S. A. Agnihotry

Studies on e-beam deposited transparent conductive films of In2O3:Sn at moderate substrate temperatures

An electron beam evaporation method is used to prepare In2O3 films with and without Sn doping. It is shown that highly transparent and conducting films can be prepared at substrate temperature as low as 200 degrees C. The characteristic feature of such films is their high carrier density and high infrared reflectivity. The lowest resistivity is found to be 2.4*10-4 Omega cm with a carrier concentration of 8*1020 cm-3 and mobility of about 30 cm2 V-1 s-1 at the doping level of 4 mol.% SnO2. These polycrystalline films show a highly preferred orientation. On the basis of Hall measurements and structural data, sources of scattering in these films are suggested.

Electrical properties and morphology of obliquely deposited electrochromic WO3 films

Abstract A thin film of WO 3 deposited obliquely (θ≈50°) improves the electro-optical response times of an electrochromic display device based on a proton insertion-extraction mechanism. The difference in the response times is attributed to morphological differences as revealed by electron microscope observations. WO 3 films were deposited normally and obliquely, by electron beam gun deposition technique so as to have comparable resistivities in vacuum of about 10 -4 Torr. The resistivity of these films was varied in different ways by exposure to atmospheric air in a controlled manner. Systematic and comparative investigations of the variation in resistivity of these films showed that the resistivity increased in the first stage (10 -4 Torr to atmospheric pressure), attained a maximum value and decreased thereafter in the second stage on long exposure to the atmosphere. The initial increase was explained as the result of oxidation leading to better stoichiometry, limited by the process of uptake of water vapour (hydration). The latter effect was much less for normally deposited film; however, for obliquely deposited film it was much greater and resulted in a decrease in resistivity by three orders of magnitude more than for normally deposited film. The large degree of hydration in an obliquely deposited film was ascribed to its more porous nature, leading to faster electro-optical response times. It was concluded that the microstructure, the stoichiometry and the water content control the electro-optical response of WO 3 films.

The preparation of an enhanced-Tc superconducting Tl2Ba2CuOx phase by using low Tl concentrations

Calcium-free high-Tc superconducting compounds of the Tl2Ba2Can-1CunOx series have been fabricated with an enhanced onset critical temperature, Tcon approximately=97.2 K. The optimisation of parameters has revealed that the choice of nominal composition plays an important role in the formation of the final phase. While a nominal 2201 composition did not yield a high-Tc phase, compounds with low thallium concentration (1201) gave a 2201 high-Tc phase. XRD studies confirm the formation of a 2201 phase from 1201 and 1/2201 phases with a approximately=3.86 AA and c approximately=23.23 AA. It has also been found that a high sintering temperature of 97 degrees C maintained for a short duration of three to five minutes is favourable for the enhancement of Tc.

The Enhancement of Tc in Calcium Free Tl-Bilayer Superconducting System (Tl2Ba2Cu1Ox)

The observation of enhanced Tc in calcium free Tl compounds Tl2Ba2Cu1Ox (2201) with varying concentration of Tl and Cu has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under the optimised conditions: sintering temperature 970°C and duration 3–10 minutes the highest Tc (onset) ~115 K and Tc (zero) ~95 K was found. The XRD studies showed the transformation of all the three nominal compositions into 2201 phase with different Tcs.

Lead-doped electron-beam-deposited BiSrCaCuO superconducting thin films☆

Abstract Superconducting thin films of the lead-doped BiSrCaCuO system have been prepared on (100) single-crystal SrTiO 3 substrates by an electron beam deposition technique using a single sintered pellet as the evaporation source. As-deposited films are amorphous and non-superconducting; post-deposition annealing at an optimized temperature in air has been found to result in crystalline and superconducting films. The superconducting characteristics of the films have been observed to be sensitive not only to the duration and temperature of post-deposition annealing but also to the lead content and the sintering parameters for the pellet to be used as the evaporation source. A pellet with nominal composition Bi 3 Pb 1 Sr 3 Ca 3 Cu 4 O y that had been sintered for 200 h zero resistivity at T c 0 = 112 K . However, films deposited using such a pellet as the evaporation source had T c 0 ≈ 73–78 K, as had the films deposited from a pellet without any lead. We investigated systemativally films deposited from pellets with more lead and sintered for different durations. It is evident from these investigations that pellets with nominal composition Bi 3 Pb 2 Sr 3 Ca 3 Cu 4 O y , i.e. with an excess of lead, and sintered for about 75 h when used as the evaporation source yield films with T c 0 ≈ 100 K when annealed between 835 and 840 °C for an optimized long duration. The films are characterized by X-ray diffraction and energy-dispersive spectroscopy techniques and have been found to be highly c axis oriented. The effect of lead in promoting a high T c 0 = 110 K phase seems to be similar to that in bulk ceramics.

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedTc in calcium-free Tl compounds of the series TlmBa2Can−1CunOx (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestTc(onset) ranges from 103 K to ∼ 115 K andTc (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentTcs.

Application of tantalum based thin film devices in active matrix addressed liquid crystal displays

A display that makes use of an electronic switch at each picture element (pixel) to control the display media is an “active matrix display”. Examples of switches are thin film field-effect transistors (TFTs) and metal-insulator-metal (MIM) non-linear thin film devices. MIMs have sufficiently high stability and controllability of characteristics, together with simpler fabrication processes than TFTs. The degree of non-linearity is optimized by critically controlling the process parameters of oxide formation. I–V characteristics of anodically formed tantalum oxide films are of the Poole-Frenkel type in the high-field region. This has been confirmed by temperature and thickness dependence of conductivity. The field-dependent conductivity, established by studies of current variation with temperature, indicates a barrier of about 1.44 eV. The value of the non-linearity coefficient (β), as obtained from the slope of the log (I/V) vs. V12 plot, ranges between 2 and 4 which seems to be adequate to activate liquid crystal pixels.