Comparison of the characteristics of TiO2 films prepared by low-pressure and plasma-enhanced chemical vapor deposition

Thin films of titanium dioxide (TiO₂) were deposited on silicon wafers at 450 °C by low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD), in the same reactor, using Ti(O-i-C₃H₇)₄ and oxygen as the reactants. The crystal structure, surface morphology, composition, chemical binding state and refractive index of the TiO₂ films, deposited with various r.f. powers, were characterized. Without plasma, LPCVD TiO₂ films show polycrystalline anatase structure, with a rough, granular surface and a typical refractive index of 2.43. On the contrary, the PECVD TiO₂ films are amorphous, with a smooth surface. By varying the r.f. power from 50 to 150 W, the refractive index of PECVD TiO₂ film increases with increasing power and becomes higher than 2.8 at 125 and 150 W. Despite the very different crystal structure, surface morphology and optical properties, the composition and chemical binding states of LPCVD and PECVD TiO₂ films are similar. The effect of plasma on the characteristics of CVD TiO₂ films is discussed.     

Optical properties of LPCVD silicon oxynitride

Low pressure chemical vapour deposition (LPCVD) silicon oxynitride films of various compositions (from pure SiO₂ to pure Si₃N₄) were deposited by changing the relative gas flow ratio. The effects of oxygen on the physical properties of the films were studied by spectroellipsometry (using Bruggeman approximation and Wemple Di Domenico model) and infrared spectroscopy. Refractive index measured by spectroellipsometry method is studied as a function of some deposition parameters: temperature of deposition, gases fluxes ratio. The high value of deposition temperature means low values in refractive index. More oxygen into films decreases the refractive index. The refractive index dispersion is studied by single-oscillator Wemple Di Domenico model. The optical band gap varies monotonically from 5 eV for silicon nitride, to 9eV for HTO LPCVD silicon dioxide and for the studied silicon oxynitride was found to be between 5 and 6 eV.     

The deposition and optical properties of Ge1−xCx thin film and infrared multilayer antireflection coatings

The effects of deposition parameters on the deposition rate, microstructure, and composition of Ge_1−xC_x thin films prepared by plasma enhanced chemical vapor deposition were studied and the films' infrared optical properties were investigated. The results show that the carbon content of these films increases as the precursor gas flow ratio of CH₄:GeH₄ increases, while the infrared refractive index of these films decreases from 4 to 2. The deposition rate increases with the radio-frequency power and reaches a constant value when the power goes above 60 W. Ge_1−xC_x/diamond-like carbon infrared antireflection coatings were prepared, and the transmittance of the coatings in the band of 8 to 14 μm was 88%, which is superior to that of Zinc Sulfide substrate by 14%.     

Optical and dielectric properties of thin SiOx films of variable composition

The effect of the composition of amorphous SiO_x films produced by the vacuum evaporation of SiO, on their optical and dielectric properties was investigated. The variation in the composition of the films was achieved by changing the deposition rate and the pressure of the residual gases.The optical band gap was observed to increase from 2.2 to 3.1 eV as the deposition rate was decreased from 50 to 5 Å s^-1 and simultaneously the refractive index, the permittiviity and the dielectric loss factor were found to decrease. The composition and structure of the films were determined from the optical absorption and IR spectroscopy.The experimental results revealed that SiO_x films produced by vacuum evaporation do not comprise a simple mixture of silicon and SiO₂ phases but they have a single-phase structure.     

Room temperature synthesis and characterization of polythiophene thin films by chemical bath deposition (CBD) method

Polythiophene thin films were deposited successfully on glass substrate by chemical bath deposition method using FeCl₃ as an oxidant and chloroform as solvent. The effect of oxidant concentration on the properties of polythiophene thin films was studied. The surface morphology was influenced by oxidant concentration and deposition time. The oxidation concentration also strongly affects the optical properties of the polythiophene thin films. The transmittance decreases while the absorption, band gap and refractive index increases due to increase in oxidant concentration.     

Advanced key technologies for magnetron sputtering and PECVD of inorganic and hybrid transparent coatings

Besides classical multilayer systems with alternating low and high refractive indices, reactive pulse magnetron sputtering processes offer various possibilities of depositing gradient films with continuously varying refractive index. Using nanoscale film growth control it is possible to achieve optical filter systems with a defined dependency of refractive index on film thickness, e.g. by sputtering a silicon target in a time variant mixture of oxygen and nitrogen. Also reactive co-sputtering of different target materials such as silicon and tantalum in oxygen is suitable as well. Rugate filters made from SiO_xN_y or Si_xTa_yO_z gradient refractive index profiles find their application in spectroscopy, laser optics and solar concentrator systems.Furthermore polymer substrates are increasingly relevant for the application of optical coatings due to their mechanical and economical advantages. Magnetron PECVD (magPECVD) using HMDSO as precursor allows to deposit carbon containing films with polymer-like properties. Results show the suitability of these coatings as hard coatings or matching layers. Multifunctional coatings with antireflective and scratch-resistant properties were deposited on polymer substrates using a combined magPECVD and sputter deposition process.     

