Nanoobjects microscopy

Thickness effect at crystallization of amorphous films of Yb2O2S

Bagmut A.G., Bagmut I.A.
Department of Physics of the National Technical University “Kharkiv Polytechnic Institute” of Ukraine. NTU “KhPI” 2, Kyrpychova str., 61002, Kharkiv, Ukraine.
E-mail: agbagmut@gmail.com

Films with layer thicknesses h = 20 - 50 nm were grown on a cleaved (001) face of KCl single crystals by the method of electron beam evaporation of the weight of high purity YbS. The structural analysis was carried out by the methods of electron diffraction and transmission electron microscopy on EM-100L and PEM-100-01 electron microscopes operating at the accelerating voltage of 100 kV.
Fig. 1 illustrates the effect of film thickness on the fraction of the crystalline phase x at crystallization of amorphous Yb2O2S upon thermal annealing in vacuum (180 min at 575ºС). At this temperature, films of thickness with h ≈ 20 nm remain amorphous. Crystallization occurs at h ≥ 25 nm (Fig. 1a, b, c). The fraction of the crystalline phase in the film increases with the growth of film thickness and on the level of higher than 50 nm is equal to 1 (Fig. 1c).

48 nm

48 nm

48 nm

Fig. 1. The effect of thickness h on the crystallization of amorphous films of Yb2O2S upon thermal annealing in vacuum (180 min at 575ºС): (a) h = 25 nm; (b) h = 40 nm; (c) d = 50 nm

a

b

c

The slowdown of crystallization at decreasing of the film thickness can be connected with increasing of the surface energy during crystallization. In addition, crystallization is associated with overcoming of the activation barrier, that separates the amorphous and crystal states. The height of the barrier increases with decreasing of the film thickness.