Room temperature sensitivity of Ta doped nanocrystalline ZnO films to NH3 exposure

Abstract

Undoped (ZnO) and doped with Ta (ZnO:Ta) films have been deposited by magnetron sputtering of a ZnO target and cosputtering of the target with Ta chips on its surface respectively. The structural and optical properties of the films have been performed by X-ray Diffraction Spectroscopy (XRD), optical transmittance and reflectance measurements. The sensitivity of the undoped ZnO thin films and those doped with Ta (ZnO:Ta) to exposure to NH3 was measured by the ratio of the resistivity in air to that in the presence of the target gas. A higher sensitivity was observed in the Ta doped ZnO film.. It is shown that ZnO thin films doped with Ta have a potential application as room temperature sensors.

Full text

JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, Vol. 11, No. 9, September 2009, p. 1371 - 1374
Room temperature sensitivity of Ta doped
nanocrystalline ZnO films to NH3 exposure♣
H. NICHEV*, O. ANGELOV, M. KAMENOVA, V. MIKLIa, D. DIMOVA-MALINOVSKA
Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko
Chaussee Blvd., 1784 Sofia, Bulgaria
aCentre for Materials Research, Tallinn Technical University, Tallinn, Estonia
Undoped (ZnO) and doped with Ta (ZnO:Ta) films have been deposited by magnetron sputtering of a ZnO target and co-
sputtering of the target with Ta chips on its surface respectively. The structural and optical properties of the films have been
performed by X-ray Diffraction Spectroscopy (XRD), optical transmittance and reflectance measurements. The sensitivity of
the undoped ZnO thin films and those doped with Ta (ZnO:Ta) to exposure to NH3 was measured by the ratio of the
resistivity in air to that in the presence of the target gas. A higher sensitivity was observed in the Ta doped ZnO film.. It is
shown that ZnO thin films doped with Ta have a potential application as room temperature sensors.
(Received November 5, 2008; accepted December 15, 2008)
Keywords: Thin films, ZnO, Optical bandgap, Gas sensors, Quartz crystal microbalance
1. Introduction
ZnO is a material widely used in industry for decades.
Because of its chemical sensitivity to different adsorbed
gases, high chemical stability, amenability to doping, non-
toxicity and low cost, it has attracted much attention as gas
sensor [1-3].
The catalytic functions of the ZnO thin film surface
have been changed by intentional introduction of different
elements into the sensing film [4, 5]. In this work, a
comparison of the room temperature sensitivity, S, of
nanocrystalline undoped (ZnO) and Ta-doped (ZnO:Ta)
thin films to NH3 exposure has been performed. The
sensitivity has been measured by the ratio of the resistivity
in air to that in the presence of the target gas. The
influence of the substrate temperature, Ts, of the
nanocrystalline thin films deposition by magnetron
sputtering on their optical and structural properties and on
the sensitivity is also analysed.
2. Experimental
Thin nanocrystalline films of ZnO and ZnO:Ta were
prepared by r.f. magnetron sputtering in an Ar (0.5 Pa)
atmosphere, from a sintered ZnO target and by co-
sputtering of a ZnO target with Ta chips on its surface,
respectively, as described earlier in [6].
The samples were deposited on glass substrates at
different Ts, in the range 1500C - 5000C. The optical and
structural properties were measured on samples 500 nm
thick.
X-ray diffraction spectra (XRD) of the samples were
obtained using a Brucker D8 Advance spectrophotometer
with CuKα radiation: λ CuKα1= 1.540560 Å and λ CuKα2 =
1.544426 Å (intensity half that of λ CuKα1). The
instrumental broadening was 0.040 in 2Θ geometry.
Optical transmittance and reflectance spectra were
measured in the range 350-1500 nm, using a CARY UV-
VIS-NIR spectrophotometer. The Ta concentration was
determined by Energy Dispersive X- Ray (EDX)
microanalysis using Link AN10000 system analysis. Co-
planar Al electrodes on the film surface were evaporated
to provide ohmic contacts for measurement of the
resisitivity changes under NH3 exposure. The gas
sensitivity, S, of the thin films on glass substrates was
determined by the ratio of the resistivity in air to that in a
NH3 atmosphere at different target gas concentrations,
calculated from the I-V measurements with a Keithley
6517 electrometer.
3. Results and discussion
The values of the Ta contents, in dependence on the
substrate temperature of the thin film
_____________________________
♣Paper presented at the International School on Condensed Matter Physics, Varna, Bulgaria, September 2008

