Surface Effects in Electron-Irradiated Ge at 80°K

Abstract

Photoconductivity, surface conductance, Hall and field effect measurements have been made before and after electron irradiations. Before irradiation, both $n$- and $p$-type specimens show a photoconductive shoulder in the range of photon energies between 0.70 and 0.55 ev, which is connected with a surface effect. Irradiation with energies below the threshold for the introduction of volume defects (about 400 kev) quenches the photoconductivity in the shoulder and causes an increase in surface conductance in $n$-type, and a decrease in $p$-type specimens. 4.5-Mev irradiations (a) extend the shoulder to about 0.49 ev and (b) introduce a sharp peak at 0.39 ev. Both features are due to electronic transitions in the surface region. Field effect measurements with steady fields and a superimposed alternating potential indicate that the shoulder is connected with transitions from the valence band into a group of slow surface states lying between 0.70 and 0.58 ev above the valence band. Irradiation below 400 kev increases the surface potential $Y$ until, in $n$-type specimens, the Fermi level passes through a high concentration of fast states within 0.05 ev of the conduction band. 4.5-Mev irradiations decrease $Y$; transitions to a second group of slow states lying between 0.4 and 0.2 ev above the valence band can now take place.