POINT-CONTACT TRANSISTOR SURFACE EFFECTS 795 



account for the increase in reverse current density* and a proportional 

 increase in the magnitude of the floating potential near the point. In 

 this case the geometry of current flow across the contact should remain 

 relatively unchanged as indicated by the floating potential measure- 

 ments. In this way the effect of the HF treatment is somewhat analogous 

 to the addition of a small donor concentration near the surface to coun- 

 teract the inversion layer. Since soluble oxide layers' have been identified 

 on etched germanium surfaces, it is not unlikely that HF (known to 

 dissolve germanium oxide)" might act to reduce the effective thickness 

 of an oxide layer. Such a hypothesis is in agreement with the results of 

 other experimenters/^ who have attributed a surface inversioji layer 

 under the point of an n-germanium rectifier to the presence of germa- 

 nium oxide. They have presumed the oxide is essential to the formation of 

 a good point contact rectifier. The fact that, for a given ambient, the 

 surface potential is determined by the oxide layer thickness has been 

 postulated b}- Ivingston.''* 



3.4.2 CPi-Etched Surfaces 



Sullivan,"" in connection with an experimental investigation of hu- 

 midity stability of electrolytically-etched and chemically-etched p-n 

 grown junction diodes, shows that CP4 chemically-etched surfaces be- 

 come more stable \\ith respect to humidity variation after humidity 

 exposure and cycling at room temperature. Referring to the fact that 

 electron diffraction studies fail to reveal a crystalline oxide film on CP4 

 chemically-polished surfaces and to the results of Law,"*" which indicate 

 that oxide films may be formed slowly at room temperature on exposure 

 1 water vapors, he attributes the changes of stability on the CP4 polished 

 surface to the building up of an oxide film. If such a change can take 

 place on the CP4 chemically-polished surface on exposure to humid room 

 air, then the results of Section 3.3 can be understood under the assump- 

 tion that the action of the HF treatment is to remove the oxide film. 



After the chemical polish, values of /c(0, —10) and average alpha for 

 the unformed units are high, as might be expected if the polishing opera- 

 tion leaves the germanium surface with no appreciable oxide film. As 

 the oxide film builds up on continued exposure to room air, both of these 

 parameters are reduced. The subsecjuent application of HF tends to 

 lestore these parameters to their original ^'alues by removal of some of 

 this oxide film. Thus, the results of this section are in accord with the 



* Evidence for an increase in surface recombination velocity on HF treated 

 surfaces is given in Section 4.2.3. 



