Vol. 37, No. 2
Journal of Semiconductors
February 2016
Effects of reductive annealing on insulating polycrystalline thin films of Nb-doped
anatase TiO2 : recovery of high conductivity
Shoichiro Nakao1; 2; Ž , Yasushi Hirose1; 2; 3 , and Tetsuya Hasegawa1; 2; 3
1 Kanagawa
Academy of Science and Technology (KAST), Kawasaki 213-0012, Japan
Science and Technology Agency, CREST, Bunkyo, Tokyo 113-0033, Japan
3 Department of Chemistry, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
2 Japan
Abstract: We studied the effects of reductive annealing on insulating polycrystalline thin films of anatase Nbdoped TiO2 (TNO). The insulating TNO films were intentionally fabricated by annealing conductive TNO films
in oxygen ambient at 400 ıC. Reduced free carrier absorption in the insulating TNO films indicated carrier compensation due to excess oxygen. With H2 -annealing, both carrier density and Hall mobility recovered to the level
of conducting TNO, demonstrating that the excess oxygen can be efficiently removed by the annealing process
without introducing additional scattering centers.
Key words: transparent conductive oxide; Nb-doped TiO2 ; pulsed laser deposition
DOI: 10.1088/1674-4926/37/2/022001
PACS: 68.60.-P; 73.61.-r
1. Introduction
Transparent conductive oxides (TCOs) have been extensively studied for this decade because TCOs are key materials for optoelectronics such as photovoltaic cells, flat panel
displays, and light-emitting devicesŒ1; 2 . Because Sn-doped
In2 O3 (ITO), being the most widely used TCO, contains a socalled “rare metal” In, many studies have been carried out to
explore In-free TCOsŒ3 . Nb-doped anatase TiO2 (TNO)Œ4; 5 is
one of such ITO alternatives. Epitaxial TNO thin films grown
on single crystalline substrates exhibit resistivity () lower than
4 10 4 cmŒ4; 6 . Furthermore, polycrystalline TNO films
with of the order of 10 4 cm have been fabricated on
glass substrates by pulsed laser deposition (PLD)Œ7 and sputteringŒ8 11 . These results demonstrate the potential of TNO as
a practical TCO, which would be listed as one of the important
applicationsŒ12; 13 of TiO2 .
An intriguing characteristic of TNO is that the conductivity of TNO films is very sensitive to the amounts of oxygen defects, such as oxygen vacancy and excess oxygen, generated during oxidative/reductive depositionŒ10; 14 and postdeposition annealingŒ15; 16 . In the previous study, we investigated the effects of oxidative and reductive annealing on the
transport properties of epitaxial TNO films. We found that
excess oxygen introduced by O2 -annealing compensated carrier electrons generated by Nb-dopingŒ16 . Here, it should be
stressed that excess oxygen in epitaxial TNO films can be removed by reductive annealing in a reversible manner. Furthermore, even highly insulating epitaxial TNO films prepared under oxidative conditions exhibited very low after annealing in
H2 ambientŒ15 . These results indicate that the removal of excess oxygen with reductive annealing is a key to achieving high
conductivity in TNO. However, the removal of excess oxygen
by reductive annealing has not been fully examined in polycrystalline TNO films on glass substrates. In the previous studies, conducting polycrystalline TNO films were prepared by re-
ductive annealing of amorphous precursor filmsŒ7 11 , where
the annealing process has two roles: crystallization and removal of excess oxygen. This makes it difficult to interpret the
annealing effect on polycrystalline TNO films. For example,
polycrystalline TNO films fabricated from oxygen-rich precursors showed much lower conductivity even after reductive
annealingŒ17 , suggesting that removal of excess oxygen from
polycrystalline films is more difficult than that from epitaxial
films. However, we cannot exclude the possibility that reduced
crystallinity and/or segregated dopants during crystallization
are responsible for the low conductivity. Thus, it is needed to
study the reductive annealing independent of the crystallization
issue.
In this study, we investigated the effects of reductive annealing on the electrical and optical properties of insulating
polycrystalline TNO films prepared by annealing low- TNO
films in oxygen ambient. Recovery of by H2 -annealing was
observed in both polycrystalline and epitaxial films, demonstrating the complete removal of excess oxygen irrespective of
film form.
2. Experimental
Highly conductive polycrystalline TNO films, hereafter
referred to as pristine films, were fabricated on alkaline-free
glass substrates (Corning, Eagle 2000) by PLD. Details of the
preparation procedure have been described elsewhereŒ7 . In
short, amorphous Ti0:94 Nb0:06 O2 thin films were deposited on
unheated glass substrates using PLD at an oxygen partial pressure of 3 10 4 Torr with a deposition rate of 0.6 nm/min,
and then crystalized into polycrystalline anatase phase by annealing at 600 ıC in H2 ambient of 700 Torr. The thickness of
the TNO films was 110 nm, which was determined by using
a stylus profiler. Insulating TNO films were prepared by annealing the pristine films at 400 ıC in O2 ambient of 350 Torr.
The insulating TNO films were further subjected to reductive
† Corresponding author. Email: [email protected]
Received 13 October 2015
© 2016 Chinese Institute of Electronics
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J. Semicond. 2016, 37(2)
Shoichiro Nakao et al.
