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2 Nuclear Instruments and Methods in Physics Research B 269 (2011) Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: Internal electron emission in metal insulator metal thin film tunnel devices bombarded with kev argon and gold-cluster projectiles Mario Marpe a, Christian Heuser a, Detlef Diesing b, Andreas Wucher a, a Fakultät für Physik, Universität Duisburg-Essen, Duisburg, Germany b Fakultät für Chemie, Universität Duisburg-Essen, Essen, Germany article info abstract Article history: Received 31 July 2010 Received in revised form 1 December 2010 Available online 17 December 2010 Keywords: Kinetic excitation Internal electron emission Kinetic electron emission Cluster bombardment Vicinage effect The kinetic excitation of a solid surface under bombardment with energetic ions is studied via internal electron emission in a metal insulator metal junction. Particular attention was focused on the dependence of the internal emission yield on the impact angle of the projectile ions, which was measured and compared to corresponding data for external emission. We find the internal and external yields to behave differently, therefore delivering complementary information regarding the depth distribution of the kinetic excitation profile. A first attempt to interpret the internal emission yield in terms of a simple electron emission model is described and shown not to be sufficient to explain the experimentally observed behavior. Besides atomic ions, measurements of the internal emission yield under bombardment with cluster projectiles were performed, which are shown to exhibit a nonlinear yield enhancement similar to that of the sputter yield. This finding constitutes a new case in the observation of vicinage effects in cluster-initiated kinetic excitation, which is attributed to the transition from a linear collision cascade to a collisional spike. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The impact of an energetic particle onto a solid surface may lead to numerous energy transfer processes within the solid. The kinetic energy of a projectile is dissipated via elastic collisions with target atoms ( nuclear stopping ), leading to a collision cascade which can result in the emission of surface particles ( sputtering ). In addition, the particle kinetics is accompanied by electronic excitation processes which occur via electronic friction of all moving particles or via electron promotion in close binary collisions. From an experimental perspective, these kinetic excitations manifest in form of ion induced electron or photon emission on one hand and electronic excitation or even ionization of sputtered particles on the other hand, both of which represent ubiquitous phenomena which are routinely observed (see, for example [1] and [2] for reviews). In particular, the external surface emission of electrons following a particle impact has been extensively studied both experimentally and theoretically, and the existing literature on this subject has been reviewed in several dedicated monographs [3 5]. Excited charge carriers produced inside the solid need to overcome the surface barrier to be emitted into the vacuum. Since the kinetic mechanisms predominantly produce low energy excitations [6], many of the hot electrons generated this way are not Corresponding author. address: andreas.wucher@uni-due.de (A. Wucher). accessible via external electron emission. Excited electrons in states between the Fermi and vacuum level are trapped within the solid and cannot be detected externally. Moreover, defect electrons ( holes ), which must of course also be produced by the excitation process, cannot be detected at all. We have therefore developed a strategy to interrogate these hot carriers by means of an internal tunnel junction which is formed by a sandwich-like structure of a two metallic films separated by a thin, insulating oxide layer. Using such a tunnel junction, low-energy electrons and holes can be detected as an internal emission current into the underlying metal substrate [7]. In addition, a bias voltage can be applied between the two metal electrodes, thus allowing to obtain information about the excitation spectrum [8,9]. In the experiments presented here, excited carriers were produced by the impact of Ar + ions with kinetic energies between 5 and 15 kev onto the top metal surface of such a metal insulator metal (MIM) device. Following our earlier experiments investigating the influence of projectile energy and charge state [9 11], particular attention has recently been devoted to the dependence of the resulting kinetic internal emission yield on the impact angle of the projectiles [12]. The present study follows this line by investigating the impact angle dependence as a function of other parameters like projectile energy and target film thickness. The philosophy behind these experiments is that a variation of the impact angle will lead to a modified excitation profile below the bombarded surface, depositing more energy closer to the surface when changing from X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi: /j.nimb

