Surface Treatments Effects for NO 2 Gas Responsivity of p-Type Surface Conductive Layer on Diamond Films
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Surface Treatments Effects for NO2 Gas Responsivity of p-Type Surface Conductive Layer on Diamond Films Kenichi Haruta1, Hideki Kimura1, and Masafumi Chiba2 1 Dept. Electrical and Electronic Engineering, Tokai Univ., Hiratsuka, Kanagawa, Japan. 2 Dept. Materials Chemistry, Tokai Univ., Numazu, Shizuoka, Japan. ABSTRACT The low resistance layer called p-type Surface Conductive Layer (PSCL) is formed when the nitrogen dioxide (NO2) was absorbed onto hydrogen-terminated surface, although the diamond is generally isolator. The PSCL conductance is dependent on NO2 concentration in the atmosphere, and this reaction has reversibility. The diamond having these characteristics can apply to gas sensor. In this study, the gas responsivity of PSCL was improved by surface treatment. The changes in the responsivity were evaluated from serial measurements of the conductance in the gas atmosphere. From this evaluation, it was observed that the adsorption and desorption of NO2 were faster via the surface conditions variation by treatment. INTRODUCTION Nitrogen dioxide (NO2) is one of the air pollution gasses, so that the emission is regulated. In recent years, the demand for a better gas sensor has been increase from the view point of environment conservation. The sensor material was required the environment resistance such as thermal and chemical stability, since the NO2 was corrosive gas and its source was usually high temperature. In this study, the diamond, which is generally as an isolator is focused. However, p-type Surface Conductive Layer (PSCL) is formed on diamond, when NO2 is adsorbed on its surface terminated by hydrogen [1]. The increase in conductance on the diamond surface was arose to induce the hole, when NO2 adsorb on it[2, 3]. This reaction has reversibility and the PSCL conductance depended on atmospheric NO2 concentration. Therefore, it is expected to use in gas sensor [4]. The PSCL can be responded to under low NO2 concentration such as about 0.01 ppm, which is sufficient to measure the air environment. PSCL is formed by limited specific gas. Oxygen, nitrogen and carbon dioxide which are contained in the air are not affect [1]. In addition, the diamond can be used in inhospitable environment because of its high chemical stability and heat resistance. However, PSCL can be used under 300 °C because the hydrogen terminated surface is oxidized in atmosphere containing oxygen[5]. The response speed is important performance for measurement the gas concentration change over time. In this study, the surface treatment for the diamond was carried out to improve the responsivity, and its effects were discussed EXPERIMENT The poly-crystalline diamond films were used for samples, which were deposited by HotFilament Chemical Vapor Deposition (HFCVD) on silicon (Si) substrate. The films were having thickness about 5 μm and roughness around 1μm. Samples were analyzed by Raman spectroscopy before the experiment. They were observed only a sharp peak at 1333cm-1. In this
results, it was indicated that non-diamond component is n
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