SURFACE PROPERTIES OF GERMANIUM 5 



contact or other, exists between the surfaces. When the proper dc potential 

 is applied between the surfaces this signal goes to zero. If no other poten- 

 tials are present this dc potential is equal and opposite to the contact 

 potential. A phase reference method* is employed to determine this balance 

 point with a relative accuracy of d=o X lO"'^ volts. A diagram of the Ge- 

 reference electrode circuit is shown in Fig. 1 . Care must be taken to shield 

 this circuit. Stray capacity reduces sensitivity and should be minimized. 

 Charged insulators inside the shield will produce an apparent c.p. All con- 

 ducting surfaces other than the Ge should be relatively far from the moving 

 reference electrode. The surface is illuminated through a compound 

 lens system by focusing the filament image, of a suitable projection 

 bulb, on the germanium surface. This light passes through the grid of 

 the reference electrode which removes about 10 per cent of the light. 

 The light can be modulated by a square wave chopper, so that (Acp.)/, 

 can be measured on an ac basis. Both Ta and Pt reference electrodes 

 have been used. The Pt electrode appears to be somewhat more constant. 

 If the Ge surface is replaced by a gold electrode the contact potential 

 difference is practically independent of the changes in the gas ambient. 

 The arrangement is such that two samples can be mounted in the bell 

 jar and the reference electrode moved from one to the other without 

 opening the bell jar. In this way two surfaces can be compared, without 

 any question arising of long time drifts in the reference electrode. 



EXPERIMENTAL RESULTS 



/. Change of c.p. between Ge and Pt reference electrodes as a function 

 of the gaseous ambient 



WTien this work was started the object was to find some means of 

 varying the c.p. It was thought that the actual values of the c.p. would 



Ge SAMPLE 



reference ^ 

 electrode' 



POTENTIOMETER 



C^ 



TO AMPLIFIER 



Fig, 1 — Schematic of experimental circuit. 



H.R. Moore designed and made the electronic equipment used to do this. 



