Applied Surface Science 257 (2011) 4593–4596 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Effect of the incident electron fluence on the electron emission yield of polycrystalline Al 2 O 3 M. Belhaj ∗ , Th. Tondu, V. Inguimbert ONERA/DESP 2, Avenue Edouard Belin, 31400 Toulouse Cedex, France article info Article history: Received 21 September 2010 Received in revised form 13 December 2010 Accepted 14 December 2010 Available online 21 December 2010 Keywords: Secondary electrons Electron emission yield Insulators Electron impact Charging Kelvin probe Al2O3 abstract The electron emission yield due to electron impact on polycrystalline Al 2 O 3 is measured with a technique based on the use of a Kelvin probe (KP method) and a pulsed electron beam. The KP method allows the clear discrimination between the external effects of charging and internal ones. The effect of the incident electron fluence on the yield in the region where the yield is higher than one is investigated. An overall drop of the electron emission yield with increasing the electron fluence is observed. This result is clearly associated to the internal effects of positive charging. Indeed, the recombination of the generated secondary electrons with the accumulated holes beneath the irradiated surface leads to the decrease of their mean free path and to the decay of the secondary electron emission yield. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The electron emission of insulator materials induced by electron irradiation plays a major role in many applications such as in scan- ning electron microscopy (SEM) [1–3], Hall Thruster technology [4,5], charging of satellites submitted to radiations [6,7], radiation detectors [8,9], etc. In most of these applications, the knowledge of “the” electron emission yield (EEY or ) of the material is highly required. The EEY is defined as the ratio of emitted electron num- ber to the incident electron number. The emitted electrons are low energy secondary (few eV) and backscattered electrons (SE and BSE). In insulators, the measurement of the EEY is made more diffi- cult as the sample charges under irradiation. To minimize charging or to maintain it at acceptable level, short electron pulses were gen- erally used [10,11]. Both, internal and external effects of charging may lead to substantial change on the EEY curves. A comprehen- sive description of the influence of charging on the EEY is given by Cazaux [12–14]. The internal effects interfere with the transport of SEs undergoing emission [12,14–22] whereas external ones con- cern the effects of the electric field produced by the trapped charge into the vacuum on the incident electrons and on the emitted ones [1–3,12–14,21]. For a better understanding of the effects of charg- ∗ Corresponding author. Tel.: +33 0 5 62 25 27 44; fax: +33 0 5 62 25 25 69. E-mail address: Mohamed.Belhaj@onera.fr (M. Belhaj). ing on the yield, it is interesting to make a clear distinction between these two effects. For this purpose, an EEY electron pulse measure- ment method based on the use of a Kelvin probe (KP) was developed recently [23]. The ability of this method to discriminate between the internal effects of charging and the externals ones was demon- strated on MgO [22]. In this paper, the internal effects of charging on the EEY of polycrystalline Al 2 O 3 are investigated. In particular, the focus is put on the influence of the incident charge fluence on the EEY curves in the 50–2000 eV incident electron energy range. 2. Experimental setup and methods 2.1. Sample The studied sample is disk-shaped polycrystalline Al 2 O 3 (2 mm thick and 20 mm in diameter). For outgassing purpose, the sample was kept under vacuum during 24 h before beginning the experi- ments. 2.2. Experimental setup A schematic diagram of the experimental arrangement is shown in Fig. 1. Cryogenic pump associated to oil-free molecular- diaphragm pumps allows the system to be maintained at vacuum level below 5 × 10 -7 Torr. The sample is mounted in a holder which can be positioned so that the electron beam strikes the entire 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.081