Ram P. Gupta

Y‐Ba‐Cu‐O superconducting film on oxidized silicon

We report thick superconducting films of Y‐Ba‐Cu‐O on oxidized silicon substrates. The critical temperatures for onset and zero resistance are 96 and 77 K, respectively. X‐ray diffraction analysis predicts 1, 2, 3 composition and orthorhombic phase of the film.

YBCO-FET room temperature ammonia sensor

Abstract A FET-ammonia sensor operating at room temperature is reported in this paper. The sensor employs a thin film of semiconducting Y:Ba:Cu:O (YBCO) compound, commonly known as 1-2-3 high temperature superconducting cuprate, as its sensing element. We observed that this material is highly and selectively sensitive to ammonia at and below room temperature. The measurements indicate that the non-amplified signal of the sensor is more than 10 mV for 5 ppm of ammonia. A typical rise time of 24 s and fall time of 250 s is measured at room temperature. The measurements on temperature dependence of sensitivity evince that the sensor signal increases if the operating temperature of the sensor is decreased. The sensor is selectively sensitive to ammonia only and exhibits a non-sensor like behaviour to hydrogen and hydrocarbon based gases. Although the exact mechanisms of gas-sensing properties of cuprates are not known, nevertheless, we tried to explain the observed sensor response to ammonia on the basis of physisorption. The demonstrative results reported here suggest that a room temperature FET ammonia sensor can be realized using semiconducting YBCO as a sensing material for the development of silicon-based integrated smart sensors.

Influence of oxidation temperature, film thickness and substrate on NO2 sensing of SnO2 ultra thin films

Thin SnO2 films of different thickness were fabricated by sputtering of tin followed by thermal oxidation. The oxidation was performed at 600, 700 and 800 8C and silicon and silicon dioxide were chosen as substrates for their compatibility to silicon CMOS technology. Work function measurements were carried out on these films upon exposure to NO2 in the concentration range 5‐100 ppm at a particular temperature under dry and humid conditions using a commercial Kelvin probe measurement set-up. Kelvin probe measures contact potential difference (CPD) resulting from change in work function of a film material on exposure to a gas. The dependence of NO2 sensitivity of films on oxidation temperature, film thickness and substrate was investigated. The influence of temperature on NO2 response from 30 to 130 8C was investigated. It has been observed that humidity has negligible effect at 130 8C. It was further observed that response of SnO2 film on silicon to NO2 increases with oxidation temperature of the film whereas for the films grown on silicon dioxide the response was parabolic. In the later case, increase in signal was exhibited by the films oxidized up to 700 8C while it was found to decrease for films oxidized at 800 8C. The films of three different thickness (10, 35 and 55 nm) prepared at 800 8C on silicon showed negligible variation in sensitivity. However, sensitivity of the films grown on silicon dioxide is observed to be thickness dependent. The films were characterized using atomic force microscopy (AFM) and Auger electron spectroscopy (AES) to study the surface morphology and depth profile respectively. The observed results are discussed to understand the NO2 sensing behavior of ultra thin SnO2 films. # 2002 Elsevier Science B.V. All rights reserved.

New gas sensing properties of high Tc cuprates

Abstract We have discovered an interesting property of high T c cuprates, which are popularly known as high temperature superconductors, for gas sensing applications. Metallic films of YBCO and BSCCO compounds are examined for gas-sensing properties by incorporating them in a FET-sensor structure. These multi-component ceramic oxides exhibited room temperature gas detection capability in comparison to conventional metal-oxides, which are sensitive to gases only at elevated temperatures. The sensor responses to NH 3, H 2, CO, CO 2, NO, NO 2, and hydrocarbon gases are presented and discussed. The results evince that thin film of high T c cuprates are highly sensitive to NH 3 and NO 2 at and around room temperature and less sensitive to other gases. These films, therefore, can be used to realize a room temperature low power FET-gas sensor.

Fluorine-implanted bismuth oxide superconductors

We report for the first time the influence of fluorine implantation in the Bi‐Sr‐Ca‐Cu‐O system. The incorporation of fluorine was found to enhance the zero resistance temperature and to sharpen the high‐temperature transition. The fluorinated sample exhibited Tc (R=0)=89 K and the high Tc phase of the composition had Tc (on) as high as 124 K with an extrapolated Tc (end) of 114 K. Resistance temperature characteristics of a sample before and after fluorine implantation are presented.

