267 research outputs found
Thermopower analysis of the electronic structure around metal-insulator transition in V1-xWxO2
Electronic structure across the metal-insulator (MI) transition of
electron-doped V1-xWxO2 epitaxial films (x = 0-0.06) grown on alfa-Al2O3
substrates was studied by means of thermopower (S) measurements. Significant
increase of |S|-values accompanied by MI transition was observed, and the
transition temperatures of S (TS) decreased with x in good linear relation with
MI transition temperatures. |S| values of V1-xWxO2 films at T > TS were
constant at low values of 23 microV K-1 independently of x, which reflects a
metallic electronic structure, whereas, those at T < TS almost linearly
decreased with logarithmic W-concentrations. The gradient of -213 microV K-1
agrees well with -kB/e*ln10 (-198 microV K-1), suggesting that V1-xWxO2 films
have insulating electronic structures with a parabolic density of state around
the conduction band bottom.Comment: Accepted for publication as a Rapid Commun. in Phys. Rev.
Thermopower analysis of metal-insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion
Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films
with controlled lattice distortion was investigated by thermopower
measurements. VO2 epitaxial films with different crystallographic orientations,
grown on (0001) alpha-Al2O3, (11-20) alpha-Al2O3, and (001) TiO2 substrates,
showed significant decrease of absolute value of Seebeck coefficient (S) from
~200 to 23 microV K-1, along with a sharp drop in electrical resistivity (rho),
due to the transition from an insulator to a metal. The MI transition
temperatures observed both in rho (Trho) and S (TS) for the VO2 films
systematically decrease with lattice shrinkage in the pseudo-rutile structure
along c-axis, accompanying a broadening of the MI transition temperature width.
Moreover, the onset TS, where the insulating phase starts to become metallic,
is much lower than onset Trho. This difference is attributed to the sensitivity
of S for the detection of hidden metallic domains in the majority insulating
phase, which cannot be detected in rho-measurements. Consequently,
S-measurements provide a straightforward and excellent approach for a deeper
understanding of the MI transition process in VO2.Comment: To be published in Physical Review
Thermopower modulation clarification of the intrinsic effective mass in a transparent oxide semiconductor, BaSnO3
Although there are so many reports on the carrier effective mass (m*) of a
transparent oxide semiconductor BaSnO3, it is almost impossible to know the
intrinsic m* value because the reported m* values are scattered from 0.06 to
3.7 m0. Here we successfully clarified the intrinsic m* of BaSnO3, m*=0.40 0.01
m0, by the thermopower modulation clarification method. We also found the
threshold of degenerate/non-degenerate semiconductor of BaSnO3; At the
threshold, the thermopower value of both La-doped BaSnO3 and BaSnO3 TFT
structure was 240 microvolt k-1, bulk carrier concentration was 1.4E19 cm-3,
and two-dimensional sheet carrier concentration was 1.8E12 cm-2. When the EF
locates above the parabolic shaped conduction band bottom, rather high mobility
was observed. On the contrary, very low carrier mobility was observed when the
EF lays below the threshold, most likely due to that the tail states suppress
the carrier mobility. The present results are useful for further development of
BaSnO3 based oxide electronics.Comment: 16 pages including 4 figure
Infrared-transmittance tunable metal-insulator conversion device with thin-film-transistor-type structure on a glass substrate
Infrared (IR) transmittance tunable metal-insulator conversion was
demonstrated on glass substrate by using thermochromic vanadium dioxide (VO2)
as the active layer in three-terminal thin-film-transistor-type device with
water-infiltrated glass as the gate insulator. Alternative positive/negative
gate-voltage applications induce the reversible protonation/deprotonation of
VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49%
modulation of IR-transmittance were simultaneously demonstrated at room
temperature by the metal-insulator phase conversion of VO2 in a non-volatile
manner. The present device is operable by the room-temperature protonation in
all-solid-state structure, and thus it will provide a new gateway to future
energy-saving technology as advanced smart window.Comment: To appear in APL Mater. (2017
Leakage-free electrolytes with different conductivity for non-volatile memory device utilizing insulator/metal ferromagnet transition of SrCoOx
The electrochemical switching of SrCoOx-based non-volatile memory with
thin-film-transistor structure was examined by using liquid-leakage-free
electrolytes with different conductivity (s) as the gate insulator. We first
examined leakage-free water, which is incorporated in the amorphous (a-) 12CaO
7Al2O3 film with nanoporous structure (CAN), but the electrochemical
oxidation/reduction of SrCoOx layer required the application of high gate
voltage (Vg) up to 20 V for a very long retention-time (t) 40 minutes,
primarily due to the low s (2.0 x 10-8 S cm-1 at RT) of leakage-free water.We
then controlled the s of leakage-free electrolyte, infiltrated in the a-NaxTaO3
film with nanopillar array structure, from 8.0 x 10-8 S cm-1 to 2.5 x 10-6 S
cm-1 at RT by changing the x = 0.01-1.0. As the result, the t, required for the
metallization of SrCoOx layer under small Vg = -3 V, becomes two orders of
magnitude shorter with increase of the s of the a-NaxTaO3 leakage-free
electrolyte. These results indicate that the ion migration in the leakage-free
electrolyte is the rate-determining step for the electrochemical switching,
compared to the other electrochemical process, and the high s of the
leakage-free electrolyte is the key factor for the development of the
non-volatile SrCoOx-based electro-magnetic phase switching device
Experimental characterization of the electronic structure of anatase TiO2: Thermopower modulation
Thermopower (S) for anatase TiO2 epitaxial films (n3D: 1E17-1E21 /cm3) and
the gate voltage (Vg) dependence of S for thin film transistors (TFTs) based on
TiO2 films were investigated to clarify the electronic density of states (DOS)
around the conduction band bottom. The slope of the |S|-log n3D plots was -20
{\mu}V/K, which is an order magnitude smaller than that of semiconductors (-198
{\mu}V/K), and the |S| values for the TFTs increased with Vg in the low Vg
region, suggesting that the extra tail states are hybridized with the original
conduction band bottom.Comment: 11 pages, 4 figure
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