1372 H. Nichev, O. Angelov, M. Kamenova, V. Mikli, D. Dimova-Malinovska
Table 1. The values of the 2Θ of (002) XRD peaks, their FWHM, the average grain size, D, the Ta content and the
optical band gap, Eg, of ZnO and ZnO:Ta thin films, deposited at different Ts.
Samples Ts,
0C
2θ, deg
(ZnO)
FWHM, deg,
(ZnO)
D, nm
(ZnO)
2θ, deg
(ZnTa2O6)
FWHM, deg
(ZnTa2O6)
D, nm
(ZnTa2O6)
Eg,
eV
E0,
meV
Ta, at.%
ZnO 150 34.30 0.52 16 - - - 3.33 63 0
ZnO 275 34.40 0.47 18 - - - 3.30 59 0
ZnO 500 34.40 0.33 25 - - - 3.27 60 0
ZnO:Ta 150 33.55 1.53 54 33.09 0.37 22 3.33 124 4
ZnO:Ta 275 33.55 1.34 62 32.95 0.48 17 3.39 95 4.5
ZnO:Ta 500 - - - 32.74 1.05 8 3.46 95 7
deposition, are displayed in Table 1. The Ta content
increased with increasing Ts. The XRD spectra shown in
Fig. 1a reveal reflections corresponding to the (002)
crystallographic plane of wurtzite ZnO, in the case of
undoped ZnO films. Only in the case of a ZnO film
deposited at a low Ts = 1500 C were the reflections of the
(100) and (101) planes observed together with the (002)
peak. The peak intensity increased, the peak position
shifted to the angle corresponding to powder ZnO (34.440)
on a 2Θ scale, and its Full Width at Half Maximum,
FWHM, decreased with increasing Ts. In the case of Ta
doped ZnO films, the bands in the XRD spectra (Fig 1 b)
of the samples deposited at lower Ts = 1500C and
2750C are very large and could be related to the presence
of different phases – ZnO and ZnTa2O6 [7].
20 30 40 50 60 70
ZnO
x25
x10
1500C
2750C
(112)
(103)
(110)
(102)
(101)
(002)
(100)
5000C
(004)
Intensity, (arb. un.)
2Θ, [deg.]
(a)
31 32 33 34 35 36
30 40 50 60 70
x2 ZnO:Ta
5000C
2750C
1500C
Intensity, (arb. un.)
2Θ, [deg.]
5000C2750C1500C
ZnO:Ta
Intensity, (arb. un.)
2Θ, (deg.)
(b)
Fig. 1. XRD spectra of ZnO (a) and ZnO:Ta(b) thin
films, deposited at different Ts.
Actually, the best fit was obtained introducing only
two bands – of ZnO (002) at 33.550 and ZnTa206 (051) in
the range 33.090 – 33.870 [7]. Fig. 2 displays the fits of the
XRD spectra (Lorentzian low was used) of ZnO:Ta films
deposited at different Ts. Increasing the Ta concentration
in the doped ZnO:Ta films with Ts increasing was
accompanying by a decrease in the intensity of the XRD
band related to ZnO. In the XRD spectrum of the sample
deposited at Ts = 5000C (the Ta concentration increased to
about 7%), this band disappears and only the band of
orthorhombic ZnTa2O6 (051) is observed [7], however its
intensity decreased. An increasing Ts resulted in a shift of
the position of the XRD peak of (051) band to a lower 2Θ,
from 33.090 to 32.740. The shift of its position to the
higher value of 2Θ than that reported for the ZnTa206
powder (2Θ =32.3730 [7]) could be due to the presence of
the compressive stress. Increasing values of Ts and of the
Ta concentration caused a decrease in the stress.
The average grain sizes, D, for ZnO and ZnTa2O6
nanocrystallites were calculated to an accuracy of 10% by
the Debye-Scherrer equation [8] to the FWHM of ZnO
(002) and ZnTa2O6 (051) peaks, and are shown in Table 1.
The increase of the Ts stimulated a better structural order
of undoped ZnO films, and led to an increase in the
average grain sizes. In the case of ZnO:Ta films, the
average grain size of the nanocrystallites of the ZnO phase
increased slightly.However, the D value of the
nanocrystallites of the ZnTa2O6 phase decreased from
about 22 to 8 nm with Ts increasing from 1500C to 5000C.
The absorption coefficient, α, in dependence on the
incident photons energy, is shown in Figs. 3 a and b. It is
calculated for direct allowed electron transitions, from the
reflectance and transmittance spectra [9]. The values of the
optical band gaps decreased from 3.33 to 3.27 eV in the
case of undoped ZnO films, and increased from 3.33 3.46
eV in the case of Ta doped ZnO films (Table 1). The
changes of Eg with Ts for undoped ZnO films can be
explained using the Burstein-Moss effect, as reported
earlier [9]. The optical band of ZnO:Ta films increased
with increasing Ts, probably due to an increasedof the Ta
concentration (as the EDX analysis shows) and with an
increasing presence of the ZnTa2O5 and other tantalum
oxide phases (according to the XRD spectra). For
example, the band gap of Ta2O5 is reported to be about
4.1-4.2 eV which value is higher than that of ZnO [10].

