The properties of transparent conductive In-Ga-Zn oxide films produced by pulsed laser deposition

To obtain TCO films for wavelengths shorter than the visible range, Ga₂O₃ was added to the In₂O₃-ZnO system as an impurity. Using pulsed laser deposition (PLD), two kinds of targets, InGaZnO₄ and InGaZn₃O_6, were deposited. Although the In-Ga-Zn-O films obtained deviated from the stoichiometry of InGaZnO₄, they were amorphous at a substrate temperature below 250 °C. We obtained the lowest resistivity of 2.77 × 10⁻³ Ω cm within the present experiment at a carrier concentration of 1.38 × 10²⁰ cm⁻³ and a Hall mobility of 16.6 cm²/Vs. The optical band gap energy shifted to higher energies and the transmittance at the blue range was improved dramatically as compared with similar amorphous IZO films.     

Anisotropic phenomena in as-evaporated amorphous chalcogenide thin films

The changes in refractive index and birefringence in as-evaporated As_xS_1−x amorphous films have been measured by means of prism-coupling technique. In particular, the time evolution, annealing and substrate temperature effects, compositional dependencies of the optical anisotropy on the fresh amorphous films are investigated. The analyse of these effects in terms of a proposed microscopic model are discussed. Such a model is shown to qualitatively explain some aspects of this phenomenon.     

High quality amorphous indium zinc oxide thin films synthesized by pulsed laser deposition

Indium zinc oxide films were grown from targets with two different In atomic concentration [In/(In + Zn)] of 40% and 80% by the pulsed laser deposition technique on glass substrates from room temperature up to 100 °C. X-ray diffraction and reflectometry investigations showed that films were amorphous and dense. Thin films (thickness < 100 nm) exhibited higher optical transmittance and resistivities than thick films (thickness > 1000 nm), probably caused by a significant decrease of oxygen vacancies due to atmosphere exposure. Films deposited from the In rich target under an oxygen pressure of 1 Pa exhibited optical transmittance higher than 85%, resistivities around 5- 7 × 10^− 4 Ω cm and mobilities in the 47-54 cm²/V s range.     

Optical and electrical properties of Te-substituted Sn-Sb-Se semiconducting thin films

Thin films of Sn₁₀Sb₂₀Se_70-XTe_X (0 ≤ X ≤ 14) composition were deposited using thermal evaporation technique. As-prepared films were amorphous as studied by X-ray diffraction. Surface morphology studies revealed that films have surface roughness ~ 2 nm and av. grain size ~ 30 nm. Optical band gap E_g showed a sharp decrease for initial substitution of Se with Te upto 2 at.%. A broad hump in the optical band gap is observed for further substitution of Se with Te. The trend of optical band gap variation with tellurium content has been qualitatively explained using band model given by Kastner. The dc-conductivity measurements showed thermally activated conduction with single activation energy for the measured temperature regime and followed Meyer-Neldel rule. The dc-activation energy has nearly half the value as that of optical band gap that revealed the intrinsic nature of semiconductor. The annealing below glass transition T_g led to decrease in optical band gap as well as dc-activation energy that might be related to increase of disorder in material with annealing.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Thermally and optically induced irreversible changes in some Ge–As–S amorphous thin films

Thermally and optically induced irreversible changes in the optical gap and refractive index were studied for sulphur rich, nearly stoichiometric and sulphur poor Ge–As–S amorphous films prepared by thermal evaporation. For films studied the optical gap in the virgin state decreases from 2.55₉ (Ge_0.121As_0.172S_0.707) to 1.63₂ eV (Ge_0.254As_0.294S_0.452) and simultaneously the refractive index increases from 2.21 to 2.87, respectively. The most sensitive composition to illumination seems to be nearly stoichiometric film (Ge_0.153As_0.201S_0.646), where the blue shift of the gap is observed close to 150 meV. Sulphur poor film (Ge_0.254As_0.294S_0.452) was found insensitive to illumination. Highest thermally induced blue shift of the gap, close to 250 meV, we observed just for Ge_0.254As_0.294S_0.452 film and for this film it was observed also nearly “giant” decrease in refractive index from 2.85 to 2.42. The behaviour of Ge_0.254As_0.294S_0.452 film is qualitatively discussed using the concept of network rigidity (insensitivity to illumination) and assuming thermally induced changes in bonding arrangement (refractive index changes).     

Microstructure and magnetic properties of zinc ferrite thin films produced by pulsed laser deposition

Zinc ferrite thin films were deposited from a target of zinc ferrite onto a MgO substrate using XeCl excimer laser operating at 308 nm and frequency of 30 Hz. The crystallographic characterizations of the films were performed using X-ray diffraction (XRD). Microstructure, surface morphology, chemical composition and grain size, as well as surface roughness were obtained from scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM). The magnetic properties of the thin films were studied in the temperature range 5–300 K and in fields of up to 5 T using SQUID magnetometry. Data on temperature and field dependence of magnetization provide a strong evidence for superparamagnetism. Paper presented at 8 AGM of MRSI, BARC, Mumbai, 1997.     

