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Effect of 130 MeV Au ion irradiation on CO2 gas sensing properties of In2Te3 thin films
Sensors and Actuators B 177 (2013) 8–13
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Effect of 130 MeV Au ion irradiation on CO2 gas sensing properties of In2 Te3 thin films P. Matheswaran a , R. Sathyamoorthy a,∗ , K. Asokan b a b
PG and Research Department of Physics, Kongunadu Arts and Science College, Coimbatore 641029, Tamil Nadu, India Materials Science Division, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 24 October 2012 Accepted 26 October 2012 Available online 3 November 2012 Keywords: In2 Te3 thin film SHI irradiation Dewetting CO2 gas sensor
a b s t r a c t CO and CO2 are harmful pollutants. The main objective of monitoring CO and CO2 is to prevent intoxication. Though these pollutants were monitored by metal oxide gas sensors, it operated at high temperature. Selectivity of metal oxides over a wide range of gas is limited. Additional contribution of sensor heaters and its associated electronics may induce poor stability of a sensor. In addition to that room temperature gas sensor is always essential to monitor the CO2 pollutant. In the present work, we have prepared In2 Te3 thin films from In/Te bilayer by SHI (Au 130 MeV) irradiation. Structural, surface morphology, elemental composition and gas sensing behavior of the as grown and irradiated samples were analyzed by XRD, SEM, RBS and I–V analysis. The observed results were discussed in connection with the SHI induced modification at the interface. As fluence increases, the crystallanity also found to increase. In addition to that dewetting structure is observed at higher fluence. The films prepared by SHI route shows better gas sensing behavior of In2 Te3 thin film than from conventional method of synthesis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thin film sensors are used as toxic gas monitor [1], humidity sensor [2], touch sensitive switches [3], and infrared detectors [4]. Gas sensors put forward to monitor and control the green house gases like CO2 , CO, nowadays which are essential to control the air pollution [5]. Synthesis of new materials in different structural forms will lead to the development of miniaturized devices. At present, material engineering of metal oxide films is well established. Though these materials are used in practice, the selectivity of a sensing material for different gas is a major issue in metal oxides and its composites. Three major factors affecting the performance of the gas sensors such as sensitivity, selectivity and response time. It was shown that deposition parameters, post deposition treatments and doping during synthesis really influence the properties of metal oxides, which are important for gas sensor applications. The surface modification by noble metals improves the sensitivity and reduces the response and recovery time. Numerous works have been carried out to sense CO2 gas using SnO and SnO2 thin films, whereas high temperature is always necessary for its operation [6–8]. Additional circuits for onsite heater and its associated electronics makes the gas sensor fabrication tedious. Moreover malfunction/failure of heater circuit also affects the performance of the gas sensor. We are in a situation
∗ Corresponding author. E-mail address: [email protected] (R. Sathyamoorthy). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.115
to meet out these challenges and a new material is in need, which works at room temperature. For sensing applications, the surface and interface interactions between the test molecules and the sensing material are important. The test molecule at the surface of the sensor induces a change in the electrical resistance. Swift heavy ion irradiation, in which an energetic ion beam is allowed to pass through a material, is a very effective technique to induce changes in microstructure and electronic energy levels, and has been used to tailor properties of various metallic, semiconducting, and insulating thin films. Not much works have been carried out by modifying the surface structure and crystallization parameters via ion beam irradiation toward the gas sensor application. Effect of SHI (Swift Heavy Ion) 75 MeV Ni ion irradiation on structure, optical, and gas sensing properties of SnO2 thin films were studied by Rani et al., but the sensing properties were investigated for ammonia as a test gas [9]. Singh et al. studied the effect of oxygen ion (100 MeV O) irradiation on ethanol sensing response of nanostructures of ZnO and SnO2 [10]. Sedghi et al. investigated the sonochemically prepared SnO2 quantum dots as a selective and low temperature CO gas sensor [11]. Electron beam irradiation does not affect the chemical composition of the semiconductor, but produces only structural defects. Electron beam irradiation does not affect the chemical composition of the semiconductor, but produces only structural defects [12]. CO and CO2 are harmful pollutants. The main objective of monitoring CO and CO2 is to prevent intoxication. This necessitates a need for development of effective and sensitive pollution monitors. A new semiconductor material for CO2 gas sensing family (i.e.
P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
9
Fig. 1. Schematic diagram of the experimental set up of CO2 gas sensor.
