Thin films, compared to single crystals, are characterized by a large surface-to-volume ratio. The film thickness determines the transport properties and the main mechanisms of charge carrier scattering. In thin films, the scattering mechanisms differ significantly from those in bulk samples, especially in the region of small thicknesses. It is necessary to take into account scattering at grain and interfacial boundaries, misfit dislocations, and other defects associated with the near-surface state. The localization of current on surface states and its capture by dangling bonds at the boundaries of crystallites entail the formation of space charge regions near them, where the concentration and mobility of current carriers can differ significantly from the corresponding parameters in the inner region. The effect of near-surface regions on the properties of films depends on their thickness, structure, doping level, and temperature. At the same time, atmospheric pressure plays an important role.
Studies were carried out on thin films of tin telluride, which were deposited on fresh chips (0001) of mica muscovite. In the work, the properties of the films were studied, in particular, the specific conductivity, the Hall coefficient, and the thermal emf. Particular attention was paid to the effect of film thickness and annealing temperature in air. The studies were carried out on tin telluride films deposited on muscovite mica, since this system is technologically attractive and has the potential to be used in various electronic devices. The research results can be useful for further improvement and optimization of the production of thin films of tin telluride and the development of new devices based on their properties.
The films were obtained by the method of open evaporation in vacuum from the vapor phase. In this case, fresh chips (0001) of muscovite mica served as substrates for deposition. The temperature of the evaporator during the deposition was 700 ºC, and the temperature of the substrates varied in the range of 150–250 ºC. The film thickness varied from 0.05 to 2 μm and was determined using an MII-4 microinterferometer.
To measure the electrical parameters of freshly grown films, experiments were carried out at temperatures from 290 to 353 K in constant magnetic fields. The sample had four Hall contacts and two current contacts. Ohmic contacts were made from silver films. The current passed through the sample was approximately 4 mA. The magnetic field was directed perpendicular to the film surface and had an induction of 1.5 Tl.
Dependence of specific conductivity, Hall coefficient and thermal emf. SnTe films versus thickness is shown in Fig. 1.
Fig. 1. Dependence of specific conductivity σ, Hall coefficient RH, thermal emf S on thickness for SnTe films. ● – measurement temperature 293 K, ○ – measurement temperature 353 K.
The measured effective parameters are integral values that depend on the distribution profile of charge carrier concentration and electrical conductivity at the local level. The observed significant increase in the surface conductivity of SnTe films in air is associated with the enrichment of the surface with major charge carriers due to the acceptor action of oxygen. Thermo emf the surface layer is slightly larger than the bulk. Annealing of films in air leads to a significant increase in the surface concentration of holes, a slight decrease in conductivity, and an increase in thermal emf. The thickness of the near-surface layer in this case is approximately doubled.
1. Prokopiv V., Horichok I., Mazur T., Matkivsky O., Turovska L. Thermoelectric materials based on samples of microdispersed PbTe and CdTe. Proceedings of the 2018 IEEE 8th International Conference on Nanomaterials: Applications & Properties (NAP - 2018) Zatoka, Odesa Region, 2018. P. 01SPN57-1 - 01SPN57-4. DOI: 10.1109/NAP.2018.8915357
2. B.S. Dzundza, I.I. Chaviak. A.I. Tkachuk. G.D. Mateyk. O.L. Sokol, Near-surface Layers and Profiles Electric Parameters of Thin Films SnTe Physics and chemistry of solid state. 2010 Vol. 11, N 3. P. 614–617.
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