Surface Phenomena of the Thin Diamond-Like Carbon Films
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ABSTRACT Diamond-like carbon films (DLC) 10-40 nm thick were deposited on quartz substrates on an interdigitated planar array of 20 pm Ni electrodes already prepared by lithographic techniques. The influence of the adsorbed molecules onte electrical properties of the thin DLC films was investigated. Current and capacitance-voltage characteristics were examined. Charge-based deep level transient spectroscopy (Q-DLTS) was used for study of adsorption and desorption processes. The strong sensitivity of Q-DLTS spectra to the presence of the vapor water and isopropyl alcohol was found. For example, the Q-DLTS signal for some deposited DLC film was changed more then in order in presence of the water vapor. Such strong surface phenomena of the thin DLC films may be exploited in novel gas sensor devices. INTRODUCTION Gas sensors are the necessary means through which an electronic system receives information from, and operates on, the outside environment. The applications for gas sensors are widespread and will have a direct impact on the economy in the near and long-term. Some noteworthy applications of gas sensors lie in the defense and automotive industries, in chemical processing and aerospace industries, in gas utilities, in the areas of medical diagnos-tics and monitoring, in environmental monitoring, and in process control in manufacturing, construction, and agriculture [1].
The majority of gas sensors are solid state sensors: solid electrolytes, catalytic combustion, and resistance modulation of semiconducting oxides. More perspectives are the semiconducting resistance gas sensors. Reactions involving gas molecules which take place at the semiconductor surface change the available charge carrier density. This, in turn, will change the conductance of the device. Oxides are used as the choice material for semiconducing gas sensors. This is done to confine the reactions (molecular chemistry) to the surface layer of atoms. At the same time the electrical properties of the material in the bulk change as the consequence of these changes on the surface layer. All present sensors usually target only one or two chemicals (gases or vapor). For sensing multiple gases, an array of sensors, each sensor providing only partial information, is used. Often the interference from other gases lowers both the sensitivity and reliability of the sensors. Therefore, a solid state single sensor that is capable of sensing multiple gases has be highly 345 Mat. Res. Soc. Symp. Proc. Vol. 555 * 1999 Materials Research Society
appreciated. A variety of surface analysis tools have been developed and applied extensively to study the influence of adsorption of different gases (vapor) on density of surface states (electrically active trapping centers). However, the electrical properties of the surface are often determined by a minute number of surface defects. Most of the surface analysis methods are unable to detect a low density of surface states. Therefore, the use of new more defect-sensitive methods are a way to establish the actual surface
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