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The secondary electron emission phenomenon often refers to the emission of electrons as a result of the interaction of impinging energetic electrons with the surface of a material. Although it is fairly well described for metals, with a typical shape of the total electron emission yield (TEEY) first increasing to reach a maximum and then decreasing along with the energy increase of the primary electrons, there is still a lack of data and detailed analysis for dielectrics, in particular thin layers. The present work proposes a new insight in the electron emission phenomenon from very thin dielectric layers. It reports on the TEEY from very thin SiO2 layers, less than 100 nm. It is found that a departure from the typical shape of the TEEY curve occurs for primary electrons with energy of around 1 keV. The TEEY curve presents a dip, a local minimum that might be as deep as below 1. This atypical shape depends substantially on the layer thickness. The measured TEEY are compared to an electron emission 1D-model in which we consider the combined effect of the space-charge electric field induced by trapped charges in the dielectric layer and of the processes of field dependent conductivity (FDC) and radiation induced conductivity (RIC) on the fate of secondary electrons. Those mechanisms govern the charge transport in the dielectric, and consequently the electron emission. The effects of the SiO2 layer thickness, incidence angle of the primary electrons and an applied external electric field on the TEEY curves are reported.

Electron emission measurements have been performed on a BN sample by using a new specific protocol and experimental setup , which allows characterizing electron emission under electron impact on resistive material in a short time and with a wide variety of extracted data: total electron emission yield, emitted electron energy distribution, elastically backscattered electron emission yield and energy efficiency of electron-surface interaction. Methodology, calibration, biases corrections and results are presented in this letter. Results are compared to that measured on another material SiO2. As there are few published data on electron emission at low incident electron energy on BN sample, it is expected that these measurements could be useful for numerous studies implying electron emission on BN surface. * * * Mr. Villemant benefits from an Onera/Cnes PhD. fellowship. Moreover, the authors are grateful to T. Gineste to allow using its calibration values and substantial experimental data for this work. This work is supported by the CNES Research and Technology Program.