Activation analysis of impurity distributions in critical layers of semiconductors

Neutron activation analysis was used as characterization technique to analyze epitaxial films for the determination of dopant concentrations. The amounts of material removed have been controlled partly by measuring the weight losses of the sample or in cases when no cooling time was used, by counting the 1.26 MeV gamma-activity of the 31Si isotope. The investigated dopants were: phosphorous, antimony and arsenic. Subsequent layers of the epitaxial films were removed by stepwise etching processes, and each dissolved layer was analyzed by means of gamma-spectroscopy. The experimental data definitely show, that concentration inhomogenities generally occur at the critical substrate (epitaxial and epitaxial) air interfaces on silicon. Fluctuations in dopant concentration are observed even in heavily doped substrated, although these can be evened out by treatment at high temperatures, which leads to the diffusion of the dopants from the nearsuperficial layers. It is probable, that this effect results from a reordering of dislocations in the crystal. (K.A.)