Performance of InSnZrO as transparent conductive oxides

Article abstract of DOI:10.1002/pssa.200925278

InSnZrO thin films were deposited on glass substrates by magnetron sputtering with an indium-tin-oxide (ITO) target and a zirconium target. X-ray diffractometry (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) revealed that InSnZrO thin films had better crystalline structure, larger grain size, and lower surface roughness than ITO thin films. Zr doping markedly improved the optical- electrical characteristics. In comparison with ITO thin films, the resistivity of InSnZrO thin films deposited at room temperature decreased from 6.24 × 10^-3 to 2.20 × 10^-3 ωcm, and the maximum optical transmittance in the visible range was enhanced from 53.7 to 66.1%. The thin films showed an obvious Burstein-Moss effect with substrate temperature. Moreover, the direct transition model showed a wider optical bandgap of InSnZrO thin films than that of ITO thin films. As a result, InSnZrO thin films prepared by cosputtering revealed better overall properties than traditional ITO thin films.

Full text of DOI:10.1002/pssa.200925278

Phys. Status Solidi A 207, No. 4, 955–962 (2010) / DOI 10.1002/pssa.200925278
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applications and materials science
Performance of InSnZrO as
transparent conductive oxides
B. Zhang*, B. Yu, J. Jin, B. Ge, and R. Yin
Department of Die and Mould Technology, Changzhou Institute of Mechatronic Technology, Changzhou 213164, China
Received 31 May 2009, revised 8 August 2009, accepted 16 October 2009
Published online 25 November 2009
PACS 61.05.cp, 68.37.Lp, 68.37.Ps, 73.61.Cw, 78.66.Li, 81.15.Cd
* Corresponding author: e-mail glass114@163.com, Phone: þ86 519 86331243, Fax: þ86 519 86331243
InSnZrO thin films were deposited on glass substrates by temperature decreased from 6.24 ꢀ 10ꢁ3 to 2.20 ꢀ 10ꢁ3 V cm,
magnetron sputtering with an indium-tin-oxide (ITO) target and and the maximum optical transmittance in the visible range
a zirconium target. X-ray diffractometry (XRD), atomic force was enhanced from 53.7 to 66.1%. The thin films showed an
microscopy (AFM), and transmission electron microscopy obvious Burstein–Moss effect with substrate temperature.
(TEM) revealed that InSnZrO thin films had better crystalline Moreover, the direct transition model showed a wider
structure, larger grain size, and lower surface roughness than optical bandgap of InSnZrO thin films than that of ITO thin
ITO thin films. Zr doping markedly improved the optical– films. As a result, InSnZrO thin films prepared by cosputtering
electrical characteristics. In comparison with ITO thin films, revealed better overall properties than traditional ITO thin
the resistivity of InSnZrO thin films deposited at room films.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The multifunctional oxides such as mentioned above [5–7]. However, it is difficult to achieve
transparent conductive oxides (TCOs) have attracted sub- better specialized properties of TCO films produced with
stantial interest for applications as transparent conductive binary and ternary compounds. Recently, some researchers
contacts. For practical applications scientists are searching have proposed the use of multicomponent oxides, whose
for new materials consisting of multicomponent oxides physical and chemical properties can be controlled by
suitable for special electrical, optical, mechanical, and changing their chemical compositions, which may resolve
chemical applications, like the production of the transparent the above problems. From this point of view, ITO thin films
conductive ZnO films by doping with Al or Ga [1] and In2O3 doped with a small amount of foreign metal, which not only
films by doping with Mo [2]. Indium-tin-oxide (ITO) is a retain the basic properties of ITO thin films but also develop
highly degenerate n-type semiconductor with a wide some special properties, have been widely studied [8–11].
bandgap (above 3.7 eV) and belongs to the classic TCOs. The transparent conductive multicomponent oxide thin films
ITO thin films have been widely used in the optoelectronics based on ITO may exhibit properties that are suitable for
industry because of their unique transparent and conducting specialized applications.
