Giant Temperature Dependence of the Work Function of GaP

Both the clean and cesiated (¯1¯1¯1) surfaces of $n$- and $p$-type GaP show very large shifts of work function upon cooling when the surface is illuminated with 1.7-eV light: $\Phi$ increases for $p$-type and decreases for $n$-type material. This is attributed to reduction of surface charge by photoexcitation of electrons into or out of surface states. The surface-state density is ∼4 × ${10}^{12}$ ${\mathrm{cm}}^{-2\mathrm{}}$ and is unaffected by submonolayer quantities of Cs.