International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
Thin film technique for preparing nano-Zno gas
sensing (O2, No2) using Plasma Deposition
Mohammed K.Khalaf1 , Baha T. Chiad2 , Ala' F. Ahmed3 & Falah A-H. Mutlak4
1
Center of Applied Physics, Ministry of Science and Technology, Baghdad, Iraq
2
Dept. of Physics, College of Science, University of Baghdad, Baghdad, Iraq
3
Dept. of Astronomy, College of Science, University of Baghdad, Baghdad, Iraq
4
Center of Applied Physics, Ministry of Science and Technology, Baghdad, Iraq
Abstract
Zinc oxide thin films in 40 nm crystalline sizes were deposited on glass plates by using DC- sputtering of Ar/O2 (10%) plasma
glow discharges. Home-made DC-glow discharge plasma (reactive ion) source was designed and implemented for this work.
Our electrodes assembly configuration is special and less complex for preparing the nanocrystalline thin films in minimum
grain size. Investigation concerned the technologies of depositing very thin film (~214-280nm) of zinc oxide and investigated
both morphological structure of the produced layers and there optical properties. The structural details and microstructure
were obtained from X-ray diffraction, optical microscope and atomic force microscope (AFM). Plasma deposited ZnO films
exhibits the uniform prefer rential growth of nano crystalline film and hexagonal type structure with good crystallinity and no
indication of amorphous components or other crystalline compounds. The minimum and average crystallite size was obtained
using atomic force microscope. The spherical ZnO nanoparticlles form a spatial homogenous film on the substrate without any
discontinuities was exhibited from the optical microscope. The energy gap of these plasma deposited ZnO films lies in the range
of 3.1-3.3eV. This work also presents investigations on the technology of gas sensing using home-made gas sensing unite and
ZnO nanocrystalline films in the O2 and NO2 gases.
Keywords: Nanotechnology, Thin Films, Gas Sensing, Zinc Oxide, Sputtering .
1. INTRODUCTION
The analysis of high band gap oxide semiconductors proved the potential possibility of applying very thin layers of zinc
oxide ZnO, titanium dioxide TiO2 and tin dioxide SnO2 both as sensor layers to detect selected lights and gases. Zinc
oxide thin films are one of the most prominent transparent conducting oxides for advanced applications such as
window layer in hetero junction solar cells, heat mirrors, piezoelectric devices, multilayer photo-thermal conversion
systems, solid state gas sensors etc. The direct optical energy gap of 3.3 eV for ZnO is large enough to transmit most of
the useful solar radiation. Further, abundant availability of ZnO in nature makes it less expensive and its sharp UV-cutoff makes it desirable in many applications. Thin films of ZnO have been prepared by using several deposition
techniques which include chemical vapor deposition, magnetron sputtering, oxidation of an evaporated metallic film,
spray pyrolysis, pulsed laser deposition, sol-gel technique etc. [1]–[6].
Among these methods, dc reactive sputtering received much attention because of sputtering from elemental target in
the presence of reactive gas for preparation of compound films with high energy of sputtered species, low pressure
operation and low substrate temperature rise, made it as an attractive technique to deposit films on different substrates.
When compared to other physical deposition techniques, sputtered films have better adhesion and greater uniformity
over large areas [1],[7]. The physical properties of ZnO films prepared by dc reactive sputtering mainly depend on the
sputtering parameters such as substrate temperature, oxygen partial pressure and sputtering pressure apart from the
target-substrate distance, sputtering power and deposition rate. In the present investigation, an attempt was made in the
deposition of ZnO films by dc reactive sputtering at various sputtering pressures and studied its effect on the some
physical properties of the films.
2. Experimental (ZnO Films Deposition)
This section is commended by the deposition procedure for films sputtering. Figure1 shows the schematic of the
sputtering chamber and the associated dc power supply of the home – built glow discharge plasma system. The
diameter of the target is (20 mm and approximated 3mm thick) and the distance between the top electrode and target is
4.6cm. The material to be sputtered is made into target; consist of electrode 99.99% purity Zn. The sputtering
procedure is commenced by evaluating the chamber to pressure lower than 1x10-6 mbar. Ar, being a noble gas which
does not react with either the target or the matunal specimen . Ar/O2 (10%) gas mixer introduce into the chamber at a
specified pressure. The dc power supply is then switched on and established to the required current and cathode bias
Volume 2, Issue 6, June 2013
Page 178
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
voltage. Surface finish and nature of the substrate used for depositing is very important since it influences the properties
of the films tremendously.
The ZnO films were deposited on glass substrates, which was ultrasonically cleaned in acetone and blown dry in air
before loading into the deposition chamber .The substrates were sputtered etched in Ar/O2 (10%) plasma at bias voltage
of 1500 volt for 30 minute. The films were allowed to deposit for 60 minute in most of the cases and then the bias
voltage was turned off along with the gas supply to the system. A small area on each sample surface was covered to
prevent deposition and used after processing to measure film thickness.
