Optical Properties and Oxide Growth during Chemical and Electrochemical Oxidation of Luminescent Porous Silicon in Liquid Electrolytes Monitored by In-Situ Photoconduction

Abstract

The progress of chemical and electrochemical oxidation of porous silicon (PSi), formed from lightly-doped p-type silicon, in aqueous electrolytes, was monitored using in situ photocurrent from a monochromatic illumination, which can be used as a signature of the optical transmission through PSi. Models are proposed to explain the results quantitatively. For anodic oxidation, the valence of reaction was obtained as ∼1.5, which is consistent with non-stoichiometric oxide growth. Electroluminescence and photoluminescence during anodic oxidation were consistent with previous reports. For chemical oxidation, monitoring can be used up to a certain stage after which the oxide grown at the PSi/substrate interface prevents further use of electrical conduction. This initial stage was exploited to get model parameters, allowing derivation of time evolution of oxidation rate, oxide thickness, porosity, and relative quantum efficiency. Concurrent dissolution of oxide was also taken into account when the oxidation rate was low. The luminescence efficiency was improved in the first stages of oxidation, and then decreases for heavy oxidation, when the silicon content had drastically decreased. The rate of oxidation seems to play a role in the luminescence efficiency, a high oxidation rate leading to lower efficiency for a given fraction of silicon content in the layer.