Humidity effect on the evolution of CsI thin films: a fractal study of rough surfaces

The present work aims at studying the morphological, micro-structural, compositional and fractal features of CsI thin films, for the thicknesses 30, 50, 300 and 500 nm, in two cases: (i) as-deposited and (ii) 1-h humid air aged. The average grain sizes, estimated from TEM technique, are varied between the range 313–1058 nm, and their values are found to get increased after exposing to the humid air. The interplanar spacing evaluated from a SAED pattern is lesser than the standard value for a strain-free CsI crystal which suggests that a compressive stress is acting in the film. The compositional analysis suggests that the atomic percentage of Cs:I is 1:1, and it increased by the factor of two with exposure to humidity. The surface morphological images, captured by atomic force microscopy, signify the fractal behavior of CsI films. The values of the roughness exponent and fractal dimensions are determined for both as-deposited and humid-aged CsI films. It is observed that fractal parameters no longer remain unchanged after exposure to humidity, but they do not show any monotonic decreasing or increasing nature. The value of the roughness exponent was found to be greater than 0.5 which indicates that the height fluctuations at neighboring pixels are positively correlated. It signifies the height variations at neighboring pixels exhibit persistent behavior and memory effect.