Paper SS2-FrM5
Dopant-enhanced Neutralization of Low Energy Li⁺ Scattered from Si(111)

Friday, November 4, 2011, 9:40 am, Room 109

The role of doping in semiconductor surface reactions is of fundamental scientific and technological importance, yet the effects of doping in atom-surface charge exchange have never been investigated directly. For example, in the dry processing of silicon, a dependence of oxidation, etching, and silicide formation rates on doping has been observed. The doping concentration changes the density of states at the surface by adding excess majority carriers, which could have a large effect on electron tunneling rates between reactants and the surface, leading to the observed doping dependences. In addition to populating the bands, the band gap of Si narrows with increasing dopant density, which may also influence charge exchange.

This work presented here demonstrates that the charge exchange between scattered low-energy Li ions and a passivated Si surface depends strongly on doping. The neutralization of 3 keV Li⁺ ions scattered from Si(111) is measured as a function of doping density, dopant type, and hydrogen coverage using time-of-flight spectroscopy. When the surfaces are saturated with hydrogen to unpin the Fermi level, the neutral fractions decrease for lightly doped samples, but become anomalously large for highly doped n-type Si. The neutralization does not correlate with the surface work function, indicating that the models used for metal surfaces are not directly applicable here.

A model is presented that includes the many-body band-gap narrowing effect, which predicts the neutralization to good accuracy using a tunneling mechanism similar to the free-electron gas jellium model normally employed for ion/metal interactions, but excluding levels in the gap. This work demonstrates that the surface of Si behaves, as far as electron transfer is concerned, like a jellium electron gas with states missing in the band gap region.