S. Tripura Sundari

Investigation of temperature dependent dielectric constant of a sputtered TiN thin film by spectroscopic ellipsometry

The temperature dependence of optical constants of titanium nitride thin film is investigated using Spectroscopic Ellipsometry (SE) between 1.4 and 5 eV in the temperature range of 300 K to 650 K in steps of 50 K. The real and imaginary parts of the dielectric functions e1(E) and e2(E) marginally increase with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillator model are carried out to describe the temperature behavior. With increase in temperature, the unscreened plasma frequency and broadening marginally decreased and increased, respectively. The parameters of the Lorentz oscillator model also showed that the relaxation time decreased with temperature while the oscillator energies increased. This study shows that owing to the marginal change in the refractive index with temperature, titanium nitride can be employed for surface plasmon sensor applications even in environments where rise in temperature is imminent.

Temperature dependent optical properties of silver from spectroscopic ellipsometry and density functional theory calculations

A detailed analysis of temperature dependence of optical constants of silver thin film measured by spectroscopic ellipsometry in the energy range 1.4 to 5 eV for temperatures ranging from 300 K to 650 K in steps of 50 K is presented in this paper. A microstructure modeling based on Bruggeman effective medium approximation is carried out on the temperature dependent optical constants. A red shift of ∼300 meV accompanied by a broadening of the interband transition is observed and an additional weak absorption edge emerges from the lower energy side of main interband absorption transition at 550 K. Drude model is applied to perform the analysis of dielectric function in the low energy region. The energy loss function −Im(1/e) shows large damping with increasing temperature and this is a consequence of increasing magnitude of e2 resulting from the composite transition in the region where e1 is very small. These experimental results are corroborated with first principles electronic structure calculations using...

Nanomechanical and optical properties of highly a-axis oriented AlN films

This paper reports optical and nanomechanical properties of predominantly a-axis oriented AlN thin films. These films were deposited by reactive DC magnetron sputtering technique at an optimal target to substrate distance of 180 mm. X-ray rocking curve (FWHM = 52 arcsec) studies confirmed the preferred orientation. Spectroscopic ellipsometry revealed a refractive index of 1.93 at a wavelength of 546 nm. The hardness and elastic modulus of these films were 17 and 190 GPa, respectively, which are much higher than those reported earlier can be useful for piezoelectric films in bulk acoustic wave resonators

Optical properties of CdTe nanoparticle thin films studied by spectroscopic ellipsometry

Thin films of CdTe nanoparticles were prepared by carrier gas evaporation technique in Ar partial pressures in the range of 10?6?10?2?mbar. The size dependent complex dielectric spectra (?(E) = ?1(E)+i?2(E)) of these cluster-deposited nanoparticle thin films were investigated in the energy range 0.6?5?eV with a spectroscopic ellipsometer. The influence of Ar partial pressure on the particle size was estimated from the shift in the fundamental absorption edge and corroborated through x-ray diffraction data. The size of the particles ranged from 18 to 13?nm and showed a systematic decrease with increase in the Ar carrier gas partial pressure (10?6?10?2?mbar). The average microstructural changes such as percentage of voids and extent of disorder occurring as a function of average particle size were analysed using a multilayer model with effective medium approximation. The correlation between the microstructure and the optical response was studied by evaluating standard sum rules. The average number of electrons per atom participating in the optical transition from valence band to conduction band decreased with reduction in particle size. Also the average energy gap, i.e. the splitting between the bonding and anti-bonding orbitals decreased as the partial pressure of Ar increased indicating the increase in surface states due to decrease in particle size. Increase in average bond-length and a decrease in the average co-ordination number were also observed in the specimens as a function of Ar partial pressure.

Disorder effects on dielectric response of Si irradiated with Ar

Irradiation induced changes in the complex dielectric spectra of single crystal Si are investigated with spectroscopic ellipsometry. The analysis of microstructural variations resulting from ion irradiation is carried out to establish the existence of a well defined amorphization threshold. Increase in fluence leads to amorphization of the crystal resulting in an order–disorder transition as inferred from peak to peak height of the first derivative spectra. The amorphous volume fraction deduced from a microstructure model using effective medium approximation is found to have a power law behavior. The variations in the critical point (CP) structure in the energy range 1.5 to 5 eV is studied as a function of fluence ranging from 6×1013 to 1×1017 Ar+ ions cm−2. The changes in the nature of the CPs is examined in detail by line shape analysis of the second derivative of the optical spectra above and below the amorphization threshold. Irradiation induced structural disorder and its effect on the interband opti...

