200
7 Electronic Defect States
without leaving their lattice site. The isomer shifts of
119 Sn in various III-V compounds are shown in
Fig. 7.22. In [615] it is concluded from these data that the tin donor is formed by a positive tin ion and
the electron charge transfer to its neighboring (group-V) atoms is rather small. For tin as an acceptor,
for the present conditions an ionized, i.e. negatively charged acceptor, the isomer shift follows closely
the trend from substitution in group-IV semiconductors. Therefore four electrons form the tetrahedral
bond, while the extra electron is located rather at the (positively charged) group-III next neighbors and
not in the impurity cell. The difference to the point charge Coulomb distribution is called central-cell
correction.
Deviation from the ideal stoichiometry introduces point defects that can be electrically active and
change conductivity type and carrier concentration. In the case of CuInSe 2 , excess Cu could go on
interstitial positions or promote selenium vacancies, both leading to n-type behavior. This material
is particularly sensitive to deviations from ideal stoichiometry for both Cu/In ratio (Fig. 7.23) and Se
deficiency [616].
7.5.6 Autodoping
If intrinsic defects such as vacancies or interstitials, possibly as a result of non-stoichiometry, or antisite defects cause electronic levels relevant for conductivity one speaks of autodoping. An example is
the role of A-B antisites in AB 2 O 4 spinels (Sect. 3.4.7). In the perfect crystal the A (B) atoms occupy
tetraeder (octaeder) places. Typical charges are A
2+ and B
3+ . Thus (without charge transfer) the A
Fig. 7.21 Background
doping of GaAs due to
carbon in MOVPE for
different ratios of the
partial pressures of AsH 3
and TMG
(trimethylgallium). The
conductivity type (blue
squares: n-type, red
circles: p-type) depends on
the incorporation of C from
CH 3 radicals on Ga- or
As-site. Lines are guides to
the eye. Experimental data
from [613]
Fig. 7.22 Isomer shift
(relative to CaSnO 3 ) of
119 Sn in various group-IV
and III-V compound
semiconductors as labeled.
Dashed line is trend from
isoelectronic substitution.
Experimental data
from [615]
7 Electronic Defect States
without leaving their lattice site. The isomer shifts of
119 Sn in various III-V compounds are shown in
Fig. 7.22. In [615] it is concluded from these data that the tin donor is formed by a positive tin ion and
the electron charge transfer to its neighboring (group-V) atoms is rather small. For tin as an acceptor,
for the present conditions an ionized, i.e. negatively charged acceptor, the isomer shift follows closely
the trend from substitution in group-IV semiconductors. Therefore four electrons form the tetrahedral
bond, while the extra electron is located rather at the (positively charged) group-III next neighbors and
not in the impurity cell. The difference to the point charge Coulomb distribution is called central-cell
correction.
Deviation from the ideal stoichiometry introduces point defects that can be electrically active and
change conductivity type and carrier concentration. In the case of CuInSe 2 , excess Cu could go on
interstitial positions or promote selenium vacancies, both leading to n-type behavior. This material
is particularly sensitive to deviations from ideal stoichiometry for both Cu/In ratio (Fig. 7.23) and Se
deficiency [616].
7.5.6 Autodoping
If intrinsic defects such as vacancies or interstitials, possibly as a result of non-stoichiometry, or antisite defects cause electronic levels relevant for conductivity one speaks of autodoping. An example is
the role of A-B antisites in AB 2 O 4 spinels (Sect. 3.4.7). In the perfect crystal the A (B) atoms occupy
tetraeder (octaeder) places. Typical charges are A
2+ and B
3+ . Thus (without charge transfer) the A
Fig. 7.21 Background
doping of GaAs due to
carbon in MOVPE for
different ratios of the
partial pressures of AsH 3
and TMG
(trimethylgallium). The
conductivity type (blue
squares: n-type, red
circles: p-type) depends on
the incorporation of C from
CH 3 radicals on Ga- or
As-site. Lines are guides to
the eye. Experimental data
from [613]
Fig. 7.22 Isomer shift
(relative to CaSnO 3 ) of
119 Sn in various group-IV
and III-V compound
semiconductors as labeled.
Dashed line is trend from
isoelectronic substitution.
Experimental data
from [615]