Room temperature sensitivity of Ta doped nanocrystalline ZnO films to NH3 exposure 1373
to
30 31 32 33 34 35 36 37
0
500
1000
1500
2000
2500
3000
3500
4000
4500
measured curve
ZnO:Ta;Ts=1500C
Curve-Fit
ZnO;(002)
ZnTa2O6;(051)
Intensity, [counts/s]
2Θ,[deg.]
(a)
30 31 32 33 34 35 36 37
0
500
1000
1500
2000
2500
3000
3500
4000
4500 ZnO:Ta;Ts=2750C
ZnO;(002)
Curve-Fit
ZnTa2O6;(051)
measured curve
Intensity,[counts/s]
2Θ,[deg.]
(b)
30 31 32 33 34 35 36 37
0
100
200
300
400
500
600
700 ZnO:Ta;Ts=5000C
ZnO;(002)
ZnTa2O6;(051)
Curve-Fit measured curve
Intensity, [counts/s]
2Θ,[deg.]
(c)
Fig. 2. Fits with two Lorentzian bands of the
experimental XRD data of the ZnO:Ta samples,
deposited at (a) 1500C, (b) 2750C and (c) 5000C.
In the lower energy range, where α varies
exponentially with photon energy, it is possible to assume
that the spectral dependence of the absorption edge
follows the Urbach formula [11]. The exponential
dependence of the absorption on hν in the Urbach region
is due to the perturbation of the parabolic density of the
states at the band edge – increasing structural disorder
results in an increase in the Urbach energy, E
0 [11]. The
energy dependences of α at lower photon energies is
displayed in the inserts of Fig. 3a and b. The calculated
values for the Urbach energy (resolution of ±4 meV) are
given in Table 1. The Ta doped ZnO films exhibited
significant band tailing in the lower energy region,
compared to undoped ZnO films. This could be attributed
to structural defects due to the mixed phase in the doped
samples, and to the lower size of the crystallites as well.
The results show that E0 decreases with increasing
substrate temperature for two sets of samples, indicating
an improvement of the structural ordering which coincides
with the XRD observation.
3,13,23,33,43,5
0
1x1011
2x1011
3x1011
4x1011
5x1011
6x1011
3,2 3,3 3,4
9
10
11
12 ZnO
5000C
2500C
1500C
lnα
Energy, [eV]
5000C
2750C
1500C
(αhν)2 , [cm-2. (eV)2]
ZnO
Energy, [eV]
(a)
3,1 3,2 3,3 3,4 3,5
0
1x1010
2x1010
3.3 3.4 3.5
6
8
10
12
5000C
ZnO:Ta
2750C1500C
lnα
Energy, [eV]
5000C
2750C
1500C
ZnO:Ta
(α.hν)2, [cm-2.eV2]
Energy, [eV]
(b)
Fig. 3. (α* hν)2 plot against the photon energy for ZnO
(a) and ZnO:Ta (b) films deposited at different Ts. The
inserts show the plot of ln α vs. hν.
2500 5000 7500 10000 12500
0,0
2,0x103
4,0x103
6,0x103
8,0x103
1,0x104
ZnO-5000C
ZnO:Ta-2750C
ZnO:Ta-5000C
ZnO:Ta-1500C
x16
Sensitivity
NH3, [ppm]
Fig. 4. Sensitivity of ZnO (Ts=5000C) and ZnO:Ta
thin films vs. NH3 concentration.

