Debajyoti Das

Properties of electron-beam-evaporated tin oxide films

Abstract Transparent conducting thin films of tin oxide were prepared by electron beam evaporation of sintered pellets of SnO 2 under controlled conditions. Variations in such parameters as the substrate temperature T s and the post-deposition annealing temperature T A and time t A were studied. Structural, electrical and optical properties were measured to characterize the films. The film structure changed gradually from amorphous to crystalline (SnO phase) as T S was varied from 150 to 350 °C. A sharp decrease in the room temperature resistivity together with the growth of a crystalline phase occured in the as-deposited films at T S ≈ 250°C. On annealing in air ( T A = 550° C , t A = 2 h ) a radical structural transformation from amorphous to crystalline occurred with a sharp fall in resistivity for T S ⩽ 225°C. For T S = 350° C the lowest resistivity achieved for the undoped annealed films was 6.6×10 -3 Ω cm, the average visible transmittance was about 90% and the structure was characteristic of pure SnO 2 .

SiC-capped nanotip arrays for field emission with ultralow turn-on field

Silicon nanotips with tip diameter and height measuring 1 nm and 1 μm, respectively, and density in the range of 109–3×1011 cm−2, were fabricated monolithically from silicon wafers by electron cyclotron resonance plasma etching technique at a temperature of 200 °C. Field emission current densities of 3.0 mA/cm2 at an applied field of ∼1.0 V/μm was obtained from these silicon nanotips. High-resolution transmission electron microscope and Auger electron spectroscopy analyses concluded that the nanotips are composed of monolithic silicon and nanometer-size SiC cap at the top. A 0.35 V/μm turn-on field to draw a 10 μA/cm2 current density was demonstrated, which is much lower than other reported materials. The excellent field emission property demonstrated by these nanotips, which were fabricated by a process integrable to the existing silicon device technology at low temperatures, is a step forward in achieving low-power field emission displays and vacuum electronic devices.

Photoluminescence phenomena prevailing in c-axis oriented intrinsic ZnO thin films prepared by RF magnetron sputtering

Substantial c-axis orientation of the hexagonal ZnO crystals with wurtzite structure demonstrates only those two preferred peaks in the first-order spectra which are permitted by the Raman scattering selection rule viz., the Ehigh2 and A1 (LO) modes, that identify the improved structural quality of the undoped ZnO film grown by magnetron sputtering in Ar ambient at an RF power of P = 200 W. The presence of a substantial amount of hydroxyl groups attached to the Zn lattice has been correlated to the dominant c-axis orientation of the ZnO crystals which exhibited a distinct UV luminescence band that arises due to the typical exciton emission or near-band-edge emission. At higher applied powers, disorder-activated Raman scattering introduces a well resolved Bhigh1 mode and gradually growing second order Raman peaks, (Ehigh2 − Elow2) and (Bhigh1 − Blow1), which are caused by the breakdown of translational symmetry of the lattice by defects or impurities and lead to deviation from preferred c-axis orientation with I002/I103 < 1. Out diffusion of oxygen from the network creates increasing oxygen vacancy states and in addition, various other defects e.g., Zn interstitial , doubly ionized Zn vacancy and oxygen antisite (OZn) as the dynamic acceptor defects, act as the origins of different visible photoluminescence components classified in the UV-violet, violet, violet-blue, blue and green regions.

Nanocrystalline silicon films prepared from silane plasma in RF-PECVD, using helium dilution without hydrogen : structural and optical characterization

The effect of RF power on the nanocrystallization of a Si:H network has been studied by PECVD at a substrate temperature of 200??C and a gas pressure of 0.5?Torr, using silane as the source gas and helium as diluent, without using hydrogen. Optical characterization of the films has been done by UV?vis spectroscopy. Structural characterization has been performed by infrared absorption, x-ray diffraction, micro-Raman studies and electron microscopy by HRTEM and FESEM. In general, a structural transformation from the amorphous to nanocrystalline phase accomplished by metastable helium atoms in the plasma has been identified at a low RF power of 80?W. With an increase in the applied RF power up to 150?W, systematic improvement in crystallinity has been shown as depicted by increased crystalline volume fraction (~77%), smaller grain size (~7?nm) reduced bonded hydrogen content (~8?at.%), enhanced polymerization in the network and gradual widening in the optical gap (~1.86?eV) obtained at a high deposition rate (107???min?1), using 1?sccm of silane as the source gas and helium as the only diluent. He dilution of the SiH4 plasma is a well-proven approach to increase the growth rate. However, a nanocrystalline network with high crystalline volume fraction and well-aligned crystallographic lattice distribution, attained in Si:H from a low-power RF plasma at a growth temperature as low as 200??C, and that obtained from purely He dilution, without using H2, is being reported for the first time. The striking feature comprises that nc-Si:H films of increasing crystalline volume fraction, reduced bonded hydrogen content and wider optical gap are produced with simultaneously increasing deposition rates, which deserves extensive technological impact.