Figure 1 Schematic of the dc- Sputtering reactor
The thickness of the ZnO film deposited was in the range 214-280 nm. The average thickness of the deposited film was
investigated and determined by three methods:
(a) Weighing Method: the film thickness was calculated by using a theoretical formula given by[1]:
t
m
2  r 2  
(1)
where t is the thickness of the film; m is the mass of material taken in the ZnO target;  is the density of the material; r
is the distance between the target and substrate.
(b) The AFM method was used to determine the prepared ZnO films thickness.
(c) Interferometric Method: Measuring interference fringes produced by reflections from two reflection surfaces
brought into close proximity in an arrangement called an interferometer.
3. Results and Discussion
Effective sputtering of the cathode target (ZnO) is possible only when both the number and the energy of bombarding
ions are large. In short, the energy of the bombarding ions depends on the voltage across the cathode dark space and the
thickness of the dark space, which is inversely proportional to the gas pressure p.
3.1 Ar/O2 Mixture Plasma Characteristics
The Ar/O2 mixture discharge current as a function of cathode potential is shown in Figure2 depends on the
bombardment ions and its energy and working pressure. The discharge current was varied by changing the cathode
potential (dc applied voltage) with a constant ballast resistor. Electric field accelerates the ions and electrons which
then collide elastically with atoms of working gas giving rise to discharge current and gas temperature .The
Ar/O2mixture discharge current as a function of working pressure is shown in Figure 2. With increasing working
pressure, the discharge current is increased which attributed to more molecules are available for the electrons to collide
with and to generate a new free electron and a positive ion. In this way, an increase in pressure would increase the
number of free electrons, truing the dc voltage more negative. The dependence of the discharge current on the applied
voltage and working gas pressure is given as following formula [8]:
Ie = B Pk Vm
(2)
where B, k and m are constant with values depending on the cathode material and working gas. This behavior
attributed to the increase of the secondary electron emission coefficient γ, when the ion energy bombarding the cathode
increases, as mentioned by Auday et al (1998).
Volume 2, Issue 6, June 2013
Page 179
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
50.00
Ar discharge plasma
Vc=-1000V
Vc=-650V
Vc=-450V
DischargeCurrent(m
A)
40.00
30.00
20.00
10.00
0.00
0.00
0.20
0.40
0.60
0.80
Working Pressure(mbar)
Figure 2 The variation of discharge current with working pressure for different applied voltage.
The characteristic of sputtered ZnO films is obtained, when the gas pressure of Ar /O2 mixture was held constant at
8x10-3, 4.6x10-2 and 8x10-2 mbar. The deposition rate (film thickness) decreases from 282nm to 214nm with increase of
sputtering pressure from 8x10-3mbar to 4.6x10-2mbar respectively as shown in table 1 and Figure3.
Table1: The ZnO film thickness and plasma sputtering conditions
Increasing the Ar/O2 pressure further increases the number of electron and ions but decreases their energy because the
voltage across the cathode dark space falls due to thermionic emission from the cathode at high current densities. The
decrease in cathode potential will also result in the decrease of sputtering power hence a decrease of thickness of
prepared films. Also, when the sputtering pressure increased the mean free path of the sputtered particles travel from
the target to the substrate, and some of the sputtered particles were back scattered towards the target. This resulted in
the decrease of the deposition thickness of ZnO prepared films. At higher gas pressure, the sputtered atoms are
prevented from reaching the substrate at the anode because of randomization due to the large number of collisions with
gas molecules. Park et al [9],[10] also observed a decrease of deposition rate of ZnO sputtered films from 12.5nm/min
to 9.0nm/min with the increase of sputtering pressure from 1.5x103- mbar to 6x10-3mbar respectively.
340.00
ZnO sputtered films
ZnOFilm
sThickness(nm
)
Current Density=0.2 - 1mA/cm2
300.00
260.00
220.00
0.02
0.06
0.10
Working Pressure (mbar)
Figure 3 The variation of ZnO film thickness with sputtering gas pressure.