Deposition Temperature Effect on the Structure and Optical Property of RF‐PACVD‐Derived Hydrogenated SiCNO Film

Amorphous hydrogenated silicon oxocarbonitride (SiCNO:H) films have been deposited by plasma‐assisted chemical vapour deposition (PACVD) using bis(trimethylsilyl)carbodiimide (BTSC) as a single source precursor in a argon (Ar) radio‐frequency plasma. In this work the SiCNO:H films deposited at different deposition temperatures were studied in terms of deposition rate, refractive index, surface roughness, microstructure, and chemical composition including bonding state. The results showed that a higher deposition temperature enhanced the formation of Si‐N bonds, and disfavoured the formation of N=C=N, Si‐NCN, C‐H and Si‐CH₃ bonds. A higher deposition temperature also decreased the deposition rate and increased the refractive index of the resulting SiCNO:H film. With increasing temperature a denser film was formed, indicating a change of the deposition mechanism, i.e., transformation from particle precipitation to heterogeneous surface reaction. Except for the coatings deposited at room temperature, the surface of the films was smooth with a roughness of around 4 nm at the centre in the range of 5 μm x 5 μm area. Moreover, the films contained 8 ～ 16 at.% oxygen bonded to Si, which originated from the remnant H₂O in the deposition chamber.     

The influence of the deposition time on morphological and optical properties of Cu x S films deposited on polypropylene substrate

Copper sulfide (Cu _x S) films deposited on polypropylene substrate were obtained by chemical bath deposition (CBD) method. The influence of the deposition time on the morphology of Cu _x S films was studied by means of scanning electron microscopy. We have found that the average particles dimension increased from 37 to 49 nm with the increase of deposition time from 20 to 30 minutes. The study of optical properties of the copper sulfide films was carried out based on optical transmission spectra recorded in the 400–1000 nm wavelength range. The optical constants, such as refractive index, extinction coefficient and dielectric constant as well as electrical and optical conductivity of Cu _x S films were calculated. The obtained values are in accordance with the ones reported in the literature:We have shown that both, morphological and optical properties of Cu _x S films are strongly affected by the deposition time.     

Evolution of optical properties with deposition time of silicon nitride and diamond-like carbon films deposited by radio-frequency plasma-enhanced chemical vapor deposition method

The paper presents investigations of the optical properties of thin high-refractive-index silicon nitride (SiN_x) and diamond-like carbon (DLC) films deposited by the radio-frequency plasma-enhanced chemical vapor deposition method for applications in tuning the functional properties of optical devices working in the infrared spectral range, e.g., optical sensors, filters or resonators. The deposition technique offers the ability to control the film's optical properties and thickness on the nanometer scale. We obtained thin, high-refractive-index films of both types at deposition temperatures below 350 °C, which is acceptable under the thermal budget of most optical devices. In the case of SiN_x films, it was found that for short deposition processes (up to 5 min long) the refractive index of the film increases in parallel with its thickness (up to 50 nm), while for longer processes the refractive index becomes almost constant. For DLC films, the effect of refractive index increase was observed up to 220 nm in film thickness.     

The influence of deposition parameters on the structure and properties of magnetron-sputtered titania coatings

TiO₂ films are widely used as high refractive index dielectric layers in multi-layer optical devices and functional films. The optical properties of titania films, and their stability, are critically dependent on the film structure, which in turn, is dependent on the deposition conditions used. Titania can exist in three forms: amorphous, anatase and rutile. Titania films were grown by reactive pulsed magnetron sputtering under systematically controlled conditions using dual variable field strength planar magnetrons. The effects of deposition conditions, including the degree of unbalance of the magnetrons and pulse frequency, on titania films were investigated. The influence of these process variables was determined in terms of their impact on both deposition rate and process stability. Film properties were analyzed in terms of structure, crystallinity and refractive index. Results to date are discussed here.     

