Out segregation of boron from the bulk of highly doped Si samples leads to surfaces which present a B-Si(111)square-root 3 X square-root 3 R(30-degrees) reconstruction (hereafter abbreviated B square-root 3). Scanning tunneling microscopy investigations show that, depending on the annealing temperatures, different boron equilibrium surface concentrations are obtained in the B square-root 3 substrates. Comparisons of Cu growth on such substrates show that copper nucleation is highly correlated to the density of dangling bonds at the surface.