In2 Te3 ) has been introduced by Desai et al. [13]. He demonstrated the room temperature CO2 gas sensor for wide range of CO2 gas concentration. With increase in the concentration, the sensitivity of the sensor is found to increase. The sensitivity of the gas sensor was found to dependent upon the thickness of the film. The In2 Te3 films deposited at the substrate temperature of 473 K, having thickness of 150 nm showed the maximum sensitivity to CO2 gas. In this present work, an attempt has been made to increase the sensitivity of In2 Te3 thin film by SHI irradiation. 2. Experimental A maiden attempt has been made to prepare In2 Te3 semiconductors, a material of interest for gas sensing behavior for specific gas (CO2 ), prepared by sequential thermal evaporation followed by ion beam irradiation. When a semiconductor thin film is exposed to particular gas, the resistance of the material differs, called as Chemiresistive gas sensor. The variation in resistance may be due to the chemical and electronic interaction between the gas and the material. Indigenously developed gas sensor measuring unit with Keithley 2612 Source-Measure Unit (SMU) is used to monitor the electrical conduction of the In2 Te3 thin film under test gas. The prepared In/Te thin films of thickness 600 nm were irradiated using Au (130 MeV) ion with different fluence (1 × 1012 , 3 × 1012 , 1 × 1013 , and 3 × 1013 ions/cm2 ) in order to study SHI irradiation induced modification over the electrical conduction properties. We found that the single-phase In2 Te3 is achieved by Au (130 MeV) with 1 × 1013 and 3 × 1013 ions/cm2 . Schematic diagram of the experimental set up of CO2 gas sensor is shown in Fig. 1. Electrical conduction of the pristine and irradiated samples was measured in a dark chamber in order to avoid photo-induced conduction during experiment. Silver electrodes were coated on In/Te thin films which are irradiated by (130 MeV) Au ion irradiation of different fluence. The electrical connections were made with fine copper wires attached to the electrodes by silver paste. All electrical connections were made by Bayonet Neill–Concelman (BNC) cables through leak proof chamber. Resistance variation of the gas sensor is monitored by Keithley 2612 SMU.
is achieved in the case of pristine sample. The sample irradiated with the fluence of 1 × 1012 ions/cm2 shows the decrease in intensity of the elemental peaks, which implies that the sample slowly leads to form nearly amorphous phase. When fluence increases to 3 × 1012 ions/cm2 , the new peaks are evident in the XRD spectra which corresponds to In2 Te3 phase at 2 = 24.94◦ , 41.39◦ and 49.0◦ along with the elemental peaks (Te (1 0 0), Te (1 0 1) and In (1 0 1)) with weak intensity. When the films are irradiated with Au 130 MeV ion of fluence 1 × 1013 ions/cm2 , the bilayer mixing starts and leads to the formation of In2 Te3 phase along with trace amount of Te phase. Further, the sample irradiated with the fluence of 3 × 1013 ions/cm2 shows the increase in intensity of In2 Te3 phase, which implies that the improvement in crystallanity of In2 Te3 phase. It implies that the fluence of 3 × 1013 ions/cm2 is required to synthesis single-phase In2 Te3 from In/Te bilayer by 130 MeV Au ion irradiation. In the present case also, the mixing in In/Te is a consequence of inter-diffusion during the transient molten state and form the In2 Te3 compound. Thermal spikes are responsible for inter mixing in In/Te bilayer [14]. From XRD pattern, the grain size of the different samples was measured using Scherrer formula. Average Grain size for the sample irradiated with 1 × 1012 , 3 × 1012 , 1 × 1013 and 3 × 1013 is found to be 24.82 nm, 24.17 nm, 21.88 nm and 20.2 nm respectively. It is observed that the grain size decreases
3. Results and discussion 3.1. Structural analysis XRD pattern of pristine and irradiated In/Te samples at different fluence (1 × 1012 , 3 × 1012 , 1 × 1013 , and 3 × 1013 ions/cm2 ) are shown in Fig. 2. Pristine sample shows the crystallographic peaks correspond to the elements In and Te. No compound formation
Fig. 2. XRD pattern of Pristine and irradiated In/Te thin films.
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P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
Fig. 3. Surface morphology of In/Te thin films [(a) pristine, (b) irradiated with 1 × 1012 ions/cm2 , (c) 3 × 1012 ions/cm2 , (d) 1 × 1013 ions/cm2 and (e) 3 × 1013 ions/cm2 ].
with increase in fluence (grain size was calculated for the In2 Te3 phase with predominant peak at 2 = 24.94◦ ).
3.2. Surface morphology analysis Surface morphology of the samples is investigated before and after irradiation by SEM analysis. Fig. 3 shows the surface morphology of In/Te thin films. Surface of the pristine sample seems to have flake like morphology which corresponds to the top layer of the In/Te system (Fig. 3(a)). This flake like morphology of Te may be due ˚ to the higher chalcogenide rate of deposition (10 A/s). Morphology of the sample irradiated with the fluence 1 × 1012 ions/cm2 also shows the presence of flake like structure but smaller in size (Fig. 3(b)). Reduction in size may be due to annihilation of Te flakes at the surface. Further increase in the fluence 3 × 1012 ions/cm2 , flake like structure disappears and lead to the formation of tiny
spherical particles (Fig. 3(c)). The film irradiated at the fluence of 1 × 1013 ions/cm2 illustrates the annihilation of tiny spherical particles (Fig. 3(d)). In addition, the morphology shows the initial state of dewetting structure along with the molten state of In2 Te3 film. At higher fluence (3 × 1013 ions/cm2 ), complete dewetting structure is clearly visible and scattered throughout the surface (Fig. 3(e)).
