Distribution and Cross Sections of Fast States on Germanium Surfaces in Different Gaseous Ambients

Abstract

Simultaneous measurements of surface recombination velocity and trapped charge density in the fast states as a function of surface potential are carried out on germanium surfaces at fixed gaseous ambients and temperatures. The surface potential is varied over a range of about 10 $\frac{\mathrm{kT}}{q}$ by the application of ac fields normal to the surface. For the surfaces studied, the distribution in energy of the fast states is found to consist of four discrete sets of levels. Only one of these sets is significant in the recombination process, being characterized by relatively high capture cross sections for holes and electrons. Another set has negligible cross sections and appears only in the field-effect data. The two remaining sets of levels lie outside the range of surface potential attained and little can be said about their properties. Detailed investigation of the surface recombination process shows that the energy level of the recombination centers is temperature-dependent, increasing slightly as the temperature is decreased. The field-effect data indicate that such is also the case for the other surface states.Repeated measurements on the same sample over a long period in vacuo and in various gaseous ambients show that the structure of the fast states is markedly affected by these ambients. Initially, with the sample in vacuo, rapid changes in interface structure take place: the energy levels, the density, the ratio of the capture cross section for holes to that for electrons, as well as the cross sections themselves, all decrease with time. After a few days in vacuo, stabilization is essentially reached; following that, repeated exposures to different ambients result in reproducible changes in structure which persist for many weeks.