594
22 Light-to-Electricity Conversion
(f)
(g)
substrate
(h)
Fig. 22.5 Principle of xerography: a charging of the selenium-covered drum, b (reflection) exposure of the Se, exposed
areas become uncharged, c toner addition, d toner transfer to paper for copy, and e fixation of the toner on the copy
and preparation of drum for the next cycle. f First xerox copy (Oct. 22nd 1938). g Schematic cross section of coating
of photo-sensitive drum. The indicated thicknesses are approximate. h Image of drum with photo-sensitive layer made
from amorphous silicon. Part (h) from [1828]
Fig. 22.6 Schematic level
diagram of a quantum well.
Optical intersubband
transitions between the first
and second quantized level
(b–b) and the ground state
and the continuum (b–c)
growth
direction
b-b
L z
0
b-c
E
For infinite barrier height the energy separation between the first and second quantized levels (in
the effective-mass theory) is (cf. (12.6))
E 2 − E 1 = 3
2
2m ∗
π
2
L 2
z
.
(22.12)
For real materials the barrier height determines the maximum transition energy. Typical absorption
and transmission spectra of a QWIP structure are shown in Fig. 22.7. The spectral response is in the
mid- or far-infrared.
The dipole matrix element = = 2 |z| 1 can be easily calculated to be
=
16
9π 2 L z .
(22.13)
The oscillator strength is about 0.96. The polarization selection rule causes the absorption to vary
∝ cos
2
φ, where φ is the angle between the electric-field vector and the z direction (Fig. 22.8). This
means that for vertical incidence (φ = 90
◦ ) the absorption vanishes. Thus schemes have been developed
to allow for skew entry of the radiation (Fig. 22.9a). The strict selection rule can be relaxed by using
asymmetric potential wells (breaking of mirror symmetry/parity), strained materials (band mixing) or
22 Light-to-Electricity Conversion
(f)
(g)
substrate
(h)
Fig. 22.5 Principle of xerography: a charging of the selenium-covered drum, b (reflection) exposure of the Se, exposed
areas become uncharged, c toner addition, d toner transfer to paper for copy, and e fixation of the toner on the copy
and preparation of drum for the next cycle. f First xerox copy (Oct. 22nd 1938). g Schematic cross section of coating
of photo-sensitive drum. The indicated thicknesses are approximate. h Image of drum with photo-sensitive layer made
from amorphous silicon. Part (h) from [1828]
Fig. 22.6 Schematic level
diagram of a quantum well.
Optical intersubband
transitions between the first
and second quantized level
(b–b) and the ground state
and the continuum (b–c)
growth
direction
b-b
L z
0
b-c
E
For infinite barrier height the energy separation between the first and second quantized levels (in
the effective-mass theory) is (cf. (12.6))
E 2 − E 1 = 3
2
2m ∗
π
2
L 2
z
.
(22.12)
For real materials the barrier height determines the maximum transition energy. Typical absorption
and transmission spectra of a QWIP structure are shown in Fig. 22.7. The spectral response is in the
mid- or far-infrared.
The dipole matrix element = = 2 |z| 1 can be easily calculated to be
=
16
9π 2 L z .
(22.13)
The oscillator strength is about 0.96. The polarization selection rule causes the absorption to vary
∝ cos
2
φ, where φ is the angle between the electric-field vector and the z direction (Fig. 22.8). This
means that for vertical incidence (φ = 90
◦ ) the absorption vanishes. Thus schemes have been developed
to allow for skew entry of the radiation (Fig. 22.9a). The strict selection rule can be relaxed by using
asymmetric potential wells (breaking of mirror symmetry/parity), strained materials (band mixing) or