3.2 Structure Analysis of ZnO Sputtered Films
In the present work, diffraction studies are carried out using X- Ray shemadzu XRD-Diffractrometer (operated at 40 kV
an accelerating potential and 30mA with filtered CuKα radiation 1.5418 Ao wavelengths) was performed to identify the
crystalline phases present in the surface layers. It was found that the XRD pattern of ZnO films prepared in this work
are similar to the corresponding patterns of A.S.T.M standard compound. In Fig. 4, the peak positions and the d
spacing calculated from them are in agreement with previous studied [11, 12]. The analysis of the peak positions and
intensities showed that Bragg reflection, of ZnO films correspond to the (002) and (101) orientation, at Bragg angles
around 34.17° and 35.9° respectively. The peaks of reflections indicate that the sputtered films are of polycrystalline
structure and the intensity of (002) reflection is higher than that of other planes, which means is a suitable plane for
crystal growth. Figure 4 shows that the peak intensity increases with decrease of sputtering pressure from 4.6x102
mbar to 8x10-3mbar, in dc reactive sputtered This intensity reduction attributed to orientation of the crystallites
changed from c-axis perpendicular the substrate to that of parallel to the substrate when the sputtering pressure increase
in dc sputtered films.
Volume 2, Issue 6, June 2013
Page 180
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
The grain size of the films can be evaluated from the Scherrer's relation [1]. In present work ,with the decrease in
sputtering pressure from 4.6x10-2 mbar to 8x10-3mbar respectively as shown in Figure 4 we expected increasing the
grain size which indicated by increasing of the peak intensity of (002)and (101) orientations. The increase of grain size
with the decreasing sputtering pressure was due to the improvement in the degree of crystalline and increasing
thickness of the films.
ZnO Sputtered Film
Thickness=283nm,P=0.008mbar
Thickness=260nm,P=0.02mbar
Thickness=214nm,P=0.046mbar
Intensity(a.u)/cps
(002)
(101)
20.00
30.00
40.00
50.00
2Theta(deg.)
Figure 4 XRD pattern of ZnO films prepared by plasma sputtering
The microstructure of the plasma treated samples was examined by optical metallographic techniques using Nikon type
120-Japan optical microscope with digital camera DXM1200F, and the images were analyzed with Act-Version
2,62,2000 program. Optical micrographs of ZnO films deposited at Ar/O2 pressure of 6.4x10-2 mbar are shown in
Fig.5. This micrograph exhibited the uniform distribution of microstructures without any cracks and voids with the
variation in particle size of the grain are regular and microstructure revealed many round shaped particles in cluster
combinations.
Figure 5 Optical micrograph of ZnO sputtered thin film of 214 nm thickness.
The pictures of morphological structure of ZnO films were obtained by using an atomic force microscopy (A° 2000
setup-AFM) as shown in Figure 6. Table 2 and Figure7 shows the surface morphology details of a film nanostructure
determined by means of the AFM method. AFM characterization of the films surfaces revealed a granular,
polycrystalline morphology with grain size and roughness of 5nm.The grain size measured as described in table 2 was
varying from 40to
Figure 6 Morphological structure of ZnO by using an atomic force microscopy (AFM)
Volume 2, Issue 6, June 2013
Page 181
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
Table 2 The grain size percentage in sputtered ZnO film.
Figure 8 The optical transmission and absorbance spectra of ZnO thin films.
3.4 Gas sensing Measurements
The sensitivity of the gas sensor is defined as the capability of the sensor to respond the presence of a given gas
concentration. Mathematically, the sensitivity S is defined by the formula;
S= Rg/Rn for redactor gas, and S= Rn/Rg , for oxidator gas, where Rg and Rn is the resistance of the sensor after and
before passing the gas and reaches the saturation[13].
Resistance of ZnO films exposed to various environmental gases at 200 and 250°C was also measured inside the home
made chamber (Figure 9). The resistance measurements of ZnO samples were carried out in the dark, so that photo
excitation of carriers should not be significant. A result of resistance change vs environmental gas change. The film
sensor is relatively inert to nitrogen and argon, but sensitive to oxidation gas. The sensitivity was found to increase with
increasing of working temperature which is attributed to decrease of mobility of carriers (resistance increased) as shown
in Figures 10, 11, 12. The samples show high sensitivity to O2 and NO2 gases at operating temperature 250 oC
1E+8
Gas senser resistance as function to
the temprature for different gases.
Rair
R N2
1E+7
R Ar
Resistance(ohm)
R O2
1E+6
1E+5
1E+4
1E+3
1.60
2.00
2.40
2.80
3.20
3.60
Temprature 1/T(1000/K)
Figure 9 The gas sensor resistance as a function of the temperature for different gases
Volume 2, Issue 6, June 2013
Page 182
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
6
Resistance ratio at 200o C
Resistance ratio at 250o C
Resistance ratio at 350o C
RO2/RN2
Resistanceratio
5
4
3
2
1
0
100
Time (sec) 200
300
Figure 10 The gas sensor resistance ratio (sensitivity of O2) for different working temperatures
300
Senstivity of ZnO specimens at 250oC
Senstivity(resistance ratio)
O2
N2
200
100
N2
O2
0
0
20
40
60
Time (min)
Figure 11 Sensitivity of ZnO Thin Films to O2
20
Senstivity of ZnO specimens at 250oC
NO2
air
Senstivity(resistanceratio)
16
12
8
4
air
NO2
0
0
20
40
60
Time (min)
Figure 12 Sensitivity of ZnO Thin Films to NO2
4 Conclusions
Prototype ZnO partial pressure (O2 and NO2) sensor has been built using Ar/O2 (10%) discharges plasma. ZnO
sputtered films is a strong function of working temperature. ZnO sputtered films in nanostructure and the thicknesses
are related to working sputtering pressure.
