Semimetal–semiconductor transition in thin Bi films

The temperature dependences of the Hall coefficient of thin bismuth films with thicknesses d = 8 to 215 nm prepared by thermal evaporation in vacuum on mica substrates were obtained in the temperature range 80–300 K. It was established that at thicknesses smaller than ∼ 25–30 nm, semimetallic conductivity is not observed. It was suggested that in this thickness range a semimetal–semiconductor transition occurs. Using this assumption and taking into account the existence of surface states, it was shown that the gap between the valence and conduction bands in the semiconductor region increases with decreasing d.