Investigation of band bending in silicon by slow positron lifetime measurements

A novel approach to the analysis of positron lifetime data in depth sensitive studies using a pulsed positron beam is applied to model the diffusion of positrons in silicon. By examining only the observable lifetime parameters, inhomogeneous effects can be studied without the need to solve the time dependent diffusion equation. In particular, we study the effect of band bending near the sample surface, which creates an internal electric field. We present our first results of this analysis on p- and n-type bulk silicon samples. No band-bending could be measured for samples covered with a natural oxide. After surface treatment with HF(aq), p-type samples exhibited band bending, but the results for n-type samples were not significantly affected. In only one case, a p-type Si(B) sample with acceptor concentration ca = 1020 cm−3 and with a H terminated surface, is evidence of Fermi level pinning observed.