7.7 Deep Levels
215
(a)
q 0 q t
E D
b
E e
E opt
E DX
E C
q
E c
E
(b)
q 0
E
q t
E e
E C
E L
q
E c
DX
L
E DX
Fig. 7.37 a Schematic configuration coordinate diagram for the DX level with large lattice relaxation. q 0 is the configuration of the empty defect, q t is the configuration of the filled defect. The donor binding energy E b
D , the barrier for
electron capture E c , the barrier for electron emission E e and the optical ionization energy E o are labeled. E C denotes the
conduction-band edge. We note that in (Al,Ga)As the DX level is associated with the L conduction band (see Fig. 6.24).
b Schematic configuration coordinate diagram for the DX level in Al 0.14 Ga 0.86 As with the DX level being degenerate
with the (-related) conduction band
2
3
4
1
0
-1
GaAs
1.0
1.2
1.4
0.8
Fig. 7.38 Absorption spectrum of GaAs at low temperatures (T = 10 K) when cooled in the dark (solid line). The
dashed (dash-dotted) line is the absorption after illuminating the sample for 1 min (10 min) with white light, leading to
quenching of the EL2-related absorption. Adapted from [675]
7.7.8 Semi-insulating Semiconductors
Semiconductors with high resistivity (10
7 –10
9
cm) are called semi-insulating (‘s.i.’ or ‘si’). Semiinsulating substrates are needed for high-speed devices. The high resistivity should stem from a small
free-carrier density at finite temperature and not from a small mobility due to poor crystal quality.
For sufficiently wide band gap, the intrinsic carrier concentration is small and such pure material is
semi-insulating, e.g. GaAs with n i = 1.47 × 10
6 cm
−3 and 5.05 × 10
8
cm [676]. Since shallow
impurities are hard to avoid, another route is used technologically. Impurities that form deep levels are
incorporated in the semiconductor in order to compensate free carriers. For example, a deep acceptor
compensates all electrons if N A > N D . Since the acceptor is deep (E
b
A kT ), it does not release holes
for reasonable temperatures. Examples of suitable impurities for compensation of electrons are Si:Au
[677], GaAs:Cr [678] and InP:Fe [679]. A deep donor, e.g. InP:Cr [680], is necessary to compensate
p-type conductivity.
Figure 7.39a shows the terms of Fe in InP [681, 682]. An overview of transition metals in III–V
semiconductors can be found in [683]. The electron configuration of neutral Fe atoms is 3d
6 4s
2 (cf.
215
(a)
q 0 q t
E D
b
E e
E opt
E DX
E C
q
E c
E
(b)
q 0
E
q t
E e
E C
E L
q
E c
DX
L
E DX
Fig. 7.37 a Schematic configuration coordinate diagram for the DX level with large lattice relaxation. q 0 is the configuration of the empty defect, q t is the configuration of the filled defect. The donor binding energy E b
D , the barrier for
electron capture E c , the barrier for electron emission E e and the optical ionization energy E o are labeled. E C denotes the
conduction-band edge. We note that in (Al,Ga)As the DX level is associated with the L conduction band (see Fig. 6.24).
b Schematic configuration coordinate diagram for the DX level in Al 0.14 Ga 0.86 As with the DX level being degenerate
with the (-related) conduction band
2
3
4
1
0
-1
GaAs
1.0
1.2
1.4
0.8
Fig. 7.38 Absorption spectrum of GaAs at low temperatures (T = 10 K) when cooled in the dark (solid line). The
dashed (dash-dotted) line is the absorption after illuminating the sample for 1 min (10 min) with white light, leading to
quenching of the EL2-related absorption. Adapted from [675]
7.7.8 Semi-insulating Semiconductors
Semiconductors with high resistivity (10
7 –10
9
cm) are called semi-insulating (‘s.i.’ or ‘si’). Semiinsulating substrates are needed for high-speed devices. The high resistivity should stem from a small
free-carrier density at finite temperature and not from a small mobility due to poor crystal quality.
For sufficiently wide band gap, the intrinsic carrier concentration is small and such pure material is
semi-insulating, e.g. GaAs with n i = 1.47 × 10
6 cm
−3 and 5.05 × 10
8
cm [676]. Since shallow
impurities are hard to avoid, another route is used technologically. Impurities that form deep levels are
incorporated in the semiconductor in order to compensate free carriers. For example, a deep acceptor
compensates all electrons if N A > N D . Since the acceptor is deep (E
b
A kT ), it does not release holes
for reasonable temperatures. Examples of suitable impurities for compensation of electrons are Si:Au
[677], GaAs:Cr [678] and InP:Fe [679]. A deep donor, e.g. InP:Cr [680], is necessary to compensate
p-type conductivity.
Figure 7.39a shows the terms of Fe in InP [681, 682]. An overview of transition metals in III–V
semiconductors can be found in [683]. The electron configuration of neutral Fe atoms is 3d
6 4s
2 (cf.