On the optical properties of bismuth oxide thin films prepared by pulsed laser deposition

The optical properties of bismuth oxide films prepared by pulsed laser deposition (PLD), absorption in the photon energy range 2.50-4.30 eV and optical functions (n, k, ?₁, and ?₂) in the domain 3.20-6.50 eV, have been investigated. As-prepared films (d=0.05-1.50 μm) are characterized by a mixture of polycrystalline and amorphous phases. The fundamental absorption edge is described by direct optical band-to-band transitions with energies 2.90 and 3.83 eV. The dispersion of the optical functions provided values of 4.40-6.25 eV for electron energies of respective direct transitions. In the spectral range 400-1000 nm, bismuth oxide films show a normal dispersion, which can be interpreted in the frame of a single oscillator model.     

Structural and optical properties of amorphous oxygenated iron boron nitride thin films produced by reactive co-sputtering

Amorphous oxygenated iron boron nitride (a-FeBN:O) thin films were prepared by reactive radio-frequency (RF) sputtering, from hexagonal boron nitride chips placed on iron target, under a total pressure of a gas mixture of argon and oxygen maintained at 1 Pa. The films were deposited onto silicon and glass substrates, at room temperature. The power of the generator RF was varied from 150 to 350 W. The chemical and structural analyses were investigated using X-ray photoelectron spectroscopy (XPS), energy dispersive of X-ray and X-ray reflectometry (XRR). The optical properties of the films were obtained from the optical transmittance and reflectance measurements in the ultraviolet-visible-near infrared wavelengths range. XPS reveals the presence of boron, nitrogen, iron and oxygen atoms and also the formation of different chemical bonds such as Fe-O, B-N, B-O and the ternary BNO phase. This latter phase is predominant in the deposited films as observed in the B 1s and N 1s core level spectra. As the RF power increases, the contribution of N-B bonds in the as-deposited films decreases. The XRR results show that the mass density of a-FeBN:O thin films increases from 2.6 to 4.12 g/cm³ with increasing the RF power from 150 to 350 W. This behavior is more important for films deposited at RF power higher than 150 W, and has been associated with the enhancement of iron atoms in the film structure. The optical band gap decreases from 3.74 to 3.12 eV with increasing the RF power from 150 to 350 W.     

Optical band gap and optical constants in amorphous Se96−xTe4Agx thin films

The optical constants (absorption coefficient (α), refractive index (n), extinction coefficient (k), real and imaginary part of dielectric constant) have been studied for a-Se_96−xTe₄Ag_x (where x=0, 4, 8, 12) thin films as a function of photon energy in the wavelength range (500-1000 nm). It has been found that the optical band gap increases while n and k decreases on incorporation of Ag in Se-Te system. The value of α and k increases, while the value of n decreases with incident photon energy. The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level. A correlation between the optical band gap and electronegativity of the alloys indicates that the optical band gap increases with the decrease of electronegativity.     

Structural and optical properties of electron beam evaporated CdSe thin films

Thin films of cadmium selenide (CdSe) as a semiconductor is well suited for opto-electronic applications such as photo detection or solar energy conversion, due to its optical and electrical properties, as well as its good chemical and mechanical stability. In order to explore the possibility of using this in optoelectronics, a preliminary and thorough study of optical and structural properties of the host material is an important step. Based on the above view, the structural and optical properties of CdSe films have been studied thoroughly in the present work. The host material, CdSe film, has been prepared by the physical vapour deposition method of electron beam evaporation (PVD: EBE) technique under a pressure of 5 × 10⁻⁵ mbar. The structural properties have been studied by XRD technique. The hexagonal structure with a preferred orientation along the (0 0 2) direction of films has been confirmed by the X-ray diffraction analysis. The films have been analysed for optical band gap and absorbed a direct intrinsic band gap of 1·92 eV.     

Structural and optical properties of tellurite thin film glasses deposited by pulsed laser deposition

Tellurite (TeO₂-TiO₂-Nb₂O₅) thin film glasses have been produced by pulsed laser deposition at room temperature at laser energy densities in the range of 0.8-1.5 J/cm² and oxygen pressures in the range of 3-11 Pa. The oxygen concentration in the films increases with laser energy density to reach values very close to that of the bulk glass at 1.5 J/cm², while films prepared at 1.5 J/cm² and pressures above 5 Pa show oxygen concentration in excess of 10% comparing to the glass. X-ray photoelectron spectroscopy shows the presence of elementary Te in films deposited at O₂ pressures ≤ 5 Pa that is not detected at higher pressures, while analysis of Raman spectra of the samples suggests a progressive substitution of TeO₃ trigonal pyramids by TeO₄ trigonal bipyramids in the films when increasing their oxygen content. Spectroscopic ellipsometry analysis combined with Cauchy and effective medium modeling demonstrates the influence of these compositional and structural modifications on the optical response of the films. Since the oxygen content determines their optical response through the structural modifications induced in the films, those can be effectively controlled by tuning the deposition conditions, and films having large n (2.08) and reduced k (< 10^− 4) at 1.5 μm have been produced using the optimum deposition conditions.     

