Surface properties of germanium 3 



flow of holes and electrons to the surface and interior is just equal to 

 the rate they are being created by the light. The sign and magnitude 

 of the potential change for a given illumination depends on the body 

 properties of the germanium and on the size of the space charge layer. 



These experimental results are direct evidence for the existence of a 

 space charge layer at the free surface of a semiconductor. They not only 

 confirm the results obtained for silicon surfaces but go much further 

 in that they enable one to determine how the layer is changed by the 

 gaseous ambient used. 



It* is kno\\Ti that the surface recombination velocity, Vs , can be 

 changed, by large factors, by surface treatment.^ For mechanically 

 treated surfaces Vs approaches thermal velocities. Every hole or electron 

 striking the surface recombines. For such a surface it is found that 

 (Ac.p.)l is too small to be measured. On the other hand Vs can be as low 

 as 100 cm/sec for chemically polished or etched surfaces such as those 

 used in the experiments where (Ac.p.)l was measured. In this case one 

 wishes to know how Vs depends on the gaseous ambient. This was meas- 

 ured for the same surface used in measuring (Ac.p.)z, and it was fou^d 

 that, for the ambients used, Vs is approximately a constant and there- 

 fore independent of the other surface changes. 



A quantitative theory, some details of which are in the Appendix, has 

 been formulated to explain the results. It is proposed that there are 

 two types of recombination traps at the surface: donor type, Na per cm^, 

 with energies, Ea , near the conduction band and acceptor type, Nb per 

 cm^, with energies, Eb , near the filled band. Surface recombination 

 takes place by electrons and holes successively going into one of the 

 two types of traps. To account for the fact that Vg is unchanged by 

 changes in ambient, it is assumed that the concentrations of these traps 

 are independent of ambient. Changes in c.p. with ambient are assumed 

 to result from adsorption and desorption of fixed ions which are at an 

 effective distance ^^2X 10"^ cm outward from the surface traps. A sche- 

 matic energy level diagram is given in Fig. 13, to be discussed later. 



The charge of the ions is compensated mainly by charges in the 

 surface traps w^hich, together with the ions, form a double layer, A 

 large part of the change in c.p. with ambient results from changes in 

 this double layer. There is also a change in barrier height, —eVsy 

 associated with the redistribution of electrons in the traps. An increase 

 in negative ions on the surface requires a decrease in number of electrons 

 in traps, and thus a higher barrier. 



Part of the change with light, (Ac.p.)l , occurs in the body of the 

 semiconductor and part occurs across the barrier layer. Changes in Vb 



