Multilayer planar nanostructured SSE cathodes

Multilayer planar nanostructured solid-state field-controlled emission (SSE) are studied. SSE is an approach to control the effective surface barrier for electron emission by monitoring the space charge value of an ultra-thin layer at the surface, or in other terms to lower the effective surface barrier by modifying the electronic properties of the underneath surface layer. A 0.15 μm-thick Si-doped AlGaN layer with Al-content graded from 40% to 15% was deposited on a SiC substrate. It served as the conducting buffer layer. The finished surface was characterized to be atomically smooth by atomic force microscope. The I-V measurements were performed with a scanning anode field emission microscope (SAFEM). Two mechanisms were found to be present, first is tunneling field emission through a lowering work function. The electrons are emitted by a field emission mechanism from the quantized sub-bands inside the GaN quantum well, given a current density J_FN. The second mechanism occurs for elevated temperatures, i.e. k_BT > 0.8 eV, when hot electrons can jump over the first barrier located between the conductive substrate and the Al_0.5Ga_0.5N ultra-thin layer. As the second barrier at the surface is lower (less than 0.5 eV due to space charge) these electrons will emit directly. This first barrier controls the variation of the emitted current J_TH with temperature. In this dual-barrier model, the measured total emission current, J_mes, will be the sum of both contributions, J_mes = J_FN + J_TH.