properties [3, 4]. However, some developments such as
Due to a high sputtering rate and good film perform-
large-area flat-panel displays, solar cells, sensors, and ances, magnetron sputtering is widely used to grow ITO thin
organic light-emitting devices (OLED) require improve- films. As is known, the characteristics of ITO strongly
ments in electrical, optical, chemical, surface properties, and depend on its oxidation state and the content of impurities.
thermal stability of ITO thin films used as transparent Carrier concentration can be modified by the dopant
electrodes. At the same time, other current TCOs beyond activation state, which is due to a donor atom to substitute
ITO films also face the existing limitations of practical the lattice site and produce some free electrons to increase
application in some special devices, such as carrier carrier concentration. In this study, zirconium is regarded as
collection towards the near red region of the visible spectrum the donor, which replaces indium in the In2O3 matrix of ITO
and how to improve overall materials mobility [2]. Some thin films. As a result, one free electron is released to
TCO films with ternary component oxide such as those based contribute the electrical conductivity. The ionic radius of
on SnO2, ZnO, and In2O3 may resolve some of the problems Zr4þ and In3þ are, respectively, 0.084 and 0.092 nm, and the
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B. Zhang et al.: Performance of InSnZrO as transparent conductive oxides
ionic radius of Zr4þ is lower than that of In3þ, which implies
that a limited solid solution can be formed. Besides, Zr
dopant can improve the activation of Sn dopant, which
promotes Sn4þ replacement of In3þ. In this paper, the
properties of ITO and InSnZrO thin films deposited at
various substrate temperatures were contrastively studied.
2 Experimental The dual sputtering sources in a two-
target sputtering system, including DC and RF power
sources, were used for cosputtering with an ITO target and
a zirconium target (99.995%). ITO and InSnZrO thin films
were deposited on glass substrates, and the deposition
process of ITO and InSnZrO thin films was the same except
for including the zirconium target for the InSnZrO thin films.
The DC sputtering power of the ITO target was 45 W, and the
RF sputtering power of the zirconium target was 10 W.
The ITO target was a hot-pressed In2O3 target containing
10 wt.% SnO2. The target-to-substrate distance was 60 mm.
Typically, the base vacuum was about 1.0 ꢀ 10ꢁ4 Pa, and the
pressure was stabilized at about 0.5 Pa during the deposition
processing by controlling the argon gas flow. The oxygen
flow rate was 0.3 sccm controlled by a mass flow controller.
After chemical cleaning and etching, the float glass
substrates were placed in the vacuum chamber. The
substrates were heated from room temperature (20 8C) to
400 8C. All sputtering processes were dynamic with the
substrate rotating in front of the targets. After 30 min
deposition, a film thickness of about 240 nm was achieved.
The averaged atomic ratios of ITO and InSnZrO thin films
deposited at room temperature were In:Sn:O ¼ 55.26:6.14:
38.60 at.% and In:Sn:Zr:O ¼ 54.35:6.04:1.21:38.40 at.%,
respectively. ITO and InSnZrO thin films showed about the
same thickness under similar deposition processing.
Figure 1 XRD patterns of (a) ITO and (b) InSnZrO thin films as a
function of substrate temperature.
The film thicknesses were measured with an alpha-step
profilometer (Dektak 6M). The sheet resistance was
determined using a four-point probe system (SDY-5). The
carrier concentration and mobility of the films were film at 150 8C shows a better crystallite structure. The XRD
measured using the Hall effect in van der Pauw mode. The spectra reveal that InSnZrO thin films have better crystalline
crystal structures were examined by a X-ray diffractometer structure than ITO thin films, and Zr doping also leads to the
(XRD; D/max 2550 V) using Cu Ka radiation. The surface preferred orientation change from the (222) to the (400)
morphology of films was evaluated by atomic force plane.
microscopy (AFM; AJ) and transmission electron micro-
Based on the intensity of the XRD peak, the variation of
scopy (TEM; JEM-100CX), respectively. Optical transmit- the I(222)/I(400) ratio for ITO and InSnZrO thin films as a
tance and reflectance spectra of the ITO/glass structure were function of substrate temperature is shown in Fig. 2. The
measured with a UV–Vis–NIR scanning spectrophotometer oxidation of the growing film is in competition with the
(Lamda950). All measurements were carried out at room oxygen-removal phenomenon, and the competition has an
temperature.
