8.3 Low-Field Transport
235
Fig. 8.10 Conductivity of
B-doped diamond as a
function of temperature.
Adapted from [746]
in a parasitic ZnGa 2 O 4 spinel phase for [Ga]=4% [632]. The onset of such segregation phenomena is
accompanied with the decrease of mobility and conductivity.
8.3.13 Superconductivity
It has been found that highly doped semiconductors do not only behave like metals in the sense that
the carrier concentration is largely independent of temperature but that they can also exhibit superconductivity. Theoretical and early experimental investigations suggested the possibility of such behavior
[740–743] even when the electron concentration is much smaller than one per atom. Experimentally,
robust superconductivity in semiconductors has been found more recently for a number of semiconductors [744, 745], namely boron-doped diamond (C:B) [746] (Fig. 8.10), Si:B [747] and Ge:Ga [748].
The preparation of superconducting semiconductors with critical temperature above 1 K typically
involves hyperdoping with impurity concentrations of several atomic percent. The detailed physics of
these materials, such as the superconductor type (type-II behavior was found for C:B) or the electron
coupling mechanism (generally, phonon-assisted pairing is assumed), are still under debate.
Another type of superconducting semiconductor structure are twisted monolayers in Van-der-Waals
heterostructures for particular values of twist angle and carrier concentration (cf. Sect. 13.3).
8.3.14 Piezoresistivity
The dependence of resistivity on stress or strain is known as piezoresistive effect, first described in
[749]. It is a consequence of the modification of the band structure upon stress and the change of
effective masses (Sect. 6.12.2). In a cubic material, the resistivity ρ i for transport in cartesian direction
i changes compared to the unstrained state in a phenomenological description according to
i
ρ i
= π i j σ j ,
(8.27)
where π is the piezoresistivity tensor (8.28) and the σ j form the six-component stress tensor (5.55),
235
Fig. 8.10 Conductivity of
B-doped diamond as a
function of temperature.
Adapted from [746]
in a parasitic ZnGa 2 O 4 spinel phase for [Ga]=4% [632]. The onset of such segregation phenomena is
accompanied with the decrease of mobility and conductivity.
8.3.13 Superconductivity
It has been found that highly doped semiconductors do not only behave like metals in the sense that
the carrier concentration is largely independent of temperature but that they can also exhibit superconductivity. Theoretical and early experimental investigations suggested the possibility of such behavior
[740–743] even when the electron concentration is much smaller than one per atom. Experimentally,
robust superconductivity in semiconductors has been found more recently for a number of semiconductors [744, 745], namely boron-doped diamond (C:B) [746] (Fig. 8.10), Si:B [747] and Ge:Ga [748].
The preparation of superconducting semiconductors with critical temperature above 1 K typically
involves hyperdoping with impurity concentrations of several atomic percent. The detailed physics of
these materials, such as the superconductor type (type-II behavior was found for C:B) or the electron
coupling mechanism (generally, phonon-assisted pairing is assumed), are still under debate.
Another type of superconducting semiconductor structure are twisted monolayers in Van-der-Waals
heterostructures for particular values of twist angle and carrier concentration (cf. Sect. 13.3).
8.3.14 Piezoresistivity
The dependence of resistivity on stress or strain is known as piezoresistive effect, first described in
[749]. It is a consequence of the modification of the band structure upon stress and the change of
effective masses (Sect. 6.12.2). In a cubic material, the resistivity ρ i for transport in cartesian direction
i changes compared to the unstrained state in a phenomenological description according to
i
ρ i
= π i j σ j ,
(8.27)
where π is the piezoresistivity tensor (8.28) and the σ j form the six-component stress tensor (5.55),