Experimental Investigation of the Luminescence Properties of Porous Silicon Fabrication and Characterization of Porous Silicon-Conducting Polymer Junctions.

Experiments investigating the origin of room temperature luminescence of porous silicon were performed. Surface titration experiments indicated that a monoprotic surface species with a pK_{rm a} of 3-4 was linked to the luminescence efficiency of porous silicon. Deprotonation of the surface by exposure to diethylamine vapor resulted in the loss of luminescence. Reprotonation of the surface by exposure to trifluoroacetic acid resulted in the restoration of luminescence. Silicon oxide formation occurred with exposure to diethylamine vapor but was not responsible for the reversible quenching and restoration of luminescence. Silicon oxide formation did cause lower energy portions of the emission spectra of fresh porous silicon samples to be quenched irreversibly. A reproducible shift in the highest energy SiH mode with deprotonation and reprotonation of the surface of porous silicon was observed. This was interpreted as indicating a shift in surface energetics that occurs with surface deprotonation/reprotonation. A modified quantum confinement tunneling model was proposed to explain the interaction of a surface species with nanocrystalline regions existing within the pore walls of porous silicon. It was hypothesized that removal of the adsorbed surface proton lowers the height of the confinement barrier and allows excited electrons to tunnel out of the confinement region upon which nonradiative decay occurs. Protonation of the surface results in higher luminescence efficiencies because it raises the height of the confinement barrier and excited electrons are confined on a time scale necessary for radiative decay. Solid state porous silicon conducting polymer devices were fabricated and analyzed. Electroluminescence was not observed. Rectifying behavior, consistent with metal p-type semiconductor junctions was observed for doped conducting polymer p-type porous silicon junctions. Current -voltage data was used to find barrier heights of.1-.2 eV for doped conducting polymer p-type porous silicon junctions. Negative differential resistance was observed for some devices but was inconsistent with resonance tunneling behavior and explained in terms of charging and discharging of deep states in porous silicon. The lack of electroluminescence was explained as the "shorting out" of the devices through current paths that did not electronically inject carriers into confinement regions.