Positive and Negative Pulse Etching Method of Porous SiliconFabrication

Abstract

We present a new method in which both positive and negative pulses areused to etch silicon for fabrication of porous silicon (PS) monolayer.The optical thickness and morphology of PS monolayer fabricated withdifferent negative pulse voltages are investigated by means ofreflectance spectra, scanning electron microscopy and photoluminescencespectra. It is found that with this method the PS monolayer is thickerand more uniform. The micropores also appear to be more regular thanthose made by common positive pulse etching. This phenomenon isattributed to the vertical etching effect of the PS monolayer beingstrengthened while lateral etching process is restrained. Theexplanation we propose is that negative pulse can help the hydrogencations (H+) in the electrolyte move into the micropores of PSmonolayer. These H+ ions combine with the Si atoms on the wallof new-formed micropores, leading to formation of Si–H bonds. Theformation of Si–H bonds results in a hole depletion layer near themicropore wall surface, which decreases hole density on the surface,preventing the micropore wall from being eroded laterally by F−anions. Therefore during the positive pulse period the etching reactionoccurs exclusively only at the bottom of the micropores where lots ofholes are provided by the anode.