Surface Electronic Structure of Nitric-oxide-treated Indium Tin Oxide
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Surface Electronic Structure of Nitric-oxide-treated Indium Tin Oxide 2 Hu Jianqiao,1 Pan Jisheng,1 Furong Zhu*1and Gong Hao 1 Institute of Materials Research and Engineering, No.3 Research Link Singapore, 117602 2 Department of Materials Science, National University of Singapore, Lower Kent Ridge, Singapore, 119260
Abstract The surface electronic properties of the nitric oxide (NO) treated indium tin oxide (ITO) are examined in-situ by a four-point probe and X-ray photoelectron spectroscopy (XPS). The XPS N1s peak emerged at a high binding energy of 404 eV indicating that NO is reactive with ITO. NO adsorption induces an increase of film sheet resistance, arising from an oxygen rich layer near the ITO surface region, with approximately 2.5 nm thick. This implies that the interaction of NO with ITO is occurred around surface region. Valence band maximum measured for NO-absorbed ITO was shifted to the low binding energy side. This is related to the upward surface band bending. 1.
Introduction
A number of semiconductor oxides such as ZnO, SnO2, In2O3 and indium tin oxide (ITO) are often used for different gas sensors [1-4]. Most of these sensors are operated based on the change in the conductivity of the films, which is induced by the target gases. Normally, the conventional resistance-type sensors are operated at an elevated temperature, usually within the range of 250-300oC [4-8]. The requirement for userfriendly sensors drives the effort to develop more portable, sensitive and cost-effective devices. In addition, it is desirable to develop sensors working at room temperature. In a previous work, a nitric oxide (NO) gas sensor using a combination of ITO and quartz crystal microbalance (QCM) was fabricated and its sensitivity and repeatability for detecting NO gas were investigated [9]. Such a sensor was sensitive to NO with good repeatability at room temperature. It was shown that the sensor had a distinct negative frequency shift when it was exposed to NO. A fundamental understanding of NO adsorption on ITO surface is helpful for getting a clear picture of novel ITO-QCM sensor for NO gas. The surface electronic properties of NO-treated ITO were examined in situ by a four-point probe and X-ray photoelectron spectroscopy (XPS). The core level spectra of O1s, N1s, In3d5/2, Sn3d5/2 and valence band maximum (VBM) were measured for ITO before and after being exposed to NO at room temperature. The ITO surface composition analyses made with XPS were used to study the interaction of ITO with NO. An in situ four-point probe monitored the increase of the sheet resistance due to the exposure of ITO to NO. The results showed that the increase in the sheet resistance correlated with the presence of a low conductivity layer on the ITO surface. This layer was several nanometers thick and was NO induced. * Corresponding author: [email protected]
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2. Experimental
A 15 nm thick ITO film was deposited on glass substrate at room temperature by radio frequency magnetron sputtering using oxidized target of In
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