120 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



argon the adjacent surfaces may be depressed by as much as 130 Ang- 

 stroms. Further investigation of this phenomenon is under way. 



STABILITY OF BOMBARDED SURFACES 



No extended test has yet been made of the stabihty of bom})arded 

 surfaces but resuUs extenchng over more than two years are enccxu'aging. 

 Similarly, rectifiers for the millimeter wavelength range, mounted with- 

 out the usual protective impregnation, show little or no change at the 

 end of a year. 



In a few instances bombarded surfaces have been subjected to rather 

 severe tests with results that suggest that under normal conditions they 

 may be even more stable than surfaces activated by more conventional 

 methods. For example, surfaces contaminated while cutting or while 

 cementing them to their mountings have subsequently been cleaned 

 with solvents such as alcohol and are substantially the same before and 

 after treatment. In other cases, they have been heated in a flame to 

 soldering temperatures with no appreciable effects. Even in the very 

 severe case where the bombarded piece was heated to a cherry red and 

 the superficially oxidized layer was removed wdth hydrofluoric acid the 

 effects of bombardment were still evident. There was, however, con- 

 siderable reduction in the tolerable reverse voltage ."'There is nothing 

 in our experience to date to suggest that bombarded surfaces treated 

 in accordance with the simple straightforward methods outlined above, 

 are in any wise temporary in character. 



CONCLUSIONS 



The expei'iments reported above have shown that rather pronounced 

 changes in the electrical properties of silicon may be produced merely 

 by bombarding the polished surface with positive ions. The ratio of 

 forward to reverse currents, for example, which for the usual untreated 

 silicon is seldom more than a few hundred, can be made more than 10,000. 

 Experiments show that the effect depends to some extent on the type 

 of ion gas used, helium being a preferred medium. The effect depends 

 also on the volocity of the bombarding particles, the total bombarding 

 charge and to a lesser extent on the temperature of the specimen tluring 

 bombardment, (lood results are obtained from bombarding potentials 

 of 30 kv with current densities of 5 microamperes per square centimeter 

 for periods of one or two minutes. The temperature should preferable 

 be about 300°C. 



Ordinarily the properties of siliccju are materiall}' affected by impiuity 



