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Pd and Pt Schottky diodes were fabricated on free-standing 2-in.-diameter GaN substrates prepared by a combination of hydride vapor phase epitaxy of ∼350μm onto sapphire, substrate removal and subsequent growth of 3μm of epi GaN by metalorganic chemical vapor deposition. Vertical diodes with Ti∕Al∕Pt∕Au back contacts annealed at 850°C for 30s showed excellent rectification with an on/off ratio of ∼100 at 1.5V∕−10V. Both forward turn-on and reverse breakdown voltages showed negative temperature coefficients. Pd and Pt diodes showed detection of 10ppm H2 in N2 at 25°C, with fast (<10s) recovery times upon removal of hydrogen from the measurement ambient. The Pt showed higher detection sensitivity than Pd. Detection of C2H4 and C2H6 required much higher temperatures (∼450°C) and concentrations (10%) of the gases in N2 than hydrogen detection.
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Voss et al. (2005) studied this question.
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