research statement 2003-2012

1. Research statement
Toru Hara, PhD
2003 – 2012
1. Gas sensor based on nano-domain modulation by gaseous molecular adsorption (granted
by the Ministry of Economy, Trade, and Industry of Japan)
August 2007 to March 2012
As the inventor, the initiator, and the Principal Investigator
- Patents = 1 JP issued, 1 USP issued.
- Publications = 11 papers (10 first author papers).
- Invited speeches = 4.
- Stoichiometric SrTiO3 or BaTiO3, which have a d0 electronic configuration, can be treated as
band insulators with a band gap of about 3.2 eV [M. Cardona, Phys. Rev. 140 (1965) A651.]. In
electron-doped SrTiO3 or BaTiO3 doped electrons enter the bottom of the empty Ti 3d
conduction band, and then these materials become n-type semiconductors; however, the
electronic properties are different from conventional semiconductors such as Si and Ge.
Because of the strong electron-phonon coupling resulting from ionic displacements (the
displacement of Ti4+ and O2- in a TiO6 octahedron), in SrTiO3 and BaTiO3 the electron effective
mass (m*) is heavier than that of free electron (m = 9.1 x 10-31 kg): m* in 0.85-at.%-electron-
doped SrTiO3 was estimated to be 1.2 m for light electrons and to be 7.0 m for heavy electrons
by Chang et al. [Y. J. Chang, A. Bostwick, Y. S. Kim, K. Horn, E. Rotenberg, Phys. Rev. B 81
(2010) 235109: they observed that the Fermi surface of SrTiO3 consists of three degenerate
ellipsoids (dxy, dyz, and dzx) above 105 K, and that dxy band has lower minimum energy than the
doubly degenerate dyz and dzx bands below 105 K]. The dynamic Jahn-Teller splitting of the 3d-
t2g states of Mo5+ in SrTiO3 was reported to be 60 meV at 1.65 K [B. W. Faughnan, Phys. Rev. B
5 (1972) 4925.]. For another example, the splitting of the 3d-t2g band in BaTiO3 increases from
400 (paraelectric cubic phase) to 330 K (ferroelectric tetragonal phase) and to be about 30 meV
[F. M. Michel-Calendini, R. N. Blumenthal, J. Am. Ceram. Soc. 54 (1971) 515, 577.].
The polaronic nature in SrTiO3 [K. A. Muller, J. Supercond. 12 (1999) 3; H. P. R. Frederikse, G.
Candella, Bull. Am. Phys. Soc. 11 (1966) 108; R. P. Feynman, R. W. Hellwarth, C. K. Iddings, P.
M. Platzman, Phys. Rev. 127 (1962) 1004; J. L. M. van Mechelen, D. van der Marel, C. Grimaldi,
A. B. Kuzmenko, N. P. Armitage, N. Reyren, H. Hagemann, I. I. Mazin, Phys. Rev. Lett. 100
(2008) 226403; D. M. Eagles, P. Lalousis, J. Phys. C 17 (1984) 655.] and BaTiO3 [S. Lenjer, O.
F. Schirmer, H. Hesse, T. W. Kool, Phys. Rev. B 66 (2002) 165106; P. Gerthsen, R. Groth, K. H.
Hardtl, D. Heese, H. G. Reik, Solid State Commun. 3 (1965) 165; E. V. Bursian, Y. G. Girshberg,
E. N. Starov, Phys. Status Solidi B 46 (1971) 529.] has been intensively studied: it has been
observed that electron doping and chemical substitutions (e.g., Ba2+-substitution into Sr2+-site)

