Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 21 5A further problem arising with semiconductivity measurements is that the surfaceconductivity may be of a different type to the bulk conductivity. Measurementsof work function or field effect mobility are then necessary to fully definethe process.Both the potential probe and the bridge methods are applicable to semiconductors,but the dc methods are limited with films and powdered or sintered materialsbecause of the high contact resistance between particles. An apparatus employinga four potential probe has been described by Anderson lS5. A method for studyingthe rates of adsorption on evaporated films has been developed by Gray et ~ 1 . l ~ ~using the apparatus shown in Fig. 20. Four platinum electrodes are fused to theinside of a hard glass or silica tube. A filament serves to evaporate a film of thematerial to be studied using the techniques described in Section 2.1.1~. The filmproduced can be from a few hundred to a few thousand angstroms in thickness.Conductivity changes as a function of gas adsorption can be studied either witha potentiometer, or ac bridges covering the frequency ranges 0.5 c. sec-' to150 Kc. sec-'. If the original material evaporated is a metal, its rate of oxidationcan be measured and further adsorption of oxygen or other gases studied.Handler has used the apparatus shown in Fig. 21 to study adsorption on thinFig. 21. Diagram of the experimental tube used for measurement of the surface conductance andfield effect mobility. From P. Handler, Semiconductor Surface Physics, Ed. R. H. Kingston, (1957)p. 29.References pp. 270-278