Ion-beam induced generation of Cu adatoms on Cu(100)

Surface Science Letters 278 (1992) L110-L114 North-Holland

surface s c i e n c e letters

Surface Science Letters

Ion-beam induced generation of Cu adatoms on Cu(100) M. B r e e m a n a n d D.O. B o e r m a Nuclear Solid State Physics, Materials Science Centre, Groningen University, Nijenborgh 4, 9747 AG Groningen, Netherlands Received 8 July 1992; accepted for publication 17 August 1992

Low-energy ion scattering was used to study on-beam induced adatom generation during irradiation of a Cu(100) surface with 6 keV Ne ions at a sample temperature of 60 K. It was found that the n u m b e r of adatoms produced per incoming ion decreases from an average of 3.5 to a saturation level of 1.8 after prolonged irradiation. Two mechanisms are believed to contribute to the adatom generation: ion-beam induced atom-replacement sequences and diffusion of self-interstitial atoms to the surface. From the adatom production as a function of irradiation dose the relative contributions of the two mechanisms were determined.

The modification of surfaces due to ion bombardment has been studied extensively throughout the last decades, because of its technological importance. One of the surface morphological effects to be expected due to ion beam irradiation is the production of adatoms on the surface. Harrison and coworkers predicted the existence of this effect theoretically [1,2]. The existence of the effect has not been unambiguously proven experimentally, until very recently. Michely and Comsa [3] reported STM experiments in which the generation and nucleation of adatoms on a P t ( l l l ) surface due to ion bombardment with low-energy Ar ions were observed. We have reported [4,5] low-energy ion scattering (LEIS) measurements, in which Cu adatoms were found to accumulate on a stepped Cu(100) surface during ion beam irradiation with the analyzing beam of 6 keV Ne ions. The aim of the experiments described in this Letter is to get a better understanding of the processes which lead to ion-beam induced adatom generation. In the experiments we used a new method to measure absolute adatom coverages with LEIS. During irradiation of a single crystal with ions with energies of a few keV, many substrate atoms are set in motion, either by collisions with primary ions, or by ion-beam induced atom-replacement sequences. Atoms from deeper layers may

penetrate through the surface layer, and, if their kinetic energy is high enough, they may escape to the vacuum. This is the well-known sputtering effect. If the kinetic energy of an atom which has reached the top surface layer is too low to overcome the surface energy barrier, it will stay on the surface as an adatom. Harrison and Webb observed this effect in the simulation of 5 keV Ar-ion bombardment of a Cu(100) surface [2]. This mechanism for ion-beam induced adatom generation should be operative at any temperature, even at 0 K. There is also another process which may lead to the formation of adatoms, but only at finite temperatures: thermal diffusion of self-interstitial atoms (SIAs) towards the surface. Below the surface, many vacancy-interstitial pairs (Frenkel pairs) are formed in the damage cascades. Due to the large cross sections for scattering at energies of a few keV, and, consequently, the small penetration depth, the Frenkel pairs are created close to the surface. The thus created SIAs can thermally diffuse in the near-surface region. This implies that the lower limit for the temperature at which this mechanism is operative is determined by the onset temperature for interstitial migration, which is 40 K for Cu [6]. Many of the SIAs will recombine almost instantaneously with the simultaneously created vacancies. Some of the SIAs may survive recombina-

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M. Breeman, D.O. Boerma / Ion-beam induced generation of Cu adatoms on Cu( l O0)

tion and migrate towards the surface and become an adatom. The probability that a SIA migrates to the surface layer strongly depends on the concentration, the spatial distribution, and the mobility of the vacancies in the near-surface region, and of course on the depth of origin of the SIA. In order to separate the contributions of the two mechanisms of adatom production mentioned above, we used a method, which is based on the fact that the probability that a SIA can migrate to the surface depends strongly on the concentration of vacancies in the near-surface region. During ion bombardment a surplus of vacancies is created close to the surface, due to sputtering, ion-bombardment induced atom-replacement sequences and out-diffusion of SIAs. If the substrate temperature is lower than 220 K, which is the onset temperature for vacancy migration in Cu [7], the vacancy concentration will increase monotonically with increasing irradiation dose. A higher vacancy concentration will increase the recombination probability for SIAs created in later damage cascades. Therefore, the number of produced adatoms is expected to decrease with increasing integrated irradiation dose. Since the ion-bombardment induced atom-replacement mechanism is not expected to be affected by a concentration of vacancies below the surface, the two effects can be separated by measurements of the adatom production as a function of irradiation dose. We used low-energy ion scattering combined with time-of-flight (LEIS-TOF) [8] to do these measurements. The L E I S - T O F experiments were performed in the main scattering chamber of a UHV-system with a base pressure of 2 x 10 -1° mbar. The main chamber is provided with L E E D and Auger electron spectroscopy to inspect the surface quality. A preparation chamber, which is connected to the main chamber, was used for baking and sputtering of the samples. The Cu sample used was a single crystal, mechanically polished parallel to a (17,1,1) plane, after aligning it into the desired direction with an accuracy of about 0.1 °, using X-ray diffraction. After being electro-chemically polished, baked and sputter-cleaned, the sample was mounted onto a three-axes goniometer. From L E E D measurements it was concluded

