7.5 Shallow Defects
193
Fig. 7.13 Filled-state
image of a phosphorus
atom underneath a Si (001)
surface at a tunneling
current of 110 pA. The
doping level is
5 × 10 17 cm −3 . a Sample
bias −0.6 V, b sample bias
−1.5 V between Si:P and
tip. Image sizes are
22 × 22 nm 2 . Reprinted
with permission from
[595], ©2004 APS. Lower
row under parts b, c:
Schematic band diagrams
for the two bias situations
(b)
(a)
E F
E F
Si
D
tip
Fig. 7.14 Boron impurity
in silicon. Boron accepts
one electron and a fixed
negative charge remains
Si
Si
Si
B
Si
Si
Si
Si
valence band (Fig. 7.14). The energy level of the impurity is in the gap close to the valence-band edge.
The latter consideration is made in the electron picture. In the hole picture, the acceptor ion has a hole
and the hole ionizes (at sufficient temperature) into the valence band. After ionization the acceptor
is charged negatively. Also, for this system a hydrogen-like situation arises that is, however, more
complicated than for donors because of the degeneracy of the valence bands and their warping.
In Table 7.4 the acceptor binding energies E
b
A for group-III atoms in C, Ge and Si are listed. The
absolute acceptor energy is given as E A = E V + E
b
A . In Table 7.5 acceptor binding energies are listed
for GaAs, GaP and GaN. While in GaAs some acceptors are close to the effective mass value of 27 meV,
in GaP the deviation from the effective-mass value ≈50 meV is large.
When the conductivity is determined by holes or electrons, the material is called p-type or n-type,
respectively. We note that some metals also show hole conduction (e.g. Al). However, for metals the