Transmission electron microscopy and scanning electron microscopy offer evidence that the purely chemicalHF:HNO3:H2O“stain etch” used to form light-emitting porous silicon is actually composed of competing etches. A localized etch forms the porous nanostructure by propagation of a discrete reaction interface into the silicon substrate. An amorphous surface layer(SiO2)that is a significant by-product of this etch has been observed and is believed to be a primary efficiency-limiting factor in attempts to fabricate chemically etched porous silicon devices. A destructive etch competes for ions in solution and removes both the porous silicon and the amorphous layers from the surface of the specimen when it becomes dominant, eventually quenching the luminescence properties.