The electronic band structure and p-type conductivity of CuAlO2 films were modified via synergistic effects of energy band offset and partial substitution of less-dispersive Cu+ 3d10 with Cu2+ 3d9 orbitals in the valence band maximum by alloying nonisovalent Cu-O with CuAlO2 host. The Cu-O/CuAlO2 alloying films show excellent electronic properties with tunable wide direct bandgaps (∼3.46–3.87 eV); Hall measurements verify the highest hole mobilities (∼11.3–39.5 cm2/Vs) achieved thus far for CuAlO2 thin films and crystals. Top-gate thin film transistors constructed on p-CuAlO2 films were presented, and the devices showed pronounced performance with Ion/Ioff of ∼8.0 × 102 and field effect mobility of 0.97 cm2/Vs.
The electrical properties of nitrogen‐containing amorphous hydrogenated carbon layers are investigated. The nitrogen concentration was between 0 and 6 at%. The electrical conductivity is found to increase with the nitrogen content. The temperature dependent conductivity in the temperature range between 150 and 350 K can be well fitted by a semi‐empirically derived equation which considers the conductivity as a superposition of two hopping mechanisms with different activation energies. Together with experimental results on the optical properties and the mass density, these data allowed to propose a structural model which explains the observed effects in terms of familiar a‐C:H cluster models.
The present study is focused on the development of a gas sensor for application in a high temperature environment. The sensor has been realised using thin ®lms prepared on silicon substrates including a high temperature stable heating and wiring system. TiO 2 acts as sensitive layer. Measurements have been carried out in synthetic gas mixtures as well as in gases in a given application. Neural networks and multivariate data analysis have been used for determining the gas concentrations. The capability to detect CO, NO x , and toluene is shown. IntroductionMicromachined gas sensors using semiconductive metal oxides have been established for some years in various applications [1±3]. The sensitivity of these devices is based on a change in the resistivity of the metal oxide ®lm depending on the surrounding atmosphere. The sensor temperature is one important parameter for the interactions between the several gases and the metal oxide ®lm. Based on experimental studies [4±7] metal oxides require temperatures of 200°C and more to be applicable as conductive sensor materials. The exact temperature depends on the used metal oxide as well as on the kind of gases to be detected. For that reason the sensors have to be heated by suited elements. Such sensors can be operated in an environment with temperatures far below up to the sensor working temperature. Conceivable applications are the monitoring and controlling of combustion processes. Especially for the application at high temperatures the major problem of these sensors is the long term stability. Respective device concepts have to be developed to ensure this stability during the whole device lifetime. These concepts include the selection of appropriate materials for the substrate, sensitive layer, and metallization, as well as the selection of a suitable housing and wiring technology.In this article we employ titania thin ®lms as material for gas sensing at different temperatures and show their features. We illustrate one way to prepare sensor devices as well as the results of their characterisation in laboratory gases and in gases in a given application. Sensor design and fabrication Sensor designThe sensor chip consists of a gas sensitive titania layer with platinum electrodes above and underneath this layer. For signal pattern analysis each sensor chip contains 3 sensor cells. To generate temperature dependent gas speci®c signal pattern the cell temperatures can be tuned independently by underlying heaters. For the temperature control platinum resistors are integrated in the electrode level underneath the sensitive layer. The gas sensors are prepared using standard silicon wafers. The sensitive elements are realised on a thin silicon membrane (50 lm) in order to reduce the required heating power. Furthermore, this leads to an optimised temperature pro®le and reduces the heat¯ow from the sensor area to the chip mounting. On the membrane 5 platinum heaters are located, one heater for each sensor cell and two supporting heaters to realise the necessary temperature grad...
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