3.3. Rutherford backscattering analysis Fig. 4 illustrates the RBS spectra of pristine and irradiated In/Te bilayer thin films. Thicknesses of the samples were analyzed using XRUMP simulation software. It is observed that the pristine sample exhibits two separate peaks, which confirms the bilayer nature of In/Te thin film. RBS spectra of Au ion irradiated (1 × 1012 ions/cm2 ) samples shows the decrease in intensity of Te and In peak, this may be attributed to the following facts: (i) intermixing of In and Te and
P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
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Fig. 4. RBS spectra of pristine and irradiated In/Te bilayer thin films. Fig. 5. Variation of the resistance of the In2 Te3 thin film with different concentrations of CO2 gas at room temperature.
(ii) loss of In and Te atoms due to Au ion induced sputtering. When the sample is irradiated with the fluence of 3 × 1012 ions/cm2 , lower energy channel of In peak shits toward the higher channel, which confirms the inward diffusion of In atoms into Te layer. If the sample is irradiated with 1 × 1013 ions/cm2 fluence, complete tailing nature of In atom peak is observed. This may be due to the disappearance of separate peak width of In. For higher ion fluence (3 × 1013 ), Te peak width diminished. In addition to that the loss of thickness (100 nm) was observed, which may be due to sputtering of the Te species. From the XRD results we have also observed that the disappearance of Te (1 0 1) peak for the higher ion fluence. Sputtering of Te species observed in the RBS spectra substantiates the XRD results. This implies that all the In atoms were mixed with Te atoms, which confirms the formation of In2 Te3 compound. It is interesting to note that XRD results infer that the compound formation increases with fluence. The results observed from RBS analysis also support the compound formation. The amount of energy dissipated in the lattice is expected to be very high in the case of heavy element Au than Ni, which heats up the sample for tens of picoseconds and relax down within few picoseconds, thus induces temperature gradient. Large difference in temperature may produce significant changes in surface tension and local pressure and thus results nuclei for dewetting. Adherence of these dry patches forms embryo of the dewetted pattern.
variation of sensitivity with different ion fluence and concentrations of CO2 gas. The sensitivity of the In2 Te3 thin film increases with increase in gas concentration. This may also be due to the increase of carrier concentration of the In2 Te3 semiconductor upon exposure to the CO2 gas. With increase in the CO2 gas concentration, the conductivity of the film found to increases, which results lower resistivity. Possible mechanism for the decrease in the resistance of the sample was reported earlier [13]. Sensitivity (S) of a gas sensor is calculated as S=
Rg − Ro Ro
(1)
where Rg is resistance at gas atmosphere and Ro is the resistance of a sample in air. Positive value of sensitivity (S) of the different samples indicates the decrease in resistance upon exposure to CO2 . The magnitude of sensitivity variation is larger in the case of the
3.4. I–V analysis Fig. 5 shows the change in resistance of the sample upon CO2 exposure. As fluence increases, the resistance of the sample also increases. It may be attributed to the resistance of the different phases of Inx Tey thin film. As evident from XRD pattern (Fig. 2), pristine and 1 × 1012 ions/cm2 fluence have Te rich phases, so that the resistance of the samples becomes quite low. As fluence increases from 3 × 1012 ions/cm2 to 3 × 1013 ions/cm2 , the resistance of the sample found to increase. It may be due to the formation of In2 Te3 phase from Te rich phase. Since In2 Te3 phase have high resistivity than Te rich phase. Resistance of the sample (3 × 1012 ions/cm2 ) is found to decrease with increase in CO2 concentration. Further increase in fluence (1 × 1013 ions/cm2 and 3 × 1013 ions/cm2 ) results the large variation in resistance upon CO2 exposure, when compared with the resistance of the sample in air. Fig. 6 shows the
Fig. 6. Variation of sensitivity with different ion fluence and concentrations of CO2 gas.
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P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
4. Outcome Au (130 MeV) SHI ion irradiated In/Te thin film samples with the fluence of 3 × 1013 ions/cm2 shows increase in sensitivity upon exposed to CO2 gas. Decrease in grain size and increase in surface area of the sample exposure to the gas may greatly enhance the sensitivity of the sample. The response time for 1000 ppm of CO2 gas is found to be approximately 15–20 s. Superimposition of the following features [(i) high degree of crystallanity, (ii) larger surface area of the test sample due to dewetting and (iii) loss of thickness due to sputtering] makes this material as a suitable candidate for CO2 gas sensor, where the ion beam irradiation place a major role to functionalize the structural and surface properties of In2 Te3 thin films. References Fig. 7. Response time of 3 × 1013 ions/cm2 Au irradiated In2 Te3 gas sensor for CO2 gas.
