A p- to n-transition on α-Fe2O3-based thick film sensors studied by conductance and work function change measurements
Introduction
Semiconductor oxides based gas sensors were classified according to the sign of the resistance change due to the exposure to reducing (H2O, CO, EtOH) gases as “n”-type (resistance decreases, e.g. In2O3, ZnO, SnO2) or “p”-type (resistance increases, e.g. Cr2O3, CuO) [1]. This classification is related to the surface conductivity of the oxides, which is determined by the nature of the major electronic carriers at the surface, e.g. electrons or holes. In some cases, the changing of the behavior of the oxides, e.g. from p to n or the opposite were recorded for single (pure or differently doped) oxides [2], [3], [4], [5], [6], [7], [8], [9], [10] or for mixtures of n- and p-type oxides [1], [11], [12], [13]. For the latter case the switching seems to be related to the dominating contribution of one of the material in the overall conductivity [12], [13]. The explanation of the switching from one type to the other is a matter of debate; some authors attribute this phenomenon to different kind of surface reactions in different conditions (humidity, temperature, composition, etc.) [5], others to the change of surface type of conduction induced either by special ambient conditions (pure oxygen for long periods [3]) or additives (Rh clusters at the surface [6]).
An increasing number of papers showed change from n- to p-behavior and vice versa has been published in the last years, e.g. the switching from n- to p-type response was observed for the different oxides: MoO3 [10], In2O3 [5], Fe-doped SnO2 [9], In2O3–Fe2O3 mixture (Fe2O3 was the predominant phase) [7], [8], [13]. It indicates the general nature of this phenomenon related to the behavior of semiconductor surfaces itself, e.g. the transition from n- to p-type conductivity on n-type semiconductor can be caused by a formation of an inversion layer at the surface and therefore to the inversion of the type of mobile carrier at the surface [14], [15], [16], [17].
In order to provide an explanation, in the present study simultaneous work function changes and conductance measurements were performed. We observed p–n switching for α-Fe2O3 based (alone or in combination with corundum-type In2O3) thick film sensors, induced by changes in the gas concentration and the operating temperature. As known, In2O3 (corundum-type hexagonal lattice, a=5.487 Å and c=14.510 Å) is a typical n-type oxide semiconductor, for α-Fe2O3 (hematite, corundum-type hexagonal lattice, a=5.035 Å and c=13.750 Å) contradicting results can be found in the literature concerning its conductivity, i.e. typically n-type conductivity for low temperatures and pure oxide and p-type for high temperatures and impure oxide are found [18], [19], [20], [21], [22].
The electrical properties of hematite have been the focus of several theoretical and experimental studies. Starting from early 1950s two models of electron conduction in α-Fe2O3 are permanently discussed, e.g. the localized electron description and band model. The conduction mechanism in α-Fe2O3 still remains controversial as to whether it is due, i.e. to band transport (the band-like transport of large polarons) or to hopping conduction (small polaron hopping) [23].
In the localized electron model, electrical conduction was attributed to spatially localized Fe(3d) electronic levels, whereby the electron is transferred from one iron atom to another by II/III valence alternation (n-type conductivity, carrying electron exist as a localized particle creating an FeII in the lattice, e.g. the small polaron model) or movement of FeIV holes (p-type conductivity) [23].
On the other hand, the origin of the band gap in α-Fe2O3 has been understood within the configuration–interaction theory of the cluster model or the impurity model as well as within the one-electron band theory [24]. Pure hematite has a charge-transfer band gap of 2.2 eV [23] and can be classified as a charge-transfer insulator (or an intermediate-type insulator) [25], [26], however, the lowest unoccupied state is not Fe 3d-like but is the bottom of the Fe 4s band [26]. Crystal field, hybridization and charge transfer will influence very strongly the electronic structure of α-Fe2O3. Especially, it means that besides the 3d5 configuration (formally a Fe3+ ion state) the 3d6L configuration (according formally to a Fe2+ ion state), too will take an important part of the ground state in this compound [26]. The hybridization of oxygen 2p states and iron 3d states and the screening of the on-site Coulomb interaction tend to lead to a band-like behavior [27].
