Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions

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Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions M. Vana, F. Aumayr, P. Varga, HP. Winter Institut fir Allgemeine Physik, Technische Uniuersitiit Wien, Wiedner Hauptstrape

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&IO, A-1040 Wien, Austria

Abstract Total electron yields have been determined from electron emission statistics measured for impact of H+, Nq+ (q = 1, 5, 6) and Arq+ (q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride. Ion impact energies have been varied from almost zero up to 10 X q keV. The obtained total electron yields deviate considerably from available data derived via ion- and electron current measurements for LiF single crystal targets. Our results are explained by comparison with a recent model for MCI induced potential electron emission from clean metal surfaces, which has been properly adapted, available theory for kinetic electron emission from alkalihalide surfaces, and by considering also measured secondary electron yields for LiF. Dependences of the electron emission statistics and -yields on projectile impact energy and -charge differ strongly from corresponding properties for clean metal surfaces, which can be explained from the different roles of potential- and kinetic emission and, in particular, a relatively stronger contribution from secondary electron emission induced by fast electrons from finally neutralising projectiles inside the LiF bulk.

1. Introduction For bombardment of insulator surfaces with slow multicharged ions (MCI) Zq+, at given impact energy E,, strong increase with the ion charge state q has been observed for the total emission yields of both electrons [l-3] and secondary ions (SI) [3]. Furthermore, microscopic surface damages induced on alkalihalide crystals (AHC) by slow MCI impact also increase with q [4]. Considering these observations, Parilis et al. have proposed the so-called “Coulomb explosion” (CE) mechanism [5] as an efficient means for MCI induced ejection of particles from insulator surfaces, following an assumed strong electron depletion in the uppermost surface layer due to multi-electron capture by the approaching MCI. This CE mechanism should manifest itself in total sputtering yields increasing with the MCI charge. However, no such behaviour could be found for atomically clean Si(100) bombarded by Arqf (q 5 9, E, = 20 keV) ions [6]. On the other hand, impact of slow highly charged ions (up to on KA13Sis0,,(0H), (mica) Th74+ ’ -% = 2 keV/amu) produced larger microscopic damages for higher q [7], and slow, doubly charged noble gas ions (q = 1, 2) caused larger SI yields for atomically clean LiF than singly charged

* Corresponding author, tel. +43 1 58801 5710, fax +43 5864203, E-mail: [email protected].

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ones [8]. In summary, MCI impact on AHC surfaces gives rise to strongly q-dependent yields for emission of neutral and/or ionized target particles as well as electrons. At very low ion impact energies, where kinetic electron emission (KE) can still be neglected, for any solid surface the potential electron emission (PE) yield strongly increases with q, because of its direct relation with the total potential energy carried by the MCI toward the surface [9]. Referring to the well established process of electron stimulated desorption from AHC surfaces [lo], we speculate that slow MCI induced electron emission from AI-K might be correlated, either directly or indirectly, with the not yet well understood processes responsible for the above-mentioned q-dependent MCI induced sputtering and SI emission. We have therefore investigated electron emission induced by slow MCI on clean LiF, for which target already numerous studies on photon-, electron- and ion stimulated sputtering and desorption have been carried out [lo]. However, absolute yields for electron emission induced by singly charged ions from AHC surfaces are rather scarce [11,12], and practically no such data are available for impact of slow MCI, apart from the already mentioned ones [1,3], which have not been obtained under ultahigh vacuum (UHV) conditions (background pressure above lop6 Pa). However, since PE processes critically depend on the target surface conditions, they need to be studied under UHV conditions. For reproducible yield measurements, it must also be taken into account that insulator surfaces can charge up under ion impact 1121. For

0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00814-0

M. Vana et al. /Nucl. Instr. and Meth. in Phys. Res. B 100 (1995) 284-289

our present studies, we have applied a special technique for determination of particle induced total electron yields, which both assured appropriate experimental conditions and delivered various useful informations on the electron emission processes of interest.