L111

that a surface was obtained, consisting of terraces with a (100) orientation and an average length of 8.5 interatomic distances, and monatomic steps, as expected for a (17,1,1) surface. The goniometer onto which the sample was mounted is connected with a copper braid to a closed-cycle He refrigerator, for in situ cooling of the sample to temperatures as low as 50 K. The temperature during the experiments was monitored by means of a c h r o m e l - A u thermocouple in contact with the surface of the crystal. In the LEIS-TOF experiments both neutral and ionized scattered Ne particles were observed at a backward angle of 155 °. The primary energy of the Ne ions was 6 keV and the scattering plane was chosen parallel to the steps. In order to be sensitive to small coverages of adatoms, the signal from atoms in the first and deeper surface layers has to be completely suppressed. This was achieved by detecting the scattered particles at a glancing polar exit angle of only 4 ° with respect to the surface. In such a geometry Ne particles scattered from Cu atoms in the first and deeper layers are blocked on their way to the detector by neighbouring surface atoms. Therefore, the only visible atoms are adatoms, provided there is no other atom in the same layer within approximately six interatomic distances in the direction of the detector. To monitor the adatom coverage as a function of irradiation dose, T O F spectra were collected continuously, with a dose of approximately 6 x 1011 i o n s / c m 2 per spectrum. The temperature during the measurements was 60 + 2 K. For each T O F spectrum the content of the peak corresponding with scattering of Ne ions by Co atoms was determined. To convert the peak contents into Co adatom coverages, an accurate calibration was used, for which use was made of the stepped structure of the surface. In this case the scattering plane was chosen perpendicularly to the steps, such that the incoming Ne ions hit ascending steps. In this geometry only step-edge atoms are visible, at the applied polar detection angle of 4 °. From the known step density, the number of atoms contributing to the measured signal was determined. As already mentioned above, adatoms are only visible if there is no other adatom within about

L112

M. Breeman, D.O. Boerma / Ion-beam induced generation of Cu adatoms on Cu(lO0)

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six interatomic distances in the direction of the detector. This implies that the probability that an adatom is visible depends strongly on the adatom coverage. The visibility of adatoms in the geometry used in the experiments was calculated, assuming a random distribution of adatoms on the surface. In fig. la the visibility is plotted as a function of actual adatom coverage. In fig. lb the visible adatom coverage, equal to the actual coverage multiplied with the visibility, is plotted. As can be seen in fig. lb, the maximum visible adatom coverage which can be measured is only 5.5% of a monolayer (ML), due to the strong decrease of visibility with increasing actual coverage. One ML corresponds with 1.53 x 1015 a t o m s / c m 2 on a Cu(100) surface. To determine the adatom production per incoming ion from the L E I S - T O F experiments the slope of the actual coverage versus irradiation dose has to be determined. From fig. 1 it is clear that an accurate determination of this slope is difficult for actual coverages in excess of 5% of a ML. To circumvent this problem

the following procedure was followed. The sample was pre-irradiated with different integrated beam doses to create different concentrations of vacancies close to the surface. In order to "remove" the adatoms produced during the preirradiations, the temperature was chosen such that Cu adatoms are mobile on a Cu(100) surface, and migrate to the step edges. In this way the adatom coverage on the terraces is drastically reduced. In earlier experiments [5] we determined that Cu adatoms become mobile on a Cu(100) surface at a temperature of 140(5) K. The pre-irradiation temperature was chosen to be 185 K. At this temperature vacancies are still immobile, so that vacancies can accumulate below the surface. In this way we were able to study the adatom production accurately for higher integrated irradiation doses without being hindered by the higher adatom coverages normally associated with these higher doses. We measured the visible adatom coverage as a function of irradiation dose for the non-pre-irradiated sample, and for pre-irradiation doses of 1, 2, 3, and 4 × 1013 i o n s / c m 2. The results of these measurements are shown in fig. 2, together with these results of calculations (solid lines), which will be discussed below shortly. The adatom production per incoming ion was obtained from a comparison between the experimental data and calculated curves. In the calculations we first calculated the depth distributions of vacancies and interstitials after Ne-ion bombardment of Cu in the geometry used in the experiments, using the computer code T R I M C A S C A D E [9]. In this program the trajectories of individual atoms (primary ions as well as recoiled target atoms) are followed until their energy has become less than the displacement energy. The subsequent diffusion of SIAs in the presence of immobile vacancies and the surface was calculated, using atomistic simulations. The number of atoms reaching the surface was monitored. Detailed knowledge of the diffusivity of SIAs is not necessary for these calculations, since the time between overlapping damage cascades ( = 100 s in our experiments) is many orders of magnitude larger than the time scale for diffusion of SIAs. Recombination of interstitials and vacancies was

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irradiation dose (1013 ions/cm 2) Fig. 3. Calculated adatom production per incoming ion, as a function of irradiation dose.

