Cadmium selenide photoluminescence as a probe for the surface adsorption of dialkyl chalcogenides

The band gap photoluminescence (PL) intensity of n-CdSe is reversibly enhanced by adsorption, from the gas phase, of two families of dialkyl chalcogenide compounds: (CH3)(2)E (E = S, Se, Te) and R2S (R = CH3, C2H5, n-C3H7, i-C3H7, and t-C4H9). A dead layer model was used to estimate adduct-induced reductions in depletion width thickness at saturation; values ranged from similar to 100 to 500 Angstrom. Binding constants of less than 10 atm(-1) to as large as 500 atm(-1) were estimated from fits to the Langmuir adsorption isotherm model. The magnitude of the PL responses and binding constants of the dialkyl sulfides increases with chain length for the straight-chain derivatives and correlates with trends in ionization potential and basicity. Branched-chain derivatives do not follow this correlation as strictly but still give substantial PL enhancements and binding constants. Varying the chalcogenide atom in the dimethyl chalcogenides has little effect on either the magnitude of the PL enhancements or the binding constants, which are relatively small. Steric and electronic factors contributing to these PL effects are discussed, as is the potential use of PL for on-line detection of these group VI precursor gases in the growth of materials by chemical vapor deposition processes.