2 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



called the work function. The work function is caused in part by a 

 charged double layer or dipole at the solid surface. In metals this dipole 

 extends over a distance of the order of 10~ cm. In semiconductors how- 

 ever part of the dipole extends into the semiconductor to a distance of 

 the order of 10~® to 10"^ cm depending on the properties of the semi- 

 conductor. This part of the surface dipole is called the space-charge 

 layer. The rest of the surface dipole has approximately the same extent 

 as in metals. 



In the space between any two conducting solids there is a contact 

 potential caused by the difference between the work functions of the 

 two surfaces. One can measure this contact potential by several meth- 

 ods. We have used an adaptation of the well known method of Kelvin. 

 If one has a reference electrode whose work function remains constant, 

 then by measuring the c.p. between this electrode and another surface 

 one can measure any changes in work function or total dipole of this 

 second surface. 



The above method has been used to study the properties of the 

 germanium surface in a gaseous ambient at atmospheric pressure. It 

 has been found that the total dipole at the germanium surface can be 

 changed by changing the ambient and further that by proper control 

 of the ambient the surface can be cycled back and forth between two 

 extremes of small or large dipole corresponding to a c.p. change or work 

 function difference of the order of one-half volt. 



If one upsets the thermal equilibrium in the germanium by creating 

 excess electron-hole pairs near the surface, the potential of the surface 

 will change until a steady state is reached. When the extra electron- 

 hole pairs are introduced by illuminating the surface with light, the 

 potential change shows up as a measurable change in contact potential 

 between the reference electrode and the Ge surface.^ It has been found 

 that this contact potential change on illumination (Ac.p.)l is large and 

 positive on n-type Ge when the surface dipole is large, then decreases 

 to zero and becomes slightly negative as the surface dipole decreases 

 to the smaller extreme. For p-type Ge the (Ac.p.)^ is large and negative 

 when the surface dipole is small and go6s through zero and becomes 

 slightly positive as the surface dipole increases to the larger extreme. 



One can describe qualitatively what is going on as follows. The extra 

 hole and electron pairs created by the light diffuse either to the interior 

 or to the surface to recombine. The recombination in the interior is 

 governed by the body life time r. The surface recombination is charac- 

 terized by a recombination velocity v, . When the surface is illuminated 

 its potential, with respect to the interior, changes until the combined 