A new approach for the preparation of in situ superconducting BSCCO films

The paper reports a new approach, based on low-pressure on-axis sputter deposition followed by high-pressure plasma treatment (OPT) for the preparation of in situ superconducting Bi-Sr-Ca-Cu-O (BSCCO) films with a high-Tc phase. It was observed that OPT is essential to lower the room-temperature resistance and to achieve superconduction in the films.

Electrical transport studies in nanocrystalline CdSe/SiO2 composite films

Abstract SiO2/CdSe/SiO2 composite films in nanocrystalline form were deposited on quartz substrates at ∼20 Pa with deposition temperatures ranging from 220 to 240 K using a multi-target magnetron sputtering system. Optical, electrical, and microstructural (TEM and AFM) studies were carried out on the composite films. Studies of the variation of conductivity with temperature indicated Efros hopping within the Coulomb gap to be the predominant carrier transport process in the composite films. Furthermore, a crossover from a ‘soft’ to a ‘hard’ Coulomb gap was noticed with lowering of temperature.

Scanning tunneling microscopy of Si/SiO2 interface roughness and its dependence on growth conditions

The interface between silicon (100) and thermal silicon dioxide grown by wet, dry, and trichloroethylene oxidation has been investigated by scanning tunneling microscopy and scanning tunneling spectroscopy. The scanning tunneling microscopy images of the silicon surface, after removal of oxide, reveal the presence of silicon bumps (protrusions) in samples prepared by wet and dry oxidation while no protrusions are seen at the interface of trichloroethylene oxidized samples. The spectroscopic measurements predict that these are silicon protrusions and are produced by oxide growth conditions. X‐ray photon spectroscopy on samples containing protrusions also supports the above prediction. Thus, our study suggests that roughness of the silicon–silicon dioxide interface depends on oxide growth conditions and a relatively smooth interface is obtained by tricholoroethylene oxidation.

The effect of sputter deposition conditions on the growth mechanism of YBa2Cu3O7- delta thin films studied by scanning tunnelling microscopy

Surface topography of as-grown films of YBa2Cu3O7- delta (YBCO) deposited by off-axis RF magnetron sputtering on MgO and SrTiO3 single-crystal (100) substrates, has been investigated by scanning tunnelling microscopy (STM)/scanning tunnelling spectroscopy (STS) operated in air at ambient temperature. Both c-axis- and a-axis-oriented YBCO films have been investigated. In the case of c axis thin films, we have directly observed spiral-shaped growth terraces, which emanate from screw dislocations of this layered superconducting oxide. The growth steps are generally seen to possess a step height close to or multiples of the unit cell height of the YBCO crystalline structure. In the case of a axis growth, the substrate as well as the deposition temperature determines the grain orientation. Furthermore, our STS data revealed that the surface layer of the film is semiconducting and the tunnelling spectrum varies its shape depending on the tip-to-sample distance. A superconducting gap appears clearly in the spectra when the STM tip is placed closer to the surface than the normal position of the scanning mode. This suggests that the semiconducting layer is confined in the topmost surface region of the as-prepared film, while the layer beneath it is superconducting in character.

High Temperature Stable Metal Contacts On GaAs Based On WSi2 As Diffusion Barrier, Characterized By XPS And Electrical Measurements

A systematic investigation of high temperature stable metal contacts on GaAs using WSi2 as a diffusion barrier between the Ge and the Au metallization of a GaAs-Ge-WSi2-Au contact is presented. The effects of temperature stressing up to 610°C regarding the contact composition were characterized by XPS sputter profil-ing techniques and supported by electrical measurements of the contact parameters. It is shown that the con-tact system remains stable for long-term operation conditions at 350°C (e.g. 2oo hours). At much higher temperatures (610°C) interdiffusion between WSi2-Au and WSi2-Ge together with an additional diffusion of Ga into the metallization constitutes the lifetime limiting mechanism of such contacts.