CONTROLLING THE WORK FUNCTION OF GAAS BY CHEMISORPTION OF BENZOIC-ACID DERIVATIVES

Control of the work function of GaAs single crystals, under ambient co nditions, was achieved by chemisorption of a series of benzoic acid de rivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surf ace coverage of about one layer and a binding constant of 2.1 10(4) M( -1) for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affi nity while band bending is not affected significantly. The direction o f the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dip ole's magnitude. Investigation of the surface composition by X-ray pho toelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the m olecules from affecting band bending.