Nuclear and Electronic Sputtering Induced by High Energy Heavy Ions

Abstract

When an energetic particle comes into a solid, collisions induce several processes such as recoil and sputtering of constituent atoms, defect formation, electron excitation and emission, and photon emission. The sputtering process, especially emission process of secondary ions, has been widely studied for various target materials under bombardment of heavy ions. Most of studies are, however, concerned with secondary ion mass spectrometry (SIMS) at nuclear-collisiondominant low energies. 1−7 In a MeV-energy range, an electronic-energy-loss process becomes dominant, 8 and the basic process of secondary ion emission is different from that in the low energy range because the electronic behavior strongly depends on the solid state property. How does the highdensity electronic excitation energy deposited by a heavy ion transfer to the atomic motion? The problem was reviewed only for frozen gases and organic molecules. 9−16 The aim of this review is to show experimental results of yields and energies of secondary ions emitted from several tightly bound metallic, semiconductive, and insulating solid targets and to summarize the dynamic mechanism of secondary ion emission in the MeV-energy range by relating it to characteristic physical properties of the target materials. 2. Energy Deposition An energetic ion incident on a solid collides with constituent atoms. Because of a large mass difference between the nucleus and the electron, the collision with the nucleus is called an elastic or a nuclear collision, where kinetic energy and momentum should be conserved during the collision. The collision with the electron is named as an inelastic or an electronic collision and results in excitation and ionization of constituent electrons in the atom. Thus, the ion passing through matter loses its kinetic energy by the electronic excitation and ionization and by the kinetic collision. The energy loss by the electron-related collision is called electronic or inelastic energy loss and the kinetic-collision induced energy loss is called nuclear or elastic energy loss. The former and the latter respectively work mainly at a high and a low velocity range. The term of stopping power is also used instead of the energy loss. The most familiar stopping power formula is the BetheBloch equation 17, 18 which is derived on the basis of the plane wave Born approximation applicable at a high velocity range. In a keV energy range, the ion velocity, especially that of a heavy ion, is very low and the nuclear collision could not be omitted. Lindhard, Scharff, and Schiott (LSS) 19 derived the universal electronic and nuclear stopping power formulae based on the Thomas-Fermi atomic model at the low velocity range. The Bethe-Bloch and the LSS formulae are underlying stopping power formulae in the high- and the low-energy range, respectively. However, obtained experimental values of stopping power depend so much on incident ion and energy, and target atom. Ziegler et al. 8 have developed a versatile program named SRIM covering stopping power, range, range straggling, damage distribution, sputtering yield, and so on and covering any incident ion-target atom combinations over an energy range between 1 keV and 2 GeV. In this review the nuclear and the electronic stopping powers are calculated with the SRIM code. Examples of the results are shown in Figure 1 for C, Si, and Ag projectiles passing through a Si target.