806 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



better control of the sensitivity. After being cast in a fused silica crucible, 

 the roughly cyhndrical piece was ground to a cylinder about 1|" di- 

 ameter, a process which removed crucible contamination and gave a 

 convenient size for shcing into wafers about 0.025" tliick. The two faces 

 were then made approximately flat and parallel after which one was left 

 rough and the other ground and polished down to a good optical surface. 

 In most cases the discs were then cleaned by soaking for approximately 

 fifteen minutes in a solution of hydrofluoric acid, rinsed in distilled water 

 and dried. 



The activation consisted of exposing the polished face to a uniform 

 beam of positive ions of helium at a pressure of 10~^ to 10~^ mm of 



ff^ 



Fig. 3 — Intermediate and large size photocells made by ion bombardment. Back 

 of the intermediate also shown. 



mercury. The energy of the particles used in different units ranged from 

 100 to 30,000 electron volts. During this bombardment the silicon sm-face 

 was kept at a favorable temperature, about 395°C. 



After activation, collector electrodes of evaporated rhodium were ap- 

 plied. Cells of three sizes have been constructed, two of which are sho^^^l 

 in Fig. 3, the intermediate and the large one, of exposed active areas 

 about 0.40 and 8.0 sq. cm. respectively. A small one had an area around 

 0.005 sq. cm. INIost of the measurements reported in this paper have 

 been made with the intermediate size. 



EFFECT OF ION VELOCITY 



That ion velocit}^ has a profound effect on the voltage current char- 

 acteristic of bombarded siu'faces is shown in Fig. 5. These characteristics 

 were obtained by placing a tungsten point contact under 10 gm of force, 