Reactive sputtering of SiO2-TiO2 thin film from composite Six/TiO2 targets

Coatings of SiO₂-TiO₂ films are frequently used in a number of optical thin film applications. In this work we present results from depositing films with variable Si/Ti ratios prepared by reactive sputtering. The different Si/Ti ratios were obtained by varying the target composition of composite single targets. Compared to co-sputtering this facilitates process control and composition uniformity of the films. Varying the oxygen supply during sputter deposition can result in films ranging from metallic/substoichiometric to stoichiometric oxides. Transmittance spectra of the different films are presented and the optical constants are determined from these spectra. Furthermore, the deposition process, films structure and composition of the films are discussed. The study shows that by choosing the right composition and working in the proper oxygen flow range, it is possible to tune the refractive index.     

Optical properties of amorphous carbons and their applications and perspectives in photonics

Amorphous carbon exhibits a wide variety of optical properties and, thus, offers substantial opportunities for various applications in photonics. The main optical properties, which should be taken into account for the design of new photonic devices, are the refractive index n, the fundamental gap E_g and the E₀₄ gap. In this work, the optical properties of the various forms of amorphous carbon films grown by plasma-enhanced chemical vapor deposition, pulsed laser deposition, sputtering and vacuum cathodic arc deposition and the crucial structural and chemical factors that determine n, E_g, and E₀₄ are reviewed. The knowledge of the optical properties of such films is exploited in order to design and implement various photonic devices such as: 1) anti-reflection (AR) coatings for various uses including photovoltaic modules, 2) interferometric sensors and indicators based on carbon-based AR layers, and 3) laser patterning of amorphous carbons and study of its photosensitivity for holographic applications.     

Variable refractive index optical coatings

The discrete and restricted values of refractive index of the bulk optical materials at present available are a serious limitation on the usefulness of these materials for optical coatings. This limitation can be overcome by utilizing the atom-by-atom condensation feature of the growth of vapour-deposited thin films, which allows the homogeneous mixing of different materials irrespective of their solubility restrictions. We have used this feature of co-deposition of different materials to form mixed optical materials of variable refractive index, the variation being determined by the composition of the source material. Measurements of the optical constants of these films, prepared by co-evaporation of mixtures of ZnS and MgF₂ of various compositions, have been made. The refractive index of the mixed films is found to be in good agreement with the values predicted on the basis of the Lorentz-Lorentz theory. In addition, the optical equivalence of alternate layers of ZnS and MgF₂ with step thicknesses ranging from 5 to 250 Å has been studied. For step thicknesses less than 100 Å, the optical properties of the composite films are equivalent to those for the homogeneously mixed films. For larger step thicknesses, considerable and complicated interference effects are observed. Thus, variable refractive index composite films can be prepared by (a) co-deposition and (b) deposition of alternate discrete layers of different materials as long as the step thickness does not exceed about 100 Å. Furthermore, these techniques of obtaining optical films of mixed materials can be extended to any combination of materials and therefore they open up a new field in materials technology.     

Titanium di-oxide films using a less hygroscopic colloidal precursor

We report the study of titanium dioxide films (TiO₂) using titanium di-isopropoxyl di-2ethyl hexanoate Ti(OC₃H₇)₂ (C₇H₁₅COO)₂ colloidal precursor. This compound is less hygroscopic in nature and easy to use with processes like spin or dip coating. Thin films of TiO₂ are made on silicon substrates and their structural and optical properties are studied. The effect of Ti content in the precursor, sintering temperature and its duration on film thickness and refractive index are investigated. Refractive index shows an increasing trend with the rise in the sintering temperature but remains unchanged with the time. The film thickness decreases with both sintering temperature and time and increases with Ti content in the precursor. Reflectivity measurements show marked reduction in the reflection losses compared to bare silicon surface wherein the film thickness is altered by spin speed. XRD results show anatase phase in the samples sintered at lower temperature (<680 °C), however, a mix of anatase, brookite and rutile phases is seen above this temperature. In the samples sintered above 1100 °C, rutile phase is dominant. These results are supported by the X-ray photoelectron spectroscopy. Atomic force microscopy reveals larger grain size at higher sintering temperature. The titanium dioxide films of desirable thickness and refractive index could be used as an antireflection coating on solar cells.     

Influence of deposition parameters on the optical and structural properties of TiO2 films produced by reactive d.c. plasmatron sputtering

We investigated the variations in the structure and optical properties of TiO₂ films produced by reactive d.c. plasmatron sputtering with the most important deposition parameters.Over a wide range, the phase composition (ratio of rutile to anatase) and the grain size of the TiO₂ films can be influenced in a controlled manner by variations in the substrate temperature and the oxygen partial pressure.Because of their high refractive index and low light-scattering losses, plasmatron-sputtered TiO₂ films are of great interest in the field of optical interference coatings, e.g. for dielectric multilayer stacks.     

