Integration of Vertical Graphene Onto a Tunnelling Cathode for Digital X-Ray Imaging

As an alternative to thermionic X-ray generators, cold-cathode X-ray tubes are being developed for portable and multichannel tomography. Field emission propagating from needle structures such as carbon nanotubes or Si tips currently dominates related research and development, but various obstacles prevent the widespread of this technology. An old but simple electron emission design is the planar tunnelling cathode using a metal-oxide-semiconductor (MOS) structure, which achieves narrow beam dispersion and low supplying voltage. Directly grown vertical graphene (VG) is employed as the gate electrode of MOS and tests its potential as a new emission source. The emission efficiency of the device is initially ≈1% because of unavoidable fabrication damage during the patterning processes; it drastically improves to >40% after ozone treatment. The resulting emission current obeys the Fowler-Nordheim tunnelling model, and the enhanced emission is attributed to the effective gate thickness reduction by ozone treatment. As a proof-of-concept experiment, a clustered array of 2140 cells is integrated into a system that provides mA-class emission current for X-ray generation. With pulsed digital excitations, X-ray imaging of a chest phantom, demonstrating the feasibility of using a VG MOS electron emission source as a new and innovative X-ray generator is realized.