Nano-crystalline Fe2O3 thin films for ppm level detection of H2S

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Abstract

We have investigated the chemiresistive gas sensing characteristics of Fe2O3 thin films for a host of gases (H2S, C2H5OH, CO, NH3, CH4, CO2, NO and Cl2). Fe2O3 films were prepared by thermal oxidation of Fe films, which were deposited on (0 0 0 1) Al2O3 substrate by electron-beam evaporation method. The results of scanning electron microscopy, atomic force microscopy and X-ray diffraction revealed that the grains of these films are composed of nano-crystallites (size ∼32 nm). We demonstrate that these Fe2O3 films show purely n-type conductivity, with high selectivity, moderate response and recovery for H2S in the concentration range of 1–50 ppm.

Introduction

Metal-oxide semiconductors (MOS) have been widely investigated for gas sensor applications because of their simplicity, ease of production, low cost and capability of detecting large number of toxic and volatile gases under different conditions [1], [2]. MOS can be classified as “n-type” or “p-type” according to the direction of conductance change on exposure to reducing or oxidizing gases. For example, on exposure to reducing gases (e.g. NH3, H2S etc.) an n-type MOS (In2O3, ZnO, SnO2 etc.) show an increase in conductance, whereas p-type MOS (e.g. Cr2O3, CuO etc.) show a decrease in conductance. Exactly opposite change in conductance takes place for oxidizing gases (e.g. NO2, Cl2 etc.) i.e. a decrease in conductance for n-type and an increase in conductance for p-type [1]. This classification of MOS is related to their surface conductivity, which is governed by the nature of the major electronic carriers at the surface i.e. electrons or holes. Interestingly, certain MOS (e.g. α-Fe2O3, ZnO, SnO2 etc.) that are either pure or doped, exhibit a changeover from n-type to p-type or vice versa [3], [4], [5], [6], [7], [8], [9], [10]. Several different explanations have been provided in literature for these changeovers, which is currently a matter of debate. Some of the explanations include different type of surface reactions under varying conditions (humidity, temperature, composition, etc.) [11], [12], change in surface type conduction induced by either nanostructuring or ambient conditions or additives [3], [4], [13], [14] etc. Such type of carrier inversion was first experimentally observed in Cr2O3 [9].

Among various MOS, α-Fe2O3 is most fascinating as the literature shows that it undergoes both “n to p” as well as “p to n” transitions. Fe2O3 is usually an n-type semiconductor with band gap of about 2.2 eV. α-Fe2O3 has a complex defect structure with three type of defect species i.e. oxygen vacancies, Fe3+ interstitials and Fe2+ interstitials [15]. The presence of oxygen vacancies (i.e. loss of oxygen) leaves behind extra electrons, and therefore, leads to n-type nature. On the other hand, extra oxygen (entering the lattice as O2−) creates a deficit of electrons (i.e. introduces electronic holes), which leads to the p-type nature. Moreover, Fe2O3 can be made either p-type through Mg doping, or n-type through Ni doping [16]. Thus, n- or p-type nature of Fe2O3 sensors can be attributed to the different types of impurities or vacancies present in the films. The “n to p” or “p to n” transitions in Fe2O3 sensors, which are induced by either a change in the gas concentration or the operating temperature, generally can be attributed to the formation of an inversion layer at the surface, which in turn, leads to the inversion of the type of mobile carrier at the surface [13]. It is therefore, evident that the chemiresistive properties of Fe2O3 sensors strongly depend on the preparation methods. This is because depending upon the deposition method and processing parameters, unintentional impurities or defects can get incorporated into Fe2O3 films, and therefore, can lead to different types of conduction behavior as well as “n to p” or “p to n” transitions discussed earlier. For instance, M. Aronniemi et al. have prepared α-Fe2O3 films using atomic layer deposition and have found its n-type response on exposure to CO [17]. Yan et al. have also observed n-type response on chemical vapor deposited α-Fe2O3 thin films [18]. Poghossian et al. have reported n-type response in Fe2O3 thin films prepared by electron-beam evaporation on exposure to reducing gases [19]. Debliquy et al. have observed n to p transition on both undoped and barium oxide doped α-Fe2O3 thin films prepared by radio-frequency sputtering on adsorption of strong oxidizing gases like O3 and NO2 at high concentrations [20]. Neri et al. have reported p-type response on liquid-phase deposited Zn and Au modified Fe2O3 thin films [21]. In order to probe the intrinsic sensing characteristics of Fe2O3 films, it is therefore essential that they are grown in pure form. In order to obtain pure metal oxide thin films, electron-beam evaporation is one of the best techniques as the deposition of high purity target is carried out under high vacuum. Unfortunately, there are hardly any significant reports that describe the gas sensing characteristics of α-Fe2O3 thin films deposited by electron-beam evaporation.

In this paper, we investigate the gas sensing characteristics of pure nano-crystalline Fe2O3 films prepared by electron-beam evaporation of Fe (99.99%) followed by annealing under flowing oxygen (99.99%) gas. We demonstrate that these Fe2O3 films exhibit n-type conductivity with a selective response for H2S. In addition, no “n to p” transition was observed either as a function of operating temperature or as a function of exposure to varying H2S concentration.