3 M. Marpe et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) normal to grazing incidence. From a comparison of the resulting external and internal emission yields, one therefore expects to gain valuable information regarding the depth dependence of the kinetic excitation profile as well as the transport properties of the hot carriers across the bombarded metal film. 2. Experiment The MIM junction was prepared as a polycrystalline silver film of 20 or 40 nm thickness stacked upon an amorphous AlO x film as an insulator of approximately nm thickness followed by an aluminum film of approximately 30 nm thickness as a substrate electrode. Details regarding the preparation, characterization and working principles of these devices [7] and the procedures to determine kinetic internal emission yields under ion bombardment have been described elsewhere [11,12] and need not be repeated here. The top metal film represents the actual target, the surface of which was bombarded with rare gas (Ar + ) or metal cluster ðau 1 ; Au 2 and Au 3 ) ions of energies between 5 and 15 kev. The impact angle with respect to the surface normal was varied between 0 (normal incidence) and 80 (grazing incidence) by tilting of the sample. In these experiments, particular care must be taken regarding the alignment of the ion beam with the active area of the MIM device. In order to minimize geometrical problems arising from the angular spread of the bombarded area at large impact angles, the lateral size of the MIM junction was increased from originally 4 mm 2 to 25 mm 2. The Ar + ion beam was generated by a commercial ion source delivering a focused beam of about 1 na current into a spot of approximately 100 lm diameter. Alternatively, negatively charged gold ions ðau 1 ; Au 2 and Au 3 Þ were generated by a home-built Cesium ion sputter gun [13] delivering an ion current of up to 1 na into a spot size of about 1 mm diameter. To clearly discern ion beam induced effects and to reduce the total ion fluence, both ion guns were used in a pulsed mode with typical pulse widths of 100 ms. The internal emission yield was determined by dividing the internal emission current (measured between both metal electrodes of the MIM junction) by the primary ion current (measured by a Faraday cup). To ensure that the dielectric properties of the device remained constant, the I V characteristics of the MIM junction were recorded before, during and after the measurements. In these experiments, the bias voltage between the two metal electrodes was constantly kept at 0 V. 3. Results and discussion internal emission yield kev 05 kev 07 kev 10 kev 15 kev Upon variation of the projectile impact angle, the depth distribution of the excitation profile generated via the particle kinetics initiated by the projectile impact is expected to change. As a consequence, one would expect that hot carriers will be produced at varying average depth, the excited sub-surface volume becoming shallower with increasing impact angle. As a result, more energy will be deposited at or close to the surface, which is expected to lead to an enhancement of the external electron emission for oblique incidence, until at grazing angles the projectile will not penetrate the solid anymore and the emission yield will therefore decrease again. On the other hand, excitation produced near the surface must in some way be transported to the buried metal oxide interface in order to contribute to internal emission. Therefore, one would expect the internal emission yield to decrease with increasing impact angle, since excited electrons and holes must travel a longer distance to reach the buried tunnel junction. The internal emission yields measured for keV Ar + impact onto a MIM device with a top layer thickness of 40 nm are shown in Fig. 1. At first, it is obvious that the total yield value drastically depends on the impact energy, a finding which has been observed before [11] and represents the signature of kinetic excitation. Moreover, it is evident that the expected overall decrease of the internal emission yield with increasing impact angle is indeed observed. In accord with the above reasoning, this is in marked contrast to the impact angle dependence observed for external emission. As an example, the lower panel of Fig. 2 shows electron emission data which has been reproduced from Ref. [14]. It is seen that external emission yields strongly rise with increasing impact angle. As a consequence, we conclude that external and internal emission yields probe different features of the electronic current(na) time (s) impact angle ( degrees ) Fig. 1. Internal emission yield measured under Ar + bombardment of a 40-nm silver film vs. impact angle of projectile ions. The inset shows the measured signal response for 4-keV bombardment under grazing impact conditions (h = 80 ), where the blue and red lines correspond to measurements with the ion beam blocked. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) internal emission yield external emission yield kev 7 kev 5 kev 7 kev 10 kev 15 kev calculated 10 kev 30 kev 50 kev Ar Ag external impact angle ( degrees) internal Fig. 2. Impact angle dependence of internal (upper panel) and external (lower panel) emission yield normalized to the respective value measured at normal incidence. Open and closed symbols in the upper panel refer to data measured for a 40-nm and 20-nm silver film, respectively. The data for external emission were reproduced from Ref. [14]. Ar + Au

