M. Ghanashyam Krishna

X-ray photoelectron spectroscopy and spectral transmittance study of stoichiometry in sputtered vanadium oxide films

Abstract The growth of magnetron sputtered vanadium oxide thin films has been investigated by means of X-ray photoelectron spectroscopy and spectral transmittance in the region from 350 to 1100 nm. It is shown that films stoichiometry is dependent on film thickness as well as oxygen partial pressure. Beyond a critical thickness of ∼ 250 nm, the films are completely stoichiometric V 2 O 5 independent of the oxygen partial pressure. At low thickness, the films are a mixtured of V 2 O 3 , VO 2 and V 2 O 5 in varying proportions depending on the oxygen pressure. The presence of the non-stoichiometry has been correlated with changes in the spectral transmittance. It has also been shown that the mixed oxide phase is stable over a large regime of thickness and oxygen partial pressure during processing and could have very useful optical properties. A mechanism for the observed stoichiometry behaviour has also been proposed.

Optical constants of nanocrystalline lanthanide-doped ceria thin films with the fluorite structure

Abstract The optical constants and structure of yttria-doped ceria and ceria doped with Nd, Sm, Gd, Dy, Er and Yb oxides are reported. These films were prepared using an inorganic sol-gel route and characterized for their refractive index, optical absorption edge, structure and crystallite size. All the films crystallized into the fluorite structure well below 450 °C, with cell constants altering as a function of ionic radius. The films were also nanocrystalline and continuous, with crystallite sizes in the range of 10–16nm after 600 °C. They have high transparency in the region 350 to 1500 nm and a reasonably high refractive index (1.7–1.79 at a wavelength of 800 nm) in the dispersion-free region. The dispersion behaviour in the refractive index has been fitted to the single oscillator model and is also shown to be dependent on the ionic size of the dopant material. It is therefore proposed that they should be suitable materials for application as transparent ion intercalation films and optoionic smart windows.

Temperature and ionic size dependence of the properties of ceria based optionic thin films

Thin films of CexLn1−xOy (Ln=La, Nd, Gd, and Yb and x=0.85) were deposited by an inorganic sol–gel technique and the dependence of the refractive index, optical absorption and absorption edge of these films on post deposition annealing temperature has been studied. It has been found that the refractive index of all the films increases with increase in temperature up to 1050°C. Significantly, the value of refractive index obtained is dopant ionic size dependent. After annealing at 1050°C for 1 h, the undoped ceria films showed the highest value of 2.13 at 800 nm while the highest value obtained in La doped films was 1.97 at the same wavelength and temperature. The values for Nd, Gd and Yb were 1.94, 1.97 and 1.85, respectively at the same wavelength and temperature. Annealing at 1050°C for 16 h increased the refractive index further but reduced the transmittance in the short wavelength region (<500 nm) for all the materials. It has also been found that both the optical absorption and absorption edge are temperature dependent.

Variable optical absorption edge in ion beam sputtered thin ytterbium oxide films

Abstract Thin films of ytterbium oxide have been deposited by ion beam sputtering from the oxide target on to fused silica substrates. The films that were in the thickness range 50 to 150 nm were characterised for spectral transmittance and reflectance and the refractive index, extinction coefficient and optical absorption edge. It is demonstrated that all these quantities are extremely sensitive to variation in thickness of the films. The refractive index increases with thickness while the extinction coefficient decreases with increase in thickness. The thickest films (∼150 nm) have a refractive index of 1.75 at 600 nm which decreases to 1.55 for the 50 nm thick film. Most significantly the optical absorption edge varies from 4.85 to 4.63 eV over the same range of thickness. The observed behaviour has been fitted in to the single effective oscillator model and it is shown that this model predicts an increase in absorption edge with decreasing thickness. It is also shown, based on this model, that the variation in refractive index and absorption edge are inter-related. The dispersion energy, which is a measure of the strength of interband transitions increases with increase in thickness.

Processing and size effects on the optical properties of sputtered oxide thin films

Abstract The variation in refractive index, extinction coefficient and optical absorption edge with thickness of magnetron sputtered vanadium and niobium oxide films and ytterbium oxide films deposited by both magnetron sputtering (MS) and ion beam sputtering (IBS) has been investigated. The thickness of the films was varied in the range 50–400 nm. In all the cases it is shown that with increase in thickness the extinction coefficient and the absorption edge decreased, while the refractive index increased. Typically, for the MS ytterbia films the variation was between 1.6 and 1.75 whereas it was between 1.7 and 1.8 for the IBS films. The absorption edge varied from 4.5 to 4.7 eV for ytterbia MS thin films while it was between 4.6 and 4.8 eV for the IBS films. It is demonstrated that independent of the material, below a critical thickness the variations in refractive index, extinction coefficient and absorption edge are interrelated. It is therefore postulated that independent of the processing technique a material dependent critical thickness has to be achieved for the films to exhibit bulk like behaviour and the absorption edge and refractive index to become independent of each other.