2. give significant influences on the polaronic nature and can cause spatially fluctuating potentials,
inducing small polarons.
At paraelectric SrTiO3 surfaces adsorbed electronegative molecules such as oxygen (O2) can
induce local distortions of TiO6 unit cells; as a result, carrier-electrons become frequently
trapped near the oxygen-adsorbed surfaces of SrTiO3.
2. Thin film capacitor
October 2003 to March 2015
- Publications = 6 papers.
- The depletion layer can become narrower when a reverse-bias voltage is applied to defective
thus semiconductive (Ba,Sr)TiO3 thin films since the dielectric constant in the depletion layer is
decreased by the reverse-bias voltage (low frequency dielectric constant must be considered for
this type of material), and since deep-level donors are ionized.
The Fermi level is the highest level occupied by conduction electrons in n-type semiconductors
in thermal equilibrium. In thermal nonequilibrium, the number of conduction electrons
temporarily increases or decreases because of current injection or current outflow, respectively.
Assuming that the thermal equilibrium state is maintained temporarily and locally, the quasi-
Fermi level become higher (or lower) when the conduction electron density increases (or
decreases) compared with that of the equilibrium state.
The Fermi level of the electron-doped (Ba,Sr)TiO3 inner layer (meaning bulk) containing oxygen
vacancies is almost at the conduction band minimum. At the surface of (Ba,Sr)TiO3, adsorbed
oxygen forms a Schottky barrier with a height of approximately 0.7 eV. The electron affinity of
oxygen molecule, which adsorbs onto (Ba,Sr)TiO3 surface, is not particularly high; however, the
surface can become more stable when the adsorbed oxygen accepts electrons to become ionized
because of the mirror potential resulting from mirror charges generated in SrTiO3. For SrTiO3
and (Ba,Sr)TiO3, which have many oxygen vacancies, a mid-gap state (MGS) having a peak at
approximately 1.5 eV below the conduction band minimum is formed and can be observed in
vacuum; however, it is not observed when there is a flow of oxygen since the adsorbed oxygen
serves as an acceptor and attracts electrons not only from shallow-level donors but also from the
MGSs around 1.5 eV. As a result, the quasi-Fermi level goes downward (not at 0.7 eV but
around 1.5 eV). The same situation can take place at Pt-(Ba,Sr)TiO3 interface where Pt surface
has adsorbed oxygen coming from the atmosphere.
Under a reverse bias the deep-level donors are ionized in a stepwise manner from shallower
levels to deeper levels, causing a relaxation current to flow in a stepwise manner. Here, the
strength of the electric field (potential gradient) increases as it approaches the surface. Therefore,
donors are ionized sequentially from the surface, which leads to the downward bending of the
quasi-Fermi level. In addition, the downward bending can also be explained by the fact that the
number of deep-level donors increases as it approaches the interface with the electrode [T. Hara,
Mater. Chem. Phys. 91 (2005) 243]. After reaching the equilibrium, the downward bending may
be lost.

3. I adopted the diffusion model proposed by Wagner [C. Wagner, Phys. Z. 32 (1931) 641],
Schottky and Spenk [W. Schottky, E. Spenk, Wiss. Veroff. Siemens Werke 18 (1939) 1], and
Mott [N. F. Mott, Proc. R. Soc. A 171 (1939) 27]. In the diffusion model, the current is restricted
by the diffusion current and drift current in the depletion layer. Therefore, this model is suitable
for a semiconductor with a low carrier mobility.
In the thermal emission model proposed by Bethe [H. A. Bethe, MIT Radiat. Lab. Rep. No. 43
(1942)], which is generally used for the Schottky contact of conventional semiconductors (e.g.,
Si), the current is restricted by the current that flows over the Schottky barrier. This model is
valid only when the mean free path of the conduction electrons is longer than the width of the
depletion layer. Strictly speaking, the mean free path longer than the width from an end of the
depletion layer to a point approximately kBT below the barrier top is acceptable. Therefore, the
thermal emission model is not appropriate for (Ba,Sr)TiO3.
In the thermal emission model proposed by Bethe, the quasi-Fermi level of the semiconductor
does not conform to that of the electrode even when a forward-bias voltage is applied. According
to Gossick [B. R. Gossick, Solid State Electron. 6 (1963) 445], who supports Bethe’s thermal
emission model, “In the diffusion model in which the Fermi level of the depletion layer matches
that of the electrode when a forward-bias voltage is applied, injected electrons are considered to
be in thermal equilibrium. The electrons flowing over the barrier, which are “hot” electrons
having approximately 1 eV or higher energy, should be backscattered or they should be injected
to the depletion layer after their energy decreases to reach thermal equilibrium (in the latter
case, the heat release process is a rate-limiting process).” Thus, he denies the validity of the
diffusion model. However, as mentioned earlier, the diffusion model is appropriate for
semiconductors with a low mobility since the rates of drift and diffusion in the depletion layer
are as low as the rate of thermal emission at the interface (the thermal emission that enables
electrons to flow over the potential barrier and the heat exchange between electrons and lattices).
Rhoderick [E. H. Rhoderick, J. Phys. D 5 (1972) 1920.] states that the downward bending of the
quasi-Fermi level under a forward bias is very small and negligible. In other words, theoretical
and experimental results should be in reasonable agreement when the quasi-Fermi level is
assumed almost flat in the depletion layer. Therefore, he concluded that the thermal emission
model is valid for Si. In the case of Si, the model is appropriate since the following conditions
are satisfied: the donor density can be assumed to be constant because of the high purity of Si
[this assumption does not hold for defect-rich, sputter-grown (Ba,Sr)TiO3 films] and the
dielectric constant is independent of the bias voltage [this does not hold for (Ba,Sr)TiO3 films].
Rhoderick attempted to raise the quasi-Fermi level of the depletion layer when a saturated
current flowed stably under a forward-bias voltage so that it matched the Fermi level of the
electrode. However, it is not necessary for the two levels to be equal when the relaxation current
flows: relaxation current results from detrapping of deep-level donors; this is non-equilibrium.