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Chemical spray pyrolysis deposition of transparent and conducting Fe doped CdO thin films for ethanol sensor
Materials Science in Semiconductor Processing 40 (2015) 879–884
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/matsci
Chemical spray pyrolysis deposition of transparent and conducting Fe doped CdO thin films for ethanol sensor K. Sankarasubramanian a, P. Soundarrajan a, K. Sethuraman a,n, K. Ramamurthi b a b
School of Physics, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India Department of Physics & Nanotechnology, SRM University, Chennai 603203, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2015 Received in revised form 7 July 2015 Accepted 29 July 2015
Transparent conducting cadmium oxide (CdO) and Fe (0.5, 1, 1.5 and 2 wt%) doped CdO thin films were deposited on glass substrate using facile chemical spray pyrolysis technique. The influence of Fe doping on the structural, morphological, optical and electrical properties of CdO thin films were systematically investigated using different characterization techniques such as X-ray diffraction (XRD), atomic force microscope (AFM), UV–vis transmittance and Hall measurement respectively. The XRD patterns clearly reveal the pure and Fe doped CdO films are polycrystalline nature along with cubic crystal structure. AFM images show the roughness of the CdO thin film decreases with respect to Fe doping concentration. The surface structure of the 1 wt% Fe doped CdO film was investigated by X-ray photoelectron spectroscopy (XPS). Optical transmittance of the CdO thin film decreases with increasing of Fe doping concentration. The optical band gap value of the CdO thin film gradually decreases from 2.61 to 2.18 eV with increasing Fe doping concentration. From Hall measurement, the minimum value of resistivity is obtained for (7.67 10 4 Ω cm) 1 wt% of Fe doped CdO thin film. Finally, the ethanol sensor behaviour was observed using home-built sensor unit. The maximum ethanol gas response of 56.12% is observed for 1 wt% of Fe doped CdO thin films. This result suggests the Fe doped CdO is one of the promising material in sensor applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Transparent conducting oxide Spray pyrolysis X-ray diffraction Electrical conductivity Ethanol sensor
1. Introduction Ethanol gas sensors have been potentially applied in various areas such as chemical, medicine and food industries. Although commercially available gas sensors are mainly based on SnO2, but their sensitivity and stability to ethanol gas are still unsatisfactory and further efforts are needed in R&D. The main advantage of ethanol based sensor is low price, high sensitivity and low power consumption. The most common application of ethanol sensors is as a breath analyser, since the ethanol vapour in human breath is correlated with the concentration in the blood. Recently, gas sensors based on the semiconducting metal-oxides such as SnO2, ZnO, CdO, ITO, WO3, and CuO have been found to be very useful for detecting ethanol vapour [1–9]. It was found that cadmium oxide based thin films having high sensitivity to ethanol gas because of very thin discontinuous layer surface of CdO films can remarkably enhance its gas-sensing characteristics [10,11]. Highly conducting and transparent semiconducting materials such as tin oxide (SnO2), indium oxide (In2O3), indium tin oxide (ITO), zinc oxide n
Corresponding author. Fax: þ 91 9445252309. E-mail address: [email protected] (K. Sethuraman).
http://dx.doi.org/10.1016/j.mssp.2015.07.090 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
(ZnO) and cadmium oxide (CdO) are widely used for many applications such as flat panel displays, smart windows, light emitting diodes, heat reflectors, photovoltaic devices and sensors [12–17]. Among these, CdO is a promising material for optoelectronic applications due to its good electrical conductivity, high optical transmittance in the visible region of the solar spectrum [18]. Electrical resistivity of CdO film in the range of 10 2–10 4 Ω cm and optical band gap lies between 2.2 and 2.7 eV [19–22]. The conduction of pure CdO is attributed to native defects of oxygen vacancies and cadmium interstitials. Therefore, it is possible to control the conductivity of CdO films by controlling that native defects [23,24]. The optical and electrical properties of the film can be controlled by doping of CdO with some metallic ions. It was experimentally confirmed that doping of CdO with metallic ions of a smaller ionic radius than that of Cd2 þ , like In, Sn, Al, Sc, and Y improves its electrical conductivity and change its optical band gap, which was elucidated through Moss–Burstein effect [25–28]. So far, 3d transition metal elements, such as Mn, Fe and Co, have been alloyed with CdO and their properties have been investigated. It has been found that the magnetic semiconductors formed by replacing the cations of III–V or II–VI nonmagnetic semiconductors by ferromagnetic Mn, and Fe exhibit a number of unique optical and electrical properties, applicable for TCO
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applications. Pure and doped CdO thin films were prepared by different techniques such as sol–gel [29], DC-Magnetron sputtering [30], activated reactive evaporation [31], chemical bath deposition [32], pulsed laser deposition [33] and spray pyrolysis [34–39]. Among these, spray pyrolysis technique is a simple and cost effective method for the deposition of thin films with large area. Recently, the spray pyrolysis technique is employed for various dopant ions in the metal oxide semiconducting materials and provides an easy way to dope the required elements in the required ratio through the precursor solution [40]. It is well known that the gas sensing properties of oxide materials are related to the surface morphology and resistivity value, both are mainly depends on their crystallite and grain size. Compare with Cd2 þ ion, ionic radius of Fe2 þ ion is low, therefore, Fe is taken as a dopant in CdO thin film and attempts to enhance the sensing property with respect to doping concentration. The main aim of the present work is deposit pure and Fe doped CdO thin films and investigates their structural, optical, electrical properties for ethanol sensing. Thus far, there is no report on ethanol gas sensing studies on Fe doped CdO thin films prepared by chemical spray pyrolysis technique.
