Structural characterization of metal oxide based gas sensor heterostructures

T. Roch, T. Plecenik, P. Durina, B. Grancic, M. Mikula, M. Truchly, M. Gregor,
L. Satrapinskyy, O. Krsko, A. Plecenik, P. Kus

Department of Experimental Physics, Faculty of Mathematics Physics and Informatics, Comenius University in Bratislava, Mlynska Dolina, 842 48 Bratislava, Slovakia
roch@fmph.uniba.sk

In recent years titanium dioxide (TiO₂) is very intensively studied due to its physical and chemical properties beneficial for many applications, for example in semiconductor metal oxide gas sensing devices. Upon exposure of TiO₂ surface to traces of hydrogen, carbon monoxide, methane or other reducing gases, its resistivity changes dramatically [1]. Sensor response is defined as S=R_air/R_gas, where R_air and R_gas are resistances measured in air without detected gas and in mixture of air and measured gas, respectively.  Typically the contact metallic structures are prepared on top of the TiO₂ film. Oxide layers are deposited on unheated sapphire substrates by reactive DC magnetron sputtering. Platinum contact layers are sputtered on top of the oxide layer. We studied the possibility to utilize stable rutile phase thin films deposited at relatively low temperature on c-cut sapphire substrates. Technological conditions have been chosen in order to obtain highly oriented TiO2 rutile thin films. Surface topography has been characterized by atomic force microscopy. Structure, texture and the strain evolution has been investigated using x-ray diffraction measurements. Thin films showed epitaxial relationship with respect to the substrate: rutile-TiO₂(100)[001] || Al₂O₃(0001)[100]. Subsequent ex-situ annealing in temperature range from 500 °C to 800 °C leads to increase of crystallite size and improvement of in-plane preferential orientation.  The response of samples to hydrogen is decreasing with increasing grain size. We have shown that the stable rutile-TiO₂ thin films can be suitable long-term alternative to metastable anatase TiO₂ sensing material [2].

However we have also proposed promising devices with combined vertically stacked bottom and top contacts of gas-sensing TiO₂ layer [3]. Within sufficiently thin oxide layer the electrons in strong electric field between electrodes get hot. This effect leads to strong increase of sensitivity, shorter dynamical response and possible decrease of operating temperature down to room temperature. If ohmic contact is desired at the metal/TiO₂ interface, the structures has to be subsequently annealed. In order to understand the sensing mechanisms and structural behaviour of such structures upon thermal processing we had to investigate in detail structural properties of such heterostructures [4]. X-ray diffraction, reflectivity, texture and non-ambient temperature measurements has been performed on reference samples of the bilayers TiO₂/Pt  and trilayers Pt/TiO₂/Pt. Ex-situ annealing at the temperature of 600 °C for 1 hour leads to increase of crystallite size and improvement of in-plane preferential orientation of Pt interlayer grown on (0001)-oriented Al₂O₃ substrate. Inner 30nm thin layer shows randomly oriented both TiO₂-rutile (R) and anatase (A) phases with the volumetric ratio of R/A ~ 2.6 and with the crystallite size of ~9 nm and ~18 nm, respectively.  These data  were important input to simulations of electron transport model, which has fit very well to measured resistance data [4].

 

1. D. E. Williams, Sens. Actuators B 57 (1999), 1.

2. A. Haidry, P. Schlosser, P. Durina, M. Mikula, M. Tomasek, T. Plecenik, T. Roch,A. Pidik, M. Stefecka, J. Noskovic, M. Zahoran, P. Kus, A. Plecenik, Cent. Eur. J. Phys. 9 (2011), 1351.

3. T. Plecenik, M. Mosko, A. A. Haidry, P. Durina, M. Truchly, B. Grancic, M. Gregor, T. Roch, L. Satrapinskyy, A. Moskova, M. Mikula, P. Kus, A. Plecenik, Sensors and Actuators B-Chemical 207, Part A, (2015), 351.

4. T. Roch, P.Durina, B. Grancic, M. Gregor, T. Plecenik, M. Truchly, M. Mikula, L. Satrapinskyy, P. Kus, A. Plecenik, Appl. Surf. Sci., 312, (2014), 192.

 

Authors would like to acknowledge a support by the Research & Development Operational Program funded by ERDF supporting the project implementations: ITMS 26240220002 and 26220220004.