218
7 Electronic Defect States
Fig. 7.42 Pressure dependence of the energy of excitons bound to isolated nitrogen impurities in GaAs (circles),
measured from the top of the GaAs valence band. The dashed lines are the pressure dependent GaAs bulk band gaps
(cmp. Fig. 6.49). The solid (dash-dotted) line is a theoretical model for the nitrogen-bound exciton (electron) level.
Adapted from [693]
4.26
4.24
4.22
2
1
0
z=1.56nm
0
100
300
200
400
(c)
(d)
(a)
(b)
Fig. 7.43 Image of a topography (z = 2.8 nm) and b work function (φ = 4.21–4.26 eV) of a surface step along
[111] on a n-GaP(110) surface cleaved in UHV. Adapted from [696]. c and d show the corresponding linescans. Adapted
from [696]
Such states can lie in the band gap and capture electrons, leading to recombination and a depletion
layer. For a brief introduction on semiconductor surface physics see Chap. 11; for more details we refer
to [695].
As an example of the formation of electronic states at surface defects we show in Fig. 7.43 the
comparison of topography and work function (measured by Kelvin probe force microscopy [696]) at
a surface step on a GaP(110) surface that has been prepared by cleaving in-situ in ultrahigh vacuum
(UHV). The depletion-type band bending of the surface is about 0.4 eV. The further increase of the
position of the vacuum level at the step edge shows the presence of trap states in the band gap causing
the conduction band to bend upwards (cf. Sect. 21.2.1). Modeling of the effect shows that the charge
density at the surface is 6 × 10
11 cm
−2 and at the step edge 1.2 × 10
6 cm
−1 .
7 Electronic Defect States
Fig. 7.42 Pressure dependence of the energy of excitons bound to isolated nitrogen impurities in GaAs (circles),
measured from the top of the GaAs valence band. The dashed lines are the pressure dependent GaAs bulk band gaps
(cmp. Fig. 6.49). The solid (dash-dotted) line is a theoretical model for the nitrogen-bound exciton (electron) level.
Adapted from [693]
4.26
4.24
4.22
2
1
0
z=1.56nm
0
100
300
200
400
(c)
(d)
(a)
(b)
Fig. 7.43 Image of a topography (z = 2.8 nm) and b work function (φ = 4.21–4.26 eV) of a surface step along
[111] on a n-GaP(110) surface cleaved in UHV. Adapted from [696]. c and d show the corresponding linescans. Adapted
from [696]
Such states can lie in the band gap and capture electrons, leading to recombination and a depletion
layer. For a brief introduction on semiconductor surface physics see Chap. 11; for more details we refer
to [695].
As an example of the formation of electronic states at surface defects we show in Fig. 7.43 the
comparison of topography and work function (measured by Kelvin probe force microscopy [696]) at
a surface step on a GaP(110) surface that has been prepared by cleaving in-situ in ultrahigh vacuum
(UHV). The depletion-type band bending of the surface is about 0.4 eV. The further increase of the
position of the vacuum level at the step edge shows the presence of trap states in the band gap causing
the conduction band to bend upwards (cf. Sect. 21.2.1). Modeling of the effect shows that the charge
density at the surface is 6 × 10
11 cm
−2 and at the step edge 1.2 × 10
6 cm
−1 .