Lead telluride and lead selenide films were prepared by the thermal evaporation technique on clean glass substrates held at room temperature in a vacuum of 3×10-5 Torr. The resistance of the films was measured at different air pressures. The resistance of the as-grown films was found to be of the order of a kilohm but increased with increasing pressure. A resistance maximum was observed at a particular air pressure after which the resistance decreased with increasing pressure. An indirect method, namely, the measurement of the conductivity variation as a function of air pressure, has been used to estimate the extent of air adsorption and the adsorption isotherms have been drawn on this basis. The dependence of adsorption on thickness is also discussed. © 1985 The American Physical Society.

K. Seetharama Bhat

V. Damodara das

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Physical Review B

Thin 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 In(R/R0) 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. © 1998 Elsevier Science Ltd. All rights reserved.

Solid State Communications

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

Thin films of Pb0.8Sn0.2Te of varying thicknesses deposited by the flash evaporation method showed considerable change in resistance with time when exposed to oxygen or atmosphere. The change in resistance is found to decrease with increase in thickness and with increase in substrate temperature. The observed behaviour is explained by an oxygen adsorption model, which is further supported by the x-ray photoelectron spectroscopy (XPS) of the films.

Semiconductor Science and Technology

Effect of oxygen adsorption on the electrical resistance of Pb0.8Sn0.2Te thin films

A two-layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconducting thin films has been developed assuming that the film is a parallel combination of resistances of the three layers: The first is the interior ''grain-boundary layer, and the other two, outer layers on opposite sides, whose conductivities are altered by the band bending (and is also affected by surface-gas interactions). The equations obtained in this model lead to an inverse thickness dependence of both the conductivity and thermoelectric power of thin films. The model is applied to analyze the conductivity and thermoelectric variation with thickness of PbTe thin films and the parameters Ug, the energy dependence of the ''grain-boundary mean free path lg, and σg the surface conductivity, have been evaluated.

Fulltext

Journal of Applied Physics

A two-layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconductor thin films and application of the model to PbTe thin films

PbSe thin films of thicknesses in the range 20 to about 170 nm have been prepared on glass substrates held at room temperature by thermal evaporation of the bulk alloy at a pressure of 5×10-5 torr. The thermoelectric power of these films has been evaluated as a function of temperature in the range 300 to 500 K from the thermal e.m.f. data. It was found that the thermoelectric power of all the films initially increases with increasing temperature, then reaches a maximum, and, with a further increase in temperature, decreases rapidly and also changes sign. The possible reasons for this peculiar behaviour have been given. It is also found that there is no systematic variation of thermoelectric power with thickness of the films. This is probably due to completely specular scattering at the external and internal surfaces of PbSe thin films or to changes in stoichiometry of different films. © 1990 Chapman and Hall Ltd.

Journal of Materials Science: Materials in Electronics

Temperature variation of thermoelectric power of vacuum deposited PbSe thin films and its thickness dependence

Thin films of PbSe of different thicknesses have been prepared on glass substrates at room temperature by vacuum deposition. It is found that the electrical resistivity of the air-exposed films is much higher (by about 2 to 3 orders) than that of the as-grown (unexposed) thin films. The electrical resistivity temperature behaviors of both the air-exposed and as-grown (unexposed) thin films of PbSe are different. These observations can be explained by considering that the desorption of adsorbed gas molecules (mainly oxygen) and creation of defects at higher temperatures during heating influence the electrical conduction. The as-grown (unexposed) thin-film conductivity exhibits the expected reciprocal thickness dependence due to thickness effects, but the air-exposed film conductivity does not. This can be explained as due to the complete masking of the thickness effect by the gas-adsorption effect in air-exposed film conductivity. The reciprocal thickness dependence observed in the case of unexposed-film conductivity has been explained by the effective mean free path model. The mean free path lg and the specularity parameter p are determined as 580 A and 0.9, respectively. © 1989 The American Physical Society.

Journal	Physical Review B
ISSN	01631829
Open Access	No