The aim of this paper is to gain insight into the observed recovery of electrical activity of B impurities occurring in Porous Silicon (PS) layers exposed to ${\mathrm{NO}}_2$ molecules, while addressing the problem of the origin of B passivation in PS. Two possible mechanisms are considered, i.e., that the extra electron needed for the fourfold coordination of B atoms be provided either by H atoms (through the formation of Si-H-B complexes) or Si dangling bonds at the surface. Experimental evidence shows unambiguously that a negligible amount of B atoms binds with H, even in posthydrogenated PS, the main passivation source being the Si surface dangling bonds. This explains both the mechanism of formation of PS under electrochemical etching and the efficiency of the ${\mathrm{NO}}_2$ molecules in restoring conduction.

• Received 22 June 2001

DOI:https://doi.org/10.1103/PhysRevB.64.205308