Photodesorption from CdS(0001) exposed to NO and CO

Abstract

Photodesorption from the (0001) surface of CdS has been studied with low-energy photons of wavelength 200–850 nm. The clean, unannealed surface was exposed to high doses (∼108 L) of CO and NO at room temperature. CO and CO2 were observed to desorb from the surface exposed to CO while NO was the only desorbing specie from the NO/CdS system. The measured photodesorption threshold energy for all species is ∼1.77 eV, which is much less than the CdS energy gap and is approximately the energy of surface states within the band gap of CdS. A second threshold is observed at the direct gap of CdS. The desorption signal is found to be proportional to the incident photon flux, with the calculated quantum efficiency (yield) of ∼10−5 molecules/photon at λ=300 nm. The desorption mechanism for this system agrees with past work on silicon where photohole creation in both the surface states and bulk valence bands was prominent.