2. Experimental sections Pure and Fe doped CdO thin films were deposited on the glass substrates using simple chemical spray pyrolysis technique. Cadmium acetate [Cd(CH3(COO)2)] was used as a source material of Cd and double distilled (DD) water was used as the solvent. For preparing final resultant solution, 0.1 M of Cd (CH3(COO)2) was dissolved in DD water and stirred for 15 min. Well-cleaned microscopic glass slides (75 25 1.2 mm3) were used as a substrate. The resultant precursor solution was sprayed on preheated substrates at a substrate temperature of 300 °C. The substrate temperature was controlled through a thermocouple with the help of PID temperature controller. The optimized deposition parameters such as substrate-spray nozzle distance (25 cm), spray angle (about 45°), spray time (3 s), spray interval (30 s), carrier gas pressure (compressed air – 40 kg/cm2) and flow rate of the solution ( 3 ml/min) were kept constant. Finally, the coated substrates were allowed to cool down to room temperature. To achieve Fe doping, ferric chloride (FeCl3) with various concentration (0.5–2 wt %) was mixed with the final precursor solution. The structural properties of the prepared films were analysed using Philips X Pert PRO X-ray diffraction (Cu Kα1 radiation; λ ¼1.54056 Å). The thickness of the pure and Fe doped CdO films were measured using stylus profilometer. Surface roughness of the prepared samples was elucidated using atomic force microscope (AFM – A100 SGS). The chemical binding states of 1 wt% Fe doped CdO thin film were measured by X-ray photoelectron spectrometer (XPS) ESCA – 3400. Optical transmittance spectrum of pure and Fe doped samples was recorded in the wavelength range of 190– 900 nm using UV–vis spectrometer (Shimadzu UV-1601). The electrical properties were then measured using Hall measurement setup in Vander Pauw configuration (Ecopia HMS-3000). The ethanol sensing properties of CdO thin films were studied in a homemade sensor unit. For electrical measurements, silver paste contacts (1 mm) were made on the CdO sample of area 1 cm 1 cm. Initially, the current–voltage characteristic was studied within 710 V and in this range, the electrical contacts showed ohmic behaviour. The electrical resistance of CdO film in air (Ra) and in the presence of ethanol (Rg) was measured to evaluate the gas response (S) defined as follows:
S (%) =
R a − Rg × 100 Rg
(1)
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1(a) shows the XRD patterns of the pure and Fe-doped CdO thin films. All the samples exhibits two major (111), (200) and two minor (220), (222) diffraction peaks respectively. This analysis revealed that the pure and Fe doped CdO films are polycrystalline in nature and having a face-centred cubic crystal structure (NaCl structure of a space group Fm-3m) according to the JCPDS file no. 05-0640. No peaks corresponding to their complex oxides did not detected that suggests the film was deposited without any phase segregation [41–44]. The calculated lattice constant value for pure CdO is a ¼4.67 Å (standard value of a ¼4.69 Å) and agrees well with standard JCPDS value. The pure CdO thin film showed a preferential orientation along (200) direction, while Fe doping the intensity of the (200) diffraction plane is decreased and the intensity of (111) peak is increased. Thus, the preferential growth orientation is shifted from (200) to (111) direction at higher (1.5, 2 wt%) Fe doping. This may be due to the incorporation of Fe ions into the Cd lattice sites change the growth environment. Fig. 1(b) shows the enlargement view of (111) and (200) diffraction planes. The Fe doped CdO films slightly shifted to higher (2θ) angle as compared to the pure CdO. The shrinkage of the lattice plane by Fe doping owing to the covalent radius of Fe2 þ (0.078 m) is smaller than that of Cd2 þ (0.095 nm) [45]. The refined unit cell parameter ‘a’ of pure CdO was found to be 4.672(2) Å and Fe doped CdO (0.5, 1, 1.5 and 2 wt%) thin films shows 4.670(6) Å, 4.668(4) Å, 4.666(5) Å and 4.661(2) Å, respectively. The X-ray crystallite size of the Fe doped CdO thin films was estimated using Scherrer's relation [46] and the calculated values are given in Table 1. Fig. 2 shows the crystallite size and strain value as a function of Fe doping and observed values are presented in Table 1. As can be seen, the strain value increases with increasing of Fe doping concentrations, which indicates that the large amount of Fe doping may create lattice distortion. Importantly, the smaller radius of Fe2 þ ions than that of Cd2 þ ions leads to decrease the lattice spacing value that is responsible for the change in the crystallite. Upto 1 wt% of Fe doping, crystallite size value is increased. At higher doping concentration the crystallite size value gradually decreased. 3.2. Surface morphological studies Fig. 3 shows the AFM images of the pure and Fe doped CdO thin films with different Fe doping (0.5, 1, 1.5 and 2 wt%) concentration. Here, the change in the surface morphology depending on the increase in Fe concentration can be clearly seen. The pure CdO thin film shows a relatively spherical shaped grains and shown in Fig. 3(a). 0.5 and 1 wt% Fe doped CdO images clearly shows the irregular shapes with different sizes particles and are shown in Fig. 3(b) and (c). At higher (1.5 and 2 wt%) Fe doped CdO thin films show that the images (Fig. 3(d) and (e)) are homogeneous surface with densely packed small spherical particles. The roughness of the pure CdO thin film is 12 nm. Continuously, the roughness of Fe doped CdO thin film decreases 11, 9, 6, 5 nm with increasing doping 0.5, 1, 1.5 and 2 wt% concentration respectively. The surface morphology of the CdO is modified by Fe doping. The reason for this morphological change is may be change in the preferred orientation of the Fe doped CdO films.
