The donor and acceptor character of surface states on germanium as affected by the adsorption of carbon monoxide

Abstract

A theory is presented which connects a change of the surface structure of a semiconducting solid caused by the (reversible) adsorption of a gas, with the shape of the adsorption isotherm. The change of the surface structure leads to a saturation of the adsorption which may be very pronounced. The theory was developed to explain the rapid saturation of the adsorption of CO on germanium surfaces at room temperature. Here a maximum adsorption of about 10 11 CO molecules per cm 2 was obtained in work by Boonstra. At the same time, there was a marked increase of the surface conductance, caused by newly formed holes in the space charge region. To explain this increase, the indicated change of the surface structure was connected with a redistribution of acceptor and donor surface states. This connection was accounted for by means of a simple order-disorder theory. In the final adsorption equation, two new energy parameters appear, but the numerical value of only one, denoted as Δ E and describing the important role of CO molecules in the redistribution, must be estimated. The physical nature of δ E was not studied, but it was shown that a value Δ E = -100 kT connects in a natural way the three aspects of the problem: the redistribution of donor and acceptor surface states, the change of the surface structure, and the shape of the adsorption isotherm.