4 974 M. Marpe et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) excitation profile, thereby providing complementary information regarding the kinetic excitation process. In order to compare the impact angle dependence measured for different energies, the yield data are normalized to the respective yield measured under normal incidence and plotted in the upper panel of Fig. 2. In order to investigate the influence of the top metal film thickness, data acquired for 20 and 40 nm have been included in the plot. Probably the most striking observation is that the measured impact angle dependence appears to be largely independent of the impact energy. This finding is important since it indicates that the observed dependence must be generated by a geometrical effect rather than by the differences in projectile range. Moreover, for angles up to 40 with respect to the surface normal, the emission yield seems to be independent of the impact geometry, a finding which is remarkable because the penetration depth of the ions (calculated with SRIM 2006 [15]) decreases by about 30% in this interval. Again, this suggests a geometric cause for the observed angle dependence. For higher impact angles, the yield is found to decrease rapidly. This decay appears to be independent of the projectile energy and of the film thickness. Therefore, we conclude that the decay cannot be simply determined by the penetration depth of the projectile (and, in particular, the probability of a projectile to penetrate all the way to the oxide interface), since in that case one would expect a pronounced difference for different film thickness and impact energies. Differences are found only under grazing incidence conditions, where projectile penetration becomes insignificant. Under these conditions, some of the curves displayed in Fig. 2 appear to level off at angles above 60, while others do not show this behavior. A detailed analysis reveals that these apparent differences are due to the normalization procedure. In fact, it appears that the impact angle dependence is superimposed by a constant background signal, which is independent of impact energy and under grazing incidence conditions becomes independent of the impact angle as well. As shown in the inset of Fig. 1, this background is clearly not due to statistical noise, but instead represents a well resolved internal emission current, which corresponds to an internal emission yield of c 0 int 0:05. This signal disappears if the ion beam is blocked by either closing the internal shutter or detuning the mass filter of the ion gun (blue and red traces in the inset of Fig. 1, respectively). While the shutter blocks all particles in the beam, the Wien filter just deflects the ions and would let fast neutral Ar atoms pass. Therefore, this current must clearly be induced by the impact of Ar + ions onto the surface. A possible explanation for such a constant signal would be a potential contribution to the internal emission current which is generated by the ionization energy of the impinging ions. However, Kovacs et al. [16] have recently shown that such a contribution, which is readily observed for external emission (where it interestingly is of the same order as the value of c 0 int observed here [16]), is practically absent in the internal emission yield measured under impact of low energy Ar + ions onto a MIM device identical to the ones employed here. Under normal incidence, they observed an internal yield clearly below 10 2 for 300 ev impact energy and argue that this is due to the low transport probability of Auger electrons generated by surface neutralization across the top metal film. Based on these data, we must therefore rule out potential emission as the cause for c 0 int observed here. The essential difference between the experiments of Ref. [16] and those performed here is that the ions impinging under grazing incidence spend a relatively long time at or very close to the surface, although their total velocity is relatively high. As a consequence, one may speculate that a kinetic excitation mechanism must exist which is restricted to the very surface and therefore only operational under grazing incidence conditions. In principle, such a mechanism could involve energy transfer into surface states or surface plasmons, which then decay to produce hot electrons propagating to the tunneling junction. Clearly, more data are needed to clarify this interesting phenomenon. In order to illustrate the fundamental difference between external and internal emission yields, the impact angle dependence measured here is compared with the external yield data measured by Ferron et al. [14]. These data correspond to emission yields measured under Ar + impact onto a gold surface, but were chosen as there are no data for silver available, while the observed trend is the same for a copper target as well [14]. It is seen that the external yield also shows a strong impact angle dependence, which is similar for all projectile energies but fundamentally different from that observed for internal emission. Similar to our observation for the internal yield, the external yield also stays quite constant for angles up to 40 but then increases with increasing impact angle. According to the reasoning above, this different behavior is in principle expected. For a qualitative analysis, one can try to apply the model proposed in Ref. [14] to calculate the external emission yield c ext ¼ C Z Rmax 0 NðRÞ exp z dr; L to a description of the internal emission yield as well. In Eq. (1), z is the coordinate normal to the surface, R is the path traversed by the ion, N(R) is the number of excited electrons produced by the ion along its trajectory path interval dr, L is the mean electron attenuation length, C a target dependent constant and R max is the penetration range up to a point where the ion has lost too much energy to excite electrons with enough energy to be emitted. For internal emission, hot electrons produced at depth z must now travel a vertical distance d z instead of z (where d is the thickness of the film) to reach the buried tunnel junction, where they must overcome an energy barrier / tunnel instead of the work function. The mean electron attenuation length L can be taken from the dependence of measured internal emission yields as a function of the top metal film thickness, resulting in values of nm [9]. The quantity N(R) can be estimated by integrating the electronic stopping power calculated, for instance, with SRIM In principle, one can calculate the depth distribution of the deposited excitation energy and convert the integral over R into one over depth z. An example of the resulting excitation distribution obtained for two impact angles is shown in Fig. 3, with the contribution of projectile and recoil stopping being plotted separately. Although the SRIM calculation de/dz electronic (ev/nm ) kev Ar + Ag calculated with SRIM penetration depth z ( nm ) 0 projectile stopping recoil stopping Fig. 3. Depth distribution of electronic energy loss calculated for 10 kev Ar + impact onto a silver surface for different impact angles. The data were calculated using the SRIM 2006 package. Solid and dotted lines: energy deposited by projectile and recoil stopping, respectively. ð1þ

