Cathodoluminescence of conducting gratings and implications for electron-beam investigations of nano-photonic devices

Cathodoluminescence (CL) spectroscopy is performed on conducting 1and 2-dimensional gratings of metals, semimetals and semi-conductors of varying periods from 0.5 to 20 microns for a range of grating amplitudes from 0.1 to 4.6 microns. The overall emission spectrum consists of a 400 nm wide band centered at ~600 nm which depends little on the grating period, grating amplitude, material, e-beam energy, or temperature. CL intensity increases and the center wavelength blue shifts with increasing excitation beam current. For the larger amplitude 1-dimensional gratings fringes appear in the emission spectrum, which is due to interference between emission from grating bars and grooves. Surface corrugation is necessary to the emission as none is observed from smooth surfaces. The same band appears weakly in CL of a Cu sample with random ~1 micron surface roughness, but this emission is strongly reduced when the same sample is highly polished. The CL signal appears even when the ~10 nm electron-beam is at least 2 mm away from the grating edge, suggesting electron-beam induced currents are important to the emission, whose precise mechanism remains unclear. Previously suggested mechanisms--electron collision with image charge, transition radiation, surface contamination, and inverse photoemission effect--all fail to explain the observed spectrum and its lack of beam-energy dependence.