higher ion fluence. Formation of highly crystalline In2 Te3 phase may be the predominant factor, affects the sensitivity in principal. The observed sensitivity variation is noticeable when compared with the previous investigation. Previous investigator [13] showed a resistance change approximately 5.8 k (from 49.8 k to 44 k) when the sample is exposed to CO2 gas. In the present investigation, Ion beam irradiated In2 Te3 thin films showed a resistance change of 25 k (from 53 k to 28 k). It is interesting to note that the irradiated samples experience a large variation in resistance than the conventionally prepared samples. It corroborates the enhancement of sensitivity of ion beam irradiated In2 Te3 thin films. The enhancement of sensitivity is not only due to crystalline nature of In2 Te3 phase with increase in ion fluence but also the increase in surface area exposed to the gas due to dewetting [15]. In general, decrease in grain size, increase in surface area/porosity of the sample exposed to the gas may greatly enhance the performance of the In2 Te3 thin film. It is observed that surface area exposed to the CO2 gas may be larger, when the samples are irradiated with higher ion fluence. One suitable explanation for the above fact is that the ion beam induced surface modification may play a dominant role to increase in sensitivity. In general, the dewetting structures have large surface area than the homogeneous surface. As evident from SEM analysis, high degree of dewetting is observed from the sample irradiated with the ion fluence 3 × 1013 ions/cm2 . In addition to that, loss of thickness (100 nm) was observed when the sample is irradiated with fluence 3 × 1013 ions/cm2 , attributed to the ion beam induced sputtering. It was well documented that the gas sensitivity of In2 Te3 gas sensor increases with decrease in film thickness. Accordingly, due to the interaction of CO2 gas molecules with In2 Te3 film surface, the energy difference between the higher energy orbital and the bottom of the conduction band of In2 Te3 increases with increase in thickness leading to decrease in the carrier generation and hence the lower electrical conductivity and reduced gas sensitivity. On the other hand, films with lower thickness providing increased overlap and hence, enhanced carrier conduction in In2 Te3 material results improved gas sensitivity. As evident from RBS spectra of the irradiated sample, the loss of film thickness of 100 nm was observed at higher ion fluence (3 × 1013 ions/cm2 ). It may favor the enhancement in sensitivity of the sample. Fig. 7 shows the typical response time curve for CO2 gas of concentration 1000 ppm. The resistance variation is observed at constant interval of time, when concentration of the CO2 gas varied from 0 to 1000 ppm. It clearly depicts the response time of approximately 15–20 s. The gas sensitivity of the test sample shows the uniform response throughout the experiment.
[1] A. Chowdhuri, P. Sharma, V. Gupta, K. Sreenivas, K.V. Rao, H2 S gas sensing mechanism of SnO2 films with ultrathin CuO dotted islands, Journal of Applied Physics 92 (2002) 2172–2180. [2] K. Arshak, K. Twomey, Thin films of In2 O3 /SiO for humidity sensing applications, Sensors 2 (2002) 205–218. [3] K. Papadopoulos, D.S. Vlachos, J.N. Avaritsiotis, Comparative study of various metal-oxide-based gas-sensor architectures, Sensors and Actuators B 32 (1996) 61–69. [4] C. Corsi, G. Cappuccio, A. D’amico, G. Petrocco, G. Vitali, Thin film infrared detector arrays for integrated electronic structures, Infrared Physics 16 (1976) 37–45. [5] U. Hoefer, G. Kühner, W. Schweizer, G. Sulz, K. Steiner, CO and CO2 thin film SnO2 gas sensors on Si substrates, Sensors and Actuators B 22 (1994) 115–119. [6] F.C. Bancolo, G.N.C. Santos, R.V. Quiroga, Fabrication and characterization of SnO2 nanomaterial as CO2 gas sensor, International Journal of Scientific and Engineering Research 3 (2012) 1–6. [7] S.A. Waghuley, Synthesis, characterization and CO2 gas sensing response of SnO2 /Al2 O3 double layer sensor, Indian Journal of Pure and Applied Physics 49 (2011) 816–819. [8] G.E. Patil, D.D. Kajale, D.N. Chavan, N.K. Pawar, P.T. Ahire, S.D. Shinde, V.B. Gaikwad, G.H. Jain, Synthesis, characterization and gas sensing performance of SnO2 thin films prepared by spray pyrolysis, Bulletin of Materials Science 34 (2011) 1–9. [9] S. Rani, M.C. Bhatnagar, S.C. Roy, N.K. Puri, D. Kanjilal, p-Type gas-sensing behaviour of undoped SnO2 thinfilms irradiated with a high-energy ion beam, Sensors and Actuators B 135 (2008) 35–39. [10] R.C. Singh, M.P. Singh, O. Singh, P.S. Chandi, R. Kumar, Effect of 100 MeV O7+ ions irradiation on ethanol sensing response of nanostructures of ZnO and SnO2 , Applied Physics A: Materials Science and Processing 98 (2010) 161–166. [11] S.M. Sedghi, Y. Mortazavi, A. Khodadadi, Low temperature CO and CH4 dual selective gas sensor using SnO2 quantumdots prepared by sonochemical method, Sensors and Actuators B: Chemical 145 (2010) 7–12. [12] Z. Jiao, X. Wan, B. Zhao, H. Guo, T. Liu, M. Wu, Effects of electron beam irradiation on tin dioxide gas sensors, Bulletin of Materials Science 31 (2008) 83–86. [13] R.R. Desai, D. Lakshminarayana, P.B. Patel, C.J. Panchal, Indium sesquitelluride (In2 Te3 ) thin film gas sensor for detection of carbon dioxide, Sensors and Actuators B 107 (2005) 523–527. [14] P. Sigmund, Energy density and time constant of heavy-ion-induced elasticcollision spikes in solids, Applied Physics Letters 25 (1974), 169(1–3). [15] T. Pai, I.-C. Leu, M.-H. Hon, A hierarchical structure through imprinting of a polyimide precursor without residual layers, Journal of Micromechanics and Microengineering 18 (2008), 105005(1–8).
