Origins of cracking in highly porous anodically grown films on InP

The nature of the surface films grown during the anodization of InP in an aqueous (NH₄)₂S electrolyte has been investigated. The previously reported cracking of these films is explicitly demonstrated to occur ex situ and not during the electrochemical treatment. The films have been identified as In₂S₃ and are shown to have a columnar morphology. The measured film thickness varies linearly with the charge density passed, and comparison between experimental measurements and theoretical estimates for the thickness indicates a porosity of 70-80% for the In₂S₃ film. Film cracking is attributed to shrinkage during drying of the highly porous film and does not necessarily imply stress in the wet as-grown film.