Anodisation of n-InP in KOH results in the formation of porous
layers with a finite thickness. We propose the reason for the
cessation of porous etching is the formation of insoluble
precipitates within the pores. Electron micrographs of mature
porous layers show significant precipitates within the porous
structure. An in-situ microscopy study of the surface of InP
electrode during anodisation reveals the formation of a layer on the
surface. This layer emerges from a point on the surface and quickly
spreads across it. A likely source of this layer is the spreading of
precipitation from the etch-products saturated solution within the
porous layer. However, as we explain, once a complete porous
layer has formed, there should be no significant increase in mass
transport requirements through the porous network, leaving the
exact mechanism of the precipitation unclear.