Molecular and Quantum Acoustics vol. 26, (2005)
273
SENSOR PROPERTIES OF CADMIUM SULPHIDE (CDS) THIN FILMS IN
SURFACE ACOUSTIC WAVE SYSTEM - PRELIMINARY RESULTS
Marian URBAŃCZYK, Wiesław JAKUBIK, Erwin MACIAK
Institute of Physics, Silesian University of Technology
ul.Krzywoustego 2, 44-100 Gliwice, POLAND
murban@polsl.pl
Presented here are the preliminary results concerning a cadmium sulfide(CdS)
thin films investigations in a Surface Acoustic Wave dual-delay line systems. The
CdS films has been prepared by an open-boat, physical vapor deposition (PVD)
process using chemically prepared CdS powder as starting material. The
thicknesses of the CdS films was about 115nm and 268nm. These CdS films have
been investigated from the point of view their sensitivity towards: nitrogen
dioxide, sulphur dioxide, ammonia and H2S gases with different concentrations in
dry air.
Keywords: cadmium sulfide film , surface acoustic wave, sensor properties
1. INTRODUCTION
Conducting polymers are a new class of materials with a potent application in a number
of growing new technologies, such as energy storage [1,2], molecular recognition [3,4] and
opto-electronic devices [5,6]. All conducting polymers exhibit highly reversible redox
behaviour with a distinguishable chemical memory and hence have been considered as
prominent new materials for the fabrication of chemical sensors [7,8].
Cadmium sulfide, the mineral Greenockite, is an hexagonal, orange crystal with specific
gravity of ~4.8g/cm3 and Mohs hardness of 3.8. Synthetic Cadmium pigments based on
Cadmium sulfide are valued for their good thermal stability in many polymers, for example in
engineering plastics. Cadmium sulfide has useful properties for optoelectronics, being used in
both photosensitive and photovoltaic devices. Cadmium sulfide is an important II–VI
semiconductor with a direct band-gap of 2.4 eV at room temperature, and it receives a wide
range of research interest because of their unique properties and their wide variety of potential
applications. For instance, for laser light-emitting diodes, solar cells, non-linear optical,
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Urbańczyk M., Jakubik W., Maciak E.
optoelectronic and electronic devices. The most important properties of CdS are shown in
Table.1.
Table.1 Properties of CdS
Application fields:
IR optics, polarizers, beamsplitters, λ
/2 and λ/4 waveplates; substrates;
Structure
Density:
Young’s Modulus:
Coef. of Thermal Expansion (500 K):
Wurtzite (Hexagonal)
4.825 g/cm3
45 GPa
Specific Heat:
Thermal conductivity (at 25 oC):
Max. Transmittance (λ =2.5-15 µm):
0.47 J/gK
0.2 W/cmK
Absorption Coef. (λ =10.6 µm):
≤ 0.007 cm-1 (including 2 surfaces)
Refractive index (λ =10.6 µm):
2.2
Max. crystal diameter/length:
∅
α 1=6.26×10-6/K; α 3=3.5×10-6/K
≥ 71 %
40×30 mm
During the past decades, various methods have been applied to fabricate cadmium
sulfide nanocrystallines, such as electrochemically induced
deposition , thermal
decomposition , laser-assisted catalytic growth method , ultrasound irradiation, solvothermal
method, and hydrothermal method [9].
In Institute of Physics SUT – vacuum evaporation technology have been applied for
manufacturing of CdS thin films 115nm and 268nm (with yellow colour) in a SAW method.
In this work, we report on sensor properties of as-deposited thin films of cadmium
sulfide (CdS) by PVD technique at about 7000C from synthesized CdS powder. The
investigations have been performed by SAW method from the point of view ofr sensitivity
towards: nitrogen dioxide, sulphur dioxide, ammonia and H2S gases with different
concentrations in dry air.
