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Unfavourable
CBM
V
el
/2
E
F
E
0
E
0
-
t
-
U/
2
R
12
R
12
~4d
VBM
(a)
(b)
Fig. 7.
The schematic energy landscape of a DB pair with each DB represented by a
potential well with the ground state in the band gap. In (a), the separation between DBs
is very large for the Coulombic interaction to be negligible and each DB is negatively
changed (doubly occupied). In (b), DBs are much closer together (d is the dimer-
dimer spacing) and a great Coulombic repulsion is associated with the doubly occupied
configuration on both DBs. The diagram also shows the relaxed situation resulting
after removal of one electron to the conduction band thus enabling tunneling of the
remaining excess electron between the DBs.
Fig. 8.
(a) Several tunnel-coupled pairs of DBs fabricated at different separations (spec-
ified in each case) on the H-Si(100) surface. (b) Average occupation probability of a
DB pair with 1 and 2 excess electrons as a function of DB separation. The three cases
labeled in (a) are marked here with blue arrows (Color figure online).
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