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Ion-Induced Auger Emission from Solid Targets Scanning Electron Microscopy Volume 1986 Number 2 Article 3 7-16-1986 Ion-Induced Auger Emission from Solid Targets Josette Mischler Université Paul Sabatier et Institut National des Sciences Appliquées Nicole Benazeth Université Paul Sabatier et Institut National des Sciences Appliquées Follow this and additional works at: https://digitalcommons.usu.edu/electron Part of the Biology Commons Recommended Citation Mischler, Josette and Benazeth, Nicole (1986) "Ion-Induced Auger Emission from Solid Targets," Scanning Electron Microscopy: Vol. 1986 : No. 2 , Article 3. Available at: https://digitalcommons.usu.edu/electron/vol1986/iss2/3 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Electron Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. SCANNING ELECTRON MICROSCOPY /1986/11 (Pages 351-368) 0586-5581/86$1.00+0S SEM Inc., AMF O'Hare (Chicago), IL 60666-0507 USA ION-INDUCED AUGER EMISSION FROM SOLID TARGETS Josette MISCHLERand Nicole BENAZETH Laboratoire de Physique des Solides, Associe au C.N.R.S. Universite Paul Sabatier et Institut National des Sciences Appliquees 118, Route de Narbonne - 31062 TOULOUSECedex (France) (Received for publication February 28, 1986: revised paper received July 16, 1986) Abstract Introduction We present a review of the Auger emission Impact of heavy ions on surfaces gives rise induced from light elements (Mg, Al, Si) bombarded to a variety of collision events leading to ejec­ by ions of intermediate energy (1 keV - 200 keV). tion of secondary or reflected ions, sputtered The different physical phenomena at the origin of atoms, electrons and photons. In principle each Auger emissions are outlined, in particular the sort of particle carries information about the mechanism of molecular excitation responsible for surface and can lead to a method of surface analy­ the production of inner-shell vacancies in colli­ sis : secondary ion mass spectroscopy (SIMS), Ru­ sions between two complex particles and the pro­ therford backscattering spectroscopy (RBS), proton cesses of Auger decay and electron transport induced X-Ray emission (PIXE). In practice, SIMS in the solid. Auger spectra partially consist of is the most frequently used technique. A different L23VVelectrons corresponding to decays in the type of study of energy distribution of electrons bulk; this part is similar to that observed from ejected from solids by photon and electron impact electron-induced Auger spectra. Superimposed on has produced many useful spectroscopies [Auger this broad structure appear different narrow lines electron spectroscopy (AES), X Ray or U.V. photo­ due to de-excitations from excited sputtered atoms. electron spectroscopies (XPS and UPS)] for charac­ These atomic-like lines are assigned to different terization of the electronic, chemical and geome­ L23MMor ITL23)2MMJAuger transitions. The depen­ trical properties of surfaces. In this paper, we dence of the width and shift of these lines on dif­ shall discuss the present understanding of Auger ferent parameters (e.g., ionic energy, emission electron spectroscopy induced by heavy ions from polar angle) is interpreted by Doppler effect. On solid targets and investigate the information such the other hand, the experimentally determined a study can bring to ion-surface interactions. L23VVand L23MMAuger yields are compared to values Under heavy ion impact, at intermediate ener­ calculated by computer simulations of collision gies E (in the keV to hundred keV range) electron cascades (from EVOLVEand MARLOWEcodes). Lastly emission is, primarily, due to ionizing colli~ons the azimuthal and polar angular distributions of undergone by primary ions, energetic recoil atoms both L23VVand L23MMAuger electrons are analysed and fast electrons. On the energy distribu­ by taking into account the important role of ion­ tion curve these "true" secondary electrons make induced surface topography. up the intense peak centred at low energy (about 2 eV),is followed by a steadily decreasing tail. In addition, in some cases, electrons of specific energy appear on the spectrum as structures super­ imposed on the continuous energy tail. These stru~ tures are due to Auger transitions involving a two­ electron process and leading to decay of the inner­ shell vacancies created in either the target atom and/or the incident ion. This ion-induced Auger emission was observed for the first time by Snoek et al. (1965). Since 1975, several research wor­ KEYWORDS : Ion-solid interactions, Auger emission, kers took up this line of investigation. The most Ion-induced surface topography, Surface. studied elements are Mg, Al and Si which give un­ der ion bombardment L Auger spectra with unexpec­ Address for correspondence : ted features. Fig. 1 gives an example of the energy distri­ J. MISCHLER, Laboratoire de Physique bution of electrons emitted by bombarding an Al des Sol ides, Associe au C.N.R.S. - Universite target with different ions. The Al Lz3 Auger spec­ Paul Sabatier, 118 Route de Narbonne trum appears