Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications
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Y11.5.1
Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications V. Tilak1, M. Ali2, V. Cimalla2, V. Manivannan1, P. Sandvik1, J. Fedison1, O. Ambacher2, D. Merfeld1 1
GE Global Research Center, Niskayuna, NY 12309, US Center of Micro- and Nanotechnologies, Technical University Ilmenau, Gustav-Kirchhoff-Str. 1, D-98693 Ilmenau, Germany
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ABSTRACT Hydrogen gas sensors based on Pt/GaN Schottky diode structures were fabricated and their responses to hydrogen were studied. These diodes were fabricated on Si doped GaN layer (ND = 1x1017). Three sets of diodes were fabricated with 80 Å, 240 Å and 400 Å of Pt for the Schottky contacts. The electronic performances of 0.25 x 0.25 mm devices were tested in up to 1 % H2 gas in synthetic air (79% N2, 21% O2) by volume. The devices were operated in constant current mode in a forward bias condition. The change in voltage was monitored with the diodes exposed to hydrogen and to dry air at varying temperatures. The responses increased as the thickness of the Schottky metal contact decreased at any given temperature up to 310 °C. The trend of increasing response with decreasing thickness was also observed in 0.5 x 0.5 mm and 1.0 x 1.0 mm size Schottky diodes. SEM studies of the microstructure showed that the thinner Pt devices had higher grain boundary densities. The increase in sensitivity with decreasing thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen through the Pt grain boundaries and the adsorption of hydrogen to the surface as a possible mechanism of sensing of hydrogen by Schottky diodes. INTRODUCTION Wide band gap semiconductors are currently being developed for various applications from lighting to high power microwave devices [1,2] due to their attractive properties including light emission in the UV, high breakdown fields and their robustness to both chemicals and high temperatures. This makes this class of semiconductors increasingly attractive for harsh environment gas sensors. GaN based Schottky diodes [3] and FETs [4] have previously been demonstrated as gas sensors. Lloyd-Spetz et al. have shown that using Pt, Pd or similar catalyst metals as the gate material can induce gas sensitivity [5]. This group has done extensive work on SiC based capacitors, diodes and FETs which have response in milliseconds and work at temperatures up to 800 °C [6]. Although SiC based devices have shown clear evidence as viable harsh environment gas sensors, there are some drawbacks. These include the difficulty in processing these materials and the lack of uniformity of epitaxial materials for large-scale fabrication of chips. GaN based devices benefit from comparably easier processing as well as higher uniformity in epitaxial material.
Y11.5.2
In this paper, we have fabricated Pt based GaN Schottky diodes and show the response of such devices to hydrogen. We will also look at how sensitivity to hydrogen is affected by thickness and the microstructure of the Pt. We investigate the
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