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Fig. 1. XRD patterns of pure Fe doped CdO thin films.
Table 1 Comparison of crystalline size, strain, resistivity, mobility and carrier concentration of pure and Fe doped CdO thin films. Sample
Resistivity (ρ) Mobility (μ) (Ω cm) (cm2 V 1 s 1)
Carrier concentration (N) (cm 3) 1020
Thickness (nm)
CdO Fe:CdO (0.5%) Fe:CdO (1%) Fe:CdO (1.5%) Fe:CdO (2%)
2.03 10 3 2.14 10 3
12 13
1.968 2.149
450 515
7.67 10 4
39
1.578
563
2.09 10 4
47
6.243
598
1.57 10 3
21
1.834
625
3.3. Chemical composition studies Fig. 4(a) shows the wide scan XPS spectrum of the spray deposited 1 wt% Fe doped CdO thin film. XPS measurements were performed to confirm the chemical binding states of the Fe doped CdO thin film. Fig. 4(b)–(d) shows the measured XPS spectra of Cd 3d3/2, 3d5/2, Fe 2p and O 1s core level. The sharp peaks observed at 407 and 413.5 eV are attributed to the existence of Cd2 þ and this exactly matched with the binding energy of Cd–O. Fig. 4(c) shows the measured XPS spectra of O 1s level, the peaks centred at 535.5 eV is clearly indicate the chemisorbed oxygen in the samples and its attributed to the existence of O2 . Fig. 4(d) confirms the existence of Fe2 þ ion in the Fe doped CdO thin film. The spectrum shows a peak centred at 711.5 eV is clearly agreeing Fe 2p1/2 in CdO structure. Thus, the XPS data gives an obvious evidence for the existence of the Fe2 þ ion in the Fe doped CdO thin films, which is in agreement with the XRD result. In addition to this, a few other signals relate to the carbon, silicon and sulphur are observed attributed from the glass substrate. 3.4. Optical properties Optical transmittance spectra of pure and Fe doped CdO thin films are depicted in Fig. 5. It is observed that all the films are highly transparent in the visible range. It is seen that the transparency of the CdO film completely depends on Fe dopant. The transmittance of the CdO thin films decreases with increasing of Fe doping concentration. This may be due to the decreasing of crystallinity and increasing of optical scattering effect caused by less densification of film crystallites and incorporation of more Fe ion in the lattice and interstitial position of CdO. The effect of Fe doping on the optical band gap of CdO thin films is calculated using Tauc's plot through transmittance spectra. The absorption coefficient (α) is calculated using the equation:
Fig. 2. Variation of crystalline size and lattice strain value of CdO thin films as a function of Fe doping level.
⎛ 1⎞ α = ln ⎜ ⎟/d ⎝T⎠
(2)
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Fig. 3. AFM images of CdO thin films as a function of Fe doping level.
T and d is the transmittance and film thickness respectively. The absorption coefficient (α) and the incident photon energy (hν) are related by the following equation.
(αhν )2 = A ( hν − Eg )
(3)
Here hν, A and Eg are corresponding to the photon energy, constant and optical band gap, respectively [47]. The direct band gap determination is based on the extrapolated linear regression of the curve resulting from a plot of photon energy hν vs (αhν)2. The variation of absorption coefficient against photon energy for direct band-to-band transition is given by Tauc's plot (αhν)2 ¼A (hν Eg) and is shown in Fig. 6. The band gap of CdO thin film is 2.61 eV. The optical band gap is found varying between 2.40 and 2.17 eV with increasing doping concentration. This decrease in optical band gap of CdO thin film by Fe doping may be increasing film thickness and related to the Burstein–Moss effect [48]. According to the Burstein–Moss model, the absorption edge of CdO is shifted towards lower energies. This effect occurs when the carrier concentration exceeds the conduction band edge density of states by Fe doping in CdO films. 3.5. Electrical studies The room temperature electrical resistivity (ρ), mobility (μ), and carrier concentration (N) of pure and Fe doped CdO films were measured using Vander–Pauw method in a constant magnetic field of 5 kilogauss and the observed results are presented in Table 1. The negative sign of hall coefficient confirms n-type conductivity. The pure CdO thin film shows resistivity 2.03 10 3 Ω cm which decreases to 7.67 10 4 Ω cm for 1 wt% Fe doping. Initially the resistivity value decreases up to 1 wt% Fe doping and then increases at higher doping concentrations of Fe (1.5 and 2 wt%) ions. At lower level Fe doping, the Fe ions are effectively placed into the Cd2 þ lattice and act as donors by supplying free electrons. After 1.5 and 2 wt% Fe doping, the resistivity values
increased efficiency due to phonon scattering and ionised impurity scattering. The pure CdO film exhibits a carrier concentration of 1.968 1020 cm 3. This value increases with increasing Fe doping level and reaches to a maximum value of 6.243 1020 cm 3 at 1.5 wt% of Fe doping. The resistivity value decrease at 2 wt% Fe doping concentration. The increase in carrier concentration caused by most of Fe2 þ ions homogeneously placed in Cd2 þ lattice to provide more charge carriers. At higher doping concentration of Fe ion (2 wt%) the carrier concentration value has been decreased. The reason is the excess amount of Fe ions cannot be accommodated into the CdO lattice due to its limited solid solubility. The hall mobility value increased from 12 to 47 cm2 V 1 s 1 with increasing the doping concentration of Fe (up to 1.5 wt%). The mobility value of 2 wt% Fe doped CdO film found to be decreases (2 wt%). This may be attributed to ionised impurity scattering centres in CdO films. At higher doping concentration of Fe ion increase the electrical resistivity and lead to build up of carrier traps in the lattice, which in turns reduce the mobility. Further, as the ionic radius of Fe smaller than that of Cd, hence here the excess Fe atoms may occupy the interstitial position which leads to deform the crystal structure.
