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Energy band engineering of metal oxide for enhanced visible light absorption 53

4.2.3 Excitation and recombination of charge carriers

The charge carriers generated in a semiconductor upon light illumination can be
extracted for further applications, that is for “optoelectronics.” Various electronic
transitions are possible upon light excitation because semiconductors' crystals are not
perfect and they contain various intrinsic defects or are manually doped with impuri-
ties for special purposes (Fig. 4.3) [17–23]. In addition to the band-band transitions
(a), an excitation of an electron from a donor state or an impurity level (b) into the CB
(or from the VB to a acceptor band, (c) is feasible. If the impurity concentration is very
small, the absorption cross section and the corresponding absorption coefficient will
be smaller by many orders of magnitude than those for a band-band transition.
The excited charge carriers are apt to relax back to their equilibrium states. This
is the recombination of charge carriers. There are three popular recombination mech-
anisms (Fig. 4.3): radiative recombination (d), trap recombination (e), and Auger re-
combination (f), respectively [19–23]. Radiative recombination is the reverse process
of the band-band absorption, where the photogenerated electron drops back to its
empty equilibrium energy band and at the same time radiates a photon. The pho-
ton emitted may have the energy of the band gap or less. This recombination oc-
curs primarily in direct band gap semiconductors. The trap recombination, also called
Shockley–Read–Hall recombination, occurs when an electron falls into an internal
band trap state caused by an alien doping or a structural defect. This type of recombi-
nation always takes place when the defect level lies near the middle of the forbidden
band. Therefore impurities that introduce energy levels near midgap (deep-level) are
very effective recombination centers. This also happens on the surface, where there is
an abundance of defects that introduce trapping states. Therefore trap recombination
by defect levels contributes significantly to charge carriers' loss at surfaces. Auger re-
combination is similar to radiative recombination, but the excess energy given off by
the electron is transferred to a second electron instead of just emitting the energy as a
photon. Photocatalysis takes place if the photogenerated charge carriers separate and
survive to the surface for chemical reactions without being recombined, which will be
discussed in the following sections.

CB

E A
b
a c d e f
E D

VB
Fig. 4.3 Excitation and recombination of electrons in a semiconductor. a: band-band
absorption, b and c: intraband absorptions, d:radiative emission, e: trap recombination,
and f: Auger recombination.