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, SiC4H12?, 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 similar to 100 to 500 Angstrom. Adduct formation constants, estimated from fits of the concentration-dependent PL changes to the Langmuir adsorption isotherm model, range from similar to 10 to 10(3) atm(-1) and are similar to 10(2) atm(-1) for most of the isomers. Two chlorinated silanes, H2SiCl2 and HSiCl3, 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).