3.6. Ethanol sensing characteristics Generally, the gas sensing mechanism of semiconducting metal oxide thin films is based on a change in electrical conductance or resistance due to the gas adsorption and desorption on the sensing surface of the film. Hence, here on the surface of the Fe doped CdO thin film, the interaction occurs between the chemisorbed ethanol and absorbed oxygen. At low operating temperature, O−2 is chemisorbed, while at higher temperatures O2 and O are chemosorbed and O2 disappear rapidly. The oxygen adsorption can be described by following equations [49]:
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883
Fig. 4. XPS spectra of 1 wt% of Fe doped CdO thin film.
Fig. 6. Plots of (αhγ)2 against hγ of CdO films as a function of Fe doping level. Fig. 5. UV–vis optical transmittance spectra of CdO thin films as a function of Fe doping level.
O2 (ads) + e− ↔ O2−(ads) ⎫ ⎪ ⎪ O2−(ads) + e− ↔ 2O−(ads) ⎬ ⎪ − ⎪ O− + e− ↔ O2(ads ) ⎭
On exposure to ethanol gas at different operating temperatures, the reaction between the ethanol vapour and oxygen that is adsorbed onto the surface of the film can be expressed by following equation [50]:
CH3 CH2 OH(adsorbed) + 6O−(adsorbed) → 2CO2 + 3H2 O + 6e− (4)
(5)
Due to the reaction, a number of free electrons are re-injected
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ethanol gas and lowered the optimum operating temperature as well. Our work demonstrates the ability to increase the response of Fe doped CdO thin films based ethanol sensors, which will have great merit in commercialized.
References
Fig. 7. Variation in Ethanol gas response of the CdO thin film as a function of Fe doping level at 1000 ppm.
into the film, so that the resistance of the films decreases as the ethanol gas flows into the chamber and is subsequently adsorbed onto the surface of the Fe doped CdO thin film. The gas sensing experiments were carried out in the operating temperature ranging from 150 to 375 °C in the exposure of 1000 ppm ethanol gas. The lower temperature operation, the surface of the sensor does not get completely desorbed which causes smaller change in gas response. As the temperature increases and reaches around 300 °C, the sudden increase in the gas response is observed. The oxidation of ethanol is much enhanced and faster at 300 °C on the surface of the Fe doped CdO thin film following the trend of a redox reaction. Fig. 7 shows the sensing response curve of Fe doped CdO thin film in the doping concentration from 0.5 to 2 wt%. It is clearly seen that, Fe doped CdO thin film shows different gas sense response at the same ethanol gas concentration. From the figure the maximum gas sensitivity 56.12% is observed for 1 wt% of Fe doped CdO thin film, which is decreased to 52% for 0.5 wt% of Fe doping and 43%, 38% of sensitivity is observed for 1.5 and 2 wt% of Fe doping, respectively. At low temperatures the sensor response is restricted by the rate of the chemical reaction, and at higher temperatures it is restricted by the rate of diffusion of gas molecules. At some intermediate temperature, the rate of these two processes become equal, and at that point the sensor response reaches its maximum [51].
