Response to hydrogen of a metal/AlN/Si thin film structure: Effects of composition and structure of a combination Pd–Cr gate

The effect of Cr addition to the gate of a Pd/AlN/Si Hydrogen gas sensor was investigated by modifying the gate composition and structure. A device with a pure Pd gate responds to hydrogen concentrations as low as 1 ppm, with saturation at about 60 ppm. However, with a Pd0.96Cr0.04 alloy gated device, although the response amplitude was reduced, the dynamic range was considerably enhanced, with useful response up to 50,000 ppm and with quite satisfactory signal to noise down to 50 ppm. Interestingly, in 10% Cr alloyed to the gate, the response was lost. To investigate the mechanism responsible for this effect, two gate structures were studied where a thin layer of pure Pd was added to the 4% alloy gate, one at its outer surface and the other at the gate/AlN interface. The response of the former saturated at 60 ppm and in general behaved comparably to the device with a pure Pd gate, whereas the latter showed the expanded dynamic range. For all these cases the maximum magnitude of the electrical response is in the range of 0.1–0.5 V, even though the hydrogen concentration range varies by three orders of magnitude. This indicates that saturating electrical response is highly sensitive to the properties of the gate outer surface, whereas hydrogen sensitivity is highly dependent on the amount of hydrogen reaching the interface between metal gate and insulator.