impact on the modification of the prefer orientation growth
from the (222) to the (400) plane [13]. Oxygen incorporation
leads to films oriented in the (222) direction, and (400)
orientation is related to oxygen deficiency. In the case of Zr
3 Results and discussion
3.1 Microstructure characterization Figure
1
shows the variations of XRD patterns of ITO and InSnZrO addition to the sputtering system, a Zr atom can lead to
thin films at various substrate temperatures. Because oxygen deficiency of the vacuum environment and
magnetron sputtering is a high-energy deposition process, Zr-doping can also lead to oxygen deficiency in the In2O3
all samples are well crystallized and show clear XRD peaks, matrix because of the oxygen-deficient state of zirconium
and they crystallize in the cubic bixbyite structure of indium oxide, which favors the preferred orientation along the (400)
oxide [12]. With the increase in substrate temperature, an direction. Besides, the change of the orientation from the
increase in intensity of the XRD peak is observed, and ITO (222) plane to the (400) plane is related to the deposition
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Phys. Status Solidi A 207, No. 4 (2010)
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compensates the force. The oxygen ions are removed and
the repulsive forces increase with substrate temperature,
which leads to an enlargement of the matrix. As substrate
temperature increases, the crystal structure tends to be
integrated and then the lattice constant decreases. The lattice
parameter of InSnZrO thin films decreases and follows by a
sudden rise with the increase in substrate temperature. Zr
doping favors a better crystalline structure of InSnZrO films,
leading to the decrease in lattice parameter. With higher
substrate temperature, the matrix vibration of a multi-
component oxide is quickened, leading to an enlargement of
the unit cell, and an increase in lattice parameter.
Furthermore, the average grain sizes of the films at room
temperature and 400 8C estimated by Scherrer’s relation and
FWHM of (222) and (400) reflection peaks are 21 and 42 nm
for ITO thin films and 26 and 54 nm for InSnZrO thin films. It
is believed that InSnZrO thin films have bigger grain size
than ITO thin films and the grain size shows an evident
increasing tendency with increasing substrate temperature.
Atomic force microscopy images of ITO and InSnZrO
thin films are shown in Fig. 3. All the films display a very
Figure 2 Variation of I(222)/I(400) ratio of ITO and InSnZrO thin
filmsasafunctionofsubstratetemperature.Insetshowsthevariation
of lattice parameter.
conditions. The variation of orientation with substrate smooth and void-free surface. The grain size shows an
temperature is also related to the energy of the sputtered evident increasing tendency with substrate temperature. It is
particles reaching the substrate surface [14]. When the evident that the substrate temperature changes both the grain
energy of the sputtering particles on the substrate surface size and the grain shape markedly, leading to the marked
attains a certain value, the thin film can form in the (400) improvement of film crystallization. The surface rough-
plane orientation.
nesses can be a barometer for the grain sizes and the number
The lattice parameters of the thin films can be calculated of grain boundaries, and the number of grain boundaries can
from the diffraction angles and the prominent d-values also be interpreted from the sizes and numbers of the peaks
corresponding to the (222) and (400) planes, and the between the height and minimum valleys. Table 1 shows the
variations of lattice parameters of all the films with substrate roughness of ITO and InSnZrO thin films. Rp-v is the average
temperature are shown in Fig. 2. The lattice parameters of roughness of peak-valley and Rrms is the root mean square
ITO and InSnZrO thin films show the contrary variations. As roughness, whose value was calculated by AFM results using
to ITO thin films, the increase in lattice parameter is a high-pass filter to reject the factor of the bare substrate
attributed to the increase in repulsive forces coming from the roughness. The surface roughness increases with substrate
extra positive charge of the positive tin ion. In the oxidized temperature, and InSnZrO thin films show lower surface
state, the interstitial oxygen’s negative ion charge roughness than the ITO thin films at the same substrate
Figure 3 (online color at: www.pss-a.com) AFM images of (a) ITO and (b) InSnZrO thin films with substrate temperature
(2000 nm ꢀ 2000 nm).