Morphological, structural and optical properties of thin SiC layer growth onto silicon by pulsed laser deposition

Crystalline silicon carbide thin layers were grown on a p-type Si(1 0 0) substrate by pulsed laser deposition (PLD) using KrF excimer laser at λ=248 nm from a 6H-SiC hot-pressed target. The target “SiC” used to elaborate our SiC films is realized from a mixture of 1SiO₂ with 3C (carbon) “1SiO₂+3C” heated in an oven at 2500 °C (the target was a hot-pressed material and supplied by Goodfellow). The morphological, structural and optical properties of SiC layers were investigated by scanning electronic microscopy (SEM), high-resolution X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and UV-visible spectrophotometer. XRD analysis of the target showed that this latter is a hexagonal structure (6H-SiC). The XRD pattern shows that a 1.6 μm crystalline SiC layer was formed. In addition, a SIMS analysis gives a ratio Si/C of the thin SiC layer around 1.15 but the ratio Si/C of the target was found equal to 1.06, whereas one should have 1.0. This is due to the degree of the sensitivity of the SIMS technique and due to the higher ionization efficiency of Si compared to C atoms, all these which give different ratios. It is known that the PLD technique reproduces the same macroscopic property (optical, mechanical, structural, etc.) of the target. An optical gap (E_Gap) of the SiC layer of about 2.51 eV was obtained by reflectance measurement. Finally, a crystalline thin SiC layer of 1.6 μm was elaborated using PLD method at low-temperature deposition.     

Effect of oxygen partial pressure on microstructural and optical properties of titanium oxide thin films prepared by pulsed laser deposition

Nanocrystalline titanium oxide (TiO₂) thin films were deposited on silicon (1 0 0) and quartz substrates at various oxygen partial pressures (1 × 10⁻⁵ to 3.5 × 10⁻¹ mbar) with a substrate temperature of 973 K by pulsed laser deposition. The microstructural and optical properties were characterized using Grazing incidence X-ray diffraction, atomic force microscopy, UV–visible spectroscopy and photoluminescence. The X-ray diffraction studies indicated the formation of mixed phases (anatase and rutile) at higher oxygen partial pressures (3.5 × 10⁻² to 3.5 × 10⁻¹ mbar) and strong rutile phase at lower oxygen partial pressures (1 × 10⁻⁵ to 3.5 × 10⁻³ mbar). The atomic force microscopy studies showed the dense and uniform distribution of nanocrystallites. The root mean square surface roughness of the films increased with increasing oxygen partial pressures. The UV–visible studies showed that the bandgap of the films increased from 3.20 eV to 3.60 eV with the increase of oxygen partial pressures. The refractive index was found to decrease from 2.73 to 2.06 (at 550 nm) as the oxygen partial pressure increased from 1.5 × 10⁻⁴ mbar to 3.5 × 10⁻¹ mbar. The photoluminescence peaks were fitted to Gaussian function and the bandgap was found to be in the range ∼3.28–3.40 eV for anatase and 2.98–3.13 eV for rutile phases with increasing oxygen partial pressure from 1 × 10⁻⁵ to 3.5 × 10⁻¹ mbar.     

Optical waveguide based on amorphous Er-doped Ga-Ge-Sb-S(Se) pulsed laser deposited thin films

Amorphous chalcogenide films play a motivating role in the development of integrated planar optical circuits due to their potential functionality in near infrared (IR) and mid-IR spectral regions. More specifically, the photoluminescence of rare earth ions in amorphous chalcogenide films can be used in laser and amplifier devices in the IR spectral domain. The aim of the present investigation was to optimize the deposition conditions for the fabrication of undoped and Er³⁺ doped sulphide and selenide thin films with nominal composition Ga₅Ge₂₀Sb₁₀S(Se)₆₅ or Ga₅Ge₂₃Sb₅S₆₇ by pulsed laser deposition (PLD). The study of compositional, morphological and structural characteristics of the layers was realized by scanning electron microscopy-energy dispersive spectroscopy, atomic force microscopy and Raman spectroscopy analyses, respectively. Some optical properties (transmittance, index of refraction, optical band gap, etc.) of prepared chalcogenide films and optical losses were investigated as well. The clear identification of near-IR photoluminescence of Er³⁺ ions was obtained for both selenide and sulphide films. The decay of the ⁴I_13/2 → ⁴I_15/2 transition at 1.54 µm in Er³⁺ doped Ga₅Ge₂₀Sb₁₀S₆₅ PLD sulphide films was studied to assess the effects of film thickness, rare earth concentration and multilayer PLD deposition on their spectroscopic properties.     