2. EXPERIMENTAL
Surface Acoustic Waves (SAW) are very attractive due to their remarkable sensitivity
in a specific configuration of the sensor structure, as well as their small size, low power
consumption and frequency measurements [10-12]. The sensor structure in SAW method is
shown in Fig.1. In such a sensor structure we can use both the acoustoelectric interaction
( between the electric potential associated with surface wave and the charge carrier in the CdS
film) and the mass effects [13-15]. For rather high resistance CdS samples (even above 200 G
Ω) we expect mainly the mass interactions.
Molecular and Quantum Acoustics vol. 26, (2005)
275
Fig.1. The sensor structure with CdS thin film in a dual delay line configuration on
piezoelectric lithium niobate substrate placed in a measuring chamber
The experimental set-up for acoustic sensor is based on frequency changes in a surface
acoustic wave dual delay line system, which is nowadays well known – Fig.2. On a
piezoelectric LiNbO3 substrate, two identical acoustic paths are formed, using interdigital
transducers. Next, a CdS thin film structure is formed in the measuring line in vacuum
deposition processes. The second path serves as a reference and can compensate small
variations of temperature and pressure. Both delay lines are placed in the feedback loop of
oscillator circuits and the response to the particular gas of the active is detected as a change
of the differential frequency ∆f, i.e. the difference between the two oscillator frequencies f
and f0.
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Urbańczyk M., Jakubik W., Maciak E.
Fig.2. Schematic diagram of the experimental set-up
The investigated CdS films with thicknesses of about 115 and 268 nm, were made by
means of the vacuum-sublimation method, using a special aluminium mask and molybdenum
boat. The source temperature was about 700 oC and the thickness was measured by the
interference method. A copper-constantan thermocouple was used to control the temperature.
The total flow rate of 1000ml/min was used during all the measurements. The volume of the
measuring chamber was about 30cm3. The sensor was tested in a computer-controlled system.
Gases of 99.999% pure hydrogen and 99.998% pure NO2, SO2, NH3 in dry air were mixed
using mass flow controllers (Bronkhorst Hi-Tech). The temperature was measured using a
thermocouple adjacent to the structure.
Molecular and Quantum Acoustics vol. 26, (2005)
277
3. RESULTS
Examples of interaction of the obtained CdS film with 115nm in SAW system in sulphur
dioxide are shown in Fig.3, with NO2 in Fig. 4, with NH3 in Fig.5 and with H2S in Fig.6.
2004­12­20
C dS ~115nm
heating
T = 52oC
dosing F3 (SO2) 600 s
plik 2004-12-20d
211,5
60
58
211,0
56
temp
210,5
52
50
∆f
210,0
48
Temperature [oC]
Frequency [kHz]
54
46
20
15
209,5
44
10
5 ppm SO2 in air
42
209,0
0
600
1200
1800
2400
3000
3600
4200
4800
40
5400
Time [s]
Fig.3. Interaction of CdS film (~115nm on LiNbO3 Y-Z substrate) with different
concentrations of sulphur dioxide (SO2) in synthetic air at temperature (~520C)
A very small interactions of the investigated film of CdS can be observed for the
measurements with SO2 gas even at higher temperatures (~520C). The interaction is on the
level of ~100Hz . The resistance of the sample is above of the range of the Electrometer
Keithley 614 (200GΩ). So monitoring of the resistance cannot be performed.
In Fig.4 we can observe an increase of differential frequency of the sample under the
influence of various concentrations of NO2 in synthetic air (between 1000 and 5000 ppm).
The regeneration of the sample at measurement temperature ~560C is quite good.
The decrease of differential frequency of the sample (but for lower frequency mode) is
observed for interaction with ammonia – Fig.5. The changes in frequency are between 250Hz
for 1000ppm to 500Hz for 3000ppm NH3 in dry air.
278
Urbańczyk M., Jakubik W., Maciak E.
plik 2004-12-21c
2004-12-21
CdS 115nm
heating
T = ~ 56oC
dosing Doz1 (NO2) 600s
60
216,2
58
56
54
215,8
52
50
215,6
48
215,4
46
Temperatureo[C]
Frequency [kHz]
216,0
44
215,2
42
1000 ppm NO2 w pow syntet
5000
2000
40
215,0
0
600
1200
1800
2400
3000
3600
4200
Time [s]
Fig.4. Interaction of CdS film (~115nm on LiNbO3 Y-Z substrate) with different
concentrations of NO2 in synthetic air at temperature (~560C)
60
38,4
58
56
temp.
