Electrical conduction in Ba, Sr)TiO 3 thin ®lm MIS capacitor under humid conditions Seema Agarwal a , G.L. Sharma a, * , R. Manchanda b a Thin ®lm laboratory, Department of Physics, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India b Solid State Physics Laboratory, Defense Research and Development Organization, Lucknow Road, Timarpur, New Delhi 110054, India Received 16 April 2001; accepted 20 June 2001 by H. Akai Abstract Barium strontium titanate BST) thin ®lms are made on n-type Si substrate by the sol±gel technique to study electrical conduction in a metal±insulator±semiconductor MIS) device under humid conditions. These ®lms are porous and adsorb the water vapour when exposed to a humid environment. Due to adsorption of the water vapor, a large shift of ,1 volt in ¯at band voltage from C±V measurements is observed. An attempt has been made to understand the current conduction mechanism in these ®lms under different humidity conditions. Impedance measurements have been made at various frequencies and at different humidity levels to distinguish individual contributions to electrical conduction, arising from different sources like bulk and grain boundaries, inter granular contact regions and electrode±sample interface regions on the basis of non-Debye theory. We have proposed a physical model and its equivalent circuit to explain the skewed semicircles in the Cole±Cole plot of the experimentally acquired data. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 73.40.Qv Keywords: A. Dielectric; A. Thin ®lms; B. Sol±gel; D. Humidity; E. Impedance 1. Introduction Humidity sensors are widely used for measurement and control of water vapor concentration in many areas such as meteorology, medicine, industrial paper, textile, electric, automotive) and agriculture. The relative humidity RH) is the ratio of actual vapor pressure to the saturation vapor pressure and is expressed as from 0% to 100% RH. The sensor materials used are either polymeric or ceramic. It is widely accepted [1] that ceramic type humidity sensors are superior to the polymer type because of their high stability towards a variety of chemical species, wide range of operating temperatures and fast response to the changes of humidity. Since water is a polar molecule, the negatively charged oxygen is electrostatically attached to the positively charged cations of the sensor material. When the cationic charge density is high, the electrostatic ®eld is high enough to form a chemical bond, between cation and oxygen of water molecules, in the initially adsorbed water vapor layer. This initial monolayer is chemisorbed. This layer, once formed, is not further affected by exposure to humidity but it can be thermally desorbed. The irreversible reaction for the ®rst layer can be given as: M 1 1 H 2 O ! M 2 OH 1 H 1 where M is a cation. Once the ®rst layer has been formed, subsequent layers of water molecules are physically adsorbed. The physisorbed water dissociates, because of the high electric ®elds in the chemisorbed water layer, as given below. 2H 2 O $ H 3 O 1 1 OH 2 It is estimated that the fraction of adsorbed water disso- ciated is of the order of 1% or a factor of 10 6 greater than that in liquid water [2]. The charge transport occurs when H 3 O 1 releases a proton to neighboring water molecules which accepts it while releasing another proton, and so on. This is known as Grotthuss chain reaction [3±5]. At high Solid State Communications 119 2001) 681±686 0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-109801)00284-8 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: 191-011-6596519; fax: 191-011- 6862037. E-mail address: sagarwal@netearth.iitd.ac.in G.L. Sharma).