10.11 Field Effect
331
between 1–10
4 cm/s [1031, 1032]. The Si-SiO 2 interface can exhibit S ≤ 0.5 cm/s. Time-resolved
measurements and detailed modeling for Si have been reported in [1033].
The recombination velocity at surfaces can be reduced using suitable passivation schemes. Typically
inert layers or chemical treatments are used. Surface passivation can be accomplished in two fundamentally different ways: Either the surface defect states themselves are removed or an internal electrical
field is established that screens excess charge carriers from the surface defects. Sulfur chemistry is a
popular treatment of GaAs surfaces. A review of the passivation schemes for III–V semiconductors
can be found in [1035].
10.12.2 Grain Boundaries
Grain boundaries can be a source of non-radiative recombination. This is technologically important for
solar cells made from polycrystalline silicon (cf. Sect. 22.4.6). The grain boundary can be understood
as an inner surface in the crystal. Modeling of recombination at a grain boundary can be done using
an interface recombination velocity [1036, 1037] or considering deep traps [1038]. The minority
carrier lifetime decreases with decreasing grain boundary area A (Fig. 10.33a). The carrier loss at
a grain boundary can be imaged directly via the efficiency of the collection of an electron beam
induced current (EBIC) as shown in Fig. 10.33b. The minority carrier lifetime is only unaffected when
the average distance to a grain boundary is much larger than the minority carrier diffusion length,
√
A L D , otherwise the entire grain volume is subject to non-radiative recombination.
The recombination velocity at grain boundaries can be reduced, similar to that at surfaces, using
suitable passivation schemes. A famous one is the chlorine treatment of polycrystalline CdTe used in
thin film solar cells [1041].
10.12.3 Dislocations
Also dislocations typically act as recombination centers, sometimes called carrier sinks. In Fig. 10.34
it can be seen that the minority carrier lifetime depends on the dislocation density n d and follows a
τ
−1
∝ n d law, as if each dislocation is a recombination center [1042]. The non-radiative recombination
GaAs
10
16
10
17
10
18
10
19
10
5
10
6
10
4
10
7
10
15
-3
Fig. 10.32 Surface recombination velocity for GaAs as a function of n-type doping concentration. Different experimental
points correspond to different surface treatment methods. Dashed line is a guide to the eye. Experimental data from [1034]
331
between 1–10
4 cm/s [1031, 1032]. The Si-SiO 2 interface can exhibit S ≤ 0.5 cm/s. Time-resolved
measurements and detailed modeling for Si have been reported in [1033].
The recombination velocity at surfaces can be reduced using suitable passivation schemes. Typically
inert layers or chemical treatments are used. Surface passivation can be accomplished in two fundamentally different ways: Either the surface defect states themselves are removed or an internal electrical
field is established that screens excess charge carriers from the surface defects. Sulfur chemistry is a
popular treatment of GaAs surfaces. A review of the passivation schemes for III–V semiconductors
can be found in [1035].
10.12.2 Grain Boundaries
Grain boundaries can be a source of non-radiative recombination. This is technologically important for
solar cells made from polycrystalline silicon (cf. Sect. 22.4.6). The grain boundary can be understood
as an inner surface in the crystal. Modeling of recombination at a grain boundary can be done using
an interface recombination velocity [1036, 1037] or considering deep traps [1038]. The minority
carrier lifetime decreases with decreasing grain boundary area A (Fig. 10.33a). The carrier loss at
a grain boundary can be imaged directly via the efficiency of the collection of an electron beam
induced current (EBIC) as shown in Fig. 10.33b. The minority carrier lifetime is only unaffected when
the average distance to a grain boundary is much larger than the minority carrier diffusion length,
√
A L D , otherwise the entire grain volume is subject to non-radiative recombination.
The recombination velocity at grain boundaries can be reduced, similar to that at surfaces, using
suitable passivation schemes. A famous one is the chlorine treatment of polycrystalline CdTe used in
thin film solar cells [1041].
10.12.3 Dislocations
Also dislocations typically act as recombination centers, sometimes called carrier sinks. In Fig. 10.34
it can be seen that the minority carrier lifetime depends on the dislocation density n d and follows a
τ
−1
∝ n d law, as if each dislocation is a recombination center [1042]. The non-radiative recombination
GaAs
10
16
10
17
10
18
10
19
10
5
10
6
10
4
10
7
10
15
-3
Fig. 10.32 Surface recombination velocity for GaAs as a function of n-type doping concentration. Different experimental
points correspond to different surface treatment methods. Dashed line is a guide to the eye. Experimental data from [1034]