38,2
54
38,0
50
48
37,8
46
∆f
44
37,6
42
40
37,4
Temperature [oC]
Frequency [kHz]
52
38
36
37,2
34
1000 ppm NH3 in air
0
600
1200
1800
32
3000
2000
37,0
2400
3000
3600
30
4200
Time [s]
Fig.5. Interaction of CdS film (~115nm on LiNbO3 Y-Z substrate) with different
concentrations of NH3 in synthetic air at temperature (~580C)
Molecular and Quantum Acoustics vol. 26, (2005)
279
For interaction with H2S a lack of regeneration at lower temperatures and good
regeneration but small interactions at higher temperatures is clearly observed – Fig.6.
In Fig.7 an example of interaction of the second film of CdS (~268nm) with sulphur
dioxide is shown. The changes in frequency ∆f are not great even for rather high gas
concentrations – between 1000 – 5000 ppm in dry air.
plik 2005-01-04a
50
48
46
46,4
42
38
46,2
36
34
46,1
32
30
46,0
o
40
Temperature C]
[
Frequency [kHz]
44
46,3
28
26
45,9
24
10 ppm H2S in air
22
20
45,8
0
900
1800
2700
20
4500
3600
Time [s]
32,6
60
58
56
32,4
temp
54
Frequency [kHz]
50
48
32,0
∆f
46
44
31,8
42
40
31,6
Temperature [oC]
52
32,2
38
36
31,4
34
10 ppm H2S in air
30
20
31,2
0
900
1800
2700
3600
4500
32
40
5400
6300
30
7200
Time [s]
Fig.6. An examples of preliminary acoustic measurements for CdS 115nm with different
concentrations of H2S at ~320C (upper) and ~530C (lower graph).
280
Urbańczyk M., Jakubik W., Maciak E.
40
plik 2005-01-12b
38
t
36
34
32
30
28
26
251.24
24
22
∆f
251.22
2005-01-12
CdS 2 h~268nm
20
o
T measurement ~ 34 C
dosing Doz1 (SO2) 600s
16
Temperature [ oC]
Frequency [kHz]
251.26
18
14
12
5000
2000
251.20
10
0
600
1200
1800
1000 ppm SO2 in Air
2400
3000
3600
4200
Time [s]
Fig.7. Interaction of CdS film (~268nm on LiNbO3 Y-Z substrate) with different
concentrations of sulphur dioxide (SO2) in synthetic air at temperature (~340C)
4. CONCLUSIONS
The cadmium sulfide films has been prepared by an open-boat, physical vapor
deposition (PVD) process using chemically prepared powder as starting material. The
thicknesses of the CdS films was about 115 and 268 nm. These CdS films have been
investigated from the point of view their sensitivity towards: nitrogen dioxide, sulphur
dioxide, ammonia and H2S gases with different concentrations in dry air. Preliminary
measurements of this two CdS films have been performed using an acoustic method with
Surface Acoustic Waves. This method can be successfully used even when the resistance of
the sample is very high – above 200GΩ.
For CdS 115nm film, good and very repeatable interaction with NH3 at higher
temperatures (above 500C) has been observed.
Whereas for interaction with H2S lack of regeneration at lower temperatures and good
regeneration but small interactions at higher temperatures is clearly observed.
For the sample with a CdS thickness of 268nm an interaction with SO 2 on the level
50Hz is observed but only for higher concentrations.
The best results were achieved after many cycles of interaction and at higher
temperatures.
Molecular and Quantum Acoustics vol. 26, (2005)
281
ACKOWLEDGEMENTS
This work was sponsored by the Polish Committee for Scientific Research within the grant
No. O T00A02726.
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