Ion induced electron emission: The effects of conduction band electrons and surface density of states

Recent plasma based measurements related to ion induced electron emission (IIEE) have suggested that conduction band electrons in semiconductors may significantly contribute to the overall electron emission. The foundation behind this hypothesis is simple: more electrons near the vacuum level equals more electron emission. Although intuitive, the hypothesis is in direct conflict with fundamental theory that has assumed since the 1960s that only valence band electrons contribute to the primary IIEE mechanism of Auger neutralization. To resolve this apparent contradiction we executed an experimental plan to test conclusively if electrons in the conduction band contribute to the IIEE yield.;The experimental plan consisted of measuring the IIEE yield from Si and Ge for p-type, intrinsic and N-type surfaces. Theoretical calculations of the bulk electron concentration indicate that from p-type to n-type Si the electron density in the conduction band varies by 15 orders of magnitude. For Ge there is 10 order of magnitude variation from p-type to n-type. We found that despite this significant increase in the bulk electron concentration the measured IIEE yield was not found to increase from p-type and n-type for Si or Ge. We verified that the independence of the IIEE yield from the conduction band electron density is not due to surface contamination, by making measurements on chemically cleaned and sputter cleaned surfaces. Both surface types showed a similar trend but sputter cleaned surfaces had larger IIEE yields. We also verified that Fermi level pinning did not cause the independence of IIEE yield from the conduction band electron density by measuring the surface Fermi level using XPS.;Furthermore, since fundamental theory of Auger neutralization assumed that only valence band electrons contributed to the IIEE, we decided to extend the theory to include conduction band electrons. We found that the measured IIEE yield increased by 0.02% when the conduction band electron density was increased by 17 orders of magnitude. We were able to conclude that the insensitivity of the IIEE yield on the conduction band electron densities is due to the larger number of electrons in the valence band compared to the conduction band. Furthermore, chemically cleaned p-type Ge IIEE yields from our modified mass spectrometer suggests that at higher energies above 100 eV the emission process may no longer be dominated by Auger neutralization. This conclusion comes from the observations that the ions have an non-Auger ion kinetic energy dependence and that p-type Ge has large IIEE yields than n-type Ge despite having measurably identical surface density of states.