JOURNAL OF APPLIED PHYSICS
VOLUME 85,NUMBER 12
15 JUNE 1999
A study of low temperature crystallization
of amorphous thin film indium-tin-oxide
指導教授：林克默
學
生：邱巧緣
2011/05/09
博士
Outline
•
•
•
•
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Introduction
Experiment
Results
Discussion
Conclusion
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Introduction
• Indium–oxide has the bixbyite crystal structure which
is a c-type rare earth, vacancy defect oxide. The
bixbyite structure is similar except that the MO8
coordination units (oxygen position on the corners of
a cube and M, a metal atom located at the center of
the cube) are replaced with units that have oxygen
missing from either the body or the face diagonal.
• The removal of two oxygen ions from the MO8 to
form the MO6 coordination units forces the
displacement of the cation from the center of the
cube.
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• Amorphous indium–oxide is likely formed during
physical vapor deposition processing when MO6
coordination units, that evolve from the source or are
formed while chemisorbed on the growth surface, are
incorrectly oriented when they are incorporated into
the growing film.
• Remarkably, the crystallization of amorphous ITO
occurs rapidly at very low homologous temperatures
(T/Tm＜0.19) with 150 °C being commonly cited as
the temperature at which crystallization occurs
rapidly. The significance of this temperature is not
known; however, it is noted that it is close to the
melting point of In metal (157 °C).
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Experiment
• Amorphous ITO films were deposited by electron
beam evaporation from a sintered indium–tin oxide
pellet containing 9.9 wt.% SnO2. The base pressure
of the deposition chamber was 3×10-6 Torr and no
gases were introduced during deposition.
• The source material was evaporated for 5 min immediately prior to deposition of 180 nm thick films on
both oxidized Si and glass substrates at a deposition
rate of approximately 0.8 nm/s. In situ measurements
of resistivity were made during annealing in air at
temperatures that ranged between 120 and 165 °C.
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Results
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• These TEM observations suggest that the ITO
in the as-deposited samples is fully amorphous.
Exposure of the amorphous ITO to the electron
beam changes the appearance of the material from a
uniform amorphous contrast to a light–dark mottled
contrast suggesting that the amorphous material is
susceptible to electron-beam damage.
• After annealing in air for 1 h at 162 °C, the sample
becomes fully crystalline and, as shown in the
cross-sectional image ,consists of large block-like
grains that are, on average, approximately 100 nm
in size.
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• Figure 2 shows that the rate
of the transformation
(change in resistivity)
increases with increasing
temperature, which indicates
that it is thermally activated.
• During the course of the
crystallization, the resistivity
gradually decreases to below 1×10-3 Ω cm.
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• A distinct crystalline peak
corresponding to bixbyite
In2O3 [222] at a 2θ of 30.7
is apparent. Further annealing sharpens the crystalline
In2O3 peaks and, while the
diffuse amorphous peak
integrated intensity drops to
zero, the crystalline [222]
peak reaches a maximum
intensity. After crystallization, a weak peak can be
resolved at a 2θ of about 33°,
which is consistent with
metallic indium.
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• Elementary electrostatics
are then used following
the procedure of
Landauer* to derive
the conductivity as a
function of the volume
fraction of the second
phase:
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• A set of reflectivity versus time data were collected
over the temperature range 110–200 °C.
• The reflectivity changes rapidly initially then
gradually approaches a constant value as shown in
Fig. 5. Unlike the resistivity data which clearly shows
two regimes, the reflectivity data appear to show only
one. This can be seen in Fig. 5 which shows both
resistivity and reflectivity data that were obtained at
135 °C plotted together. This plot shows that 80% of
the reflectivity change occurs during regime I while,
during the same interval, the resistivity changes by
only 20% of the total.
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Discussion
• ITO is a degenerate n-type semiconductor with a band
gap that varies with carrier density between 3.5 and 4.0
eV due to the Burstein–Moss effect.
• It is well established that free carriers in ITO are
contributed by two principal donors: four valent tin
substituting in the crystalline lattice for In and doubly
charged oxygen vacancies (conventionally
represented by [Sn• ] and V••, respectively).
• In our experiments electronic transport
measurements before and after crystallization
revealed that the change in resistivity seen in Fig.2 is
due primarily to an increase in carrier concentration
rather than to an increase in carrier mobility.
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• Inspection of the resistivity versus time data Fig.
2 suggests that there processes which occur during
the annealing of amorphous indium–oxide both of
which affect the carrier density in the oxide. We
speculate that the first regime is associated with the
relaxation of the amorphous structure; the
realignment of In–O bonds to generate a locally
ordered structure that is smaller than a single bixbyite
unit cell (1 nm) but has sufficiently organized InO6
structural units to allow the creation of oxygen
vacancies which contribute carriers and results in a
sharp drop in the film resistivity.
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Avrami–Johnson–Mehl
Equation
• This analysis reveals that the activation energy for
processes is approximately 1.360.2 eV. Based on this,
and the TEM observation that shows grain size and
hence nucleation rate to be independent of the
annealing temperature, it is likely that the
microstructure is controlled by the same atomistic
process—short range atomic rearrangement during
the relaxation of the amorphous structure and similar
short range movements across the amorphous /
crystalline interface during crystallization.
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Conclusion
1. 結果表明在不同的時間常數中，電阻率的變化會
隨著材料在熱激活反應而改變，類似如活化能的
現象。
2. 由實驗得到的ITO薄膜，經Avrami–Johnson–Mehl
方程式計算可得知其具有二-三維成長方向的特
徵。
3. 由此研究的這些過程，更完整了解ITO特性，有
利於今後沉積低電阻、高穿透性之ITO導電膜的
發展，使用於低動能及低基板溫度。
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Thanks for your attention!!
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