2. Experiment The LiF target has been prepared as a polycrystalline 50 pg/cm2 film (thickness about 200 nm), by deposition of LiF under high vacuum conditions on a stainless steel backing, and transferring the latter into a setup for measuring particle-induced electron emission statistics (ES) as described in Ref. [9]. For these measurements, the LiF target was kept at room temperature and regularly cleaned by Ar+ sputtering (ion energy and flux typically 1 keV and 10’4/cm2s, respectively) between subsequent runs with the projectile ions of actual interest. The latter ion fluxes were kept below 103/s, which both was necessary for the proper working of our ES technique and avoided any target charging-up effects, which otherwise could only be achieved by target heating up to 400°C. As projectile species, H+, Nqt (4 = 1, 5, 6) or Arqf (q = 1, 3, 6, 9) ions have been delivered from our 5 GHz ECR source [13]. Impact energy E, on the target surface could be varied from almost zero up to 10 X q keV. Absolute total electron yields y were derived from the measured ES as described in our earlier work for a clean Au surface [9]. The target could also be bombarded by electrons extracted from the sputter gun for target cleaning, after reversing its bias voltage. Total electron yields obtained from ES measurements with atomically clean metal surfaces have been shown to be accurate within f 4% 191. However, in the present study we observed some unusually large data scattering from different experimental runs under apparently equal conditions. This resulted mainly from the limited q/M resolution in primary MCI beam analysis, where selected MCI beams were somewhat contaminated by small amounts of other ion species with nearly equal q/M. Especially at higher impact energies the ES measurements for LiF could be disturbed from overlapping ES for the admixed ion species with similar q/M. This effect is more severe than in measurements with clean Au [14], since the ES for LiF are generally broader (see below). Furthermore, preparation of the LiF surface by Ar+ sputtering without subsequent thermal annealing created radiation damages with a slightly variable surface- and bulk exciton population [lo]. This could have caused slightly different ES for measurements under otherwise equal experimental conditions, since more PE could be produced from the interaction of MCI with exciton states than with the much deeper LiF valence band states. Because of these reasons, the here presented total electron yields for LiF are accurate within typically

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f 10% only, which still is sufficiently precise for our conclusions to be presented. We like to point out that, in comparison with the common way of measuring total electron yields from currents of projectile ions and emitted electrons, our here applied technique not only delivers total electron yields with comparable accuracy (see above), but also delivers the respective electron emission statistics, which themselves are quite useful for investigating the basic mechanisms for the observed electron emission. Furthermore, the ES technique is much more sensitive and thus avoids excessive radiation damage and charging-up of the target surface. It also rules out erroneous total electron yield measurements due to negative ion desorption [8], because pulses from our surface barrier detector following the impact of fast negative ions are much broader than the ones from impact of electrons with the same kinetic energy.

3. Results Fig. 1 shows total electron yields y versus ion energy E, for impact of Ar@ (q = 1, 3, 6, 9) on LiF over the full impact energy range covered by our present measurements. Fig. 2 presents ES measured for Arg+ impact with E, = 0.9, 13.5 and 63 keV, respectively. In contrast to the symmetric, Gaussian-shaped ES resulting from MCI impact on clean Au [9,15], at higher projectile energies the here presented ES become asymmetric with a tail toward higher emission multiplicities. This could have been caused by a superposition of several ES distributions, resulting from electron emission from target regions with different work functions W+. Variation of the projectile flux within a factor of 30 showed, that the asymmetric ES were not caused by pile-up effects from the ES detector and its subsequent electronics, as has been

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tions as in the present experiments, the formation of metallic lithium on the LiF surface could be ruled out. Therefore, the observed ES asymmetry could not have resulted from Li island formation, which is observed for electron impact on AHC at room temperature 117,181. We thus concluded, that the observed asymmetric ES structure is typical for the particular target material and can be explained in. a similar way as for impact of singly charged ions on metal oxide surfaces [19], see also discussion in Section 4. Fig. 3 shows total electron yields for impact of H+, Ar+ and electrons on LiF, respectively, versus projectile energy E,. Note that our present measurements for electron impact have been carried out with such low electron fluxes (typically less than 103/s), that no formation of a lithium metal film on the LiF surface (see above) could take place during data accumulation. Also plotted in Fig. 3 were total electron yields for impact of the same projectile species on clean polycrystalline gold (for H+ and Ar+ from Ref. [20], and for electrons from Ref. [21]). In Fig. 4, measured total electron yields y have been plotted versus projectile energy Er, for impact of Nq+ (q = 1, 5, 6) on LiF and Au [22].

