We numerically studied the effect of the geometric structure and Al component on the optical capture performance of gradient Al component AlxGa1 − xN photocathodes. The effects of geometric parameters, such as base radius (R), wire-to-wire spacing, cone rate, and angle of incident light, on the optical response were systematically studied based on the finite element method. In the radial direction, we study the optical response of rectangular periodic structure and hexagonal periodic structure. Simulation results show that pencil nanostructure can achieve omnidirectional and broadband light absorption of AlxGa1 − xN nanowires with hexagonal periodic structure. In addition, we used the Spicer three-step emission model to establish the photoemission efficiency of the AlxGa1 − xN nanostructure. As a result, the photocathode achieves optimal quantum efficiency when the Al component is in the range of 0 to 0.75 and sublayer thickness of 240, 180, 120, and 60 nm.
We numerically analyzed the influence of geometric structures of gradient Al component AlxGa1 − xN nanowire on their light-trapping properties, ranging from nanopillars, inverted conical frustums, to inverted hexagonal frustums. COMSOL® Multiphysics package based on the finite element method is used to systematically study the effects of geometric parameters such as base radii (R), pillar height (H), period (P), and angle of incident light on the optical absorption. The simulation results show that compared with the other nanostructure counterparts, the inverted hexagonal frustum can effectively couple photons into the nanoarrays to achieve wide spectrum and effective optical absorption for AlxGa1 − xN nanowire-based UV photocathode. The inverted hexagon frustum with optimum height can obtain an optical absorption above 95% over a wide wavelength of 200 to 380 nm and a broad angle of incident light between 0 deg and 70 deg. All these findings not only show that the gradient Al component AlxGa1 − xN material has a great potential advantage for the UV photocathode, but it also provides an efficient broadband and omni-directional light trappers for the UV photocathode.
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