin films by pulsed laser deposition

Magnesium and aluminum doped CdO thin films were deposited on quartz substrate using pulsed laser deposition technique. Magnesium is used to widen the band gap and aluminum is used to increase carrier concentration of CdO films. The effect of growth temperature on structural, optical, and electrical properties was studied. These films are crystalline in nature and their preferred orientation depends on growth temperature. These films are highly transparent (∼86%) in visible region. The band gap of the films varies from 3.1 eV to 3.4 eV. The electrical conductivity and carrier concentration were found to decrease with increase in growth temperature.     

Growth and characterization of rocksalt MnS/GaAs epilayers by hot-wall epitaxy

Epitaxial growth characteristics of α-MnS on GaAs(1 0 0) substrates have been investigated by X-ray diffraction and double crystal rocking curve measurements. Growth of stoichiometric α-MnS films has been performed by hot-wall epitaxy using Mn and ZnS as a source of sulfur. The films on GaAs(1 0 0) at low substrate temperature exhibit multiphase crystal structures of zincblende and rocksalt, and the main structure is changed to rocksalt with increasing substrate temperature. Photoluminescence spectrum of the α-MnS epilayer at 5 K exhibits broad emission bands, which are attributed to Mn²⁺ ions. The band gap energy of the α-MnS epilayer at room temperature was also estimated to be about 3.3 eV by reflection.     

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.     

Substrate effect on electronic sputtering yield in polycrystalline fluoride (LiF, CaF2 and BaF2) thin films

Influence of substrate on electronic sputtering of fluoride (LiF, CaF₂ and BaF₂) thin films, 10 and 100 nm thin, under dense electronic excitation of 120 MeV Ag²⁵⁺ ions irradiation is investigated. The sputtering yield of the films deposited on insulating (glass) and semiconducting (Si) substrates are determined by elastic recoil detection analysis technique. Results revealed that sputtering yield is higher, up to 7.4 × 10⁶ atoms/ion for LiF film on glass substrate, than that is reported for bulk materials/crystals (∼10⁴ atoms/ion), while a lower value of the yield (2.3 × 10⁶ atoms/ion) is observed for film deposited on Si substrate. The increase in the yield for thin films as compared to bulk material is a combined effect of the insulator substrate used for deposition and reduced film dimension. The results are explained in the framework of thermal spike model along with substrate and size effects in thin films. It is also observed that the material with higher band gap showed higher sputtering yield.     

Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD

Zinc oxide films of 40 nm thickness have been deposited on glass substrates by pulsed laser deposition using an excimer XeCl laser (308 nm) at different substrate temperatures ranging from room temperature to 650 °C. Surface investigations carried out by using atomic force microscopy have shown a strong influence of temperature on the films surface topography. UV-VIS transmittance measurements have shown that our ZnO films are highly transparent in the visible wavelength region, having an average transmittance of ∼90%. The optical band gap of the films was found to be 3.26 eV, which is lower than the theoretical value of 3.37 eV. Besides the normal absorption edge related to the transition between the valence and the conduction band, an additional absorption band was also recorded in the wavelength region around 364 nm (∼3.4 eV). This additional absorption band may be due to excitonic, impurity, and/or quantum size effects. Photoreduction/oxidation in ozone of the ZnO films lead to larger conductivity changes for higher deposition temperature. In conclusion, the ozone sensing characteristics as well as the optical properties of the ZnO thin films deposited by pulsed laser deposition are strongly influenced by the substrate temperature during growth. The sensitivity of the films towards ozone might be enhanced significantly by the control of the films deposition parameters and surface characteristics.     

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.     

Influence of substrate temperature on electrical and optical properties of p-type semitransparent conductive nickel oxide thin films deposited by radio frequency sputtering

Nickel oxide thin films were deposited on fused silica and Si(1 0 0) substrates at different substrate temperatures ranging from room temperature to 400 °C using radio frequency reactive magnetron sputtering from a Ni metal target in a mixture of O₂ and Ar. With the increase of substrate temperature, nickel oxide films deposited on the Si substrates exhibit transition from amorphous to poly-crystalline structures with different preferred orientations of NiO(2 0 0) and (1 1 1). The films deposited at higher temperature exhibit higher Ni²⁺/Ni³⁺ ratio. With substrate temperature increasing from room temperature to 400 °C, the electrical resistivities of nickel oxide films increase from (2.8 ± 0.1) × 10⁻² to (8.7 ± 0.1) Ω cm, and the optical band-gap energies increase from 3.65 to 3.88 eV. A p-nickel oxide/n-zinc oxide heterojunction was fabricated to confirm the p-type conduction of nickel oxide thin film, which exhibited a steadily rectifying behavior.     

Preparation and properties of spray-deposited ZnIn2Se4 nanocrystalline thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO)-coated glass substrates using a spray pyrolysis technique. Aqueous solution containing precursors of Zn, In, and Se has been used to obtain good quality deposits at different substrate temperatures. The preparative parameters such as substrate temperature and concentration of precursors solution have been optimized by photoelectrochemical technique and are found to be 325 °C and 0.025 M, respectively. The X-ray diffraction patterns show that the films are nanocrystalline with rhombohedral crystal structure having lattice parameter a=4.05 Å. The scanning electron microscopy (SEM) studies reveal the compact morphology with large number of single crystals on the surface. From optical absorption data the indirect band gap energy of ZnIn₂Se₄ thin film is found to be 1.41 eV.     