5 M. Marpe et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) neglects collisional excitations by electron promotion processes and must therefore be regarded to be of only qualitative nature, it is clearly seen that the energy deposition is centered closer to the surface under oblique incidence conditions. While the excitation energy density produced under normal incidence is approximately constant up to a depth of about 5 nm below the surface and gradually decreases for larger depths, the excitation profile generated under oblique incidence is strongly peaked at the surface and falls much more rapidly with increasing depth. Moreover, it is seen that recoils produced by nuclear stopping significantly contribute to the excitation profile, since they experience inelastic energy losses in the same way as the projectile. In fact, if the curves displayed in Fig. 3 are integrated, one finds that about 36% of the kinetic impact energy is converted into electronic excitation, although nuclear stopping clearly dominates the projectile energy dissipation at the bombarding conditions applied here [17]. The sum of both contributions displayed in Fig. 3 constitutes a total energy deposition profile de=dz, which can as a first approximation be assumed to be proportional to the quantity N(R) in Eq. (1), yielding Z d de c int ¼ C 0 dz exp d z dz k eff The impact angle dependence of the internal electron yield calculated using Eq. (2) for a top layer thickness of d = 20 nm and for k eff = 10 nm is included as a dotted line in Fig. 2 (upper panel). It is seen that this simple model correctly predicts the fact that the internal yield does not depend on the impact geometry for angles up to 40 with respect to the surface normal and falls for larger impact angles. However, the calculated angle dependence is much weaker than that observed experimentally, demonstrating that this model is clearly oversimplified. In order to explain the experimentally observed behavior, a more sophisticated treatment of both the kinetic excitation processes and the transport of excitation across the top metal film is needed. In principle, one could follow the strategy of Roesler and Brauer [18,19], who used a set of differential particle electron and electron electron collision cross sections along with the Boltzmann transport equation to treat the excitation and transport [3,18,19]. Alternatively, one could use Monte Carlo schemes to follow the generation and ballistic transport of each excited electron separately. Calculations of this type have been successfully performed to predict surface emission yields, [3,20], but are still lacking for a description of internal emission phenomena. Kovacs et al. [16] have recently published a study where the production of hot carriers is treated in terms of a hot spot model, followed by a ballistic treatment of the transport across the metal film in terms of a Monte Carlo approach and an over-the-barrier model describing the transmission through the tunneling junction. One of the important conclusions of this work is that the spatial inhomogeneity of the electron excitation spectrum must be taken into account in order to understand the ratio between internal and external emission yields obtained under normal incidence. In fact, electronic friction, involving direct scattering of conduction band electrons with the moving particle, must lead to a pronounced anisotropy since the largest possible energy transfer occurs in a head-on collision with an electron moving with the Fermi velocity in opposite direction as the moving particle. Therefore, the kinetically induced excitation spectrum will be peaked in the direction of the moving particle, the effect being the more pronounced the higher the excitation energy. As a consequence, electrons with the highest excitation energies will predominantly propagate along the direction of the projectile impact, thereby greatly increasing their average travel distance to the buried tunnel junction. We presume that this is the reason for the more ð2þ strongly decreasing internal emission yield with increasing impact angle. A way to decrease the penetration depth of the projectiles while keeping their kinetic impact energy constant is to change their nuclearity, i.e., use cluster projectiles instead of atomic ions. The philosophy behind this approach is that upon impact, a cluster projectile breaks up into its constituent atoms, each of which now has only a fraction of the total impact energy and therefore penetrates less deep into the solid. In that context, the question arises as to whether the yield produced by the impact of such a cluster exhibits nonlinear effects, i.e., is different from the sum of the yields produced by each of its constituent atoms impinging independently with the same impact velocity. In order to examine for such effects, one should therefore compare yield data measured under conditions where kinetic energy per constituent atom should be kept constant. In case of our experiments, we measured the internal emission yield of MIM