Kev Ion Beam Induced Surface Modification of Sic Hydrogen Sensor
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propylene, H 2 mixed in an inert gas etc.) passes across the surface containing palladium, the hydrogen is separated from its previous chemical bond in the gas and linked to palladium. This results in changing the space charge region surrounding the metal clusters, which in turn affect the conductivity of the crystal. This change in conductivity is measured and can be correlated to surface concentrations of catalyst and to the concentration of the sampled gas in the environment. EXPERIMENT Rather than depositing palladium onto the surface of silicon carbide as in ref. [3], for this study we implanted palladium ions through the silicon face of 6H, n-type (nitrogen doped), 3.5' off-axis orientation, silicon carbide samples provided by Cree Research, Inc. The samples were implanted at 500'C, in order to minimize the induced implantation damages, to fluences between 3x 104 at/cm2 and 3.2x 1016 at/cm 2 , and implantation energies of 70 keV and 130 keV. The implantation energies were chosen using the SRIM code [4] in order to get high surface concentrations of Pd ions. Also, from the same code, the approximated implantation range was established as being 40 nm and 65 nm (see figure 6). The current measurements were done using a Keithley model 595 IV/CV meter interfaced to a computer, for several temperatures near 23, 70, 145, 215'C in a closed gas 135 Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
environment. Voltages of ±IV were applied on the backside of the sample, and the current was measured using a probe on several places on the implanted side. The temperature was also monitored in order to correct for current fluctuations due to temperature changes. For this experiment, air was cycled with an H 2-Ar gas mixture with 4% hydrogen, and the cycles were chosen to be 2 minutes in length (a compromise between the signal rise time and the length for a complete measurement of 7 cycles). A drawing of the experimental setup is shown in Fig. 1. The metallic contacts were made out of gilt copper, and the insulator was thick Teflon, so that the capacitance with the bottom side of the aluminum enclosure (connected to the common ground as a Faraday cage) was negligible. T monitor
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Figure 1. Schematic view of the setup. RESULTS Figures 2 through 5 show that at room temperature (23°C) the current increases in the presence of the 4% H 2 in the H2-Ar mixture, for all of the SiC implanted samples. When the H 2 Ar environment is replaced by air, the current reverts to its initial value. At higher substrate temperatures, the difference between the current read-out for H 2-Ar ambient and air ambient decreases as we approach 60°C to 84°C where it becomes zero. By increasing the temperature, one can clearly see from these figures that the current in the presence of H 2-Ar gas becomes definitely smaller than when air is present. For comparison, measurements performed on blank SiC samples revealed no current fluctuation with the gas change for any temperature in the range 23°C - 240'C
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