However, irrespective to the nature of the band gap, the semiconducting behavior of hematite can be described in terms of band-like theory. α-Fe2O3 is known to be n-type semiconductor with band gap of about 2.2 eV [28]. α-Fe2O3 has tendency to become oxygen-deficient (and thus n-type) with oxygen vacancies [29]. The defect formation can be described as [21]:andIt means, that hematite has a complex defect structure in which three types defects species (oxygen vacancies, Fe3+ interstitials, Fe2+ interstitials) are present [29]. The presence of the defects give rise to semiconducting properties. Loss of oxygen leaves behind extra electrons and produces an n-type semiconductor; extra oxygen (entering the lattice as O2−) creates a deficit of electrons (i.e. introduces electronic holes), which produces p-type behavior.
Three values of the band gap of α-Fe2O3 are given in [30]. The most reliable seems to be 2.34 eV at 270 °C. Experimental band gap obtained from the electrical conductivity measurements is about 2.0-2.7 eV [25]. The surface properties of α-Fe2O3 are basically unknown and also for other metal oxides an understanding is only badly developed [31].
Although bulk conductivity measurements [21], [22] on powdered samples show n–p transition, this transition was attributed to and explained in terms of defect bulk chemistry. The electrical properties of α-Fe2O3 depend on the nature of the precursors and sintering conditions that influence the defect chemistry of the hematite. α-Fe2O3 annealed in air was found to be a n-type semiconductor, and annealed in oxygen a p-type one [20]. The p-type conductivity is mainly due to the presence of cationic impurities [22], e.g. presence of Mg2+ or Ni2+ (0.019–0.17%) leads to p-type behavior [19]. As these defects are associated to donor states a n-type conductivity becomes predominant. The Seebeck voltage shows α-Fe2O3 is n-type below 800 °C and p-type above this temperature [21]. The conversion from n- to p-type behavior was due to the different motilities of electrons and holes [19], [26]. The conversion from n- to p-type behavior was because the electrons are the most mobile carrier below ≈800 °C but the hole is more mobile above 800 °C.
Section snippets
Experimental
The thick film sensors based on the individual oxides (α-Fe2O3 and In2O3) and on the mixed oxides α-Fe2O3–In2O3 (Fe:In = 1:9 and 9:1) were investigated by conductance measurements. Thick film sensors were prepared by screen printing of the corresponding oxide paste on alumina substrates. The oxide pastes were prepared from the individual oxides (α-Fe2O3 and In2O3) and mixed oxides (α-Fe2O3–In2O3) and an organic binder. Mixed oxide compositions were prepared by the ball-milling of the
Conductance measurements
The main results of the conductance measurements can be summarized as follows:
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resistance in air increases in the order: In2O3 < α-Fe2O3–In2O3 (Fe:In = 1:9) < α-Fe2O3–In2O3 (Fe:In = 9:1) < α-Fe2O3;
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all the compositions show much higher response to ethanol (20–500 ppm) and propanal vapors (10–300 ppm) than for CO (500–10,000 ppm);
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In2O3 and α-Fe2O3–In2O3 (Fe:In = 1:9) show typical n-type response both in dry and humid air in the temperature range 250–350 °C; the resistance in the humid air is lower in
Conclusions
We give the first experimental evidence for the origin of the n–p switching on a n-type α-Fe2O3 which is shown to be due to the oxygen adsorption and formation of an inversion layer at the surface. The obtained results indicate that basically the same surface reaction with the gas is accompanied with different transducing of chemical change on the surface into bulk electrical signal. This allows the possibility to distinguish between these two contributions into overall sensor signal and
Acknowledgements
This work was supported in the frame of the INCO Copernicus GASMOH (environmental control by means of a new gas detection principle: gas detection by means of metal oxide heterojunctions, ICA2-CT-2000-10041) project. The oxide powders were prepared in Istituto per la Microelettronica e i Microsistemi, IMM-CNR, Sezione di Lecce, Via Arnesano, 73100 Lecce, Italy. We are indebted to Mauro Epifani and Dr. Pietro Siciliano (both IMM-CNR) for providing the samples used in this study. We would also
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