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Table 1 Tmax, index of refraction (n) and roughness of (a) ITO and (b) InSnZrO thin films as a function of substrate temperature (Tmax
maximum transmittance in the visible wavelength range; n: index of refraction at 500 nm wavelength; Rrms: root mean square roughness;
p-v: average roughness of peak-valley).
:
R
temperature (8C)
parameters
optical data
Tmax (%)
AFM data
index of refraction
n (at 500 nm)
Rrms (nm)
Rp-v (nm)
(a)
(b)
(a)
(b)
(a)
(b)
(a)
(b)
4.12
4.20
8.25
20
53.7
78.7
87.2
87.5
87.9
66.1
82.7
86.4
88.1
89.5
2.40
2.05
1.99
1.98
1.95
2.21
2.07
2.01
1.99
1.93
1.03
1.73
2.78
3.28
5.33
1.01
1.61
1.89
1.95
2.80
5.31
8.58
9.50
14.37
19.24
150
250
300
400
9.31
10.74
temperature. Surface morphologies and surface roughness InSnZrO thin films are all 300 8C, and the thicknesses are
can directly affect the electrical properties of the films, about 60 nm. All the films were peeled off from the substrate
especially the carrier mobility.
by dipping them into deionized water and were picked up
KCl (100) single crystal was used as a substrate for TEM with a copper grid to prepare samples. Figure 4 shows the
analysis. The deposited temperatures of both ITO and high-resolution electron microscopy (HREM) images,
bright-field images, and the selected-area diffraction pattern
(SADP). The HREM and bright-field image show that
InSnZrO thin films have a better crystalline structure and
larger grain size than ITO thin films. The SADPs show the
polycrystalline structure of all the thin films, and the
diffraction spots of InSnZrO thin films are scattered because
its orientations and larger grain size. Some of the weaker
reflections may not be crystalline structure corresponding to
the hexagonal phases of In2O3 [15].
3.2 Electronic and optical properties Figures 5
and 6 show the sheet resistance, resistivity, carrier
concentration, and mobility as a function of substrate
temperature for ITO and InSnZrO thin films. The sheet
resistance and resistivity of ITO and InSnZrO thin films
decrease with the increase in substrate temperature and reach
about 10 V/sq and 2.4 ꢀ 10ꢁ4 V cm at 400 8C, respectively.
The carrier concentration and mobility, as a whole, increase
with increase in the substrate temperature. The increase in
carrier concentration with substrate temperature may be due
to an increase in the diffusion of Sn atoms from interstitial
locations and grain boundaries into the In sites in the In2O3
matrix. Sn atoms act as donors in ITO films, therefore the
increase in Sn diffusion with substrate temperature results in
higher electron concentration. The variation of mobility is
regarded as the result of better crystallinity of the films,
which promotes the increase in the grain size and reduced
grain-boundary scattering and increase conductivity [16].
The mobility of the ITO films at 150 8C is higher, which
corresponds well with the better crystallite structure of ITO
films at 150 8C. The improved crystal performance and the
decrease in the grain boundary reduce the absorption of
donor SnO2 in dislocations and crystal defects. Therefore,
the carrier concentration and mobility increase, reducing the
Figure 4 HREMimages,bright-fieldimages,andSADPof(a)ITO
and (b) InSnZrO thin films deposited on KCl substrates (for all
films deposition temperature at 300 8C and thickness of 60 nm).
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Phys. Status Solidi A 207, No. 4 (2010)
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concentration [17]. The behaviors of the mobility in
polycrystalline films are dependent on different electron-
scattering mechanism including grain-boundary scattering,
neutral defect scattering, and ionized-impurity scattering.
The grain-boundary scattering decreases with the increase in
substrate temperature and the growth of grain size. The
neutral-defect scattering is enhanced with the incorporation
of oxygen and thermal instability at high temperature. Due to
high carrier concentration of crystalline films, the ionized-
impurity scattering always plays a more important role than
other scattering mechanisms to limit the carrier mobility.
Indeed, it is necessary to expect and realize the enhancement
of the mobility with the increase of carrier concentration.
