7.4 Doping
185
Fig. 7.5 Comparison of
ZnO, NiO and MgO on a
common energy scale,
comparing conduction
band and valence band
edges and n-type (red) and
p-type (blue) pinning
energies (determined for
metal-rich and oxygen-rich
conditions, respectively).
Adapted from [571]
ZnO
NiO
MgO
E-E
V
found for changes in the anion concentration and conductivity in CuI [38] (p-type) and ZnO [80] (ntype). The modification of CuI by exposure to different partial pressure of iodine in organic solutions
with different iodine concentration [41] and subsequently various concentrations of copper vacancies
[568] can be considered the first doping of a semiconductor (1909).
The electronic levels of a defect or an impurity can exist within the forbidden gap of the bulk host
material. These levels can lie close to the band edges or in the vicinity of the middle of the band gap.
In a simplified approach, the first stem from shallow defects (Sect. 7.5), the latter from deep defects
(Sect. 7.7).
7.4.2 Doping Principles
In [569] various doping principles are formulated. Essentially, the amount of impurities that lead to
electrically active dopants is limited by the increasingly probable formation of compensating defects.
In the case of donors, these are electron killers, e.g. n-type doping of Si:As is limited by the formation
of V Si [570]. In the case of acceptors, the compensating defects are hole killers. The so-called n-type
pinning energy E
n,pin
F
is the Fermi level at which such killer defect (e.g. a cation vacancy) forms. When
the Fermi level reaches the pinning energy, no further progress in n-type doping can be made, since the
spontaneously generated electron killers will negate the introduced (impurity) donors. As a tendency,
materials with low lying conduction band, i.e. large electron affinity (difference between vacuum level
and conduction band) can be doped n-type. Similarly, p-type doping by acceptors, shifting the Fermi
level towards the valence band, will meet at some point E
p,pin
F
, called p-type pinning energy, when
native hole killers, such as anion vacancies or cation interstitials form spontaneously. At this point,
further p-type doping is no longer possible. p-type doping is facilitated by materials whose valance
band maximum is close to the vacuum level [569].
A comparison of the wide gap materials ZnO, NiO and MgO is depicted in Fig. 7.5. The position of
the pinning levels is marked on a common energy scale. From the position of E
n,pin
F
it can be understood,
that ZnO can be highly n-doped while NiO and MgO cannot [571]. From E
p,pin
F
, NiO can be doped
p-type, while MgO cannot be doped at all.
For dopability, generally, it is also important that the ionized charges from impurities are free and
thus contribute to the free charge carrier density and do not form localized states, e.g. due to polaronic
effects (Sect. 8.7).
185
Fig. 7.5 Comparison of
ZnO, NiO and MgO on a
common energy scale,
comparing conduction
band and valence band
edges and n-type (red) and
p-type (blue) pinning
energies (determined for
metal-rich and oxygen-rich
conditions, respectively).
Adapted from [571]
ZnO
NiO
MgO
E-E
V
found for changes in the anion concentration and conductivity in CuI [38] (p-type) and ZnO [80] (ntype). The modification of CuI by exposure to different partial pressure of iodine in organic solutions
with different iodine concentration [41] and subsequently various concentrations of copper vacancies
[568] can be considered the first doping of a semiconductor (1909).
The electronic levels of a defect or an impurity can exist within the forbidden gap of the bulk host
material. These levels can lie close to the band edges or in the vicinity of the middle of the band gap.
In a simplified approach, the first stem from shallow defects (Sect. 7.5), the latter from deep defects
(Sect. 7.7).
7.4.2 Doping Principles
In [569] various doping principles are formulated. Essentially, the amount of impurities that lead to
electrically active dopants is limited by the increasingly probable formation of compensating defects.
In the case of donors, these are electron killers, e.g. n-type doping of Si:As is limited by the formation
of V Si [570]. In the case of acceptors, the compensating defects are hole killers. The so-called n-type
pinning energy E
n,pin
F
is the Fermi level at which such killer defect (e.g. a cation vacancy) forms. When
the Fermi level reaches the pinning energy, no further progress in n-type doping can be made, since the
spontaneously generated electron killers will negate the introduced (impurity) donors. As a tendency,
materials with low lying conduction band, i.e. large electron affinity (difference between vacuum level
and conduction band) can be doped n-type. Similarly, p-type doping by acceptors, shifting the Fermi
level towards the valence band, will meet at some point E
p,pin
F
, called p-type pinning energy, when
native hole killers, such as anion vacancies or cation interstitials form spontaneously. At this point,
further p-type doping is no longer possible. p-type doping is facilitated by materials whose valance
band maximum is close to the vacuum level [569].
A comparison of the wide gap materials ZnO, NiO and MgO is depicted in Fig. 7.5. The position of
the pinning levels is marked on a common energy scale. From the position of E
n,pin
F
it can be understood,
that ZnO can be highly n-doped while NiO and MgO cannot [571]. From E
p,pin
F
, NiO can be doped
p-type, while MgO cannot be doped at all.
For dopability, generally, it is also important that the ionized charges from impurities are free and
thus contribute to the free charge carrier density and do not form localized states, e.g. due to polaronic
effects (Sect. 8.7).