dependent on the electronic carrier density. In this contribution, we focus on
nanoionic devices enabling physical property tuning and enhancement which are
promising derivative of the atomic switch technology.
The electronic structure near Fermi level is the origin of many physical properties
of materials (e.g., electronic conductivity, superconductivity, thermal conductivity,
magnetic susceptibility, color, optical reflectance) [25]. Control of the electronic
structure, or more specifically electronic carrier density, has long been the most
important issue in materials science because so many physical properties can be
widely tuned by varying the electronic carrier density [26].
Two carrier doping methods have mainly been used for controlling the electronic
carrier density in materials. One is chemical doping accompanied by generation of
defects in materials (e.g., hetero-atoms, vacancies, and interstitials). Electronic
(hole) carriers are added through substitution of the original atoms of a material by
hetero-atoms and compensation of excess charge induced by the defects. Not only
the hetero-atom doping but also nonstoichiometry, caused by atomic vacancies or
interstitials, induces excess charge and the resultant electronic carrier addition. The
chemical doping enables the control of electronic carrier density over a very wide
range as long as the materials can accept such defects, and this method has been used
by researchers to explore various physical properties.
The other method is electrostatic carrier doping using a dielectric that works as a
capacitor. In this method, since electronic carriers are electrostatically added without
generating defects, the electronic structure of the material is less affected than in the
case of chemical doping: electrostatic carrier doping can be understood as a Fermi
level shift in a rigid band model rather than that with defect level generation. This
method has been exploited in field-effect-transistors (FETs), wherein the electronic
carrier density is tunable in situ by gate voltage modulation, although in this case, the
controllability of carrier density, e.g., 10
13 cm
À2 , is not as good as that by chemical
doping because of the small capacitance of dielectric materials (e.g., 0.7 μF/cm
2 for
SiO 2 ) [26–30].
While the ability to control electronic carrier density had produced advances in
the study of material properties and in semiconductor technology, two goals have
remained unrealized: (1) a high electronic carrier density cannot be tuned in situ, and
(2) a high electronic carrier density cannot be achieved by doping without generating
defects.
If a high electronic carrier density could be tuned in situ, a wider range of physical
properties could be exploited for the development of novel functional devices
beyond the reach of conventional semiconductor technology. Moreover, if high
densities could be reached by doping without generating defects, it would lead to
remarkable advances in exploiting physical properties that are sensitive to disturbance in the electronic structure, such as superconductivity. Therefore, the realization of these two goals would have a huge impact on both fundamental research and
practical applications by overcoming limitations in conventional materials synthesis
and semiconductor technology. Is this possible?
Nanoionic devices are promising candidates to overcome these limitations. In the
devices, ionic transport is used to achieve (1) electrochemical carrier doping using a
162
T. Tsuchiya et al.
nanoionic devices enabling physical property tuning and enhancement which are
promising derivative of the atomic switch technology.
The electronic structure near Fermi level is the origin of many physical properties
of materials (e.g., electronic conductivity, superconductivity, thermal conductivity,
magnetic susceptibility, color, optical reflectance) [25]. Control of the electronic
structure, or more specifically electronic carrier density, has long been the most
important issue in materials science because so many physical properties can be
widely tuned by varying the electronic carrier density [26].
Two carrier doping methods have mainly been used for controlling the electronic
carrier density in materials. One is chemical doping accompanied by generation of
defects in materials (e.g., hetero-atoms, vacancies, and interstitials). Electronic
(hole) carriers are added through substitution of the original atoms of a material by
hetero-atoms and compensation of excess charge induced by the defects. Not only
the hetero-atom doping but also nonstoichiometry, caused by atomic vacancies or
interstitials, induces excess charge and the resultant electronic carrier addition. The
chemical doping enables the control of electronic carrier density over a very wide
range as long as the materials can accept such defects, and this method has been used
by researchers to explore various physical properties.
The other method is electrostatic carrier doping using a dielectric that works as a
capacitor. In this method, since electronic carriers are electrostatically added without
generating defects, the electronic structure of the material is less affected than in the
case of chemical doping: electrostatic carrier doping can be understood as a Fermi
level shift in a rigid band model rather than that with defect level generation. This
method has been exploited in field-effect-transistors (FETs), wherein the electronic
carrier density is tunable in situ by gate voltage modulation, although in this case, the
controllability of carrier density, e.g., 10
13 cm
À2 , is not as good as that by chemical
doping because of the small capacitance of dielectric materials (e.g., 0.7 μF/cm
2 for
SiO 2 ) [26–30].
While the ability to control electronic carrier density had produced advances in
the study of material properties and in semiconductor technology, two goals have
remained unrealized: (1) a high electronic carrier density cannot be tuned in situ, and
(2) a high electronic carrier density cannot be achieved by doping without generating
defects.
If a high electronic carrier density could be tuned in situ, a wider range of physical
properties could be exploited for the development of novel functional devices
beyond the reach of conventional semiconductor technology. Moreover, if high
densities could be reached by doping without generating defects, it would lead to
remarkable advances in exploiting physical properties that are sensitive to disturbance in the electronic structure, such as superconductivity. Therefore, the realization of these two goals would have a huge impact on both fundamental research and
practical applications by overcoming limitations in conventional materials synthesis
and semiconductor technology. Is this possible?
Nanoionic devices are promising candidates to overcome these limitations. In the
devices, ionic transport is used to achieve (1) electrochemical carrier doping using a
162
T. Tsuchiya et al.