Composite-layered solid-state field controlled emitter for a better control of the cathode surface barrier

摘要

Experimental measurements have shown that electron emission was obtained from metallic planar surfaces covered with ultrathin wide band gap semiconductor layers. To get a better control of the effective surface barrier, we proposed a composite-layer nanostructured solid-state field controlled emitter with two ultrathin layers of 4 nm GaN and 2 nm Al_(0.5)Ga_(0.5)N. This composite structure defined a quantum well at the cathode surface. The threshold of the applied field to obtain electron emission was in the range of 100 V/μm. To interpret these experimental results, we propose a dual-barrier model related to the nanostructured layers and a serial two-step mechanism for the electron emission. In a first step, under the polarization, the electrons are injected into the ultrathin surface layer from the cathode substrate, creating a concentration of electrons in the GaN quantum well. This electron concentration or space charge formation induced an energy shift leading to a relative lowering of the vacuum level compared to the Fermi level of the substrate. We have measured the electron emission dependence with field and temperature of these cathodes and have determined an effective surface tunnelling barrier ≤ 0.5 eV consistent with an effective thermal activation energy of ～0.85 eV. Estimation of the effective barrier due to space charge formation from to the occupation of the localized bands in the quantum well is in good agreement with the experimental data.