Biographies
P. Matheswaran has completed his Ph.D. in 2012 at Kongunadu Arts and Science College affiliated by Bharathiar University, Coimbatore. His thesis entitled “Effect of annealing and SHI irradiation on Indium Chalcogenide bilayer thin film”. The research work was carried out at Inter University Accelerator Centre New Delhi through UFUP-40304 and BTR-2 (UFUP-49222) Project. His research interest is ion beam processing techniques for Phase change memory and CO2 gas sensor.
P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
R. Sathyamoorthy has completed his Ph.D. in 1992 at Bharathiar University and presently employed as Associate Professor and Head at the Department of Physics in Kongunadu Arts and Science College, Coimbatore. He has received UGC research award, Commonwealth award and so. Presently he is working in Nanomaterials synthesis and ion beam irradiation for gas sensing applications. He has over 93 publications in his credit.
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K. Asokan received his Ph.D. from University of Rajasthan, Jaipur in 1992. He joined as scientist in the 1994 and presently in Materials Science Division, Inter-University Accelerator Centre, New Delhi. He has visited Taiwan and South Korea for his post doctoral study and other countries to present his research work. He collaborates with various academic research institutes and universities mainly on ion beam interaction in materials and its applications. His special interest is in the understanding the electronic structure of materials. He has over 150 publications in his credit.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Effect of 130 MeV Au ion irradiation on CO2 gas sensing properties of In2 Te3 thin films P. Matheswaran a , R. Sathyamoorthy a,∗ , K. Asokan b a b
PG and Research Department of Physics, Kongunadu Arts and Science College, Coimbatore 641029, Tamil Nadu, India Materials Science Division, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 24 October 2012 Accepted 26 October 2012 Available online 3 November 2012 Keywords: In2 Te3 thin film SHI irradiation Dewetting CO2 gas sensor
a b s t r a c t CO and CO2 are harmful pollutants. The main objective of monitoring CO and CO2 is to prevent intoxication. Though these pollutants were monitored by metal oxide gas sensors, it operated at high temperature. Selectivity of metal oxides over a wide range of gas is limited. Additional contribution of sensor heaters and its associated electronics may induce poor stability of a sensor. In addition to that room temperature gas sensor is always essential to monitor the CO2 pollutant. In the present work, we have prepared In2 Te3 thin films from In/Te bilayer by SHI (Au 130 MeV) irradiation. Structural, surface morphology, elemental composition and gas sensing behavior of the as grown and irradiated samples were analyzed by XRD, SEM, RBS and I–V analysis. The observed results were discussed in connection with the SHI induced modification at the interface. As fluence increases, the crystallanity also found to increase. In addition to that dewetting structure is observed at higher fluence. The films prepared by SHI route shows better gas sensing behavior of In2 Te3 thin film than from conventional method of synthesis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thin film sensors are used as toxic gas monitor [1], humidity sensor [2], touch sensitive switches [3], and infrared detectors [4]. Gas sensors put forward to monitor and control the green house gases like CO2 , CO, nowadays which are essential to control the air pollution [5]. Synthesis of new materials in different structural forms will lead to the development of miniaturized devices. At present, material engineering of metal oxide films is well established. Though these materials are used in practice, the selectivity of a sensing material for different gas is a major issue in metal oxides and its composites. Three major factors affecting the performance of the gas sensors such as sensitivity, selectivity and response time. It was shown that deposition parameters, post deposition treatments and doping during synthesis really influence the properties of metal oxides, which are important for gas sensor applications. The surface modification by noble metals improves the sensitivity and reduces the response and recovery time. Numerous works have been carried out to sense CO2 gas using SnO and SnO2 thin films, whereas high temperature is always necessary for its operation [6–8]. Additional circuits for onsite heater and its associated electronics makes the gas sensor fabrication tedious. Moreover malfunction/failure of heater circuit also affects the performance of the gas sensor. We are in a situation
∗ Corresponding author. E-mail address: [email protected] (R. Sathyamoorthy). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.115
to meet out these challenges and a new material is in need, which works at room temperature. For sensing applications, the surface and interface interactions between the test molecules and the sensing material are important. The test molecule at the surface of the sensor induces a change in the electrical resistance. Swift heavy ion irradiation, in which an energetic ion beam is allowed to pass through a material, is a very effective technique to induce changes in microstructure and electronic energy levels, and has been used to tailor properties of various metallic, semiconducting, and insulating thin films. Not much works have been carried out by modifying the surface structure and crystallization parameters via ion beam irradiation toward the gas sensor application. Effect of SHI (Swift Heavy Ion) 75 MeV Ni ion irradiation on structure, optical, and gas sensing properties of SnO2 thin films were studied by Rani et al., but the sensing properties were investigated for ammonia as a test gas [9]. Singh et al. studied the effect of oxygen ion (100 MeV O) irradiation on ethanol sensing response of nanostructures of ZnO and SnO2 [10]. Sedghi et al. investigated the sonochemically prepared SnO2 quantum dots as a selective and low temperature CO gas sensor [11]. Electron beam irradiation does not affect the chemical composition of the semiconductor, but produces only structural defects. Electron beam irradiation does not affect the chemical composition of the semiconductor, but produces only structural defects [12]. CO and CO2 are harmful pollutants. The main objective of monitoring CO and CO2 is to prevent intoxication. This necessitates a need for development of effective and sensitive pollution monitors. A new semiconductor material for CO2 gas sensing family (i.e.
