SURFACE PROPERTIES OF GERMANIUM 



11 



the chopping frequency, the improvement in signal to noise enables one 

 to read very small changes. 



A plot of (Ac.p.)l in volts versus the contact potential for samples A 

 n-type and D p-type is shown in Fig. 8. For the n-type sample the signal 

 is large and positive when the contact potential is small and becomes 

 small and negative as the contact potential increases. Except for the 

 shift in the contact potential where (Ac.p.)/, goes through zero, the re- 

 sults for the p-type sample are practically the opposite of those for the 

 n-type sample. Similar curves have been obtained cycle after cycle in 

 many complete runs. Within experimental error the curves have the 

 same shape for a given sample for all cycles in all runs. Sometimes the 

 curve for the first cycle in a run will differ in shape from the rest. How- 

 ever there are shifts such that the c.p. for which the light goes through 

 zero (c.p.)o does vary from cycle to cycle throughout a run. Fig. 9 is a 

 plot of (c.p.)o versus cycle number for p-type sample D and n-type 

 sample A. The data are plotted for two distinct runs. Both of these 

 units were in the bell jar when the measurements were taken. Consistent 

 results of this kind give one confidence that the Pt reference electrode 

 is staying constant. 



These experimental results can be summarized as follows. All data 

 for a given sample can be superimposed by shifts in contact potential 

 scales for the different cycles. A plot of (Ac.p.)l versus c.p. — (c.p.)o 

 can be represented by a single curve for each unit. Moreover all the 

 curves for all units both n- and p-type have quite similar shapes provided 

 in the latter we plot [ — (Ac.p.)i,] versus (c.p.)o — c.p. The shape of this 

 curve is shown in Fig. 8 for a given sample and Fig. 9 shows how (c.p.)o 

 varies from cycle to cycle. These plots adequately describe the results. 



20 24 28 32 



TIME IN MINUTES 



Fig. 7 — Contact potential cycles for silicon. 