The optical transmittance and reflection spectra of ITO
and InSnZrO thin films (including glass substrate) as a
function of the wavelength are represented in Fig. 7. The
transmission in the visible region increases with the increase
in substrate temperature. The transmission in the visible
region of InSnZrO thin films is higher than that of ITO thin
films, and a maximum transmission of about 89% is achieved
Figure 5 VariationofresistivityofITOandInSnZrOthinfilmsasa
function of substrate temperature. Inset shows the variation of sheet
resistance.
resistivity of all the films. By contrast with ITO thin films,
InSnZrO thin films show better electrical properties. We can
see that the sheet resistance of ITO and InSnZrO films
deposited at room temperature are 260.12 and 91.65 V/sq
(resistivity, 6.24 ꢀ 10ꢁ3 and 2.20 ꢀ 10ꢁ3 V cm), which show
that higher electrical quality ITO films can be obtained at
lower substrate temperature with Zr doping. The carrier
concentration of InSnZrO thin films is higher than that of
ITO thin films, because zirconium is regarded as the donor
that replaces indium in the In2O3 matrix of ITO thin films. In
the meantime, the mobility of the films increases due to Zr
doping to some extent. The electrical properties of ITO films
deposited at lower substrate temperature can be markedly
improved with Zr doping by cosputtering technology.
Concerning the electron transport in this multicompo-
nent system, the mobility is closely related with the carrier
Figure 7 Optical transmittance and reflection spectra of (a) ITO
Figure 6 Variation of carrier concentration and mobility of ITO and (b) InSnZrO thin films as a function of substrate temperature in
and InSnZrO thin films as a function of substrate temperature.
the wavelength range 200–2500 nm.
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in the visible region for InSnZrO thin films (shown in values of the index of refraction (n) according to 500 nm
Table 1). As the film thicknesses of all samples under wavelength are given in Table 1, which show the decrease in
investigation are equal within small variation, the trans- index of refraction with substrate temperature for ITO and
mission and reflection behaviors in the 200–2500 nm InSnZrO thin films.
wavelength region are due to a variation of the complex
The reflection in the higher-wavelength region is closely
index of refraction of the films. To examine these behaviors related with the carrier concentration of the films. Above the
in detail we calculated the complex index of refraction for all 1000 nm region, the reflection is strongly related to the free-
the films by using simulation software. Figure 8 shows the electron densities, and the free electrons can produce
index of refraction (n) and extinction coefficient (k) of ITO reflecting behavior of the photons in the near-IR region,
and InSnZrO thin films with substrate temperature. The the free-electron carrier reflection becomes critical in the
absorption coefficient (a) and extinction coefficient (k) have near-IR region. Therefore, the continuous increases in the
the relation, a ¼ 4pnk/l. On the basis of the condition of the reflection or the decreases in the absorption in the near-IR
interference pattern, the peak of transmittance appears when region with the substrate temperature imply the sequential
the thin-film thickness (d) is suitable for 4nd ¼ ml, where l is increases in the carrier concentrations, and the same
the incident wavelength, and m is the interference order (an behavior is to InSnZrO thin films. From the transmittance
odd integer). Because the d is invariable at different substrate spectrum in Fig. 7, we observe the fact that the absorption
temperatures, a decrease of refractive index resulted in the edge shifts toward shorter wavelength with increasing
shift of transmittance maximum (shown in Fig. 7). The temperature. According to Drude’s theory, ITO film is
transparent until the fundamental absorption starts at
wavelengths close to the corresponding optical bandgap
energy. The widening of optical bandgap energy is due to the
Burstein–Moss effect [18]. Assuming that the conduction
band and valence band are parabolic in nature and the B–M
shift is the predominant effect, the carrier concentration can
roughly be estimated from the optical bandgap shift due to
the B–M effect, and the optical bandgap is closely related
with the carrier concentration.
According to the direct allowed transitions equation, the
direct transition model of (aE)2 versus photon energy E was
established, aE ¼ A(E ꢁ Eg)1/2 and optical bandgap energy
Eg was obtained by linear extrapolation to (aE)2 ¼ 0 (insets
of Fig. 9) [19]. The absorption coefficient (a) is determined
by the relation, a ¼ (1/d) ln(1/T) where d is the thickness of
the films, T is the transmittance of the samples. The optical
bandgap of ITO and InSnZrO thin films increase with
substrate temperature (in Fig. 9), which may be due to an
Figure 8 Index of refraction (n) and extinction coefficient (k) of Figure 9 Variation of optical bandgap of ITO and InSnZrO thin
(a) ITO and (b) InSnZrO thin films as a function of substrate films with substrate temperature. Insets show a typical plots of E vs.
temperature in the wavelength range 200–2500 nm.
