216
7 Electronic Defect States
(a)
Fe
3+
E V
1.34
Fe
2+
InP:Fe
0.78
1.14
5
T 2
0
5
E
E C
0.25
(b)
10
16
10
17
0
0.2
0.4
0.6
8
.
0
0
.
1
-3
InP:Sn
InP:Sn,Fe
InP:Sn
n
Fig. 7.39 a Schematic band diagram of InP with levels of Fe impurities in the 3+ and 2+ charge states at low temperature.
All energies are given in eV. The arrow denotes capture of an electron (from the conduction band or a shallow donor)
on the deep acceptor. Compare this figure also with Figs. 9.36 and 10.25. b Depth profile of electron concentration
in an InP:Sn/InP:Sn,Fe/InP:Sn structure. The change n ≈ 4.5 × 10 16 cm −3 of electron concentration is due to the
compensation by Fe and corresponds to the chemical iron concentration determined by SIMS, [Fe] = 4.9 × 10 19 cm −3 .
Part b adapted from [688]
Table 17.2). The Fe is incorporated on the In site and thus has a Fe
3+ state as a neutral acceptor (A
0 ).
The Fe
3+ state has the electron configuration 3d
5 . The arrow in Fig. 7.39a represents the capture of
an electron from the conduction band or from a shallow donor. The charge state of the Fe becomes
Fe
2+ (charged acceptor, A
− ) with the electron configuration 3d
6 . The cubic crystal field (T d symmetry)
splits this
5 D Fe state
8 into two terms [684] that exhibit further fine structure [682]. The large thermal
activation energy of 0.64 eV found in the Hall effect on semi-insulating InP:Fe [679] corresponds to
the energy separation of the
5 E level and the conduction band.
The maximum electron concentration that can be compensated in this way is limited by the solubility of Fe in InP [685], about 1 × 10
17 cm
3 . Higher Fe incorporation leads to the formation of Fe (or
FeP) precipitates and degrades the crystal quality. Only a fraction of the incorporated Fe may then be
electrically active and contribute to the compensation. The maximum electrically active Fe concentration is found to be 5–6×10
16 cm
−3 [686]. The compensation can be directly visualized via the depth
profile of the electron concentration in a n-si-n structure (Fig. 7.39b). The poor thermal stability of Fe,
i.e. high diffusion coefficient, has evoked proposals for more stable dopants such as InP:Ru [687].
7.7.9 Isoelectronic Impurities
Isoelectronic impurities, generally represent a deep level with a short range potential. The isoelectronic
trap introduces a bound state for an electron or a hole. Once a carrier has been captured, the defect
becomes charged. The other carrier type is then easily trapped, forming a bound exciton (Sect. 10.3.2).
The theory of isoelectronic impurities is outlined in [689]. A detailed theoretical treatment of N in
GaAs and GaP is given in [544].
In GaP:N, an electron is spatially localized on the N impurity. Most of the wave function is at the
X-point. The nitrogen-bound electron level in GaP (A 1 symmetry) is close to the conduction band edge
and within the band gap. Important for the energy position is the lattice relaxation, leading to an inward
relaxation of the surrounding Ga atoms (Fig. 7.41). Due to the spatial localization of the wave function
it is delocalized in k-space (Fig. 7.40a) and obtains a sizeable component at the -point, facilitating
zero-phonon absorption from the valence band. This effect is present only when the lattice relaxation
around the impurity is considered; without relaxation the -component is zero, with relaxation about
1% [544]. The -component of the wave-function is larger for localization at an isoelectronic impurity
8 The notation is 2S+1 J (multiplicity), with S being the total spin and J being the total angular momentum.
7 Electronic Defect States
(a)
Fe
3+
E V
1.34
Fe
2+
InP:Fe
0.78
1.14
5
T 2
0
5
E
E C
0.25
(b)
10
16
10
17
0
0.2
0.4
0.6
8
.
0
0
.
1
-3
InP:Sn
InP:Sn,Fe
InP:Sn
n
Fig. 7.39 a Schematic band diagram of InP with levels of Fe impurities in the 3+ and 2+ charge states at low temperature.
All energies are given in eV. The arrow denotes capture of an electron (from the conduction band or a shallow donor)
on the deep acceptor. Compare this figure also with Figs. 9.36 and 10.25. b Depth profile of electron concentration
in an InP:Sn/InP:Sn,Fe/InP:Sn structure. The change n ≈ 4.5 × 10 16 cm −3 of electron concentration is due to the
compensation by Fe and corresponds to the chemical iron concentration determined by SIMS, [Fe] = 4.9 × 10 19 cm −3 .
Part b adapted from [688]
Table 17.2). The Fe is incorporated on the In site and thus has a Fe
3+ state as a neutral acceptor (A
0 ).
The Fe
3+ state has the electron configuration 3d
5 . The arrow in Fig. 7.39a represents the capture of
an electron from the conduction band or from a shallow donor. The charge state of the Fe becomes
Fe
2+ (charged acceptor, A
− ) with the electron configuration 3d
6 . The cubic crystal field (T d symmetry)
splits this
5 D Fe state
8 into two terms [684] that exhibit further fine structure [682]. The large thermal
activation energy of 0.64 eV found in the Hall effect on semi-insulating InP:Fe [679] corresponds to
the energy separation of the
5 E level and the conduction band.
The maximum electron concentration that can be compensated in this way is limited by the solubility of Fe in InP [685], about 1 × 10
17 cm
3 . Higher Fe incorporation leads to the formation of Fe (or
FeP) precipitates and degrades the crystal quality. Only a fraction of the incorporated Fe may then be
electrically active and contribute to the compensation. The maximum electrically active Fe concentration is found to be 5–6×10
16 cm
−3 [686]. The compensation can be directly visualized via the depth
profile of the electron concentration in a n-si-n structure (Fig. 7.39b). The poor thermal stability of Fe,
i.e. high diffusion coefficient, has evoked proposals for more stable dopants such as InP:Ru [687].
7.7.9 Isoelectronic Impurities
Isoelectronic impurities, generally represent a deep level with a short range potential. The isoelectronic
trap introduces a bound state for an electron or a hole. Once a carrier has been captured, the defect
becomes charged. The other carrier type is then easily trapped, forming a bound exciton (Sect. 10.3.2).
The theory of isoelectronic impurities is outlined in [689]. A detailed theoretical treatment of N in
GaAs and GaP is given in [544].
In GaP:N, an electron is spatially localized on the N impurity. Most of the wave function is at the
X-point. The nitrogen-bound electron level in GaP (A 1 symmetry) is close to the conduction band edge
and within the band gap. Important for the energy position is the lattice relaxation, leading to an inward
relaxation of the surrounding Ga atoms (Fig. 7.41). Due to the spatial localization of the wave function
it is delocalized in k-space (Fig. 7.40a) and obtains a sizeable component at the -point, facilitating
zero-phonon absorption from the valence band. This effect is present only when the lattice relaxation
around the impurity is considered; without relaxation the -component is zero, with relaxation about
1% [544]. The -component of the wave-function is larger for localization at an isoelectronic impurity
8 The notation is 2S+1 J (multiplicity), with S being the total spin and J being the total angular momentum.