Fabrication And Electron Emission Performance Of Metal-Insulator-Nanostructures Cathode

Nanostructured films with good conductivity were used in the Metal-Insulator-Nanostructures (MIN) cathode instead of the top metal electrode used in the Metal-Insulator-Metal (MIM) cathode, working as gate to modulate the emission of electrons. MIM is a film-type field emission cathode, which possess good emission uniformity, strong antipollution ability, small beam divergence and high resolution.However, the main drawback of MIM cathode is that the emission efficiency is too low, generally less than 0.1%. In this study, three methods were put forward to improve the efficiency, i.e. optimizing insulatingproperty of dielectric layer, utilizing composite structure of gate and designing a new cathode structure of Metal-Insulator-Nanostructures (MIN).Firstly, SiO2/PI layered films were designed and prepared according to the dielectric breakdown theory. SiO2 thin films were deposited by radio-frequency (RF) magnetron sputtering, and the polyimide (PI) thin films were prepared by sol-gel method. The effects of heat treatment on the morphology, crystalline characteristics, electrical properties of the films were studied. The results showed that the insulating property of layered films was much better than each phase-pure film with same thickness. The breakdownfield of a SiO2-PI-SiO2 film reached to 3.78MV/cm, and increased slowly with increasing the SiO2/PI interfaces (stacking layers).Secondly, the electron emissivity was increased by using Cu/Ag layered nano-films as the gates. Both Cu and Ag films are of excellent electrical conductivity and large electron mean free path, and these two metals show a contact potential difference, which can reduce the workfunction of the gate. It was found that the electron emissivity of the MIM cathode with Cu/Ag gate increased nearly fourfold, reaching to 0.3%, compared to Cu gate. Moreover, the thickness ratio of Cu to Ag films played an important role on the electron emission performance, and the highest emission efficiency was obtained at a thickness of 1:1. Annealing treatment could remove the internal stress of Cu/Ag films, and improved the field emission stability of MIM cathode.In order to further improve emission efficiency, a new cathode structure of MIN was designed, where the gates were made of carbon nanotubes (CNTs) and graphene films, respectively. CNTs films were deposited by the spraying method. The effects of thickness of CNTs films on the photoelectric property, and thus on the emission efficiency of MIN cathode were studied. The results indicated that at a thickness of 40nm, the CNTs films were of a sheet resistances of 95Q/sq and a transmittance of 75%. The MI-CNTs cathodes showed low turn-on voltage (20V), high electron emissivity of 0.7%, good electronic focusing performance, and high reliability and stability. Graphene gates were prepared by chemical vapor deposition method and transferred to the insulating layer using substrate etching method. It showed that MI-Graphene had excellent field emission performance with a low turn-on voltage (15 V) and a high emission efficiency(0.83%). The anode current density of MI-CNTs and MI-Graphene reached to 100μA/cm2 and did not decay after 2 hours testing.