Figure 1. (Color online) (a) Resistivity () of epitaxial (squares) and
polycrystalline (circles) Nb-doped TiO2 (TNO) films. The pristine
polycrystalline film was post-annealed in O2 and H2 at 400 ıC repeatedly. The data of the epitaxial film are from Reference [16]. (b)
Transmittance spectra of the polycrystalline TNO films (pristine and
after post-deposition annealing in O2 and H2 ambient).
annealing in H2 ambient of 700 Torr at various temperatures
above 400 ıC. Annealing duration was fixed at 1 h throughout
this study. Crystal structures were characterized using X-ray
diffraction (XRD). Transport properties, such as , carrier density (ne /, and Hall mobility (), were evaluated using a standard six-probe method. Transmittance spectra were measured
using a UV–visible–near-infrared spectrophotometer. All the
characterizations were carried out at room temperature.
3. Results and discussion
First, we examined the effects of O2 -annealing on the pristine TNO films. Figure 1 shows values of the pristine and annealed TNO films. The figure includes the data for the epitaxial
TNO film prepared with the same annealing procedureŒ14 for
comparison. The pristine TNO films exhibited low of 5.7 10 4 cm (ne D 1.4 1021 cm 3 and D 8.1 cm2 /(Vs)),
which are typical values for polycrystalline TNO films deposited by PLDŒ7 . O2 -annealing resulted in a drastic increase
of to 10–100 cm, which are two or three orders of magnitude higher than those for the O2 -annealed epitaxial TNO
films. Unfortunately, Hall measurements on the O2 -annealed
polycrystalline films were unsuccessful. Instead, we investi-
Figure 2. (Color online) (a) , (b) carrier density (ne /, and (c) Hall mobility () of the polycrystalline TNO films annealed at various temperatures in H2 ambient. Before H2 -annealing, the TNO films showed
high values of 10–100 cm due to excess oxygen. The dashed lines
represent the pristine values before O2 -annealing.
gated free carrier absorption observed in transmittance spectra.
As shown in Figure 1(b), O2 -annealing caused a significant increase in transmittance in the near infrared region (> 1300 nm).
Such a behavior, i.e., reduced free carrier absorption, can be explained by the carrier compensation by excess oxygen, as has
been observed in the epitaxial TNO filmsŒ16 .
Next, we investigated the effects of reductive annealing
on the transport properties of insulating polycrystalline TNO
films containing excess oxygen. As seen in Figure 1(a), the insulating polycrystalline TNO film recovered high conductivity
after H2 -annealing at 400 ıC. However, the values after H2 annealing were of the order of 10 3 cm, which is somewhat
higher than that of the pristine value, 5.7 10 4 cm. The
higher in the H2 -annealed polycrystalline TNO film mainly
resulted from lower ; H2 -annealed polycrystalline TNO film
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J. Semicond. 2016, 37(2)
Shoichiro Nakao et al.
4. Conclusion
We studied the effects of reductive annealing on the insulating polycrystalline TNO thin films containing excess oxygen. With H2 -annealing at 600 ıC, the transport properties fully
recovered, demonstrating the complete removal of excess oxygen from the polycrystalline TNO films.
Acknowledgements
S. Nakao gratefully acknowledges Ms. Reiko Nagashima
of The University of Tokyo for her encouragement. This work
was partially supported by JSPS KAKENHI Grant Number
15K04687.
Figure 3. X-ray diffraction (XRD) patterns of the pristine film and
the film annealed at 600 ıC in H2 ambient. Before H2 -annealing, the
films showed high values due to excess oxygen.
exhibited ne D 1.2 1021 cm 3 and D 2.5 cm2 /(Vs), which
correspond to 86% of ne and 31% of for the pristine film,
respectively. It should be noted that the epitaxial TNO film
showed complete recovery of the transport properties with the
same annealing procedureŒ16 . We speculate that, in polycrystalline TNO films, excess oxygen is more strongly bound to
grain boundaries than to the grain interior. Such excess oxygen might partially remain even after H2 -annealing at 400 ıC,
resulting in suppressed .
Practically, it is very important whether excess oxygen can
be completely removed from polycrystalline TNO films. Figure 2 shows transport properties of the polycrystalline TNO
films annealed at various temperatures in H2 ambient. Before
H2 -annealing, the TNO films showed high values of 10–100
cm. With the increase of annealing temperature, the transport properties of the annealed films approached the pristine
values. Notably, the TNO film annealed at 600 ıC exhibited a
low of 6.0 10 4 cm (ne D 1.33 1021 cm 3 and D 7.8
cm2 /(Vs)), which is very close to that of the pristine film, 5.7 10 4 cm (ne D 1.36 1021 cm 3 and D 8.1 cm2 /(Vs)).
Fully recovered ne demonstrates complete removal of excess
oxygen even in polycrystalline TNO films. The value identical to that of the pristine film indicates that, accompanied with
the removal of excess oxygen, no additional scattering centers
such as reduced crystallinity and impurity phase were incorporated. In order to confirm this, we compared the XRD pattern of the film annealed at 600 ıC with that of the pristine
film, as shown in Figure 3. All the diffraction peaks were indexed to anatase TiO2 , showing that the films were polycrystalline anatase with no impurity phase. In addition, the intensities of the 101 diffraction peaks for the films were very similar
to each other, indicating the similar crystallinity of the films.
These results clearly show that it is possible to remove excess
oxygen even in polycrystalline TNO films without reducing
crystallinity. We speculate that the lower conductivity in polycrystalline TNO films grown from oxygen-rich precursors after
reductive annealingŒ17 is a phenomenon related to non-ideal
features of the crystallization, which might be reduced crystallinity and/or segregated dopants during crystallization.
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