288
9 Optical Properties
(a)
4
40
50
60
-1
2p
2p 0
3p 4p
E D
2p
3p
2s
3p 0
Si
Sn
Pb
GaAs
30
5
6
7
2p
1.9T
0T
(b)
48
50
52
54
)
-1
GaAs
1s-3p
1s-3p 0
dark
illuminated
Fig. 9.34 a Far-infrared photoconductivity response (Lyman-type s→p series) of not intentionally doped GaAs with
residual donors Pb, Sn, and Si, N A = 2.6 × 10 13 cm −3 , N D − N A = 8 × 10 12 cm −3 . The upper (lower) curve is
for a magnetic field of 0 (1.9) T. Measurement temperature is 4.2 K. b Photoconductive response of a (different) GaAs
sample with the same impurities (N D = 1 × 10 13 cm −3 ) with (upper curve) and without (lower curve) illumination with
above-bandgap light (B = 1.9 T, T = 4.2 K). Adapted from [905]
causes the p states (and states with higher orbital angular momentum) to split into p 0 and p ± states.
Such an effect is absent in a direct semiconductor with an isotropic conduction-band minimum such
as GaAs (Fig. 9.34). Optical transitions between the 1s and various p states can be directly observed
in absorption, e.g. for Si:P in [904]. These transitions are also observed in photoconductivity because
the missing energy to the ionization into the continuum is supplied by a phonon at finite temperature
(photothermal ionization) (Fig. 9.33b) [903]. The splitting of the 2p transition in Fig. 9.34a is the
chemical shift due to different donors incorporated in the GaAs (Si, Sn, and Pb). Peak broadening is
mostly due to Stark broadening due to neighboring charged impurities. The application of a magnetic
field induces Zeeman-like splittings and increases the sharpness of the peaks. The peak width can be
500
N (cm )
D
-3
0
50
100
-1
-1
GaAs:Te
E D
b
0
2.1 10
16
1.0 10
15
6.7 10
15
1.0 10
14
Fig. 9.35 Low-temperature (T = 1.35 K) absorption spectra of highly doped n-type GaAs:Te with doping concentrations
as labeled (circles: N D = 2.1 × 10 16 cm −3 , stars: 6.7 × 10 14 , triangles: 1.0 × 10 15 ). A sharp photoconductivity spectrum
(in arbitrary units) from low-doped GaAs:Te (crosses, N D = 1.0 × 10 14 cm −3 ) is shown for comparison (cf. Fig. 9.34a).
The energy of the 1s→2p transition and the donor binding energy (onset of continuum absorption) are indicated. Adapted
from [906]
9 Optical Properties
(a)
4
40
50
60
-1
2p
2p 0
3p 4p
E D
2p
3p
2s
3p 0
Si
Sn
Pb
GaAs
30
5
6
7
2p
1.9T
0T
(b)
48
50
52
54
)
-1
GaAs
1s-3p
1s-3p 0
dark
illuminated
Fig. 9.34 a Far-infrared photoconductivity response (Lyman-type s→p series) of not intentionally doped GaAs with
residual donors Pb, Sn, and Si, N A = 2.6 × 10 13 cm −3 , N D − N A = 8 × 10 12 cm −3 . The upper (lower) curve is
for a magnetic field of 0 (1.9) T. Measurement temperature is 4.2 K. b Photoconductive response of a (different) GaAs
sample with the same impurities (N D = 1 × 10 13 cm −3 ) with (upper curve) and without (lower curve) illumination with
above-bandgap light (B = 1.9 T, T = 4.2 K). Adapted from [905]
causes the p states (and states with higher orbital angular momentum) to split into p 0 and p ± states.
Such an effect is absent in a direct semiconductor with an isotropic conduction-band minimum such
as GaAs (Fig. 9.34). Optical transitions between the 1s and various p states can be directly observed
in absorption, e.g. for Si:P in [904]. These transitions are also observed in photoconductivity because
the missing energy to the ionization into the continuum is supplied by a phonon at finite temperature
(photothermal ionization) (Fig. 9.33b) [903]. The splitting of the 2p transition in Fig. 9.34a is the
chemical shift due to different donors incorporated in the GaAs (Si, Sn, and Pb). Peak broadening is
mostly due to Stark broadening due to neighboring charged impurities. The application of a magnetic
field induces Zeeman-like splittings and increases the sharpness of the peaks. The peak width can be
500
N (cm )
D
-3
0
50
100
-1
-1
GaAs:Te
E D
b
0
2.1 10
16
1.0 10
15
6.7 10
15
1.0 10
14
Fig. 9.35 Low-temperature (T = 1.35 K) absorption spectra of highly doped n-type GaAs:Te with doping concentrations
as labeled (circles: N D = 2.1 × 10 16 cm −3 , stars: 6.7 × 10 14 , triangles: 1.0 × 10 15 ). A sharp photoconductivity spectrum
(in arbitrary units) from low-doped GaAs:Te (crosses, N D = 1.0 × 10 14 cm −3 ) is shown for comparison (cf. Fig. 9.34a).
The energy of the 1s→2p transition and the donor binding energy (onset of continuum absorption) are indicated. Adapted
from [906]