PHOTOELECTRIC PROPERTIES OF lONICALLY BOMBARDED SILICON 805 



A typical spectral response curve of such a barrier is shown in Fig. 2a 

 while Fig. 2b gives its open circuit voltage, short-circuit current and 

 resistance when illuminated by a tvinslcii light of 2848°K color tempera- 

 ture. This cell resistance was taken as e(iual to that of an added series 

 resistance which reduced the short-circuit photocurrent to one-half. 

 The value so obtained is somewhat higher than the corresponding ratio 

 of the voltage and current given in the figure. Fig. 2c giv(\s the })ehavior 

 as a rectifier in the dark and with a stated light on the barrier. 



Cells whose barrier was near the surface were made by cutting close 

 to the natural one as shown in Fig. lb. This cut exposed large photoactive 

 areas. Surface barrier activity was occasionally found on the top surface 

 of some melts. These surface type cells showed a wider spectral response 

 toward the visible than the internal barrier type. This was found to be 

 due to the spectral absorption characteristics of the bulk sihcon. A 

 further discussion of this appears later in the paper. 



These early barrier cells showed remarkable stability under severe 

 temperature conditions. For instance, they could be heated to redness 

 in air and quenched in water with no serious change in their character- 

 istics. They were tested in hquid nitrogen, under water and in oil without 

 injury. They could be illuminated with direct sunlight with no injury 

 to their response characteristics other than the temporary effect of the 

 increased temperature. Several of these internal barrier cells have been 

 in use in test circuits for more than ten years with no serious change in 

 their photoresponse properties. These observations seemed to indicate 

 clearly that a very high degree of stability could be expected from sihcon 

 photocells. 



However, there were serious practical disadvantages to the early 

 cells. Those sho\vn in Fig. la were active near the exposed barrier itself 

 which was usually a strip along the surface about ^ mm wide. On the 

 other hand, the surface types as shown in Fig. lb showed irregular re- 

 sponsiveness over the surface area. 



From these early studies it was clear that if a good method could be 

 found to activate large areas of silicon surfaces uniformly, cells could 

 be made which might compete with other kinds of surface barrier type 

 cells already available. The search for such a process resulted in the 

 ionic bombardment method of activating silicon surfaces. Such surfaces 

 also have desirable rectifying properties.^ 



METHOD OF PREPARATION 



Hyper-purity sihcon was used for bombardment type cells to avoid 

 the formation of natural barriers due to minute impurities and to give 



