Transmission electron microscopy studies of bismuth films
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D. L. Partin and J. Heremans General Motors Research Laboratories, Warren, Michigan 48090-9055 (Received 12 September 1989; accepted 28 December 1989) We have studied the structure of Bi films grown on BaF2 substrates as a function of the distance from the interface with the substrate. Close to the interface the films show a polycrystalline structure and probably contain some amorphous regions. The crystallite size increases as the distance from the interface increases. At distances >90 nm the film becomes single crystalline and has the (111) trigonal direction normal to the film surface. Our structural data relate very well to mobility measurements performed on samples grown under the same conditions. Our results suggest that the carrier mobility is higher at distances 3=90 nm from the interface, where the film is single crystalline, than close to the interface, where it is polycrystalline.
Bi films have been successfully grown using techniques such as sputtering, thermal evaporation, and molecular beam epitaxy (MBE). There have been several measurements of the physical properties of Bi films that have shown a dependence on the thickness of the film as well as on their structure and crystalline perfection. For example, the mobility of Bi films grown on the (111) plane of BaF2 using MBE has been observed to increase with increasing film thickness.1 Also, the mobility, conductivity, and transverse magnetoresistance of Bi films have been predicted to depend on film thickness in an oscillatory manner.2 An oscillatory dependence of the resistivity ratio and galvanomagnetic coefficients on the film thickness at T = 78 K and 4.2 K has been observed experimentally on Bi films grown on hot mica.3 The dependence of the physical properties of Bi on film thickness has been explained in terms of the quantum size effect (QSE).4'5 The QSE is observed when the sample thickness is comparable to the de Broglie wavelength of the conduction electrons. In Bi where the electron effective mass is very small, the electron de Broglie wavelength is relatively large (=40 nm). Therefore, it is easy to grow Bi films of thickness less than the de Broglie wavelength using thin film techniques. In semi-metallic Bi where the conduction band overlaps the valence band, QSE will result in the appearance of a band gap at sufficiently small thickness, and the semi-metal is expected to show semiconductor-like behavior.6 There is also experimental evidence for a dependence of the physical properties of Bi films on structure a)Previously
known as L. Salamanca-Young.
J. Mater. Res., Vol. 5, No. 4, Apr 1990
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and crystalline perfection. For example, experimental values of Hall coefficient and transverse magnetoresistance at T = 78 K and 4.2 K for thicknesses below 70 nm of Bi films grown on hot mica do not agree with the theoretical predictions.7 These disagreements have been attributed to the role played by the polycrystalline structure of thin Bi films.7 In addition, measurem