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4. Discussion Projected ion ranges have been estimated (G. Betz, private communication) for impact of H+, N+ and Ar+, respectively, on polycrystalline LF, by applying the computer codes MARLOWE [23] and TRIM [24]. The polycrystalline LiF target was simulated, respectively, by an appropriately scaled NaCl structure, screened Coulomb potentials for the Li- and F atoms, and random projectile impact directions (MARLOWE), and by “amorphous” LiF (TRIM). Projected ranges for 500 eV impact energy are 12, 2 and 1.5 nm, respectively, and increase approximately linearly with E, toward higher impact energy.

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Consequently, at least up to E, = 20 keV for both Nq+ and Ar qf the projected ranges are much smaller than the thickness of our LiF film target (about 200 nm, cf. Section 2). Our present total electron yields may be compared with data from earlier investigations for impact of 5-25 keV Ar+ [ll], 0.1-l keV Krqf (q 5 5) [l] and 2 50 eV MCI from metal species (q I 8) [3], respectively, on heated (above 300°C) monocrystalline LF targets. All these data have been derived from measured projectile ion- and emitted electron currents, while no corresponding ES data are known. In general, the published total electron yields are smaller than ours by more than a factor of 2, which might be caused by different experimental situations in the earlier studies, which involved other target composition and vacuum conditions and, in particular, much higher projectile currents. Consequently, a more detailed comparison with the earlier published data is not very meaningful. In Ref. [9], our present understanding of electron emission from a clean metal surface under impact of slow MCI has been described within a scenario basically consisting of three steps. During step #l the MCI approaches the surface and rapidly captures electrons from the latter via resonance neutralisation (RN), in which way it is turned into a so-called “hollow atom”, i.e. a neutral, multiply excited particle. This hollow atom looses electrons both into vacuum due to auto-ionisation (AI) and into the solid via resonance ionisation (RI), but these electron losses are quickly replaced by the ongoing RN, until surface impact takes place. During the following step #2, all highly excited electrons carried by the hollow atom into the surface are “peeled off” (PO) due to screening. In the subsequent step #3 the particle moves further into the solid and becomes reneutralized, after which it will relax into its neutral ground state via different transitions, among which ejection of fast projectile Auger electrons in the course of inner shell neutralisation is especially noteworthy. These fast electrons may either escape into vacuum or induce slow secondary electrons inside the solid, which can contribute to the total electron yield. Depending on the MCI species and its charge state and impact energy, and on the type and surface conditions of the solid, the various above mentioned contributions to the total electron yield may greatly differ [9]. Referring now to our Fig. 4, we observe that in comparison with MCI induced electron emission from clean gold (I+$ = 5.1 eV), for LiF the PE contribution to the total electron yield dominates only at considerably lower ion impact energy over the respective KE contributions, and is also comparably less important. This clearly different role can be explained by a lower threshold and a higher importance for the concomitant KE processes (see below), and by the much higher effective work function (W+ = 12 eV, [8,17,25]) of LiF. For impact of singly charged ions, the electron emission apparently starts at E, = 100 eV, without a clearly

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defined threshold. It increases first gently and then rather steeply up to about 10 keV, above which it gradually levels off. As shown in detail in Ref. [2], this behaviour results from conditions relevant for KE from AI-X, in particular the projectile energy dependence of electron production via quasi-molecular autoionisation in close collisions of the projectiles with the negative halogen ions, and the rather large mean free paths for electrons diffusing through the AHC (typically 5-10 nm [2,26]). Saturation of KE at high impact energy results from an increasingly deeper projectile penetration into the target bulk, which causes a correspondingly stronger attenuation of electrons to be ejected into vacuum. In contrast to KE from AEIC, for a clean metal surface principally different collision processes contribute to KE. In particular, the threshold impact energy for dominant KE processes in a metal is much higher (e.g. E, = 1 keV for Ar+ on Au [20]), and KE yields are considerably smaller (for Ar+ on Au cf. Fig. 3, and for N+ on Au y = 3 at 30 keV [27]). The particular KE mechanisms within the AHC bulk are also responsible for the fact that, at a given E,, this KE becomes less efficient with higher primary ion charge [2]. As already shown for MCI impact on NaCl [3], “crossings ’ ’ of y versus E, curves related to the different ion charge states can be observed in our measurements, see curves for N+ and N5+ in Fig. 4, and curves for Arf and Ar3+, and for Ar3+ and Ar6+ in Fig. 1, respectively. Whereas for q = 1 projectiles practically no PE arises from LiF, at high E, for q = 1 the more efficient KE compensates for the PE contributions induced by multiply charged ions. The change of PE yields when going from N5+ to N6+ (cf. Fig. 4) is considerably larger for LiF than Au. As explained in Ref. [29], for impact of N6+ inside the target bulk one projectile K-shell vacancy gives rise, via respective Auger transitions, to production of comparably fast electrons (E, = 350 eV). Fig. 3 shows that for the electron impact energy range of 100-500 eV, secondary (i.e. electron-induced) electron emission yields are typically by a factor of 3 larger for LiF than for Au. Consequently, secondary emission induced by fast electrons inside the AHC is more efficient than inside a metal target, from which fact the above noted, relatively larger change in the PE yields for LiF can be explained. A similar, however less pronounced behaviour is apparent for Arq+ when changing q from 8 to 9, since then one L-shell vacancy can give rise to emission of fast (E, = 200 eV) Auger electrons [28]. This change in PE yields is probably less pronounced due to the influence of the above-surface electron emission processes, which follow the principally different surface-densities-of-states (S-DOS) for Au and LiF, respectively. In summary, the largely different work functions and S-DOS make MCI induced “above-surface” PE contributions for LiF less important than for Au, whereas the subsequent “below-surface” PE- and KE contributions