4. Conclusion Pure and Fe doped CdO thin films were successfully prepared using facile spray pyrolysis technique. The cubic crystal structure with no evidence of Fe2O3 or mixed phases is observed. The slight variation of lattice parameter is due to the replacement of Cd by Fe ions. The roughness of the CdO thin film decreases with increasing Fe doping concentration. The chemical binding states of the 1 wt% Fe doped CdO thin film confirmed by the XPS measurements. With respect to Fe doping concentrations, the optical band gap value is decreased from 2.61 to 2.18 eV. The minimum electrical resistivity of 7.67 10 4 Ω cm is obtained for the 1 wt% of Fe doped CdO thin film, which is attributed to the increase of carrier concentration due to the substitution of Fe2 þ ions in the CdO film. The maximum ethanol gas response of 56.12% is observed for 1 wt% of Fe doped CdO thin film at an operating temperature of 300 °C and the presence of the thin layer of Fe doped CdO (1 wt% of Fe CdO) thin film surface greatly enhanced the sensitivity of the sensor to
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/matsci
Chemical spray pyrolysis deposition of transparent and conducting Fe doped CdO thin films for ethanol sensor K. Sankarasubramanian a, P. Soundarrajan a, K. Sethuraman a,n, K. Ramamurthi b a b
School of Physics, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India Department of Physics & Nanotechnology, SRM University, Chennai 603203, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2015 Received in revised form 7 July 2015 Accepted 29 July 2015
Transparent conducting cadmium oxide (CdO) and Fe (0.5, 1, 1.5 and 2 wt%) doped CdO thin films were deposited on glass substrate using facile chemical spray pyrolysis technique. The influence of Fe doping on the structural, morphological, optical and electrical properties of CdO thin films were systematically investigated using different characterization techniques such as X-ray diffraction (XRD), atomic force microscope (AFM), UV–vis transmittance and Hall measurement respectively. The XRD patterns clearly reveal the pure and Fe doped CdO films are polycrystalline nature along with cubic crystal structure. AFM images show the roughness of the CdO thin film decreases with respect to Fe doping concentration. The surface structure of the 1 wt% Fe doped CdO film was investigated by X-ray photoelectron spectroscopy (XPS). Optical transmittance of the CdO thin film decreases with increasing of Fe doping concentration. The optical band gap value of the CdO thin film gradually decreases from 2.61 to 2.18 eV with increasing Fe doping concentration. From Hall measurement, the minimum value of resistivity is obtained for (7.67 10 4 Ω cm) 1 wt% of Fe doped CdO thin film. Finally, the ethanol sensor behaviour was observed using home-built sensor unit. The maximum ethanol gas response of 56.12% is observed for 1 wt% of Fe doped CdO thin films. This result suggests the Fe doped CdO is one of the promising material in sensor applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Transparent conducting oxide Spray pyrolysis X-ray diffraction Electrical conductivity Ethanol sensor
1. Introduction Ethanol gas sensors have been potentially applied in various areas such as chemical, medicine and food industries. Although commercially available gas sensors are mainly based on SnO2, but their sensitivity and stability to ethanol gas are still unsatisfactory and further efforts are needed in R&D. The main advantage of ethanol based sensor is low price, high sensitivity and low power consumption. The most common application of ethanol sensors is as a breath analyser, since the ethanol vapour in human breath is correlated with the concentration in the blood. Recently, gas sensors based on the semiconducting metal-oxides such as SnO2, ZnO, CdO, ITO, WO3, and CuO have been found to be very useful for detecting ethanol vapour [1–9]. It was found that cadmium oxide based thin films having high sensitivity to ethanol gas because of very thin discontinuous layer surface of CdO films can remarkably enhance its gas-sensing characteristics [10,11]. Highly conducting and transparent semiconducting materials such as tin oxide (SnO2), indium oxide (In2O3), indium tin oxide (ITO), zinc oxide n
Corresponding author. Fax: þ 91 9445252309. E-mail address: [email protected] (K. Sethuraman).
http://dx.doi.org/10.1016/j.mssp.2015.07.090 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
(ZnO) and cadmium oxide (CdO) are widely used for many applications such as flat panel displays, smart windows, light emitting diodes, heat reflectors, photovoltaic devices and sensors [12–17]. Among these, CdO is a promising material for optoelectronic applications due to its good electrical conductivity, high optical transmittance in the visible region of the solar spectrum [18]. Electrical resistivity of CdO film in the range of 10 2–10 4 Ω cm and optical band gap lies between 2.2 and 2.7 eV [19–22]. The conduction of pure CdO is attributed to native defects of oxygen vacancies and cadmium interstitials. Therefore, it is possible to control the conductivity of CdO films by controlling that native defects [23,24]. The optical and electrical properties of the film can be controlled by doping of CdO with some metallic ions. It was experimentally confirmed that doping of CdO with metallic ions of a smaller ionic radius than that of Cd2 þ , like In, Sn, Al, Sc, and Y improves its electrical conductivity and change its optical band gap, which was elucidated through Moss–Burstein effect [25–28]. So far, 3d transition metal elements, such as Mn, Fe and Co, have been alloyed with CdO and their properties have been investigated. It has been found that the magnetic semiconductors formed by replacing the cations of III–V or II–VI nonmagnetic semiconductors by ferromagnetic Mn, and Fe exhibit a number of unique optical and electrical properties, applicable for TCO
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applications. Pure and doped CdO thin films were prepared by different techniques such as sol–gel [29], DC-Magnetron sputtering [30], activated reactive evaporation [31], chemical bath deposition [32], pulsed laser deposition [33] and spray pyrolysis [34–39]. Among these, spray pyrolysis technique is a simple and cost effective method for the deposition of thin films with large area. Recently, the spray pyrolysis technique is employed for various dopant ions in the metal oxide semiconducting materials and provides an easy way to dope the required elements in the required ratio through the precursor solution [40]. It is well known that the gas sensing properties of oxide materials are related to the surface morphology and resistivity value, both are mainly depends on their crystallite and grain size. Compare with Cd2 þ ion, ionic radius of Fe2 þ ion is low, therefore, Fe is taken as a dopant in CdO thin film and attempts to enhance the sensing property with respect to doping concentration. The main aim of the present work is deposit pure and Fe doped CdO thin films and investigates their structural, optical, electrical properties for ethanol sensing. Thus far, there is no report on ethanol gas sensing studies on Fe doped CdO thin films prepared by chemical spray pyrolysis technique.