Tailoring of room temperature excitonic luminescence in sol-gel zinc oxide-silica nanocomposite films

Abstract Zinc oxide–silica (ZnO/SiO 2 =20:80, molar ratio) nanocomposites consisting of ZnO nanoparticles embedded in a dielectric matrix were prepared by a sol–gel technique (spin coating). Optical transmittance, Raman effect and photoluminescence measurements of the composites indicated effective capping of the ZnO nanoparticles (radii 1.4–1.5 nm) in the host showing practically no variation of particle size with the post-deposition-annealing treatments. The blue shift of the band gap (4.23–4.29 eV) from that of bulk ZnO (3.3 eV) indicated strong carrier confinement for samples annealed at T ≤773 K. Highly intense UV emission (approx. 4.2 eV) at room temperature could be obtained by annealing the composites in static oxygen atmosphere, while the visible defect-related luminescence (approx. 2.62 and 2.43 eV) could be reduced, resulting in a high intensity ratio (approx. 27) of the two.

Properties of tin oxide films prepared by reactive electron beam evaporation

Tin oxide films were prepared by electron beam evaporation of pellets of Specpure SnO2 in the presence of added oxygen. By optimizing the deposition conditions, transparent and conducting tin oxide films exhibiting the structural characteristics of a predominant SnO2 phase were produced. The effect of annealing the films in air was also studied. The lowest resistivity obtained was 7.5 × 10−4 Ω cm, with a visible transmittance of over 90%. The properties studied to characterize the films were (1) structure by X-ray diffraction and transmission electron microscopy; (2) resistivity, Hall mobility and carrier concentration; (3) optical transmission and band gap. Films were also subjected to reduction tests by exposure to a hydrogen plasma to determine their suitability as electrodes for hydrogenated amorphous silicon solar cells.

Characterization of the Si:H network during transformation from amorphous to micro- and nanocrystalline structures

Optical, structural, and electrical characterizations of the Si:H network have been performed during its changes occurred by the increasing H2 dilution to the SiH4 ensemble in hot-wire chemical vapor deposition (HWCVD). A rapid structural transformation from a mostly amorphous phase to comprehensive micro/nanocrystallinity was attained at a relatively low H2 dilution, even at a low substrate temperature, because of the associated abundance of atomic H in HWCVD. However, elevated H2 dilution induces enormous polyhydrogenation and formation of lesser dense network full of voids, mostly around the tiny micro/nanocrystallites. This highly defective grain boundary zone provides high density of electronic trapping centers and contributes significantly to the transport of carriers. The overall electrical transport in the amorphous-micro-nano-crystalline heterogeneous Si:H network has been accounted in the framework of a three-phase model comprised of amorphous and crystalline (micro- and nano-) components while ...

Characterization of tin doped indium oxide films prepared by electron beam evaporation

Abstract Tin doped indium oxide (ITO) films have been prepared by electron beam evaporation of hot pressed powder of 90% In 2 O 3 10% SnO 2 by weight. The parameters varied for optimization of film properties have been the substrate temperature and the partial pressure of the oxygen added. Properties which have been studied for characterization are the resistivity, Hall effect, transmittance and optical band gap. The structural studies have been made by X-ray diffraction and transmission electron microscopy. D.c. resistivity in the range 10 −3 −10 −4 Ω cm and visible transmittance in excess of 90% have been obtained for the films, with proper parametric adjustments. A 〈111〉 texture has been generally exhibited by the ITO films, using X-ray diffraction. This has been corroborated by electron diffraction studies.

Characterization of undoped μc-SiO:H films prepared from (SiH4+CO2+H2)-plasma in RF glow discharge

Undoped hydrogenated microcrystalline silicon oxygen alloy films (μc-SiO:H) have been prepared from (SiH4+CO2+H2)-plasma in RF glow discharge at a high H2 dilution, moderately high RF power and substrate temperature. A detailed characterization of the films has been done by electrical, optical as well as structural studies, e.g., IR absorption spectroscopy, Raman scattering and transmission electron microscopy. The presence of a very small amount of oxygen induces the crystallization process, which fails to sustain at a higher oxygen dilution. At higher deposition temperature and in improved μc-network H content reduces, however, O incorporation is favoured. Sharp crystallographic rings in the electron diffraction pattern identify several definite planes of c-Si and no such crystal planes from c-SiOX is detected.

Infrared vibrational spectra of hydrogenated amorphous silicon carbide thin films prepared by glow discharge

Abstract The IR vibrational spectra of a-SiC:H films prepared by rf glow discharge decompositon of a mixture of silane and methane gases have been studied. The films have been prepared under different conditions which include variation of methane concentration in the gas mixture, rf power and substrate temperature. The structures of the a-SiC:H films depend very sensitively on the deposition parameters. The spectra of p-type a-SiC:H films prepared with boron doping have also been studied. The effect of annealing on the IR spectra of the films has been investigated. Attempts have been made to analyse all these data in order to obtain information about different bonded configurations in the films.