Crystallisation of selenium thin films doped with iodine after evaporation

Abstract

Amorphous selenium thin films deposited under vacuum have been doped with iodine either during or after crystallisation. It is shown that when the films are first crystallised at 363 K for 6 h and then submitted to iodine atmosphere at 363 K for 1 h, the structural properties of the films are not modified while their conductivity increases by a factor of 8. Iodine atmosphere induces post crystallisation of amorphous selenium films even at room temperature by increasing the selenium atom mobility at the surface of the films, which induces growth of crystalline spherulites. With annealing, when the heating rate is high (>15 K/min), constraints appear in the films, the density of spherulites increases and the films are inhomogeneous. When the heating rate is small and constant (1 K/min) the interaction between iodine and selenium takes place all over the sample and there is only a small density of small spherulites, while the crystallisation of the whole sample is more homogeneous. XPS and microprobe analysis that the iodine is equally repartitioned in the selenium film show it. Moreover there is a mixture of neutral iodine andS I₃ ⁻ as shown by XPS and Raman studies. The high crystalline quality of the films can explain the high conductivity (>10⁻³ Ω⁻¹ cm⁻¹) of these selenium doped films