214
7 Electronic Defect States
Carriers can be removed from the DX center by optical absorption of photons with energy larger than
about 1.2 eV. If carriers are removed by optical excitation at low temperatures the (re-)capture is so slow
(σ < 10
−30 cm
2 ) that the carriers remain in the conduction band and cause persistent photoconductivity
(PPC). The PPC is only reduced upon increasing the sample temperature. The concentration of the DX
center is about the same as the net doping concentration.
The properties of the DX center are reviewed in [670, 671]. So far, no definite microscopic model
of the DX center has been agreed on. Lang [672] proposed that the DX center involves a donor
and an unknown defect (probably a vacancy). It probably involves large lattice relaxation as in the
configuration coordinates model of Fig. 7.37 where the donor binding energy E
b
D with respect to the
conduction-band minimum, the barrier for electron capture E c , the barrier for electron emission E e and
the optical ionization energy E o are labeled. The donor binding energy is measured with Hall effect
(cf. Sect. 15.2.1) at temperatures sufficient to overcome the capture and emission barriers, the emission
barrier is measured with deep level transient spectroscopy (DLTS). The capture barrier manifests itself
in PPC experiments. We note that the DX center is related to the L-conduction band. For small Al mole
fraction, the DX level is degenerate with the -related conduction band (see Fig. 7.37b).
Theoretical models and experimental evidence hint at a vacancy-interstitial model for the Si-DX
center [673]. The donor (Si) is displaced along the 111 direction from the Ga substitution site. The
displacement is predicted to be 0.117 nm and the distorted geometry can be viewed as a Ga vacancy and
a Si interstitial. The charge state of the (filled) DX center is proposed to be a two-electron negative-U
state.
7.7.7 EL2 Defect
The EL2 defect is a deep donor in GaAs. It is not related to impurities but occurs for intrinsic material,
in particular grown under As-rich conditions. It has physical properties similar to the DX center.
The bleaching of absorption due to EL2, i.e. the optical removal of electrons from the defect at low
temperatures, is shown in Fig. 7.38. The microscopic model [674] describes the EL2 defect as an
arsenic antisite defect, i.e. an arsenic atom on a Ga site, As Ga . In the charged state the arsenic atom is
displaced from the lattice position and a complex of a Ga vacancy (symmetry T 3d ) and an interstitial
As (symmetry C 3v ) with 0.14 nm displacement along 111 forms (V Ga -As i ). The charged state is filled
with two electrons.
Fig. 7.36 Energy barrier
for electron capture E c at
the Si-DX center in
Al x Ga 1−x As for various
compositions.
Experimental data
from [669]
0.5
0.4
0.3
0.2
0.1
0.3 0.4 0.5 0.6 0.7
Al Ga As:Si
x
1-x
7 Electronic Defect States
Carriers can be removed from the DX center by optical absorption of photons with energy larger than
about 1.2 eV. If carriers are removed by optical excitation at low temperatures the (re-)capture is so slow
(σ < 10
−30 cm
2 ) that the carriers remain in the conduction band and cause persistent photoconductivity
(PPC). The PPC is only reduced upon increasing the sample temperature. The concentration of the DX
center is about the same as the net doping concentration.
The properties of the DX center are reviewed in [670, 671]. So far, no definite microscopic model
of the DX center has been agreed on. Lang [672] proposed that the DX center involves a donor
and an unknown defect (probably a vacancy). It probably involves large lattice relaxation as in the
configuration coordinates model of Fig. 7.37 where the donor binding energy E
b
D with respect to the
conduction-band minimum, the barrier for electron capture E c , the barrier for electron emission E e and
the optical ionization energy E o are labeled. The donor binding energy is measured with Hall effect
(cf. Sect. 15.2.1) at temperatures sufficient to overcome the capture and emission barriers, the emission
barrier is measured with deep level transient spectroscopy (DLTS). The capture barrier manifests itself
in PPC experiments. We note that the DX center is related to the L-conduction band. For small Al mole
fraction, the DX level is degenerate with the -related conduction band (see Fig. 7.37b).
Theoretical models and experimental evidence hint at a vacancy-interstitial model for the Si-DX
center [673]. The donor (Si) is displaced along the 111 direction from the Ga substitution site. The
displacement is predicted to be 0.117 nm and the distorted geometry can be viewed as a Ga vacancy and
a Si interstitial. The charge state of the (filled) DX center is proposed to be a two-electron negative-U
state.
7.7.7 EL2 Defect
The EL2 defect is a deep donor in GaAs. It is not related to impurities but occurs for intrinsic material,
in particular grown under As-rich conditions. It has physical properties similar to the DX center.
The bleaching of absorption due to EL2, i.e. the optical removal of electrons from the defect at low
temperatures, is shown in Fig. 7.38. The microscopic model [674] describes the EL2 defect as an
arsenic antisite defect, i.e. an arsenic atom on a Ga site, As Ga . In the charged state the arsenic atom is
displaced from the lattice position and a complex of a Ga vacancy (symmetry T 3d ) and an interstitial
As (symmetry C 3v ) with 0.14 nm displacement along 111 forms (V Ga -As i ). The charged state is filled
with two electrons.
Fig. 7.36 Energy barrier
for electron capture E c at
the Si-DX center in
Al x Ga 1−x As for various
compositions.
Experimental data
from [669]
0.5
0.4
0.3
0.2
0.1
0.3 0.4 0.5 0.6 0.7
Al Ga As:Si
x
1-x