Effect of thickness on the optical absorption edge of sputtered vanadium oxide films

Abstract A study of the optical absorption edge of dc magnetron sputtered vanadium oxide films is reported. It is shown that the absorption edge is strongly dependent on the thickness of the films. From the spectral transmittance characteristics and optical absorption edge behaviour it is demonstrated that there is a critical thickness (250 nm) above which the films become completely stoichiometric V 2 O 5 . At thickness greater than 250 nm the absorption edge is 2.24 eV corresponding to stoichiometric V 2 O 5 and below that the values range between 2.4 and 2.5 eV indicating the possible existence of non-stoichiometry in thinner films. The most significant result of the present study is to show that thickness of the films in conjunction with oxygen partial pressure can be used to modulate optical band gap of vanadium oxide films.

A study of nanocrystalline CeO2/PrOx optoionic thin films: temperature and oxygen vacancy dependence

Abstract The structure and optical constants of praseodymia doped ceria thin films are reported with and without oxygen annealing. These films were prepared using an inorganic sol-gel route and crystallized into the cubic fluorite structure well below 400°C, with cell constants altering as a function of dopant concentration and oxygen treatment/vacancy concentration. The films were also nanocrystalline, continuous and highly transparent in the region between 350–1500 nm. Optical transmission decreased with increased crystallite size due to scattering, below 600 nm. The refractive indices of the films at 800 nm in the dispersion free region were shown to vary in direct proportion to the lattice parameter/vacancy concentration and temperature treatment, and varied between 1.77 and 2.26. It is therefore proposed that they should be suitable materials for application as transparent ion intercalation films and optoionic smart windows.

Low temperature preparation of orthoferrite thin-films by an inorganic sol–gel process

Abstract Aqueous precursor sols of orthoferrites were prepared by room temperature processing of inexpensive commercially available metal salts. Crystalline thin-films of orthoferrites of general formula, LnFeO3(Ln=La, Nd, Sm, Gd, Dy, Er, Yb and Y) were prepared by dip-coating with the precursor sol and heating at 650°C. All the films showed >80% transmittance independent of the rare earth ion. The optical band-gap was 3.1 eV and the refractive index was 1.61±0.02 independent of the rare earth ion. The films exhibited uniformity in thickness, mechanical and thermal integrity upon heating.

Temperature and ionic size dependence on the structure and optical properties of nanocrystalline lanthanide doped zirconia thin films

Abstract Sols of lanthanide-doped zirconia with the general formula Zr0.90Ln0.10O2−x were prepared by an inorganic sol–gel method. The sols were characterized for particle size distribution, and the gels and heated gels were characterized for structure crystallite size and lattice parameter. Thin films of these materials were deposited on quartz substrates, and were crystalline, continuous and single phase as-deposited, with a cubic fluorite structure as shown by X-ray diffraction. The films were annealed to 600 and 1050°C after deposition and found to be transparent in the region between 400 and 1100 nm, the crystalline structure becoming tetragonal at 1050°C. The refractive index increased with increase in annealing temperature and decreased with ionic size of the dopant, varying between 1.53 for the largest cation, and 1.65 for the smallest cation at 600°C, and between 1.77 and 1.90 over the same range at 1050°C. Optical band gap calculations also showed a similar ionic size and temperature dependence with the optical band gap varying from 5.55 to 5.70 eV.

On the effects of optical constants and physical parameters on the emittance of thin-film selective emitters for thermophotovoltaic applications

A parametric study of the thermal radiative properties of thin-film selective emitters used in thermophotovoltaic applications has been performed. It has been found that the effective directional emittance depends strongly on the physical thickness, refractive index and spectral extinction coefficient of the films. The emissivity exhibits a saturation, for an optical depth greater than unity, which is dependent on the refractive index and extinction coefficient of the material, namely the higher the refractive index and extinction coefficient, the lower the saturation value. For transparent films, the thickness required to achieve saturation is of the order of tens of micrometres whereas much lower thicknesses are required for absorbing films. Significantly, an anomalous increase in emittance at low thickness for high-index films has been observed, which might be of particular interest in thermophotovoltaic applications. The emittance has also been found to be strongly dependent on the emissivity and reflectivity of the substrates and on the direction of propagation of radiation. The total hemispherical emittance of the films has been found to increase with increasing temperature and total directional emittance, for the wavelength range . The data presented provide a design model for the optimum thickness of a thin-film thermophotovoltaic emitter.