7.8 The Charge Neutrality Level
219
7.8 The Charge Neutrality Level
The charge neutrality level (CNL) of a semiconductor is defined as the maximum occupied surface
state energy at a neutral surface. In this case it is identical with the Fermi level (which renders the
surface without a net charge). The CNL is also termed Fermi level stabilization energy [700] or
‘branch point energy’ [697] and marks the energy at which the character of intrinsic defects changes
from predominantly donor-like (below CNL) to predominantly acceptor-like (above CNL). If Fermi
level in the bulk and CNL deviate, surface charges appear; when the Fermi level is above (below) the
charge neutrality level, the surface is negatively (positively) charged. Whether this means a depletion
or accumulation layer depends on the conductivity type of the semiconductor. Band bending and space
charge regions are discussed in more detail further below (Sect. 12.3.4, Sect. 21.2.1). The position
of the CNL can be calculated from the Brillouin zone average of the conduction-to-valence band
difference [698, 699].
Experimentally, the Fermi level will be established at the CNL when a lot of deep defects are
introduced, e.g. by irradiation. For many semiconductors, the CNL is close to the middle of the band
gap (Si, GaAs). Notable exceptions are e.g. InAs or In 2 O 3 with a CNL within the conduction band
leading to n-type surface conduction.
7.9 Hydrogen in Semiconductors
The role of hydrogen in semiconductors was first recognized in studies of ZnO [701]. It is now clear
that hydrogen plays an important role in the passivation of defects. As a ‘small’ atom, it can attach
easily to dangling bonds and form an electron-pair bond. Thus, surfaces, grain boundaries, dislocations
and shallow (donor and acceptor) and deep impurity levels become passivated. A good overview and
many details of the physics and technological use of hydrogen in semiconductors can be found in [702,
703]. The hydrogen must be typically introduced as atomic species into semiconductors, e.g. from a
plasma in the vicinity of the surface or by ion irradiation.
With regard to silicon it is important to note that the Si–H bond is stronger than the Si–Si bond. Thus
a silicon surface under atomic hydrogen exhibits Si–H termination rather than Si–Si dimers [704]. Due
to the stronger bond, the hydrogenation leads to an increase of the silicon band gap, which can be used
for surface passivation [705], leading to reduced reverse diode current.
The hydrogen concentration in amorphous Si (a-Si) can be as high as 50% [706]. Electronic grade
a-Si contains typically 10–30 atomic% hydrogen and is thus rather a silicon–hydrogen alloy.
Hydrogen in crystalline silicon occupies the bond-center interstitial position (see Fig. 3.18b) as
shown in Fig. 7.44a. The complexes formed by hydrogen with shallow acceptors and donors have been
studied in detail. It is now generally accepted that for acceptors (e.g. boron) in silicon the hydrogen
is located close to the bond-center position of the Si–B pair (BM, bond minimum) as sketched in
Fig. 7.45a. The boron atom forms an electron-pair bond with three silicon atoms of the tetrahedra, the
fourth silicon bonds to the hydrogen atom. The complex therefore no longer acts as an acceptor. The
silicon atoms and the acceptor relax their positions. The adiabatic potential energy surface of hydrogen
in Si:B is shown in Fig. 7.44b. The hydrogen can sit on four equivalent sites (BM) along the 111
directions of the initial B–Si 4 tetrahedron. This reduces the symmetry, e.g. of H–B vibrations [708].
The energetic barrier for the hydrogen orientation has been determined to be 0.2 eV theoretically [707]
for a hydrogen motion along the path BM–C–BM in Fig. 7.44b. Stress (along [100] and [112]) reduces
the symmetry and leads to splitting of the local vibrational modes, now showing axial symmetry [709].
However, this preferential orientation disappears with an activation energy of 0.19 eV, close to the
theoretical value.
219
7.8 The Charge Neutrality Level
The charge neutrality level (CNL) of a semiconductor is defined as the maximum occupied surface
state energy at a neutral surface. In this case it is identical with the Fermi level (which renders the
surface without a net charge). The CNL is also termed Fermi level stabilization energy [700] or
‘branch point energy’ [697] and marks the energy at which the character of intrinsic defects changes
from predominantly donor-like (below CNL) to predominantly acceptor-like (above CNL). If Fermi
level in the bulk and CNL deviate, surface charges appear; when the Fermi level is above (below) the
charge neutrality level, the surface is negatively (positively) charged. Whether this means a depletion
or accumulation layer depends on the conductivity type of the semiconductor. Band bending and space
charge regions are discussed in more detail further below (Sect. 12.3.4, Sect. 21.2.1). The position
of the CNL can be calculated from the Brillouin zone average of the conduction-to-valence band
difference [698, 699].
Experimentally, the Fermi level will be established at the CNL when a lot of deep defects are
introduced, e.g. by irradiation. For many semiconductors, the CNL is close to the middle of the band
gap (Si, GaAs). Notable exceptions are e.g. InAs or In 2 O 3 with a CNL within the conduction band
leading to n-type surface conduction.
7.9 Hydrogen in Semiconductors
The role of hydrogen in semiconductors was first recognized in studies of ZnO [701]. It is now clear
that hydrogen plays an important role in the passivation of defects. As a ‘small’ atom, it can attach
easily to dangling bonds and form an electron-pair bond. Thus, surfaces, grain boundaries, dislocations
and shallow (donor and acceptor) and deep impurity levels become passivated. A good overview and
many details of the physics and technological use of hydrogen in semiconductors can be found in [702,
703]. The hydrogen must be typically introduced as atomic species into semiconductors, e.g. from a
plasma in the vicinity of the surface or by ion irradiation.
With regard to silicon it is important to note that the Si–H bond is stronger than the Si–Si bond. Thus
a silicon surface under atomic hydrogen exhibits Si–H termination rather than Si–Si dimers [704]. Due
to the stronger bond, the hydrogenation leads to an increase of the silicon band gap, which can be used
for surface passivation [705], leading to reduced reverse diode current.
The hydrogen concentration in amorphous Si (a-Si) can be as high as 50% [706]. Electronic grade
a-Si contains typically 10–30 atomic% hydrogen and is thus rather a silicon–hydrogen alloy.
Hydrogen in crystalline silicon occupies the bond-center interstitial position (see Fig. 3.18b) as
shown in Fig. 7.44a. The complexes formed by hydrogen with shallow acceptors and donors have been
studied in detail. It is now generally accepted that for acceptors (e.g. boron) in silicon the hydrogen
is located close to the bond-center position of the Si–B pair (BM, bond minimum) as sketched in
Fig. 7.45a. The boron atom forms an electron-pair bond with three silicon atoms of the tetrahedra, the
fourth silicon bonds to the hydrogen atom. The complex therefore no longer acts as an acceptor. The
silicon atoms and the acceptor relax their positions. The adiabatic potential energy surface of hydrogen
in Si:B is shown in Fig. 7.44b. The hydrogen can sit on four equivalent sites (BM) along the 111
directions of the initial B–Si 4 tetrahedron. This reduces the symmetry, e.g. of H–B vibrations [708].
The energetic barrier for the hydrogen orientation has been determined to be 0.2 eV theoretically [707]
for a hydrogen motion along the path BM–C–BM in Fig. 7.44b. Stress (along [100] and [112]) reduces
the symmetry and leads to splitting of the local vibrational modes, now showing axial symmetry [709].
However, this preferential orientation disappears with an activation energy of 0.19 eV, close to the
theoretical value.