P. Matheswaran et al. / Sensors and Actuators B 177 (2013) 8–13
9
Fig. 1. Schematic diagram of the experimental set up of CO2 gas sensor.
In2 Te3 ) has been introduced by Desai et al. [13]. He demonstrated the room temperature CO2 gas sensor for wide range of CO2 gas concentration. With increase in the concentration, the sensitivity of the sensor is found to increase. The sensitivity of the gas sensor was found to dependent upon the thickness of the film. The In2 Te3 films deposited at the substrate temperature of 473 K, having thickness of 150 nm showed the maximum sensitivity to CO2 gas. In this present work, an attempt has been made to increase the sensitivity of In2 Te3 thin film by SHI irradiation. 2. Experimental A maiden attempt has been made to prepare In2 Te3 semiconductors, a material of interest for gas sensing behavior for specific gas (CO2 ), prepared by sequential thermal evaporation followed by ion beam irradiation. When a semiconductor thin film is exposed to particular gas, the resistance of the material differs, called as Chemiresistive gas sensor. The variation in resistance may be due to the chemical and electronic interaction between the gas and the material. Indigenously developed gas sensor measuring unit with Keithley 2612 Source-Measure Unit (SMU) is used to monitor the electrical conduction of the In2 Te3 thin film under test gas. The prepared In/Te thin films of thickness 600 nm were irradiated using Au (130 MeV) ion with different fluence (1 × 1012 , 3 × 1012 , 1 × 1013 , and 3 × 1013 ions/cm2 ) in order to study SHI irradiation induced modification over the electrical conduction properties. We found that the single-phase In2 Te3 is achieved by Au (130 MeV) with 1 × 1013 and 3 × 1013 ions/cm2 . Schematic diagram of the experimental set up of CO2 gas sensor is shown in Fig. 1. Electrical conduction of the pristine and irradiated samples was measured in a dark chamber in order to avoid photo-induced conduction during experiment. Silver electrodes were coated on In/Te thin films which are irradiated by (130 MeV) Au ion irradiation of different fluence. The electrical connections were made with fine copper wires attached to the electrodes by silver paste. All electrical connections were made by Bayonet Neill–Concelman (BNC) cables through leak proof chamber. Resistance variation of the gas sensor is monitored by Keithley 2612 SMU.
is achieved in the case of pristine sample. The sample irradiated with the fluence of 1 × 1012 ions/cm2 shows the decrease in intensity of the elemental peaks, which implies that the sample slowly leads to form nearly amorphous phase. When fluence increases to 3 × 1012 ions/cm2 , the new peaks are evident in the XRD spectra which corresponds to In2 Te3 phase at 2 = 24.94◦ , 41.39◦ and 49.0◦ along with the elemental peaks (Te (1 0 0), Te (1 0 1) and In (1 0 1)) with weak intensity. When the films are irradiated with Au 130 MeV ion of fluence 1 × 1013 ions/cm2 , the bilayer mixing starts and leads to the formation of In2 Te3 phase along with trace amount of Te phase. Further, the sample irradiated with the fluence of 3 × 1013 ions/cm2 shows the increase in intensity of In2 Te3 phase, which implies that the improvement in crystallanity of In2 Te3 phase. It implies that the fluence of 3 × 1013 ions/cm2 is required to synthesis single-phase In2 Te3 from In/Te bilayer by 130 MeV Au ion irradiation. In the present case also, the mixing in In/Te is a consequence of inter-diffusion during the transient molten state and form the In2 Te3 compound. Thermal spikes are responsible for inter mixing in In/Te bilayer [14]. From XRD pattern, the grain size of the different samples was measured using Scherrer formula. Average Grain size for the sample irradiated with 1 × 1012 , 3 × 1012 , 1 × 1013 and 3 × 1013 is found to be 24.82 nm, 24.17 nm, 21.88 nm and 20.2 nm respectively. It is observed that the grain size decreases
3. Results and discussion 3.1. Structural analysis XRD pattern of pristine and irradiated In/Te samples at different fluence (1 × 1012 , 3 × 1012 , 1 × 1013 , and 3 × 1013 ions/cm2 ) are shown in Fig. 2. Pristine sample shows the crystallographic peaks correspond to the elements In and Te. No compound formation
Fig. 2. XRD pattern of Pristine and irradiated In/Te thin films.