Section snippets

Experimental

α-Fe2O3 thin films were deposited in following two steps. In the first step, Fe films (thickness ∼100 nm) were deposited onto pre-cleaned (0 0 0 1) Al2O3 substrates using electron-beam evaporation under a base vacuum of ∼1.3 × 10−4 Pa. High purity (99.99%) Fe metal powder was cold pressed into a pellet and was used as the source material for deposition. In the second step, deposited Fe films were subjected to thermal oxidation at 800 °C under flowing high purity oxygen (flow rate: 50 sccm) for 2 h. The

Morphology and structural characterization

Fig. 1 shows the photographs of the Fe films before and after thermal oxidation at 800 °C (2 h). It is seen that as-deposited Fe films exhibit metallic luster and are black in color, whereas after oxidation the luster disappears and the color changes to bright red, which is an indication of the formation of α-Fe2O3 phase [23].

Fig. 2(a) shows SEM image of the Fe2O3 films. The film exhibits a granular morphology. Fig. 2(b) shows a typical AFM image, which was recorded on 2 × 2 μm2 area in contact

Conclusions

We presented a systematic study of gas sensing properties of phase pure α-Fe2O3 thin films prepared by electron-beam evaporation of Fe followed by thermal oxidation in oxygen ambience. Fe2O3 films prepared using this method show purely n-type conductivity without any n–p transition as a function of gas concentration and/or operating temperature. It is therefore suggested that n to p or p to n transition reported in literature are largely due to the presence of impurity states. Our films show

Acknowledgements

V.B. thanks CSIR, New Delhi for the award of senior research fellowship. Authors also acknowledge Dr. Jagannath, Technical physics division, B.A.R.C. for his help with XPS measurement. This work is partly supported by “DAE-SRC Outstanding Research Investigator Award” (2008/21/05-BRNS/2743) and “Prospective Research Funds” (2008/38/02-BRNS/2858) granted to D.K.A. The authors are also thankful to BRNS-DAE for providing financial assistance vide sanction no. 2008/37/4/BRNS.

Mr. Vishal Balouria received his M.Sc. degree in Physics from Dr. B.R. Ambedkar National Institute of Technology, Department of Physics, Jalandhar, India in the year 2008. He is working toward his Ph.D. degree at Guru Nanak Dev University. His research interests are fabrication and characterization of thin films and nanostructures for gas sensor applications.

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    Mr. Vishal Balouria received his M.Sc. degree in Physics from Dr. B.R. Ambedkar National Institute of Technology, Department of Physics, Jalandhar, India in the year 2008. He is working toward his Ph.D. degree at Guru Nanak Dev University. His research interests are fabrication and characterization of thin films and nanostructures for gas sensor applications.

    Dr. Arvind Kumar is a Ph.D. from Homi Bhabha National Institute, Mumbai in 2012. His area of interest is the investigation of various organic semiconductors for gas sensing application.

    Dr. Soumen Samanta joined BARC in 2008 through 50th Batch of Training School after completing M.Sc. (Physics) from IIT Kanpur. He completed his Ph.D. from Homi Bhabha National Institute, Mumbai in 2012. He is presently working as a Scientific Officer (D) in the Technical Physics Division of BARC. His current research interest is to understand the charge transport in organic semiconductors.

    Dr. Ajay Singh joined BARC in 1999 through 42nd Batch of Training School after completing M.Sc. (Physics) from Garhwal University. He completed his PhD from Mumbai University in 2004. Presently he is working on charge transport studies of metal phthalocyanine thin films, development of thermoelectric devices and chemiresistive gas sensors. He is a recipient of Humboldt Post-doctoral fellowship (2005–2007), DAE Young Scientist Award for excellence in Science and Technology - 2007, National Academy of Sciences India (NASI) Young Scientist Platinum Jubilee Award–2010, DAE Solid State Physics symposium Young Achiever Award 2010 and Indian National Science Academy (INSA) medal for Young Scientist 2012.

    Dr. A.K. Debnath is presently working as Scientific Officer (F) at Technical Physics Division of BARC. He has extensively worked on oxide materials based gas sensor, particularly for H2S detection. His current research interest is to understand the charge transport and gas sensing properties of ultra thin films of organic semiconductor grown using MBE.

    Dr. Aman Mahajan received M.Sc. degree (1997) and Ph.D. (2003) in Physics at Guru Nanak Dev University, Amritsar, India. He is presently working as Assistant Professor in the same university. His main interests are preparation and characterization of organic thin films for OLEDs, sensors and photovoltaic applications.

    Dr. R.K. Bedi is a Ph.D. and retired professor from the department of Physics, Guru Nanak Dev University, Amritsar, India. He has held academic positions such as Head of Department, Dean, Science faculty and other administrative positions in the university. His research interests are Material Science, Thin films, Photovoltaic, Sensors. He is a fellow of The Institution of Electronics and Telecommunication Engineers (IETE).

    Dr. D.K. Aswal joined BARC in 1986 and is currently Head, Thin Films Devices Section. His area of scientific interest is condensed matter physics, specializing in device-oriented research leading to hybrid molecule-on-Si nanoelectronics, thermoelectric devices, and gas sensors. He is a recipient of several international fellowships including, JSPS fellowship, Japan (1997–1999), IFCPAR fellowship, France (2004–2005), BMBF fellowship, Germany (2006) and CEA fellowship, France (2008). He is recipient of several awards, including “MRSI Medal 2010”, “Homi Bhabha Science and Technology Award -2007”, “DAE-SRC Outstanding Research Investigator Award-2008”, and “Paraj: Excellence in Science Award, 2000”.

    Dr. S.K. Gupta joined Bhabha Atomic Research Center in 1975 and is presently Head, Technical Physics Division of BARC. Over the years, he has worked on space quality silicon solar cells, high temperature superconductor thin films and single crystals, gas sensors and thermoelectric materials. He has carried out extensive studies on vortex dynamics in superconductors. He is a member of the National Academy of Sciences, India.

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