7.5 Shallow Defects
201
Fig. 7.23 Carrier
concentration and
conductivity type (blue
squares: n-type, red circles:
p-type) as a function of
stoichiometry for CuInSe 2
thin films. Lines are guides
to the eye. Experimental
data from [616]
Fig. 7.24 Schematic
position of electronic levels
of A B (blue, 0/− transition
level) and B A (red, +/0
transition level) defects in
AB 2 O 4 spinels and
resulting material
properties (compensated,
n- or p-type or
semi-insulating).
After [617]
VB
CB
B Td
A oh
E F
comp. p-type n-type
s.i.
atom on octaeder site (A Oh ) acts like a donor and the B atom on a tetraeder site (B Td ) as an acceptor.
Such defects have been classified in [617] as being able to create compensated, semi-insulating, ntype or p-type material depending on the defect formation energies and the position of the electronic
levels of the A B and B A defects in the band gap (Fig. 7.24). An example for a p-type spinel oxide is
ZnCo 2 O 4 [618].
7.5.7 High Doping
For low doping concentrations, the impurity atoms can be considered to be decoupled. At low temperature, only hopping from one impurity to the next is possible due to thermal emission or tunneling and
the semiconductor becomes an insulator.