junctions with a 40 nm thick silver top layer bombarded with single negatively charged Au projectiles. The kinetic energy was chosen as 5 kev for the atomic ion Au 1, 10 kev for the gold dimer Au 2 and 15 kev for the gold trimer Au 3. Because of geometrical limitations in the experimental setup the angle of incidence was kept fixed at 45. Fig. 4 depicts the resulting internal emission yield data. Comparison with a linear prediction (indicated by the straight line) reveals that the yields produced by these clusters clearly exhibit a nonlinear yield enhancement, i.e., the yield observed for Au n is larger than n times that measured for Au. Interestingly, the enhancement is of similar magnitude as that observed for the total sputtering yield [21]. Vicinage effects of this type have often been observed for cluster bombardment induced kinetic electron emission [22 25], albeit mostly at much higher impact energies where the process is dominated by electronic stopping of the projectile constituents. In most cases, a sublinear effect was observed where the electron yield induced by a cluster projectile is smaller than that generated by the cluster constituents impinging independently. At first sight, the overlinear enhancement observed here therefore appears unusual, but one should keep in mind that our experiment was performed at much lower impact energy, where the energy dissipation is dominated by nuclear collision dynamics. Based on the similarity of the effects observed for sputtering and electron emission, we do not believe that the vicinage effect depicted in Fig. 4 is caused by a modification of the electronic stopping power as discussed, for instance, by Nagy internal emsission yield kev Au kev Au projectile nuclearity n 15 kev Au - 3 Fig. 4. Internal emission yield under bombardment with Au n projectile ions of 5 kev/atom impinging under 45 with respect to the surface normal. The data were normalized to the yield measured under Au bombardment. 3

6 976 M. Marpe et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) et al. [26]. Instead, the nature of the collision dynamics changes from a linear collision cascade (with only few moving atoms) to a collisional spike with a high density of moving atoms [21]. In addition, most of the kinetic excitation is generated by target recoil motion rather than by the projectile constituents themselves. As a consequence, more electronic excitation and, hence, more internal electron emission is generated under cluster bombardment. 4. Conclusion The kinetic internal emission observed in MIM tunnel junctions under bombardment with energetic particles shows an interesting dependence on the impact angle. For impact angles up to 40 with respect to the surface normal, the yield is mostly independent of the impact geometry, whereas it decreases rapidly for higher impact angle and drops to about 10% of the value for normal incidence at impact angles around 80. This behavior is exactly opposite to that observed for the external emission yield, thereby proving the expectation that external and internal yields probe different features of the kinetic excitation profile. Attempts to interpret this behavior using a modification of the simple model proposed in Ref. [14] to describe external electron emission in combination with SRIM 2006 calculations reproduce the qualitative trends of the experimental data, but are clearly not sufficient to accurately calculate the internal electron yield. For a quantitative explanation of the experimental results it is therefore obvious that a more sophisticated theoretical model is needed, which takes into account the local and temporal excitation profile as well as the transport of excitation to the buried tunnel junction. As mentioned above, such calculations are possible but certainly beyond the scope of this paper. The results obtained under gold cluster bombardment show a nonlinear enhancement of internal electron emission yields in dependence on the projectile cluster size, i.e., the yield observed for a correlated impact of several atoms is larger than that observed for these impacts occurring independently. The observed effect is similar to that measured for the total sputter yield. We attribute this finding to a change of the collision dynamics from a linear collision cascade under atomic bombardment to a collisional spike under cluster bombardment. The preliminary results reported here therefore constitute a new case in the observation of vicinage effects in kinetic excitation induced by slow heavy cluster ions. A more detailed investigation looking at the impact angle dependence of the measured yields is currently under way and will be the subject of a forthcoming publication. Acknowledgement The authors are greatly indebted to the Deutsche Forschungsgemeinschaft for financial support in the frame of the Sonderforschungsbereich 616 Energy Dissipation at Surfaces. Intensive discussions with D. Kovacs are gratefully acknowledged. References [1] R.A. Baragiola, Nucl. Instrum. Methods B 78 (1993) 223. [2] M.L. Yu, in: R. Behrisch, K. Wittmaack (Eds.), Sputtering by Particle Bombardment III, Springer, Berlin, 1991, p. 91. [3] M. Roesler, W. Brauer, J. DeVooght, J.C. Dehaes, A. Dubus, M. Cailler, J.-P. Ganachaud, Particle Induced Electron Emission I, Springer, [4] D. Hasselkamp, H. Rothard, K.-O. Groeneveld, J. Kemmler, P. Varga, H. Winter, Particle Induced Electron Emission II, Springer, [5] H.P. Winter, J. Burgdoerfer, Slow Heavy-Particle Induced Electron Emission from Solid Surfaces, Springer, [6] Z. Sroubek, Appl. Phys. Lett. 45 (1984) 849. [7] D. Diesing, G. Kritzler, M. Stermann, D. Nolting, A. Otto, J. Solid State Electrochem. 7 (2003) 389. [8] D. 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