Doping induced structural and compositional changes in ZnO spray pyrolysed films and the effects on optical and electrical properties

Highly transparent, conductive ZnO films have been deposited by spray pyrolysis of a zinc acetate based solution. Quality films are yielded as our process is analogous to an aerosol assisted chemical vapour deposition rather than a droplet deposition spray pyrolysis technique. The properties of the films are governed by the additives to the base precursor solution. When aluminium acetylacetonate is added to the precursor solution, ZnO:Al films are grown with free charge carrier concentrations of more than 10²⁰ cm⁻³. The carrier density and mobility are measured by both Hall probe and near infrared spectroscopy. Film growth and grain size, morphology and orientation are altered using an increased percentage of ZnCl₂ in the precursor, which results in a 10 fold increase in charge carrier mobility. An investigation is presented correlating the composition of the precursor solution with the chemical, structural, electrical and optical properties of the grown films.     

Pressure dependent structural and optical properties of silicon carbide thin films deposited by hot wire chemical vapor deposition from pure silane and methane gases

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition technique from silane (SiH₄) and methane (CH₄) gas precursors. The effect of deposition pressure on structural and optical properties of SiC films was investigated. Various spectroscopic methods including Fourier transform infrared spectroscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and UV–Vis–NIR spectroscopy were used to study these properties. Films deposited at low deposition pressure were Si-rich, and were embedded with nano-crystals of silicon. These films showed strong absorption in the visible region and had low energy band gaps. Near stoichiometric SiC film, were formed at intermediate deposition pressure and these films were transparent in the visible region and exhibited a wide optical band gap. High deposition pressures caused inhomogeneity in the film as reflected by the increase in disorder parameter and low refractive index of the films. This was shown to be due to formation of sp ² carbon clusters in the film structure.     

Structural, chemical, and optical properties of tin sulfide thin films as controlled by the growth temperature during co-evaporation and subsequent annealing

Tin sulfide thin films on soda-lime glass substrate were prepared by co-evaporation. This technique uses a vapor phase procedure involving chemical reactions between the precursor species evaporated simultaneously. The influence of the substrate temperature in the crystal structure and chemical composition were determined by X-ray diffraction and energy dispersive analysis of X-rays, showing that thin films crystallized in SnS, SnS₂, and Sn₂S₃ phases. Scanning electron microscope shows thin films with homogenous and uniform surface. Some of the samples were annealed to study the variation of structural, chemical, and optical properties. The variation of refractive index (n), extinction coefficient (k), and dielectric constant (ε) with wavelength and photon energy are reported. The energy band gap was calculated from optical transmittance and reflectance measurements in the range 300–1500 nm. The calculated energy band gap values were between 1.75 and 2.3 eV, depending on the phase in which crystallized the different thin films.     

Structural, optical and electrical peculiarities of r.f. plasma sputtered indium tin oxide films

In this work the influence of the deposition conditions on the structural, electrical and optical properties of the ITO films was studied. Films were deposited by r.f. plasma sputtering technique in Ar and varying Ar + O₂ gas mixtures, with and without substrate heating.Transmittance and reflectance of the films were measured in the range 350-2500 nm; the refractive index (n) and the extinction coefficient (k) were calculated by the spectral data simulation. The sheet resistance of the films was measured by four-point probe method. X-ray diffraction analysis was performed to study the texture of the films. Threshold behaviour was observed in the optical and electrical properties of ITO films deposited in Ar + O₂ atmosphere at a certain oxygen concentration determined by a fix combination of all other deposition conditions. A schematic diagram for the change of the film properties versus composition was suggested, which explains the obtained results.     

Controlled synthesis and characteristics of antireflection coatings of TiO2 produced from a organometallic colloid

Antireflection titanium dioxide (TiO₂) coatings have been developed on monocrystalline silicon by a sol–gel spin-coating process using titanium di-isopropoxidebis(acetylacetonate) colloidal precursor solution. The effect of titanium content in the precursor, spin rate, sintering duration and temperature have been studied and their effect on coating thickness and optical properties (i.e., refractive index and reflectivity) were investigated. The influence of post-deposition sintering temperature on the optical characteristics, composition and the microstructure of the coatings have been evaluated by UV–vis spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction techniques. Solar cells made on silicon wafers with TiO₂ as antireflection layer showed enhancement of more than 20% in short circuit current density in comparison to a cell devoid of the TiO₂ coating.