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Fig. 3. Surface morphology of In/Te thin films [(a) pristine, (b) irradiated with 1 × 1012 ions/cm2 , (c) 3 × 1012 ions/cm2 , (d) 1 × 1013 ions/cm2 and (e) 3 × 1013 ions/cm2 ].
with increase in fluence (grain size was calculated for the In2 Te3 phase with predominant peak at 2 = 24.94◦ ).
3.2. Surface morphology analysis Surface morphology of the samples is investigated before and after irradiation by SEM analysis. Fig. 3 shows the surface morphology of In/Te thin films. Surface of the pristine sample seems to have flake like morphology which corresponds to the top layer of the In/Te system (Fig. 3(a)). This flake like morphology of Te may be due ˚ to the higher chalcogenide rate of deposition (10 A/s). Morphology of the sample irradiated with the fluence 1 × 1012 ions/cm2 also shows the presence of flake like structure but smaller in size (Fig. 3(b)). Reduction in size may be due to annihilation of Te flakes at the surface. Further increase in the fluence 3 × 1012 ions/cm2 , flake like structure disappears and lead to the formation of tiny
spherical particles (Fig. 3(c)). The film irradiated at the fluence of 1 × 1013 ions/cm2 illustrates the annihilation of tiny spherical particles (Fig. 3(d)). In addition, the morphology shows the initial state of dewetting structure along with the molten state of In2 Te3 film. At higher fluence (3 × 1013 ions/cm2 ), complete dewetting structure is clearly visible and scattered throughout the surface (Fig. 3(e)).
3.3. Rutherford backscattering analysis Fig. 4 illustrates the RBS spectra of pristine and irradiated In/Te bilayer thin films. Thicknesses of the samples were analyzed using XRUMP simulation software. It is observed that the pristine sample exhibits two separate peaks, which confirms the bilayer nature of In/Te thin film. RBS spectra of Au ion irradiated (1 × 1012 ions/cm2 ) samples shows the decrease in intensity of Te and In peak, this may be attributed to the following facts: (i) intermixing of In and Te and
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Fig. 4. RBS spectra of pristine and irradiated In/Te bilayer thin films. Fig. 5. Variation of the resistance of the In2 Te3 thin film with different concentrations of CO2 gas at room temperature.
(ii) loss of In and Te atoms due to Au ion induced sputtering. When the sample is irradiated with the fluence of 3 × 1012 ions/cm2 , lower energy channel of In peak shits toward the higher channel, which confirms the inward diffusion of In atoms into Te layer. If the sample is irradiated with 1 × 1013 ions/cm2 fluence, complete tailing nature of In atom peak is observed. This may be due to the disappearance of separate peak width of In. For higher ion fluence (3 × 1013 ), Te peak width diminished. In addition to that the loss of thickness (100 nm) was observed, which may be due to sputtering of the Te species. From the XRD results we have also observed that the disappearance of Te (1 0 1) peak for the higher ion fluence. Sputtering of Te species observed in the RBS spectra substantiates the XRD results. This implies that all the In atoms were mixed with Te atoms, which confirms the formation of In2 Te3 compound. It is interesting to note that XRD results infer that the compound formation increases with fluence. The results observed from RBS analysis also support the compound formation. The amount of energy dissipated in the lattice is expected to be very high in the case of heavy element Au than Ni, which heats up the sample for tens of picoseconds and relax down within few picoseconds, thus induces temperature gradient. Large difference in temperature may produce significant changes in surface tension and local pressure and thus results nuclei for dewetting. Adherence of these dry patches forms embryo of the dewetted pattern.
variation of sensitivity with different ion fluence and concentrations of CO2 gas. The sensitivity of the In2 Te3 thin film increases with increase in gas concentration. This may also be due to the increase of carrier concentration of the In2 Te3 semiconductor upon exposure to the CO2 gas. With increase in the CO2 gas concentration, the conductivity of the film found to increases, which results lower resistivity. Possible mechanism for the decrease in the resistance of the sample was reported earlier [13]. Sensitivity (S) of a gas sensor is calculated as S=
Rg − Ro Ro
(1)
where Rg is resistance at gas atmosphere and Ro is the resistance of a sample in air. Positive value of sensitivity (S) of the different samples indicates the decrease in resistance upon exposure to CO2 . The magnitude of sensitivity variation is larger in the case of the
3.4. I–V analysis Fig. 5 shows the change in resistance of the sample upon CO2 exposure. As fluence increases, the resistance of the sample also increases. It may be attributed to the resistance of the different phases of Inx Tey thin film. As evident from XRD pattern (Fig. 2), pristine and 1 × 1012 ions/cm2 fluence have Te rich phases, so that the resistance of the samples becomes quite low. As fluence increases from 3 × 1012 ions/cm2 to 3 × 1013 ions/cm2 , the resistance of the sample found to increase. It may be due to the formation of In2 Te3 phase from Te rich phase. Since In2 Te3 phase have high resistivity than Te rich phase. Resistance of the sample (3 × 1012 ions/cm2 ) is found to decrease with increase in CO2 concentration. Further increase in fluence (1 × 1013 ions/cm2 and 3 × 1013 ions/cm2 ) results the large variation in resistance upon CO2 exposure, when compared with the resistance of the sample in air. Fig. 6 shows the
Fig. 6. Variation of sensitivity with different ion fluence and concentrations of CO2 gas.