Origin of bending in uncoated microcantilever - Surface topography?

We provide direct experimental evidence to show that difference in surface topography on opposite sides of an uncoated microcantilever induces bending, upon exposure to water molecules. Examination on opposite sides of the microcantilever by atomic force microscopy reveals the presence of localized surface features on one side, which renders the induced stress non-uniform. Further, the root mean square inclination angle characterizing the surface topography shows a difference of 73° between the opposite sides. The absence of deflection in another uncoated microcantilever having similar surface topography confirms that in former microcantilever bending is indeed induced by differences in surface topography.

Effect of surface stress on microcantilever resonance frequency during water adsorption: influence of microcantilever dimensions

This paper reports the effect of dimensions of microcantilever (MC) on its resonance frequency and bending upon adsorption of water molecules. Study is conducted on three MCs having the dimensions of 450 × 40 × 2.5 μm(3) (MC1), 225 × 30 × 3 μm(3) (MC2) and 125 × 35 × 4.5 μm(3) (MC3). The measured resonant frequency showed the expected negative shift in MC1, initially positive followed by a negative shift in MC2 and only positive shift in MC3 during adsorption. This behavior is attributed to changes in the stiffness of the MC associated with the surface stress. The surface stress generated on the MC has been derived from its bending measurements upon water adsorption. The change in the stiffness of MC evaluated from an independent estimate of expected frequency shift showed that the relative stiffness change of MC increases linearly with the surface stress scaled with cube of width to height ratio of MCs, confirming the dimensional dependence of adsorption induced stiffness change.

Reactive Pulsed Laser Deposition of Titanium Nitride Thin Films: Effect of Reactive Gas Pressure on the Structure, Composition, and Properties

Titanium nitride (TiN) thin films were deposited by reactive pulsed laser deposition (RPLD) technique. For the first time, the composition evaluated from proton elastic backscattering spectrometry, in a quantitative manner, revealed a dependence on the partial pressure of nitrogen from 1 to 10 Pa. Grazing incidence-XRD (GI-XRD) confirmed the formation of predominantly nanocrystalline TiN phase with a crystallite size of around 30 nm. The hardness showed maximum value of ~30 GPa when the composition is near stoichiometric and the friction coefficient was found to be as low as 0.3. In addition, a systematic optical response was observed as a function of deposition pressure from the surface of the TiN films using spectroscopic ellipsometry.

Evolution of temperature dependent properties of oriented YBaCo4O7 thin films

Bulk YBaCo4O7 was synthesized using a ceramic route in air. Thin films of this compound were grown on (100) oriented LaAlO3 substrate using a Pulsed Laser Deposition technique. X-ray diffraction results of thin film indicate that the film has (110) orientation. Electrical resistivity measurements, in 300K to 100K temperature range showed that the film is insulating, similar to that of the bulk. Spectroscopic ellipsometric measurements on thin films in the energy range 1.5 to 5 eV and at temperatures from 300 K to 400 K showed that the direct band gap s 2.45 eV at ambient temperature while its value increases with increase in temperature. The increase in band gap with temperature is attributed to the Moss-Burstein effect.

Phase selective gas sensing properties of nanostructured TiO 2 thin films

TiO2 thin films were prepared by r.f. magnetron sputtering and spray pyrolysis. Films were grown on Si (100) and interdigitated gold on alumina substrates. The film morphology and roughness were determined using atomic force microscopy. The films obtained by r.f. sputtering were smoother with rms surface roughness of about 0.8 nm (over a 5µm × 5 µm scan area) whereas the rms roughness of the films prepared by spray pyrolysis was around 3.9 nm (over 5 µm × 5 µm scan area). The crystal structure was determined using glancing incidence x-ray diffraction and it was observed that the r.f. sputtered films consisted of rutile phase while the films obtained by spray pyrolysis were anatase phase. The optical parameters namely, refractive index (n) and extinction coefficient (k) were estimated using a variable angle spectroscopic ellipsometry. Gas sensing studies with ammonia revealed that rutile phase is sensitive while the anatase phase showed no response. A correlation is given between optical, sensing properties and microstructure.