Figure 11

STM images of the Si(001)- c (4 × 4) surface: a empty states, 19 × 14 nm, U _s = +2.5 V, I _t = 120 pA; b empty states, 19 × 12 nm, U _s = +2.0 V, I _t = 120 pA; c filled states, 22 × 15 nm, U _s = -3.9 V, I _t = 150 pA; d filled states, 19 × 20 nm, U _s = -3.9 V, I _t = 150 pA. As it is clearly observed in a, the structure is composed by a mixture of the α-c(4 × 4) and β-c(4 × 4) modifications; it is seen in c that the c(4 × 4) and (2 × 1) reconstructions coexist on the surface; location of dimers forming the c(4 × 4) structure with respect to the dimers of the (2 × 1) structure is also seen; ad-dimers in both epitaxial and non-epitaxial orientations are seen in c. The β-c(4 × 4) modification prevails in d which is only partially occupied by c(4 × 4).