M. Vana et al. / Nucl. Insrr. and Meth. in Phys. Rex B 100 (1995) 284-1189

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y (e’iion) Fig. 5. Full widths at half maximum (FWHM) of measured electron emission statistics versus resulting total electron yields y, for impact of Nqt and Arq+ (q = 1, 6) on LiF (data points connected by thin lines), and for impact of various slow multicharged ions on clean gold (thick solid curve), respectively (for further details cf. text).

behave just in the opposite way, because of the considerably larger electron mean free paths in the LiF bulk, and also the much larger secondary electron yields. However. in a more detailed treatment we should consider all processes relevant for electron production before, at and after the MCI impact on the LiF surface, just in the same way as for a clean Au surface 191. Such a refined treatment might show, e.g., that the MCI induced above-surface electron production from LiF is changed due to blocking of resonance ionsation (RI) of the intermediately formed “ hollow atoms” in front of the surface, since the LiF band gap constitutes forbidden final states for RI. At last, with reference to Fig. 2 we shortly comment on the observed electron emission statistics (ES). For impact of Ar9’, at lower impact energies the PE contribution is dominant or still important, and we observe relatively narrow, Gaussian shaped (i.e. symmetric) ES. However, for higher E, (i.e. dominant KE) considerably broader and clearly asymmetric ES are produced. At a given total electron yield, in comparison with ES for MCI impact on clean Au [ls], for LiF the FWHM of ES is generally larger. Fig. 5 compares the FWHM of ES measured for Au and LiF, respectively, for given total yields y, which shows that the FWHM for Au is smaller than for LiF, and that for LiF the ES is also broader for singly than for multiply charged ions. From this observation we conclude that at given values of -y the ES will be the broader, the less PE contributes to the respective total electron emission. Recent investigations [ 19,29,30] have explained the formation of asymmetric ES from a larger projected range of ions and larger mean free paths of electrons in the LiF bulk, which both favour contributions from backscattered projectiles and recoil particles to primary electron production inside the solid, and the generation of secondary

We have presented total electron yields derived from electron emission statistics measured for impact of singly and multiply charged ions on clean polycrystalline LiF. Ion impact energies have been varied from the exclusive potential emission (PE) regime up to the regime for dominant kinetic emission (KE). The applied experimental technique is almost ideally suited for PE and KE studies with insulator target surfaces. Our results on KE can be explained with an existing theory for KE from alkali halide crystal surfaces. At high impact energy asymmetric electron emission statistics were found, in agreement with published results for singly charged ion impact on metal oxide surfaces. In order to explain the observed PE, we have adapted our earlier developed model for slow MCI induced electron emission from clean metal surfaces, by taking into account characteristic differences in total electron emission above, at and below the target surface, respectively. As a particularly important deviation from metallic targets, for LiF and probably other alkalihalides the role of subsurface secondary electron emission is much more pronounced, which might be of interest for the not yet well understood MCI-induced sputtering and secondary ion emission processes from alkalihalide crystals.

Acknowledgements This work has been supported by Austrian Fonds zur Fiirderung der wissenschaftlichen Forschung and by Kommission zur Koordination der Kernfusionsforschung at the Austrian Academy of Sciences. The authors acknowledge very useful contributions to different parts of this work from G. Betz and C. Lemell.

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