(aE)2 for ITO and InSnZrO thin films.
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Phys. Status Solidi A 207, No. 4 (2010)
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increase in carrier concentration with substrate temperature
and as a result, the absorption edge shifts towards the near
UV range. The increase in optical bandgap can be explained
on the basis of B–M effect. The direct transition models show
wider optical bandgap of InSnZrO thin films than that of ITO
thin films, which implies that Zr-doping improves the carrier
concentration of the films and widens the corresponding
optical bandgap.
In order to evaluate the optoelectronic quality of the
films, the figure of merit FTC for transparent conductors in
photovoltaic applications is given by FTC ¼ T1a0/Rsq, where
Ta is the average transmittance in the visible range and Rsq is
the sheet resistance of the films [20]. Figure 10 shows the
figure of merit of all the films with substrate temperature. The
figure of merit of ITO and InSnZrO thin films increase with
substrate temperature, which shows that the figure of merit of
ITO films increases from 0.003 ꢀ 10ꢁ3 to 15.03 ꢀ 10ꢁ3 Vꢁ1
,
and the values of InSnZrO films vary from 0.15 ꢀ 10ꢁ3 to
18.14 ꢀ 10ꢁ3 Vꢁ1. The role of temperature in the improve-
ment of the figure of merit may be related to the increase in
the grain size and mobility. The higher value of figure of
merit of InSnZrO thin films than that of ITO thin films are
observed for the films deposited at a lower substrate
temperature and above 300 8C, which reveals that InSnZrO
thin films have better optoelectronic properties. The
zirconium-doped ITO films indicate that the doping with a
small amount of foreign metal increases the thermal and
chemical stability, which are shown in Fig. 11. Based on the
measurements of the contact angles, the zirconium-doped
ITO films enhances surface polarity, increases surface
energy, and decreases contact angle, therefore, hole carriers
can easily be injected into the OLED active layers at a lower
driving voltage with a modified surface of the zirconium-
doped ITO electrodes.
Figure 11 DR/Rasafunctionofimmersiontimeforfilms(a)under
350 8C thermal environment and (b) after immersion in 0.1 M HCl
solution, DR is the change of sheet resistance.
4 Conclusion The properties of ITO and InSnZrO thin
films deposited at different substrate temperatures were
contrastively studied. XRD analysis revealed a change in
preferential orientation of polycrystalline structure from the
(222) to the (400) plane with Zr doping; it was the result of
the oxygen-removal mechanism competing with the oxi-
dation phenomenon. InSnZrO thin films had better crystal-
line structure, larger grain size, and lower surface roughness
than ITO thin films. The resistivity of ITO and InSnZrO thin
films deposited at room temperature were 6.24 ꢀ 10ꢁ3 and
2.20 ꢀ 10ꢁ3 V cm, which showed that better properties of
ITO thin films could be obtained at lower substrate
temperature with Zr-doping. The transmission in the visible
region of InSnZrO thin films was higher than that of ITO thin
films. The Zr doping improved the carrier concentration of
the films and widened the corresponding optical bandgap.
The widening of optical bandgap was due to the Burstein–
Moss effect. The higher value of the figure of merit of
InSnZrO films was observed, which revealed that InSnZrO
Figure 10 Figure of merit of ITO and InSnZrO thin films with
substrate temperature. The figure of merit is given by FTC ¼ T1a0/Rsq.
Ta istheaveragetransmittanceinthevisiblerangeandRsq isthesheet
resistance.
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B. Zhang et al.: Performance of InSnZrO as transparent conductive oxides
thin films had better optoelectronic properties as well as the
increases in the thermal and chemical stability. InSnZrO
films could find extensive application.
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Acknowledgements This work has been supported by the
Basic Research Fund for Changzhou Institute of Mechatronic
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