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taken into account by a spherical recombination volume around a vacancy. The recombination radius was adjusted, optimally reproducing the experimental data. Its value was determined to be 5.4 +_ 0.9 A, in reasonable agreement with theoretical estimates [10,11]. Details of the calculations will be presented elsewhere [12]. The calculation results in a curve of the adatom production per ion versus irradiation dose. Using this curve, and the visibility as shown in fig. la, we then calculated the visible adatom coverages for the different pre-irradiation treatments. As can be seen in fig. 2, the calculated curves are consistent with the measured data. From the calculated adatom-production curve, shown in fig. 3, it can be seen that the adatom production decreases from 3.5 adatoms per incoming ion on a freshly prepared sample, to 1.8 adatoms after irradiation with 8 × 1013 i o n s / c m z. The value of 1.8 adatoms per incoming ion is approached almost asymptotically. This shows that for these irradiation doses

the vacancy concentration is already so high that hardly any SIA can reach the surface any more, implying that the contribution of diffusion of SIAs to the total adatom production is almost zero. Therefore, we conclude that the contributions of the two mechanisms for adatom production as mentioned in the introduction, are determined separately, and are found to be about equal in our case, for ion bombardment of a freshly prepared sample. An additional experiment was performed to test the model of vacancy accumulation close to the surface. The sample was pre-irradiated with a relatively high dose of 4.5 × 1013 i o n s / c m z at room temperature. Since vacancies are mobile at this temperature, we do not expect them to accumulate close to the surface. Therefore, no effect of this pre-irradiation on the adatom production is to be expected, despite the high pre-irradiation dose. In fig. 4 the experimental data for this

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irradiation dose ( 1013 ions/cm 2) Fig. 4. Visible adatom coverage as a function of irradiation dose, measured for a sample, pre-irradiated at room temperature. The solid line represents the result of a calculation in which it was assumed that the pre-irradiation has no effect.

L 114

M. Breeman, D.O. Boerma / Ion-beam induced generation of Cu adatoms on Cu (100)

pre-irradiation treatment are shown, together with the calculated curve for zero pre-irradiation dose. As can be seen, the agreement between experiment and calculation is perfect, which is consistent with the discussion above. In summary, it was observed that adatoms appear on a Cu(100) surface during ion-bombardment with 6 keV Ne ions. L E I S - T O F experiments were performed to determine separately the contributions of two mechanisms which explain this ion-beam induced adatom generation. The total adatom production was found to be 3.5 adatoms per incoming ion for a freshly prepared sample. This number decreases to 1.8 per incoming ion after prolonged irradiation. A pre-irradiation at room temperature showed no effect on the adatom production, which is consistent with the presented model. Based on the present results, we conclude that the process of ion-beam induced adatom generation is well understood. The technical assistance of J.A. Reinders and L. Venema is highly appreciated. This work is part of the research program of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (NWO).

References [1] D.E. Harrison, Jr., P.W. Kelly, B.J. Garrison and N. Winograd, Surf. Sci. 76 (1978) 311. [2] R.P. Webb and D.E. Harrison, Jr., Radiat. Eft. Lett. 86 (1983) 15. [3] Th. Michely and G. Comsa, Phys. Rev. B 44 (1991) 8411. [4] M. Breeman, G. Dorenbos and D.O. Boerma, Nucl. Instrum. Methods B 64 (1992) 64. [5] M. Breeman and D.O. Boerma, Surf. Sci. 269/270 (1992) 224. [6] W. Schilling, G. Burger, K. Isebeck and H. Wenzl, ill: Vacancies and Interstitials in Metals, Eds. A. Seeger, D. Schumacher, W. Schilling and J. Diehl (North-Holland, Amsterdam, 1970) p. 255. [7] Th. Wichert, M. Deicher, O. Echt and E. Recknagel, Phys. Rev. Lett. 41 (1978) 1659. [8] D.O. Boerma, Nucl. Instrum. Methods B 50 (1990) 77. [9l The computer program TRIM-CASCADE is written by Dr. J.P. Biersack. For more details about TRIM programs, the reader is referred to: J.F. Ziegler, J.P. Biersack and U. Littmark, in: The Stopping and Range of Ions in Solids (Pergamon Press, New York, 1985). [10] K. Drittler, H.J. Lahann and H. Wollenberger, Radiat. Eft. 2 (1969) 51. [11] A. Scholz and C. Lehmann, Phys. Rev. B 6 (1972) 813. [12] A paper about the calculation of the adatom production is in preparation.