Modifications in structural and optical properties of Mn-ion implanted CdS thin films

In this paper, we report on modifications in structural and optical properties of CdS thin films due to 190 keV Mn-ion implantation at 573 K. Mn-ion implantation induces disorder in the lattice, but does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. The optical band gap was found to decrease with increasing ion fluence. This is explained on the basis of band tailing due to the creation of localized energy states generated by structural disorder. Enhancement in the Raman scattering intensity has been attributed to the enhancement in the surface roughness due to increasing ion fluence. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (⁴T₁ → ⁶A₁) transition of tetrahedrally coordinated Mn²⁺ ions.     

Optical characterization of PLD grown nitrogen-doped TiO2 thin films

Nitrogen-doped TiO₂ thin films were prepared by pulsed laser deposition (PLD) by ablating metallic Ti target with pulses of 248 nm wavelength in reactive atmospheres of O₂/N₂ gas mixtures. The layers were characterized by UV-VIS spectrophotometry and variable angle spectroscopic ellipsometry with complementary profilometry for measuring the thickness of the films. Band gap and extinction coefficient values are presented for films deposited at different substrate temperatures and for varied N₂ content of the gas mixture. The shown tendencies are correlated to nitrogen incorporation into the TiO_2-xN_x layers. It is shown that layers of significantly increased visible extinction coefficient with band gap energy as low as 2.89 eV can be obtained. A method is also presented how the spectroscopic ellipsometric data should be evaluated in order to result reliable band gap values.     

Study of temperature dependence and angular distribution of poly(9,9-dioctylfluorene) polymer films deposited by matrix-assisted pulsed laser evaporation (MAPLE)

Poly(9,9-dioctylfluorene) (PFO) polymer films were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique. The polymer was diluted (0.5 wt%) in tetrahydrofuran and, once cooled to liquid nitrogen temperature, it was irradiated with a KrF excimer laser. 10,000 laser pulses were used to deposit PFO films on 〈1 0 0〉 Si substrates at different temperatures (−16, 30, 50 and 70 °C). One PFO film was deposited with 16,000 laser pulses at a substrate temperature of 50 °C. The morphology, optical and structural properties of the films were investigated by SEM, AFM, PL and FTIR spectroscopy. SEM inspection showed different characteristic features on the film surface, like deflated balloons, droplets and entangled polymer filaments. The roughness of the films was, at least partially, controlled by substrate heating, which however had the effect to reduce the deposition rate. The increase of the laser pulse number modified the target composition and increased the surface roughness. The angular distribution of the material ejected from the target confirmed the forward ejection of the target material. PFO films presented negligible modification of the chemical structure respect to the bulk material.     

Synthesis of molybdenum silicide by both ion implantation and ion beam assisted deposition

Two groups of Mo/Si films were deposited on surface of Si(1 0 0) crystal. The first group of the samples was prepared by both ion beam assisted deposition (IBAD) and metal vapor vacuum arc (MEVVA) ion implantation technologies under temperatures from 200 to 400 °C. The deposited species of IBAD were Mo and Si, and different sputtering Ar ion densities were selected. The mixed Mo/Si films were implanted by Mo ion with energy of 94 keV, and fluence of Mo ion was 5 × 10¹⁶ ions/cm². The second group of the samples was prepared only by IBAD under the same test temperature range. The Mo/Si samples were analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), sheet resistance, nanohardness, and modulus of the Mo/Si films were also measured. For the Mo/Si films implanted with Mo ion, XRD results indicate that phase of the Mo/Si films prepared at 400 and 300 °C was pure MoSi₂. Sheet resistance of the Mo/Si films implanted with Mo ion was less than that of the Mo/Si films prepared without ion implantation. Nanohardness and modulus of the Mo/Si films were obviously affected by test parameters.     

Structure and optical properties of chemically synthesized titanium oxide deposited by evaporation technique

Titanium oxide inorganic ion exchange material was synthesized by hydrolysis with water and ammonia solution. Structural feature of the synthesized titanium oxide was analyzed using X-ray diffraction, X-ray fluorescence and infrared spectrometer technique. Tentative formula of titanium oxide was determined and written as TiO₂·0.58H₂O. Titanium oxide films were deposited on glass substrates by means of an electron beam evaporation technique at room temperature from bulk sample. The films were annealed at 250, 350, 450, and 550 °C temperatures. Transmittance, reflectance, optical energy gap, refractive index and extinction coefficient were investigated. The transmittance values of 85% in the visible region and 88% in the near infrared region have been obtained for titanium oxide film annealed at 550 °C. Kubelka-Munk function was used to evaluate the absorption coefficient which was used to determine the optical band gap. It was found that the optical band gap increases with increasing annealing temperature whereas the refractive index and extinction coefficient decreases.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.