The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films

Over the last century, society has benefited greatly from technological advances. However, this progress has come with many unintended consequences such as pollution of air and water. Hence the need to monitor air pollution, e.g., by nitrogen dioxide (NO2) from Diesel cars or ozone (O3) produced in hazardous concentrations by sunlight on some hot days, is obvious. In many cases monitoring local variations in concentration of these gases by gas sensor networks is necessary, requiring cheap and reliable gas sensors. Conductometric gas sensors (Fig. 1) made of semiconducting metal oxides, detecting gases by a change of electrical conductance upon gas adsorption, are a promising candidate and cheap devices are already on the market.

 

N-type semiconducting oxides, like indium oxide (In2O3) used for these sensors, show a decrease in conductance due to a charge transfer between the oxide surface and the adsorbed oxidizing gas (Fig 1). The vast majority of these sensors in application and research are based on polycrystalline (grainy) material, making a detailed understanding of the correlation of surface and volume phenomena and the underlying sensing mechanisms difficult: As indicated by arrows in Fig. 2(a), conductance is due to a complex interplay of contributions from the bulk of the grains, the surface, and across & along grain boundaries. Moreover, doping (addition of small amounts) of various metals into the oxide has been empirically shown to enable sensing improvements.

 

Fig 1: Sketch of a conductometric gas sensor consisting of a film of gas-sensitive material on a substrate and two electrical contacts.

Fig 2: Schematics of: (a) a polycrystalline (grainy) film and (b) a single-crystalline film (without grains). Arrows indicate possible contributions to conductance.

Fig 3: Conductance measured over time of three samples: undoped, undoped & plasma-treated and Mg-doped. During the first 120 s gas desorbs and during the second 120 s (grey area in the graph) gas adsorbs. The gas-response, the relative conductance change upon gas-adsorption, is highest for the Mg-doped film and almost non-existent for the plasma-treated film.

Our study of the fundamental principle of gas sensing and its improvement by doping is based on single-crystalline In2O3 films (without grains) as simple model systems. In these films only two electrically parallel conductance contributions, bulk and surface (Fig. 2b), add up to the measured conductance. Systematically disabling the bulk or surface conduction by controlled bulk doping with the deep acceptor Mg or by oxygen-plasma surface treatments, respectively, allowed us to show that the gas response on oxidizing gases (Fig. 3) is fundamentally based on the modulation of the surface conductance whereas the bulk conductance remains unaffected. With these results we could rationalize the increased gas sensitivity after bulk Mg-doping (Fig. 3) by disabling the gas-insensitive parallel bulk conductance. Our concept of using the surface conduction for gas sensing and removing the bulk conductance (or intra-grain conductance in poly-crystalline material) by deep acceptor doping can be generally applied to other n-type oxides, such as the widely-used gas-sensor material tin dioxide (SnO2).

1 Autor J. Rombach , A. Papadogianni , M. Mischo , V. Cimalla , L. Kirste , O. Ambacher , T. Berthold , S. Krischok , M. Himmerlich , Sören Selve , O. Bierwagen
Titel

The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films

Source Sens. Actuators B Chem. , 236 , 909 ( 2016 )
DOI : 10.1016/j.snb.2016.03.079 | 2784 Cite : Bibtex RIS
J. Rombach, A. Papadogianni, M. Mischo, V. Cimalla, L. Kirste, O. Ambacher, T. Berthold, S. Krischok, M. Himmerlich, Sören Selve, and O. Bierwagen