Protective Layer Process of Graphene-Oxide-Semiconductor Electron Emission Devices for Low Earth Orbit Applications

Graphene-oxide-semiconductor (GOS) planar electron sources achieve high electron emission densities at low applied voltages. However, in low earth orbit, graphene is easily etched by atomic oxygen (AO). This erosion makes it challenging to use GOS as neutralizers in ion engines. To improve the oxidation resistance, hexagonal boron nitride (h-BN) was deposited as a protective layer by chemical vapor deposition (CVD) methods, and aluminum oxide was deposited by the atomic layer deposition method. After exposure to AO by oxygen plasma ashing, we measured the Raman spectra and optical transmittance of the protective films. The results showed that h-BN protective film deposited by the thermal CVD method protected the graphene even after 30-minute ashing, corresponding to 100.4 hours of operation in low earth orbit. Thus, the thermal CVD method was found to be the best at present.