Performance of a MIS Type Pd-Cr/AlN/Si Hydrogen Sensor
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A5.4.1
Performance of a MIS Type Pd-Cr/AlN/Si Hydrogen Sensor Linfeng Zhang1, Ibrahim A. Al-Homoudi2, Md H. Rahman3, Erik F. McCullen1, Lajos Rimai1, Ron J. Baird1, Ratna Naik4, Golam Newaz2, Gregory W. Auner1, K.Y. Simon Ng3 1 Department of Electrical and Computer Engineering 2 Department of Mechanical Engineering 3 Department of Chemical Engineering and Materials Science 4 Department of Physics and Astronomy Wayne State University, Detroit, MI 48202 ABSTRACT An MIS Hydrogen sensor with a Pd0.96Cr0.04/AlN/Si structure was fabricated, exhibiting the dynamic range considerably wider than that of analogous devices with pure Pd gates. A useful response could be obtained for Hydrogen concentrations as large as 50,000 ppm. Although the response amplitude was much reduced at the lower concentrations, satisfactory signal to noise down to 50 ppm could be obtained. The saturating magnitude of the electrical response is in the range of 0.1 to 0.5 V, which is the same as that for the pure Pd gated devices, inspite of the 3 orders of magnitude difference in the saturation hydrogen concentration. This result will be discussed in terms of the response mechanism of these devices.
INTRODUCTION When biased in depletion the Pd/AlN/Si metal/insulator/semiconductor (MIS) structure behaves as a voltage dependent capacitor [1, 2]. At constant capacitance, the presence of hydrogen in the surrounding gas causes a voltage shift, which increases in magnitude with increasing hydrogen concentration. This type of sensor shows high sensitivity and selectivity to hydrogen down to 1 ppm. However it saturates at around 60 ppm [3]. It also has a very slow response time and exhibits baseline drift. Such a sensor may present a problem to quantify hydrogen in high concentrations. Moreover, it is known that at high hydrogen concentrations (>1%), Pd goes through a phase transition from α to β. The Pd gate could be easily separated from the insulator surface, and the morphology of the Pd gate would change significantly, possibly negatively affecting the sensors performance [4, 5]. To make a more stable catalytic metal gate, other elements could be added to the Pd to suppress the phase transition and increase the dynamic range. In our study, Pd-Cr alloy was used as the metal gate. Aluminum nitride (AlN) was used as insulator since its band gap (6.2 eV) is much wider than the band gap of Si (1.2 eV). Furthermore, AlN has high chemical durability and its thermal expansion coefficient matches well that of Si(111) substrates thus improving the stability of the sensor This paper, describes the preparation of devices with Pd-Cr gates. Their morphology, dynamic range, sensitivity and stability were compared to the corresponding properties of sensors with pure Pd gates. These results might provide a better understanding the effect of Cr on the sensor’s response, and guidance towards the design of a MIS type sensor with improved performance.
A5.4.2
EXPERIMENTAL Pd-Cr/AlN/n-Si thin film devices (1 mm diameter gates) were fabricated by a combination of plasma so
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