Growth of Anodic Films on Compound Semiconductor Electrodes: InP in Aqueous (NH4)2S

Summary.  Film formation on compound semiconductors under anodic conditions is discussed. The surface properties of InP electrodes were examined following anodization in an (NH₄)₂S electrolyte. The observation of a current peak in the cyclic voltammetric curve was attributed to selective etching of the substrate and a film formation process. AFM images of samples anodized in the sulfide solution revealed surface pitting. Thicker films formed at higher potentials exhibited extensive cracking as observed by optical and electron microscopy, and this was explicitly demonstrated to occur ex situ rather than during the electrochemical treatment. The composition of the thick film was identified as In₂S₃ by EDX and XPS. The measured film thickness varies linearly with the charge passed, and comparison between experimental thickness measurements and theoretical estimates for the thickness indicate a porosity of over 70%. Cracking is attributed to shrinkage during drying of the highly porous film and does not necessarily imply stress in the wet film as grown. During the growth of the thick porous film, spontaneous current oscillations have been observed. The frequency of oscillation was found to be proportional to the current density, regardless of whether the measurements were carried out during a potential sweep or at constant potential. Thus, the charge passed per oscillation remained constant. A characteristic value of approximately 0.3 C · cm⁻² was measured under potential sweep conditions, and a similar value was obtained at constant potential. Received October 16, 2001. Accepted (revised) December 21, 2001