Reversible Ion Induced Modification of Consequent Secondary Electron Emission in Porous Silicon

Abstract

Abstract:We report measurements of secondary electron emission (SEE) induced by electron and ion bombardment on porous silicon (PS). We found that electron induced emission is strongly reduced by ion bombardment, and that this reduction is reversible. The reduction effect is large even for ion fluxes much lower compared to that of the electron beam. We attribute this effect to changes in the charge distribution of the surface dipole originated in the difference between ion and electron charge deposition depths. The nanostructure of PS plays an important role in this effect as well as in the reversibility of the process. We think that this effect could be useful in the dynamic centering and monitoring of ion and electron beams in electron spectroscopy.The discovery of the efficient luminescence of porous silicon (PS) by Canham [1] was the starting point for a wide interest and extensive studies devoted to this material. The tailoring of PS in the range of microns to nanometers confers a peculiar behavior to this material, where quantum effects play an important role. Arrays of field emitters made of ntype porous silicon show an enhanced electron emission [2]. PS diodes can operate as surface-emitting cold cathodes [3]. On the other hand, plasma generation inside the pores [4], turns PS into a base material for flat-panel field emission [5] and plasma displays. Although there are several studies about the physics of SEE dielectric films, size effects on the electronic properties of nanoporous silicon (NPSi) may introduce new and exciting peculiarities. The study of secondary electron emission (SEE) under ion bombardment is then of interest, from both basic physics and technological points of view.The penetration depth (range) of electrons in NPSi is large than in bulk crystalline Si due to the high porosity of this material. On the other hand, since the range of ions is comparable to the characteristic dimensions of the NPSi, i.e. quite smaller than electron ranges, the density that the ions see is essentially the same as that of the bulk materials. The reason is that within these structures the atoms remain in their original crystalline sites. Thus, the natural differences among electron and ion ranges, for similar kinetic energies, are increased due to the porous nature of NPSi. These differences promote the enhancement of the dipole moment generated by electron beam induced SEE near the surface [6] when the ion beam impinges on the surface. On the other hand, in the presence of potential neutralization mechanisms, like Auger effect [7], positive charge is also released at the very surface. This dipole moment reaches a stationary value as a result of the balance between near surface electron *Address correspondence to this author at the INTEC, CONICET -UNL Güemes 3450 -3000 Santa Fe, Argentina; Tel: +54 342 4559174; Fax: +54 342 4550944; E-mail: rkoro@intec.ceride.gov.ar depletion due to SEE, charge coming from the electron and ion beams, and relaxation by recombination through charge transport along the tortuou...