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4. Outcome Au (130 MeV) SHI ion irradiated In/Te thin film samples with the fluence of 3 × 1013 ions/cm2 shows increase in sensitivity upon exposed to CO2 gas. Decrease in grain size and increase in surface area of the sample exposure to the gas may greatly enhance the sensitivity of the sample. The response time for 1000 ppm of CO2 gas is found to be approximately 15–20 s. Superimposition of the following features [(i) high degree of crystallanity, (ii) larger surface area of the test sample due to dewetting and (iii) loss of thickness due to sputtering] makes this material as a suitable candidate for CO2 gas sensor, where the ion beam irradiation place a major role to functionalize the structural and surface properties of In2 Te3 thin films. References Fig. 7. Response time of 3 × 1013 ions/cm2 Au irradiated In2 Te3 gas sensor for CO2 gas.
higher ion fluence. Formation of highly crystalline In2 Te3 phase may be the predominant factor, affects the sensitivity in principal. The observed sensitivity variation is noticeable when compared with the previous investigation. Previous investigator [13] showed a resistance change approximately 5.8 k (from 49.8 k to 44 k) when the sample is exposed to CO2 gas. In the present investigation, Ion beam irradiated In2 Te3 thin films showed a resistance change of 25 k (from 53 k to 28 k). It is interesting to note that the irradiated samples experience a large variation in resistance than the conventionally prepared samples. It corroborates the enhancement of sensitivity of ion beam irradiated In2 Te3 thin films. The enhancement of sensitivity is not only due to crystalline nature of In2 Te3 phase with increase in ion fluence but also the increase in surface area exposed to the gas due to dewetting [15]. In general, decrease in grain size, increase in surface area/porosity of the sample exposed to the gas may greatly enhance the performance of the In2 Te3 thin film. It is observed that surface area exposed to the CO2 gas may be larger, when the samples are irradiated with higher ion fluence. One suitable explanation for the above fact is that the ion beam induced surface modification may play a dominant role to increase in sensitivity. In general, the dewetting structures have large surface area than the homogeneous surface. As evident from SEM analysis, high degree of dewetting is observed from the sample irradiated with the ion fluence 3 × 1013 ions/cm2 . In addition to that, loss of thickness (100 nm) was observed when the sample is irradiated with fluence 3 × 1013 ions/cm2 , attributed to the ion beam induced sputtering. It was well documented that the gas sensitivity of In2 Te3 gas sensor increases with decrease in film thickness. Accordingly, due to the interaction of CO2 gas molecules with In2 Te3 film surface, the energy difference between the higher energy orbital and the bottom of the conduction band of In2 Te3 increases with increase in thickness leading to decrease in the carrier generation and hence the lower electrical conductivity and reduced gas sensitivity. On the other hand, films with lower thickness providing increased overlap and hence, enhanced carrier conduction in In2 Te3 material results improved gas sensitivity. As evident from RBS spectra of the irradiated sample, the loss of film thickness of 100 nm was observed at higher ion fluence (3 × 1013 ions/cm2 ). It may favor the enhancement in sensitivity of the sample. Fig. 7 shows the typical response time curve for CO2 gas of concentration 1000 ppm. The resistance variation is observed at constant interval of time, when concentration of the CO2 gas varied from 0 to 1000 ppm. It clearly depicts the response time of approximately 15–20 s. The gas sensitivity of the test sample shows the uniform response throughout the experiment.
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Biographies
P. Matheswaran has completed his Ph.D. in 2012 at Kongunadu Arts and Science College affiliated by Bharathiar University, Coimbatore. His thesis entitled “Effect of annealing and SHI irradiation on Indium Chalcogenide bilayer thin film”. The research work was carried out at Inter University Accelerator Centre New Delhi through UFUP-40304 and BTR-2 (UFUP-49222) Project. His research interest is ion beam processing techniques for Phase change memory and CO2 gas sensor.
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R. Sathyamoorthy has completed his Ph.D. in 1992 at Bharathiar University and presently employed as Associate Professor and Head at the Department of Physics in Kongunadu Arts and Science College, Coimbatore. He has received UGC research award, Commonwealth award and so. Presently he is working in Nanomaterials synthesis and ion beam irradiation for gas sensing applications. He has over 93 publications in his credit.
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K. Asokan received his Ph.D. from University of Rajasthan, Jaipur in 1992. He joined as scientist in the 1994 and presently in Materials Science Division, Inter-University Accelerator Centre, New Delhi. He has visited Taiwan and South Korea for his post doctoral study and other countries to present his research work. He collaborates with various academic research institutes and universities mainly on ion beam interaction in materials and its applications. His special interest is in the understanding the electronic structure of materials. He has over 150 publications in his credit.