Thickness Dependence of Conductivity due to the Polycrystalline Structure in Evaporated CdS Thin Films

Abstract

In investigations of conductivity of semiconducting thin films it i s generally supposed that the concentration of electrons is independent of film thickness and that the mobility of electrons does not depend on film thickness if the mean free path of the electrons i s smaller than film thickness. Experimental results, e. g. on thin films of germanium (l), lead telluride (2). tellurium (3), and cadmium sulphide (4). in which a dependence of carrier concentration and mobility on film thickness was observed for t h i c h e s s e s up to 10 p m, show that this assumption is not always satisfied for thicker films, too. In the mentioned papers the investigations were performed on several specimens of different thickness obtained from different evaporations. Such specimens a r e commonly not identical in real structure.Therefore we have measured the conductivity in dependence on growing film thickness during evaporation. The thickness was determined by optical interference fringes. The glass substrate was held at a temperature of 160 O C .

Recrystallization of cadmium sulphide films

“…Our electron microscopic investigations [ 5 ] and those of Shallcross [B], showing a n increase in crystallite size with growing film thickness, and the electron diffraction investigations of very thin ( t = 1000 A) CdS films by Addiss [3],…”

“…The thicknesses of the polycrystalline films obtained by evaporation were 0.1 to 5 pm, their deposition rates varied from 1 to 20 Kjs. The mean crystallite size increased with film thickness t and reached values of about 1 pm [ 5 ] . All the films annealed a t 700 "C in argon as well as in vacuum showed after recrystallization, independent of deposition rate and film thickness, a considerable increase in mean crystallite size together with a change in orientation of the crystallites.…”

Electrical Properties of Polycrystalline Semiconductor Thin Films

“…Berger [62,87] showed that the Hall coefficient and the carrier concentrations also exhibit exponential dependences, similar to that of Eq.…”

“…PbS [39,57], CdS [29,30,[58][59][60][61][62][63], CdSe [64], CdTe [65], GaAs [66,67], and several other semiconductor thin films [68][69][70][71][72]. For the first example, Fig.…”

Electrical properties of CdS and CdSe films deposited on vibrating substrates

“…In the early work, the importance of all the parameters which influence the properties of an evaporated film was not fully appreciated. In evaporated CdS and CdSe films, size dependence of resistivity was suggested by Berger et al 4 Bartran et al 5 have studied the dependence of transport properties on film thickness in polycrystalline CdS films through Hall effect measurements and showed that the variation of carrier concentration and mobility with film thickness is similar to the grain size dependence on thickness. Ray et al 6 studied the temperature dependence of electrical resistivity of CdS films and discussed the role of excess cadmium formed in the film during deposition.…”