1374 H. Nichev, O. Angelov, M. Kamenova, V. Mikli, D. Dimova-Malinovska
The variation in the sensitivity of ZnO, deposited at
Ts=5000C and ZnO:Ta, deposited at different Ts, with NH3
concentrations up to 12,500 ppm is shown in Fig. 4. The
sensitivity of ZnO films deposited at 1500C and 2750C is
not shown, because it is lower than the used NH3
concentration. The sensitivity of ZnO:Ta films to NH3
increased with increasing Ts. It has to be pointed out that
the ZnO:Ta film had a sensitivity 3 times higher than that
of the ZnO film deposited at the same Ts = 5000C with
12,500 ppm of NH3.
It is well accepted that NH3 reacts with the surface
species and the trapped electrons are returned to the
conduction band, causing an increase in the conductivity
of the material and the sensitivity of the sensor [3].
Usually, the gas sensitivity increases with decreasing grain
sizes in the films [12].
The higher sensitivity of the Ta doped ZnO films
compared to the undoped ones reported here could be due
to the presence of a higher concentration of defects at the
surface of the grains (as suggested from the observed band
tailing), where higher quantity of O2- and OH- could be
absorbed. Lower crystallite sizes in the Ta doped ZnO film
compared to undoped ZnO deposited at Ts = 5000C result
in an increase in their effective surface and as a
consequence the concentration of point and packing
defects. The presence of the phases of ZnTa2O6 could be a
reason for the higher sensitivity of the ZnO:Ta, as well.
4. Conclusions
Study on the influence of the substrate temperature of
nanocrystalline ZnO and ZnO:Ta thin films deposited by
r.f. magnetron sputtering on their structural and optical
properties, and sensitivity to ammonia, has been
performed. The optical band gap decreased with Ts for the
undoped ZnO films and increased in the case of ZnO:Ta
films. The sensitivity to NH3 of ZnO and ZnO:Ta films
increased with increasing Ts. The ZnO:Ta film
demonstrated a higher sensitivity than the undoped ZnO
film deposited at the same Ts = 5000C. A higher sensitivity
was observed in ZnO:Ta films, compared to the undoped
ZnO films. The higher concentration of defects and
different phases (ZnO and ZnTa2O6), or only one phase of
ZnTa2O6 in Ta-doped ZnO films, could be one of the
reasons for the different behaviours of the sensors with
undoped and Ta doped ZnO films. It is shown that
nanocrystalline Ta doped ZnO thin films have a potential
application as room temperature sensors.
Acknowledgements
This work is being performed with financial support
from the Bulgarian National Scientific Fund by a project
UF 05/2005. H. Nichev gratefully acknowledges the
financial support of European Social Fund, Human
Resources Development Programme, under the contract
BG 051PO001/07/3.3-02/58/ 17.06.2008
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*Corresponding author: nitschew@yahoo.de