10.5 Self-Absorption
321
Fig. 10.22 Photoluminescence (solid line) and absorption (dashed line) spectra of excitons bound to substitutional
oxygen in ZnTe at T = 20 K. The energy position is relative to the A-line at 1.9860 eV. The vertical dashed lines have a
separation of 25.9 meV. Adapted from [1001]
separation of about 26 meV, the optical phonon energy in ZnTe. The Huang-Rhys parameter is about
3–4. Other peaks are due to acoustic phonons.
10.5 Self-Absorption
Luminescence that is emitted within the semiconductor can be (re-)absorbed before it may reach the
surface and can leave the crystal. This effect is called self-absorption. It is particularly strong for
radiation with an energy where the absorption α(ω) is high, i.e. above the band gap of a direct
semiconductor. Similarly to the penetration depth 1/α for radiation entering the crystal, emission
approximately occurs only from a layer of such thickness. For typical values of α in the range of
10
5 cm
−1 , the ‘skin’ of the semiconductor that emits light with energy above the band gap is 100 nm.
For light at the low energy side of the band gap or with energy within the band gap (deep levels), the
emission depth can be much larger.
After re-absorption, the energy has another chance to relax non-radiatively, thus reducing the quantum efficiency. Alternatively it can be reemitted, either at the same energy or at a lower energy. Possibly
several re-absorption processes occur before a photon eventually leaves the semiconductor (‘photon
recycling’). Such processes are important in LED structures where photon extraction has to be optimized (Sect. 23.3.4). Emission on phonon replica (Sect. 10.4) is red-shifted from the energy range of
strong absorption and thus suffers no (or only little) self-absorption. This can be seen from the spectrum of a thick ZnO crystal excited homogeneously (via two-photon absorption with a red Ruby laser),
Fig. 10.23. The zero phonon line (at E X ), originating from the ≈100 nm skin of the samples and being
by far the strongest in thin films (Fig. 10.5), is practically absent and emission on the phonon replica
collected from the entire volume dominates the spectrum.
10.6 Donor–Acceptor Pair Transitions
Optical transitions can occur between neutral donors and acceptors. The (spatially indirect) donor–
acceptor pair (DAP) recombination is present in (partially) compensated semiconductors and follows
321
Fig. 10.22 Photoluminescence (solid line) and absorption (dashed line) spectra of excitons bound to substitutional
oxygen in ZnTe at T = 20 K. The energy position is relative to the A-line at 1.9860 eV. The vertical dashed lines have a
separation of 25.9 meV. Adapted from [1001]
separation of about 26 meV, the optical phonon energy in ZnTe. The Huang-Rhys parameter is about
3–4. Other peaks are due to acoustic phonons.
10.5 Self-Absorption
Luminescence that is emitted within the semiconductor can be (re-)absorbed before it may reach the
surface and can leave the crystal. This effect is called self-absorption. It is particularly strong for
radiation with an energy where the absorption α(ω) is high, i.e. above the band gap of a direct
semiconductor. Similarly to the penetration depth 1/α for radiation entering the crystal, emission
approximately occurs only from a layer of such thickness. For typical values of α in the range of
10
5 cm
−1 , the ‘skin’ of the semiconductor that emits light with energy above the band gap is 100 nm.
For light at the low energy side of the band gap or with energy within the band gap (deep levels), the
emission depth can be much larger.
After re-absorption, the energy has another chance to relax non-radiatively, thus reducing the quantum efficiency. Alternatively it can be reemitted, either at the same energy or at a lower energy. Possibly
several re-absorption processes occur before a photon eventually leaves the semiconductor (‘photon
recycling’). Such processes are important in LED structures where photon extraction has to be optimized (Sect. 23.3.4). Emission on phonon replica (Sect. 10.4) is red-shifted from the energy range of
strong absorption and thus suffers no (or only little) self-absorption. This can be seen from the spectrum of a thick ZnO crystal excited homogeneously (via two-photon absorption with a red Ruby laser),
Fig. 10.23. The zero phonon line (at E X ), originating from the ≈100 nm skin of the samples and being
by far the strongest in thin films (Fig. 10.5), is practically absent and emission on the phonon replica
collected from the entire volume dominates the spectrum.
10.6 Donor–Acceptor Pair Transitions
Optical transitions can occur between neutral donors and acceptors. The (spatially indirect) donor–
acceptor pair (DAP) recombination is present in (partially) compensated semiconductors and follows