A.K. Pal

Bandgap and optical transitions in thin films from reflectance measurements

Abstract A new formulation and method are presented for evaluating bandgap, optical transitions and optical constants from the reflectance data for films deposited onto a non-absorbing substrate.

Studies on electron transport properties and the Burstein-Moss shift in indium-doped ZnO films

Abstract Electrical conductivity, Hall mobility, thermoelectric power and optical properties were studied for indium-doped zinc oxide films produced by the magnetron sputtering technique. The data were analysed in the light of the existing theories and it was observed that scattering due to neutral impurities together with optical phonons is operative in these films. The optical data indicated a distinct Burstein-Moss shift. The values of carrier concentration, plasma frequency and optical band gap were correlated with each other, indicating the effective mass of carriers to be equal to 0.35me.

Grain boundary scattering in aluminium-doped ZnO films

Abstract Electrical conductivity and Hall mobility of ZnO:Al films were measured in the temperature range 170–350 K. The effects of post-deposition heat treatment in vacuum and hydrogen atmosphere on the above properties were studied to derive meaningful information on the scattering mechanisms in these films. Grain boundary scattering was found to be the predominant mode of scattering at lower temperatures whereas the contribution from ionized impurities together with phonon scattering become significant at higher temperatures. The density Q t of trap states and its position was determined.

Synthesis of DLC films with different sp2/sp3 ratios and their hydrophobic behaviour

Diamond-like carbon (DLC) films were deposited on glass substrate by sputtering of a vitreous carbon target in Ar + H2 plasma. The sp2/sp3 content in the films depended on the relative amount of hydrogen in the Ar + H2 plasma. The films were characterized by Fourier transformed infrared studies, Raman studies, scanning electron microscopy, atomic force microscopy and optical measurements. Hydrophobicity in these films was studied by measuring the contact angles of the water droplets and it was found that the films were extremely hydrophobic. The results are interpreted in terms of hybridization of carbon in these DLC films.

Surface plasmon resonance in nanocrystalline silver particles embedded in SiO2 matrix

Nanocomposite films containing silver particles embedded in SiO2 matrix were prepared by high pressure (~40 Pa) d.c. sputtering technique in an argon plasma on fused silica substrate. Particle size and metal volume fraction were tailored by varying the substrate temperature (Ts~233-300 K) and deposition time (15-240 s). Reduction in size and volume fraction of metal particles culminated in blue-shift of the surface plasmon resonance peak in the optical absorbance spectra of the films. Absence of surface plasmon peak in the absorption spectra below a critical particle size and metal concentration of the nanocomposite films and appearance of sharp absorption edge in the absorbance spectra within the UV-VIS range indicated the semiconducting behaviour of the ultrafine silver particles. Experimental absorbance spectra were theoretically simulated by Mie scattering theory and Maxwell-Garnett effective medium theory.

Grain boundary scattering in CuInSe2 films

Electrical conductivity and Hall mobility of CuInSe2 films were measured in the temperature range of 77–400 K. The films were deposited with different Cu/In ratios ranging from 0.7–0.9 and at substrate temperatures of 620–720 K. Effects of grain boundary scattering on the electron transport properties were studied carefully and it was observed that scattering at the grain boundaries is a predominant factor controlling the electron transport properties at lower temperatures while complex scattering mechanisms become operative at higher temperatures. The energy values of trap levels and the densities of trap states were also obtained.

Nano-diamond films produced from CVD of camphor

Abstract Nano-diamond films were deposited by chemical vapour deposition (CVD) of camphor (C 10 H 16 O) and hydrogen (∼ 75 vol.%) on glass (at 523–623 K) and quartz/Si substrate (at 523–713 K). The films had low values of surface roughness (∼ 17 nm on quartz) along with high values of hardness and band gap. IR studies of the films deposited on Si substrate indicated the films to be transparent in the IR region. Optical properties of the films deposited on quartz (fused silica) and glass (Corning) substrates were studied. The absorption data in the below band gap region were utilized to estimate the strain (2–12 × 10 −3 ), stress (1.9–8.4 GPa) and hardness (47–62 GPa) in the films. The FTIR studies of the films did not show absorption around 2900 cm −1 , while the Raman spectra indicated a sharp peak (with FWHM ∼8 cm −1 ) at 1337 cm −1 corresponding to good quality diamond films.

Optical properties of nanocrystalline ZnS films prepared by high pressure magnetron sputtering

Abstract Nanocrystalline ZnS films with different thickness (10–40 nm) were deposited onto quartz and NaCl substrates by magnetron sputtering of a ZnS target in argon plasma. All the films showed a zinc blende structure and the photoluminescence peak positions depended on the surface to volume ratio of the films. The optical absorption in these films could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites.

Some aspects of surface roughness in polycrystalline thin films : optical constants and grain distribution

A modified formulation and new method have been suggested to derive meaningful information on the optical constants, grain size and grain distribution of polycrystalline thin films where surface roughness plays an important part through the contributions arising out of scattered transmission and diffuse reflection. It was observed that this method successfully described the experimental observations in polycrystalline CuInSe2 and CdSe films.

Effect of particle shape distribution on the surface plasmon resonance of Ag?SiO2 nanocomposite thin films

Silver?silica nanocomposite thin films were prepared by high pressure d.c.?sputtering technique. Films deposited at lower substrate temperature showed a narrow distribution of nanoparticles with nearly spherical shape. Increase in substrate temperature resulted in films with a non-uniform size and shape due to the agglomeration of the nanoparticles. This size and shape distribution has a profound effect on the optical absorbance spectra and results in a broad and asymmetric surface plasmon band. A?shape distribution introduced in the Maxwell?Garnett or Bruggeman effective medium theory was found to give a reasonable description of the experimentally observed optical absorption spectra.