The contact potential (c.p.) and the change of contact potential with illumination (Δc.p.) L of several germanium surfaces have been measured. The reference electrode used was platinum. It was found that the c.p. could be cycled between two extremes about 0.5 volts apart by changing the gaseous ambient. Ozone or peroxide vapors gave the c.p. extreme corresponding to the largest dipole at the Ge surface. Vapors with OH radicals produced the other extreme. There is a one to one correlation between c.p. and (Δc.p.) L . For 12-ohm cm n-type Ge (Δc.p.) L was large and positive when the surface dipole was largest, decreased to zero and became slightly negative as the surface dipole decreased to its smallest value. The variation for 12-ohm cm p-type Ge was just opposite as regards both sign and dependence on surface dipole. The surface recombination velocity was found to be independent of c.p. For a chemically prepared surface it was 50–70 cm/sec and 180–200 cm/sec for n and p-type surfaces respectively. A theory is given that explains the results in terms of surface traps, N a per cm ² donor-type traps near the conduction band and B b per cm ² acceptor-type traps near the filled band. A quantitative fit with experiment is obtained with only one free parameter. The results are direct evidence for the existence of a space charge layer at the free surface of a semiconductor.

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