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Journal ArticleDOI

Time dependent electrical resistance of Bi2(Te0.4Se0.6)3 thin films in vacuum and on exposure to atmosphere

13 Nov 1998-Solid State Communications (Pergamon)-Vol. 108, Iss: 11, pp 873-877

AbstractThin films of different thicknesses have been vacuum (2×10 −5  torr) deposited by the flash evaporation technique onto glass plates held at room temperature. In situ measurement of electrical resistance in vacuum (2×10 −5  torr) as a function of time was carried out until the resistance stabilized. Then, on exposure to air by controlled admittance of air into the vacuum chamber, it was found that the resistance of the films increased as pressure increased. After air was admitted up to atmospheric pressure, the resistance was also found to increase as a function of time in the atmosphere. Also, it was observed that the resistance of the film during re-evacuation does not retrace the previous path during the exposure of the films and thus the resistance variation is not reversible. The variation of resistance taking place in the films in vacuum with time is attributed to the coalescence process. The agglomeration rates for the films of different thicknesses were obtained from the linear behavior of ln( R / R 0 ) vs time plot. The resistance variation of the films during the exposure to air is attributed to oxygen adsorption. It was found that the annealed films are less influenced by the exposure compared to the as-grown films. Adsorption of oxygen takes place on the external and internal surfaces of the films. The films were found to be ESR active. Structural analysis was carried out using the XRD and TEM techniques.

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Citations
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Journal ArticleDOI
Abstract: Optical, structural, morphological and photoelectrochemical properties were investigated of ternary MoBi 2 Se 5 thin film prepared by simple arrested precipitation technique (APT). The precursors used were molybdenum, bismuth, triethanolamine complexes (TEA) along with organic additives. Ammonium molybdate, Bismuth nitrate and sodium selenosulphite were used as sources of Mo 4+ , Bi 3+ and Se 2− ions. The optical band gap of thin film was estimated to be 1.78 eV. X-ray diffraction data reveals that the grown MoBi 2 Se 5 thin film was highly nanocrystalline with orthorhombic structure. Scanning electron microscopy studies reveal that porous layer having elongated fibrous morphology with high surface area. The film was obtained with a well-defined composition, very close to the expected one. PEC application of prepared thin film were checked in Sulphide / Poly sulphide electrolyte which revealed that MoBi 2 Se 5 thin film deposited on FTO coated glass exhibited maximum values of fill factor (FF) and conversion efficiency (η) with n type semiconductor nature.

15 citations

Journal ArticleDOI
Abstract: : This work is to present results of flash evaporation of PbTe directly over single crystals p-type Si substrates, in order to produce heterojunction infrared detectors (HIRD), working at 4.3 micrometers IR wavelength. The evaporation was performed on modified JEOL vacuum equipment, model JEE4B (a), working with vacuum pressure around 10(exp -5) torr, using diffusion pump. The HIRDs produced with this method presented the same detectivity (D*) values of HIRDs made with Hot Wall Epitaxial System (HWE) (b), in which PbTe epitaxial layers were grown directly over the same Si substrates, where an ionic pump reached about 10(exp -9) torr as vacuum pressure. The best results. were obtained with PbTe layers grown with Molecular Beam Epitaxial (MBE) method (c) directly over Si substrates, where the vacuum pressure is around 10(exp -9) torr, also using an ionic pump. The advantage of growing PbTe directly over Si wafers is that the HIRDs perform at room temperature. The detectivity values of HIRDs obtained with methods (a) and (b), were D* approx. 4, 8 x 10(exp 5) cm.Hz(sup 1/2)/W and with method (c), D* approx. 6, 7 x 10(exp 6) cm.Hz(sup 1/2/W. Different technologies: (a) very low costs, (c) high technology; not very different results.

References
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Journal ArticleDOI
Abstract: The field of thermoelectric energy conversion is reviewed from both a theoretical and an experimental standpoint. The basic theory is introduced and the thermodynamic and solid state views are compared. An overview of the development of thermoelectric materials is presented with particular emphasis being placed on the most recent developments in high-temperature semiconductors. A number of possible device applications are discussed and the successful use and suitability of these devices for space power is manifest.

845 citations

Journal ArticleDOI
Abstract: The valence band density of states and core levels have been measured for Bi 2 O 3 , Bi 2 S 3 , Bi 2 Se 3 , and Bi 2 Te 3 by X-ray photoelectron spectroscopy. The results for Bi 2 Se 3 and Bi 2 Te 3 are compared with theoretical band structure calculations. The low ionization energy bands are interpreted as arising from p-like bands of bismuth and the Group VI atom. The core levels of bismuth have been found to shift to higher ionization energy with increasing electronegativity of the Group VI element in the compound. Evidence for two inequivalent selenium atoms in the spectrum of the Bi 2 Se 3 structure has been observed.

104 citations

Journal ArticleDOI
G. A. Somorjai1
Abstract: The interaction of oxygen with CdSe surfaces has been studied with thin films of CdSe evaporated in ultra high vacuum in the temperature range 0–360°C and pressure range 10−4−10mm Hg. The effect was monitored via conductivity measurements, carried out in situ. When oxygen is introduced, the initially “clean” film undergoes an instantaneous irreversible chemisorption followed by a slow uptake which is also irreversible in the studied temperature range. This interaction brings about a 3–5 orders of magnitude decrease in the conductivity of the n-type film, i.e. oxygen exhibits acceptor-like properties. The rate of the slow irreversible oxygen uptake can be expressed as d N d t = kP 1 4 o 2 exp ( − bN) and the activation energy for the process has been measured. Donor type weakly adsorbed oxygen was also found to be present on CdSe films which could be reversibly removed from the surface; this reaction was also accompanied by charge transfer and was strongly pressure dependent. The heat of reaction for this process is 12 kcal mole . The boundary layer theory was evoked to explain the kinetics of the slow uptake of the acceptor-like oxygen. Attempts have been made to explain the mechanism of the different surface reactions.

70 citations

Journal ArticleDOI
Abstract: We examine the effect of ambients on the electrical properties of a number of PbSe epitaxial films. These properties are compared with the calculations based on existing theoretical models. We conclude that the existing surface-mobility calculations for degenerate semiconductors are inadequate. The most important result is that the scattering of carriers at the surface appears to be completely specular in character.

44 citations

Journal ArticleDOI
Abstract: The adsorption of oxygen on clean silicon surfaces produced by crushing in vacuo has been examined in the pressure range 30–200 μ Hg of oxygen at room temperature. The initial adsorption to the extent of about a monolayer was very rapid but kinetic data were obtained for the subsequent slow adsorption. The results were complex and probably sensitive to the crystal faces exposed during crushing. The adsorption process was found to terminate at an oxygen takeup equivalent to about 1.5 oxygen atoms per surface silicon atom. A unique configuration, with an inherent termination at a coverage of 1.5, can be constructed for oxygen adsorbed on the {111} and {110} faces of silicon. Prolonged crushing of the silicon led to an enhanced take-up of oxygen which may result from the heating of very small particles during the fast adsorption stage. The surface areas of the powders were determined by the B.E.T. method, using krypton at liquid nitrogen temperature.

44 citations