RESEARCH ARTICLE
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoelectronics and Optoelectronics
Vol. 9, 1–5, 2014
Impact of Nanoporous Metal Oxide Morphology
on Electron Transfer Processes in Ti–TiO
2
–Si
Heterostructures
Y. S. Milovanov
1 ∗
, I. V. Gavrilchenko
1
, V. Y. Gayvoronsky
2
,
G. V. Kuznetsov
1
, and V. A. Skryshevsky
1
1
Institute of High Technologies, Taras Shevchenko National University of Kyiv, 64 Volodymyrska, Kyiv, 01601, Ukraine
2
Institute of Physics, NASU, pr. Nauki 46, 03028 Kiev, Ukraine
By sol–gel synthesis a nanosized porous layers of TiO
2
with different porosities and specific surface
area were fabricated on silicon substrates. The created Ti–TiO
2
-(np)Si heterostructures were stud-
ied by the impedance spectroscopy. The significant influence of structure of porous TiO
2
and type of
silicon substrate on the dependence of capacity and active conductivity on an applied voltage bias
is observed. In the frame of the proposed equivalent circuit model the analysis of the contribution
of nanocrystals and intercrystalline boundaries into electron transfer processes is carried out. It is
observered the effect of negative differential conductivity that is governed by the changing of the
capture of injected electrons and holes in porous-TiO
2
.
Keywords: Heterostructure, TiO
2
, Differential Conductivity, Porous Semiconductor.
1. INTRODUCTION
Recently, much attention is paid to the investigation of
wide gap nanocrystalline oxide materials which are char-
acterized by unique properties and opportunities for appli-
cation. These compounds include titanium oxide TiO
2
that
is a chemically stable and inexpensive material and now
is used for gas sensors,
1 2
thin film coatings,
3
injection
solar cells.
4 5
It has unique photocatalytic properties,
6–8
can be applied as photoanode in a photoelectrochemical
cells,
9
and it is a promising material for nonlinear optical
applications,
10
for photonic crystals,
11
UV photodiodes,
12
and light-controlled memory cells.
13
Modern soluble techniques of nanocrystalline titanium
oxide fabrication provide a stability of chemical composi-
tion, reproducibility and controllability of the microstruc-
ture (crystallite size, size and pore volume, specific surface
area). Methods of sol–gel technology are widely used, they
allow to synthesize the fine-grained oxide films with a
wide size distribution of nanocrystals.
14 15
Reduction of the grain size to the threshold value (d<
10 nm) is accompanied by a sharp increase of the sur-
face energy role and by the corresponding modification
of physical and chemical properties. Processes of carriers
transport were investigated for the porous titanium oxide,
∗
Author to whom correspondence should be addressed.
drift mobility and lifetime of electrons and holes were
determined.
16–19
The processes of injection and capture of
charge carriers in direct tunneling or tunneling with energy
levels in band gap of TiO
2
significantly affect on the elec-
tronic transport in wide-oxide materials.
11 19 20
In nanos-
tructured films a large number of electron traps on the
developed internal surface with the concentration up to
10
18
cm
-3
is present and defines the electron transport
properties.
12 21 22
Features of injection mechanisms of carrier transport in
the contact of nanostructured TiO
2
with other semiconduc-
tors, first of all, with silicon, are not adequately explored.
23
In the present work we study the electrophysical charac-
teristics of surface-barrier structures based on sol–gel pre-
pared nanocrystalline TiO
2
films on silicon substrates.
2. EXPERIMENTAL DETAILS
Titanium dioxide films were deposited by method of
sol–gel synthesis. To isopropanol (C
3
H
7
OH), which was
used as a solvent, tetraisoprooxide titanium (TIRT) and
-terpineol (-Tr) were added that provide supplemen-
tary solution viscosity. As a complex-forming reagents
the polyethylene glycol with molecular masses M =
300 g/mole (PEG300) and M = 1000 g/mole (PEG1000)
were used. Further we will denote the created samples of
PEG300 as TiO
2
(300), and of PEG1000 as TiO
2
(1000)
J. Nanoelectron. Optoelectron. 2014, Vol. 9, No. 3 1555-130X/2014/9/001/005 doi:10.1166/jno.2014.1593 1