References
[1] K. Chopra and I. Kaur, Thin Film Device Application, Pllenum press, New York and London, 20, 1983.
[2] T. K. Subramanyam , B. Srinivasulu Naidu, S. Uthanna , "Physical Properties of Zinc Oxide Films Prepared by dc
Reactive Magnetron Sputtering at Different Sputtering Pressures", Cryst. Res. Technol. 35, 10, p. 1193, 2000.
[3] K. Golaszewskaa , E. Kaminskaa, T. Pustelny, and K. Gut," Planar Optical Waveguides for Application in
Optoelectronic Gas Sensors", ACTA PHYSICA POLONICA A, 114, No. 6-A, A-223 , 2008.
[4] T. Pustelny, Physical and Technical Aspects of Optoelectronic Sensors, SUT, Gliwice, p. 86, 2005.
Volume 2, Issue 6, June 2013
Page 183
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
[5] S. M. Rossnagel, "Thin film deposition with physical vapor deposition and related technologies", J. Vac. Sci.
Technol. A, 21, 5, pp. S74-S87, September, 2003.
[6] L.I. Maissel, R. Glang (eds.), Handbook of Thin Film Technology, New York, McGraw Hill pp. 3-23, 1970.
[7] H. Meixner, J. Gerblinger, U. Lampe, and M. Fleischer, Thin-film Gas Sensors Based on Semiconducting Metal
Oxides, Sensors and Actuators B, 23, pp.119-125, 1995.
[8] A. M. Suhail," Electron Beam Pumped Carbon Dioxide", Ph.D. Thesis, Dep. Phys., University of Essex, U.K., ,
1983.
[9] G. Auday,Guillot Ph. Galy J."Expermental Study of the Effective Secondary emission Coefficient for Rare Gases
and Copper Electrodes", J.Appl.Phys. 83, 11, p.5917, 1998.
[10] D.Y. Park, K.H. Ma, Kim, "RF Magnetron Sputtering on Glass Substrate" J. Appl. Phys., 81, p. 7764, 1997.
[11] B.T. Kneri, G.S. Kino, "Studies of the Optimum Condition for Growth of RF Sputtered ZnO Films" J. Appl. Phys.,
46, p. 3266, 1975.
[12] K. Matsubara, T. Yamada, "Properties of ZnO Films Prepared by Reactive Ionized Cluster Beam Deposition" Surf.
Sci., 86, 290, 1979.
[13] P. P. Sahay , S. Tewari , S. Iha , M. Shamsuddin ,"Thin Film for Gas Sensor Application", J. Mater. Sci. 40,
p.1791 2005.
Author
Dr. Mohammed K.Khalaf, received a B. Sc. in Department of Physics in 1993, M.Sc. in Physics Science degree in
1998 and Ph.D. degrees in plasma physics in 2010, from University of Baghdad, College of science, Department of
Physics. Presently, he is a researcher at the Applied Physics Center and member of plasma application Department,
Ministry of Science and Technology, Baghdad, Iraq
Dr. Baha T. Chiad, completed his Ph.D. at the physics department in laser spectroscopy from Baghdad
University – Baghdad-Iraq in 1993. His research interests lie in the field of organic semiconductor and
molecular spectroscopy. He is currently a chief of the molecular spectroscopy and laser Research Group at
the Physics department of Baghdad University.
Dr. Falah A-H. Mutlak, received a B. Sc. in Department of Physics in 1998, M.Sc. in spectroscopy physics degree
in 2002 and Ph.D. degrees in molecular spectroscopy physics in 2011, from University of Baghdad, College of
science, Department of Physics. Presently. he is a lecturer at the Department of Physics and member of renewable
energy research group Department of Phyics, College of Science of Baghdad University, Baghdad, Iraq.
Dr. Ala' F. Ahmed Alrashidy. received a B. Sc. in Department of Physics in 1999, M.Sc. in Astronomy Science
degree in 2002 and Ph.D. degrees in plasma physics in 2011, from University of Baghdad, College of science,
Department of Physics. Presently. She is a lecturer at the Department of Astronomy & Space and member of plasma
research group Department of Astronomy & Space, College of Science from Baghdad University, Baghdad, Iraq.
Volume 2, Issue 6, June 2013
Page 184