V. S. Waman

Bio: V. S. Waman is an academic researcher from Savitribai Phule Pune University. The author has contributed to research in topics: Thin film & Raman spectroscopy. The author has an hindex of 10, co-authored 29 publications receiving 349 citations.

Determination of the optical parameters of a-Si:H thin films deposited by hot wire–chemical vapour deposition technique using transmission spectrum only

Abstract: Three demonstration samples of intrinsic hydrogenated amorphous silicon (a-Si:H) films were deposited using hot wire–chemical vapour deposition (HW–CVD) technique. The optical parameters and the thickness were determined from the extremes of the interference fringes of transmission spectrum in the range of 400–2500 nm using the envelope method. The calculated values of the refractive index (n) were fitted using the two-term Cauchy dispersion relation and the static refractive index values (n 0) obtained were 2.799, 2.629 and 3.043 which were in the range of the reported values. The calculated thicknesses for all samples were cross-checked with Taly-Step profilometer and found to be almost equal. Detailed analysis was carried out to obtain the optical band gap (E g) using Tauc’s method and the estimated values were 1.99, 2.01 and 1.75 eV. The optical band gap values were correlated with the hydrogen content (C H) in the samples calculated from Fourier transform infrared (FTIR) analysis. An attempt was made to apply Wemple–DiDomenico single-effective oscillator model to the a-Si:H samples to calculate the optical parameters. The optical band gap obtained by Tauc’s method and the static refractive index calculated from Cauchy fitting are in good agreement with those obtained by the single-effective oscillator model. The real and the imaginary parts of dielectric constant (e r, e i), and the optical conductivity (σ) were also calculated.

Hydrogenated Silicon Carbide Thin Films Prepared with High Deposition Rate by Hot Wire Chemical Vapor Deposition Method

Abstract: Structural, optical, and electrical properties of hydrogenated silicon carbide (SiC:H) films, deposited from silane (SiH4) and methane (CH4) gas mixture by HW-CVD method, were investigated. Film properties are carefully and systematically studied as function of deposition pressure which is varied between 200 mTorr and 500 mTorr. The deposition rate is found to be reasonably high (9.4 nm/s 15.54 nm/s). Formation of SiC:H films is confirmed by FTIR, Raman, and XPS analysis. XRD and Raman analysis revealed that with increasing deposition pressure amorphization occurs in SiC:H films. FTIR spectroscopy analysis shows that bond density of C–H decreases while Si–C and Si–H bond densities increase with increasing deposition pressure. Total hydrogen content increases with increasing deposition pressure and was found to be <20 at.%. The absence of band ~1300–1600 cm−1 in the Raman spectra implies negligible C–C bond concentration and formation of nearly stoichiometric SiC:H films. The band gap shows increasing trend with increasing deposition pressure. The high value of Urbach energy suggests increased structural disorder in SiC:H films. Finally, it has been concluded that CH4 can be used as effective carbon source in HW-CVD method to prepare stoichiometric SiC:H films.

Influence of RF power on structural, morphology, electrical, composition and optical properties of Al-doped ZnO films deposited by RF magnetron sputtering

Abstract: In this study, influence of RF power on the structural, morphology, electrical, composition and optical properties of Al-doped ZnO (ZnO:Al) films deposited by RF magnetron sputtering have been investigated. Films were systematically and carefully investigated by using variety of characterization techniques such as low angle X-ray diffraction, UV–visible spectroscopy, Raman spectroscopy, Hall measurement, X-ray photoelectron spectroscopy, field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy etc. Low angle X-ray diffraction analysis showed that the films are polycrystalline with hexagonal wurtzite structure and which was further confirmed by Raman spectroscopy analysis. Its preferred orientation shifts from (102) to (002) with increase in RF power. The average grain size was found in the range of 15–21 nm over the entire range of RF power studied. The FE-SEM analysis showed that grain size and surface roughness of ZnO:Al films increase in with increase in RF power. The UV–visible spectroscopy analysis revealed that all films exhibit transmittance >85 % in the visible region. The optical band gap increases from 3.37 to 3.85 eV when RF power increased from 75 to 225 W. Hall measurements showed that the minimum resistivity has been achieved for the film deposited at 200 W. The improvement in the electrical properties may attribute to increase in the carrier concentration and Hall mobility. Based on the experimental results, the RF power of 200 W appears to be an optimum sputtering power for the growth of ZnO:Al films. At this optimum sputtering power ZnO:Al films having minimum resistivity (8.61 × 10−4 Ω-cm), highly optically transparent (~87 %) were obtained at low substrate temperature (60 °C) at moderately high deposition rate (22.5 nm/min). These films can be suitable for the application in the flexible electronic devices such as TCO layer on LEDs, solar cells, TFT-LCDs and touch panels.

Large-area high-throughput synthesis of monolayer graphene sheet by Hot Filament Thermal Chemical Vapor Deposition

Abstract: We report hot filament thermal CVD (HFTCVD) as a new hybrid of hot filament and thermal CVD and demonstrate its feasibility by producing high quality large area strictly monolayer graphene films on Cu substrates. Gradient in gas composition and flow rate that arises due to smart placement of the substrate inside the Ta filament wound alumina tube accompanied by radical formation on Ta due to precracking coupled with substrate mediated physicochemical processes like diffusion, polymerization etc., led to graphene growth. We further confirmed our mechanistic hypothesis by depositing graphene on Ni and SiO(2)/Si substrates. HFTCVD can be further extended to dope graphene with various heteroatoms (H, N, and B, etc.,), combine with functional materials (diamond, carbon nanotubes etc.,) and can be extended to all other materials (Si, SiO(2), SiC etc.,) and processes (initiator polymerization, TFT processing) possible by HFCVD and thermal CVD.

Tuning the absorption of ultraviolet spectra and optical parameters of aluminum doped PVA based solid polymer composites

Abstract: In this research work PVA:xAl (0.01 ≤ x ≤ 0.05) based solid polymer composites have been prepared by solution cast technique. The transmittances of the samples are decreased with increasing Al powder concentration. The absorption spectra of the samples are shifted to higher wavelengths which indicate their importance for shielding. The increase of absorption intensity in UV region was observed. The clear shift of absorption edge upon the addition of Al powder content indicated the decrease of energy band gap. The increase of extinction coefficient at high wavelengths for the samples containing Al powder revealed the loss of photon energy. The refractive indexes of the doped samples are increased compared to that of pure PVA. The linear relationship between the refractive index and volume fraction indicates a good dispersion of Al powder in PVA host polymer. The direct optical band gap measured from the plot of (αhυ)2 versus photon energy (hυ) showed a decreasing trend with increase of Al content. The Urbach energy was found to increase with increasing aluminum concentration. The complex optical dielectric functions were used to determine the type of electron transition.

The one phonon raman spectrum in microcrystalline silicon

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.