2. Experimental sections Pure and Fe doped CdO thin films were deposited on the glass substrates using simple chemical spray pyrolysis technique. Cadmium acetate [Cd(CH3(COO)2)] was used as a source material of Cd and double distilled (DD) water was used as the solvent. For preparing final resultant solution, 0.1 M of Cd (CH3(COO)2) was dissolved in DD water and stirred for 15 min. Well-cleaned microscopic glass slides (75 25 1.2 mm3) were used as a substrate. The resultant precursor solution was sprayed on preheated substrates at a substrate temperature of 300 °C. The substrate temperature was controlled through a thermocouple with the help of PID temperature controller. The optimized deposition parameters such as substrate-spray nozzle distance (25 cm), spray angle (about 45°), spray time (3 s), spray interval (30 s), carrier gas pressure (compressed air – 40 kg/cm2) and flow rate of the solution ( 3 ml/min) were kept constant. Finally, the coated substrates were allowed to cool down to room temperature. To achieve Fe doping, ferric chloride (FeCl3) with various concentration (0.5–2 wt %) was mixed with the final precursor solution. The structural properties of the prepared films were analysed using Philips X Pert PRO X-ray diffraction (Cu Kα1 radiation; λ ¼1.54056 Å). The thickness of the pure and Fe doped CdO films were measured using stylus profilometer. Surface roughness of the prepared samples was elucidated using atomic force microscope (AFM – A100 SGS). The chemical binding states of 1 wt% Fe doped CdO thin film were measured by X-ray photoelectron spectrometer (XPS) ESCA – 3400. Optical transmittance spectrum of pure and Fe doped samples was recorded in the wavelength range of 190– 900 nm using UV–vis spectrometer (Shimadzu UV-1601). The electrical properties were then measured using Hall measurement setup in Vander Pauw configuration (Ecopia HMS-3000). The ethanol sensing properties of CdO thin films were studied in a homemade sensor unit. For electrical measurements, silver paste contacts (1 mm) were made on the CdO sample of area 1 cm 1 cm. Initially, the current–voltage characteristic was studied within 710 V and in this range, the electrical contacts showed ohmic behaviour. The electrical resistance of CdO film in air (Ra) and in the presence of ethanol (Rg) was measured to evaluate the gas response (S) defined as follows:
S (%) =
R a − Rg × 100 Rg
(1)
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1(a) shows the XRD patterns of the pure and Fe-doped CdO thin films. All the samples exhibits two major (111), (200) and two minor (220), (222) diffraction peaks respectively. This analysis revealed that the pure and Fe doped CdO films are polycrystalline in nature and having a face-centred cubic crystal structure (NaCl structure of a space group Fm-3m) according to the JCPDS file no. 05-0640. No peaks corresponding to their complex oxides did not detected that suggests the film was deposited without any phase segregation [41–44]. The calculated lattice constant value for pure CdO is a ¼4.67 Å (standard value of a ¼4.69 Å) and agrees well with standard JCPDS value. The pure CdO thin film showed a preferential orientation along (200) direction, while Fe doping the intensity of the (200) diffraction plane is decreased and the intensity of (111) peak is increased. Thus, the preferential growth orientation is shifted from (200) to (111) direction at higher (1.5, 2 wt%) Fe doping. This may be due to the incorporation of Fe ions into the Cd lattice sites change the growth environment. Fig. 1(b) shows the enlargement view of (111) and (200) diffraction planes. The Fe doped CdO films slightly shifted to higher (2θ) angle as compared to the pure CdO. The shrinkage of the lattice plane by Fe doping owing to the covalent radius of Fe2 þ (0.078 m) is smaller than that of Cd2 þ (0.095 nm) [45]. The refined unit cell parameter ‘a’ of pure CdO was found to be 4.672(2) Å and Fe doped CdO (0.5, 1, 1.5 and 2 wt%) thin films shows 4.670(6) Å, 4.668(4) Å, 4.666(5) Å and 4.661(2) Å, respectively. The X-ray crystallite size of the Fe doped CdO thin films was estimated using Scherrer's relation [46] and the calculated values are given in Table 1. Fig. 2 shows the crystallite size and strain value as a function of Fe doping and observed values are presented in Table 1. As can be seen, the strain value increases with increasing of Fe doping concentrations, which indicates that the large amount of Fe doping may create lattice distortion. Importantly, the smaller radius of Fe2 þ ions than that of Cd2 þ ions leads to decrease the lattice spacing value that is responsible for the change in the crystallite. Upto 1 wt% of Fe doping, crystallite size value is increased. At higher doping concentration the crystallite size value gradually decreased. 3.2. Surface morphological studies Fig. 3 shows the AFM images of the pure and Fe doped CdO thin films with different Fe doping (0.5, 1, 1.5 and 2 wt%) concentration. Here, the change in the surface morphology depending on the increase in Fe concentration can be clearly seen. The pure CdO thin film shows a relatively spherical shaped grains and shown in Fig. 3(a). 0.5 and 1 wt% Fe doped CdO images clearly shows the irregular shapes with different sizes particles and are shown in Fig. 3(b) and (c). At higher (1.5 and 2 wt%) Fe doped CdO thin films show that the images (Fig. 3(d) and (e)) are homogeneous surface with densely packed small spherical particles. The roughness of the pure CdO thin film is 12 nm. Continuously, the roughness of Fe doped CdO thin film decreases 11, 9, 6, 5 nm with increasing doping 0.5, 1, 1.5 and 2 wt% concentration respectively. The surface morphology of the CdO is modified by Fe doping. The reason for this morphological change is may be change in the preferred orientation of the Fe doped CdO films.
