Modulation of Cadmium Selenide Photoluminescence Intensity by Adsorption of Silapentanes and Chlorinated Silanes

The band gap photoluminescence (PL) intensity of n -CdSe single crystals is reversibly enhanced by adsorption of seven of the nine silapentane isomers, SiC 4 H 12, from the gas phase onto the (0001) surface of this substrate; only tetramethylsilane and tert -butylsilane elicit no PL response. The adsorbate-induced increase in PL intensity is consistent with the species acting as Lewis bases toward the surface. Quantitatively, the PL changes can be fit by a dead-layer model, allowing estimation of the adduct-induced reduction in depletion width as ranging from ∼100 to 500 Å. Adduct formation constants, estimated from fits of the concentration-dependent PL changes to the Langmuir adsorption isotherm model, range from ∼10 to 10 3 atm -1 and are ∼10 2 atm -1 for most of the isomers. Two chlorinated silanes, H 2 SiCl 2 and HSiCl 3, yielded reversible PL quenching after an initial conditioning period, although the magnitude of the quenching was not in accord with the dead-layer model. Possible surface binding modes are discussed for these silanes, as are implications for their on-line detection as precursor molecules for growth of semiconductors by chemical vapor deposition (CVD).