between 35 and 80 eV. With proton 31062 Toulouse Cedex bombardment, it is essentially composed of a broad FRANCE Phone N° 61.55.68.21 structure assigned to an L23VVtransition invol- 351 J. Mischler, N. Benazeth ving two electrons belonging to the valence band ions can transfer a sufficient amount of kinetic (V) which is similar to that observed in electron­ energy to the target atoms to displace them from aluminium collisions. In addition, under heavy ion their lattice site (primary knock-on atom). If impact, the spectra obtained are mainly composed the energy transferred to the primary knock-on of three narrow peaks, superimposed on the broad atom is high enough, it can displace and ionize structure ; their amplitude depends on the ion another target atom. These secondary recoiling mass. This spectrum in no way looks like that in­ atoms may in turn displace further atoms resulting duced by binary collisions of 50 keV Al+ striking in a displacement cascade. A surface atom becomes Ar gas which exhibits a great number of lines, as sputtered if the energy transferred to it has a shown in Fig. 2. component normal to the surface which is larger Because the ion-induced Auger spectra of these than the surface binding energy. light elements look like neither the spectra ob­ So, from medium energy heavy ion-solid target tained in electron (or proton) -solid target col­ interactions, asymmetric collisions between the lisions nor those induced in binary gas collisions, incident ion and the target atom, and symmetric their interpretation has been the subject of con­ collisions between two target atoms can occur. troversy. Although the narrow lines are now inter­ Moreover, but with a lower probability symmetric preted by most authors as the decay of excited collisions between an implanted ion and an inci­ sputtered atoms, an Auger model does not yet exist dent ion can also take place. which totally explains all the features of these Production of inner-shell vacancies in ion-atom Auger spectra. collisions (Kessel and Fastrup, 1973) In the next section, we will outline the dif­ Either of two extreme approximations are gene­ ferent mechanisms which intervene in the ion-indu­ rally used to deal with the creation of ced Auger spectra. Then, after a brief survey of inner-shell vacancies in ion-atom coll is ions : the the experimental techniques used for the electron direct Coulomb interaction and molecular excita­ spectroscopy studies, we will review the more re­ tion (Garcia et al., 1973). cent results obtained and the interpretations pro­ The former is valid when the projectile is a posed by the different authors. high-velocity point charge (e.g., H+) which, by direct Coulomb interaction with the electrons of Different physical processes the target may ionize an inner-shell . To solve involved in ion-induced Auger emission and this problem different models have been proposed computer simulations such as : the plane wave Born approximation (P.W. B.A.) and the Binary Encounter approximation(B.[A.). Collisions in solids The molecular excitation applies to collisions When a sufficiently high-energy particle pe­ between two complex atomic systems at a velocity netrates a crystal, it interacts with the atomic high enough to form a quasi-molecule. During this nuclei, core electrons and conduction electrons collision, the internuclear separation, R, varies (Lehmann, 1977). The projectile loses its initial from infinity to approximately zero and back to kinetic energy along its path by successive colli­ infinity. When R is large. the atoms do not inter­ sions and suffers many successive deflections from act and may be referred to as separated atoms. When its initial direction. It describes a trajectory R approaches zero, the electron shells see a sin­ which ends where the kinetic energy has dropped to gle nucleus called the united atom, the atomic practically zero and the particle comes to rest. number of which is the sum of the atomic numbers, This is true for electron, proton and heavier ion Z1 and Zz, of the colliding particles. Electronic projectiles. However, an electron has, in contrast transitions occur from an initially-filled to a heavy ion, a constant charge and a very small molecular orbital to another empty or only par­ mass. These features give rise to differences in tially-filled orbital if the approach of both nu­ the penetration behaviour. A fast electron loses clei is close enough to allow an interaction bet­ practically all of its energy in ionizing colli­ ween these two molecular orbitals. Such a process sions and only a fraction of its energy is lost has been extensively studied with gaseous targets. (e.g., 10-5 for electrons of 3 MeVin Cu) in elas­ Diabatic correlation diagrams, drawn using the one­ tic collisions with atomic nuclei. Energy loss of electron molecular orbital approximation, provide projectiles heavier than electrons is due to nu­ the variation of the energy of the one-electron clear collisions (i.e., elastic or quasi-elasTic states with the distance between the two nuclei.
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