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Fig. 1. XRD patterns of pure Fe doped CdO thin films.
Table 1 Comparison of crystalline size, strain, resistivity, mobility and carrier concentration of pure and Fe doped CdO thin films. Sample
Resistivity (ρ) Mobility (μ) (Ω cm) (cm2 V 1 s 1)
Carrier concentration (N) (cm 3) 1020
Thickness (nm)
CdO Fe:CdO (0.5%) Fe:CdO (1%) Fe:CdO (1.5%) Fe:CdO (2%)
2.03 10 3 2.14 10 3
12 13
1.968 2.149
450 515
7.67 10 4
39
1.578
563
2.09 10 4
47
6.243
598
1.57 10 3
21
1.834
625
3.3. Chemical composition studies Fig. 4(a) shows the wide scan XPS spectrum of the spray deposited 1 wt% Fe doped CdO thin film. XPS measurements were performed to confirm the chemical binding states of the Fe doped CdO thin film. Fig. 4(b)–(d) shows the measured XPS spectra of Cd 3d3/2, 3d5/2, Fe 2p and O 1s core level. The sharp peaks observed at 407 and 413.5 eV are attributed to the existence of Cd2 þ and this exactly matched with the binding energy of Cd–O. Fig. 4(c) shows the measured XPS spectra of O 1s level, the peaks centred at 535.5 eV is clearly indicate the chemisorbed oxygen in the samples and its attributed to the existence of O2 . Fig. 4(d) confirms the existence of Fe2 þ ion in the Fe doped CdO thin film. The spectrum shows a peak centred at 711.5 eV is clearly agreeing Fe 2p1/2 in CdO structure. Thus, the XPS data gives an obvious evidence for the existence of the Fe2 þ ion in the Fe doped CdO thin films, which is in agreement with the XRD result. In addition to this, a few other signals relate to the carbon, silicon and sulphur are observed attributed from the glass substrate. 3.4. Optical properties Optical transmittance spectra of pure and Fe doped CdO thin films are depicted in Fig. 5. It is observed that all the films are highly transparent in the visible range. It is seen that the transparency of the CdO film completely depends on Fe dopant. The transmittance of the CdO thin films decreases with increasing of Fe doping concentration. This may be due to the decreasing of crystallinity and increasing of optical scattering effect caused by less densification of film crystallites and incorporation of more Fe ion in the lattice and interstitial position of CdO. The effect of Fe doping on the optical band gap of CdO thin films is calculated using Tauc's plot through transmittance spectra. The absorption coefficient (α) is calculated using the equation:
Fig. 2. Variation of crystalline size and lattice strain value of CdO thin films as a function of Fe doping level.
⎛ 1⎞ α = ln ⎜ ⎟/d ⎝T⎠
(2)
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Fig. 3. AFM images of CdO thin films as a function of Fe doping level.
T and d is the transmittance and film thickness respectively. The absorption coefficient (α) and the incident photon energy (hν) are related by the following equation.
(αhν )2 = A ( hν − Eg )
(3)
Here hν, A and Eg are corresponding to the photon energy, constant and optical band gap, respectively [47]. The direct band gap determination is based on the extrapolated linear regression of the curve resulting from a plot of photon energy hν vs (αhν)2. The variation of absorption coefficient against photon energy for direct band-to-band transition is given by Tauc's plot (αhν)2 ¼A (hν Eg) and is shown in Fig. 6. The band gap of CdO thin film is 2.61 eV. The optical band gap is found varying between 2.40 and 2.17 eV with increasing doping concentration. This decrease in optical band gap of CdO thin film by Fe doping may be increasing film thickness and related to the Burstein–Moss effect [48]. According to the Burstein–Moss model, the absorption edge of CdO is shifted towards lower energies. This effect occurs when the carrier concentration exceeds the conduction band edge density of states by Fe doping in CdO films. 3.5. Electrical studies The room temperature electrical resistivity (ρ), mobility (μ), and carrier concentration (N) of pure and Fe doped CdO films were measured using Vander–Pauw method in a constant magnetic field of 5 kilogauss and the observed results are presented in Table 1. The negative sign of hall coefficient confirms n-type conductivity. The pure CdO thin film shows resistivity 2.03 10 3 Ω cm which decreases to 7.67 10 4 Ω cm for 1 wt% Fe doping. Initially the resistivity value decreases up to 1 wt% Fe doping and then increases at higher doping concentrations of Fe (1.5 and 2 wt%) ions. At lower level Fe doping, the Fe ions are effectively placed into the Cd2 þ lattice and act as donors by supplying free electrons. After 1.5 and 2 wt% Fe doping, the resistivity values
increased efficiency due to phonon scattering and ionised impurity scattering. The pure CdO film exhibits a carrier concentration of 1.968 1020 cm 3. This value increases with increasing Fe doping level and reaches to a maximum value of 6.243 1020 cm 3 at 1.5 wt% of Fe doping. The resistivity value decrease at 2 wt% Fe doping concentration. The increase in carrier concentration caused by most of Fe2 þ ions homogeneously placed in Cd2 þ lattice to provide more charge carriers. At higher doping concentration of Fe ion (2 wt%) the carrier concentration value has been decreased. The reason is the excess amount of Fe ions cannot be accommodated into the CdO lattice due to its limited solid solubility. The hall mobility value increased from 12 to 47 cm2 V 1 s 1 with increasing the doping concentration of Fe (up to 1.5 wt%). The mobility value of 2 wt% Fe doped CdO film found to be decreases (2 wt%). This may be attributed to ionised impurity scattering centres in CdO films. At higher doping concentration of Fe ion increase the electrical resistivity and lead to build up of carrier traps in the lattice, which in turns reduce the mobility. Further, as the ionic radius of Fe smaller than that of Cd, hence here the excess Fe atoms may occupy the interstitial position which leads to deform the crystal structure.
