Delta doping in MBE has attracted considerable attention since its inception in 1980 , as a vehicle for studies of 2D phenomena and for its potential applications in, for example, FET devices  and I.R. detectors . Boron delta doping in silicon is a recent development . In previous papers we have reported on the growth, TEM, CV profiling, SIMS profiling and XRD characterisation of B delta layers [4,5,6]. It is of interest to know if the present growth method  produces a dopant sheet a monolayer wide or, in any event, what is the true width. Previous techniques [4,5,6] have yielded values between 0.4nm and 3nm for the width. Schottky barrier tunnelling spectroscopy provides further information on this question, but is also of interest for a study of the electronic structure of the delta layer. It has been used previously for a study of delta doping in III-V materials  and also for Sb delta doping in Si [8,9], but there have so far been no reports of its application to Si:B delta layers.