Influence of argon neutral beam energy on the structural properties of amorphous carbon thin films grown by neutral particle beam assisted sputtering

The effects of argon neutral beam (NB) energy on amorphous carbon (a-C) films were investigated, the a-C films were deposited by a neutral particle beam assisted sputtering (NBAS) system. The energy of the neutral particle beam can be directly controlled by a reflector bias voltage as a unique operating parameter of the system. The results from the analysis by Raman spectra, Fourier transform infrared (FT-IR), UV-visible spectroscopy and electrical conductivity indicate the properties of the amorphous carbon films can be manipulated by simply adjusting the NB energy (or reflector bias voltage) without changing any other process parameters. By increasing the reflector bias voltage, the amount of cross-linked sp² clusters as well as the sp³ bonding in the a-C film coating from the NBAS system can be increased effectively and the composition of carbon thin films can be changed from a nano-crystalline graphite phase to an amorphous carbon phase. In addition, the deposition rate increases with reflector bias voltage due to additional sputtering at the carbon reflector without any variation of physical and electrical properties of the a-C film.     

Low-temperature deposition of iridium thin films by pulsed laser deposition

Extremely smooth iridium (Ir) thin films were deposited on Si(1 0 0) substrate at lower temperature than 300 °C by pulsed laser deposition (PLD) technique using Ir target in a vacuum atmosphere. The crystal orientation, surface morphology, and resistivity of the Ir thin films were systematically determined as a function of substrate temperature. Well-crystallized and single-phase Ir thin films with (1 1 1) preferred orientation were obtained at substrate temperature of 200-300 °C. The surface roughness increased with the increasing of substrate temperature. Likewise, the room-temperature resistivity of Ir thin films decreased with increasing substrate temperature, showing a low value of (10.7±0.1) μΩ cm at 300 °C.     

Annealing and recrystallization of amorphous ZnO thin films deposited under cryogenic conditions by pulsed laser deposition

This article deals with the annealing of amorphous ZnO thin films prepared by pulsed laser deposition (PLD) under cryogenic conditions. The substrate holder was cooled by liquid nitrogen. X-ray diffraction analysis evidenced that as-deposited films had amorphous structures: analysis by scanning electron microscopy (SEM) revealed their fine grained surface and inner structure. Annealing at temperatures in the range of 200-800 °C resulted in a transition in the thin film crystal structure from amorphous to polycrystalline. Various properties of the ZnO films were found depending on the recrystallization temperature. In depth investigations employing SEM, X-ray diffraction, atomic force microscopy and secondary ion mass spectroscopy provided comparisons of the recrystallizations of undoped ZnO thin films during the phase transition processes from amorphous to hexagonal wurtzite structures.     

A study of microstructural and optical properties of nanocrystalline ceria thin films prepared by pulsed laser deposition

Thin films of cerium oxide (CeO₂) have been deposited on (100) Si substrates using pulsed laser deposition technique at various substrate temperatures from room temperature (RT) to 973 K at an optimized oxygen partial pressure of 3 Pa. Structural, morphological and optical properties have been carried out using X-ray diffraction (XRD), Raman, ellipsometry and atomic force microscopy techniques. XRD results showed that the deposited films are polycrystalline with cubic structure. At room temperature, the film showed preferred orientation along (111) plane, while at higher temperatures, it exhibited preferred orientation along (200). The crystallite sizes were calculated and were found to be in the range 17-52 nm. The texture coefficient for (200) reflection increased until 573 K, and then decreased in the temperature range 673-973 K. The Raman peak appeared at 463 cm^− 1 due to the F_2g active mode also confirmed the formation of CeO₂ with a cubic structure. There was a systematic variation in the Raman peak intensity, frequency shift and line broadening with the increase of temperature. The ellipsometry studies showed that the refractive index and band gap increased from 2.2 to 2.6 and 3.4 to 3.6 eV, respectively with increasing substrate temperature from RT to 973 K.     

Optical characterization of vacuum evaporated a-Se80Te20−xCux thin films

Thin films of a-Se₈₀Te_20−xCu_x (where x=2, 6, 8 and 10) were deposited on glass substrates by vacuum evaporation technique. The absorbance, reflectance and transmittance of as-deposited thin films were measured in the wavelength region 400-1000 nm. The optical band gap and optical constants of amorphous thin films have been studied as a function of photon energy. The optical band gap increases on incorporation of copper in Se₈₀Te_20−xCu_x system. The value of refractive index (n) decreases while the value of the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states.