3.6. Ethanol sensing characteristics Generally, the gas sensing mechanism of semiconducting metal oxide thin films is based on a change in electrical conductance or resistance due to the gas adsorption and desorption on the sensing surface of the film. Hence, here on the surface of the Fe doped CdO thin film, the interaction occurs between the chemisorbed ethanol and absorbed oxygen. At low operating temperature, O−2 is chemisorbed, while at higher temperatures O2 and O are chemosorbed and O2 disappear rapidly. The oxygen adsorption can be described by following equations [49]:
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883
Fig. 4. XPS spectra of 1 wt% of Fe doped CdO thin film.
Fig. 6. Plots of (αhγ)2 against hγ of CdO films as a function of Fe doping level. Fig. 5. UV–vis optical transmittance spectra of CdO thin films as a function of Fe doping level.
O2 (ads) + e− ↔ O2−(ads) ⎫ ⎪ ⎪ O2−(ads) + e− ↔ 2O−(ads) ⎬ ⎪ − ⎪ O− + e− ↔ O2(ads ) ⎭
On exposure to ethanol gas at different operating temperatures, the reaction between the ethanol vapour and oxygen that is adsorbed onto the surface of the film can be expressed by following equation [50]:
CH3 CH2 OH(adsorbed) + 6O−(adsorbed) → 2CO2 + 3H2 O + 6e− (4)
(5)
Due to the reaction, a number of free electrons are re-injected
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ethanol gas and lowered the optimum operating temperature as well. Our work demonstrates the ability to increase the response of Fe doped CdO thin films based ethanol sensors, which will have great merit in commercialized.
References
Fig. 7. Variation in Ethanol gas response of the CdO thin film as a function of Fe doping level at 1000 ppm.
into the film, so that the resistance of the films decreases as the ethanol gas flows into the chamber and is subsequently adsorbed onto the surface of the Fe doped CdO thin film. The gas sensing experiments were carried out in the operating temperature ranging from 150 to 375 °C in the exposure of 1000 ppm ethanol gas. The lower temperature operation, the surface of the sensor does not get completely desorbed which causes smaller change in gas response. As the temperature increases and reaches around 300 °C, the sudden increase in the gas response is observed. The oxidation of ethanol is much enhanced and faster at 300 °C on the surface of the Fe doped CdO thin film following the trend of a redox reaction. Fig. 7 shows the sensing response curve of Fe doped CdO thin film in the doping concentration from 0.5 to 2 wt%. It is clearly seen that, Fe doped CdO thin film shows different gas sense response at the same ethanol gas concentration. From the figure the maximum gas sensitivity 56.12% is observed for 1 wt% of Fe doped CdO thin film, which is decreased to 52% for 0.5 wt% of Fe doping and 43%, 38% of sensitivity is observed for 1.5 and 2 wt% of Fe doping, respectively. At low temperatures the sensor response is restricted by the rate of the chemical reaction, and at higher temperatures it is restricted by the rate of diffusion of gas molecules. At some intermediate temperature, the rate of these two processes become equal, and at that point the sensor response reaches its maximum [51].
4. Conclusion Pure and Fe doped CdO thin films were successfully prepared using facile spray pyrolysis technique. The cubic crystal structure with no evidence of Fe2O3 or mixed phases is observed. The slight variation of lattice parameter is due to the replacement of Cd by Fe ions. The roughness of the CdO thin film decreases with increasing Fe doping concentration. The chemical binding states of the 1 wt% Fe doped CdO thin film confirmed by the XPS measurements. With respect to Fe doping concentrations, the optical band gap value is decreased from 2.61 to 2.18 eV. The minimum electrical resistivity of 7.67 10 4 Ω cm is obtained for the 1 wt% of Fe doped CdO thin film, which is attributed to the increase of carrier concentration due to the substitution of Fe2 þ ions in the CdO film. The maximum ethanol gas response of 56.12% is observed for 1 wt% of Fe doped CdO thin film at an operating temperature of 300 °C and the presence of the thin layer of Fe doped CdO (1 wt% of Fe CdO) thin film surface greatly enhanced the sensitivity of the sensor to
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