Defects in Cu and Ag overlayers epitaxially grown on a Ru(0001) substrate studied by slow positrons

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surface science ELSEVIER

Applied Surface Science 116 (1997) 222-227

Defects in Cu and Ag overlayers epitaxially grown on a Ru(0001) substrate studied by slow positrons C. Hahn a, R. Krause-Rehberg a,*, M. Heiler a, H. Wolter a, H. Neddermeyer a, K. Wandelt h, A. Otto c a Fachbereich Physik, Martin-Luther-Universit~it Halle, D-06099 Halle, Germany b Inst. fiir Phys. und Theor. Physik, Universitiit Bonn, Wegeler Str. 12, D-53115 Bonn, Germany c Inst. fdr Physik der kond. Materie, Universitiit Diisseldo~ Universitiitsstr. 1, D-40225 D~sseldo~ Germany

Received 5 June 1996; accepted 15 July 1996

Abstract Slow positrons were used to study buried open-volume defects in epitaxially grown Cu and Ag overlayers on Ru(0001). Vacancy complexes (voids) were found after cryo-condensation of thick (100 nm) Cu and Ag films. These complexes anneal below room temperature. An Ar-sputtered Ru(0001) surface was covered with a 32 nm Cu overlayer (deposition at 300K). The buried defect layer was studied in the as-deposited and as-annealed state as well. It was found that the defect concentration in the Cu layer is distinctly increased after annealing at 500 K. Thus, migration of sputter defects towards the surface through the epitaxially grown metallic overlayer is directly observed. PACS: 78.70B; 61.70A; 68.55 Keywords: Epitaxial growth; Cryo-condensedlayers; Sputter defects

1. Introduction Slow positrons are a unique tool for investigating near-surface open-volume defects [1]. However, applications of the method for studying defects in metallic epitaxial overlayers are still rather rare [2]. On the other hand, there is hardly any method to study buried defects in metallic overlayers and interfaces when the film thickness exceeds a few monolayers. Thus, systematical studies on the ability to

detect open-volume defects in thin epitaxial films are worth performing. In this paper we will first show that defects can be detected in coldly condensed Ag and Cu layers on metallic substrates and, secondly, we will demonstrate that thin buried defect-rich layers can be detected in a depth of more than a hundred monolayers. Furthermore, it will be demonstrated that their annealing behaviour can be studied.

2. Experimental * Corresponding author. Tel.: +49-345-5525567; fax: +49345-5527160; e-mail: [email protected].

The magnetically guided slow positron beam system at the University Halle was used to perform the

0169-4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PHS0169-4332(96)01058-6

C. Hahn et al. / Applied Surface Science 116 (1997) 222-227

experiments. The positron beam diameter was about 5 mm and a count rate of 4000 e + / s was obtained in a pure Ge y-detector with 25% efficiency. Different sample holders allow experiments between 10 and 1500 K. A differential pumping station enables UHV conditions in the sample chamber with a base pressure better than 10-10 Torr. Metallic layers were epitaxially grown in situ by evaporating Cu and Ag in a temperature-controlled UHV-compatible resistance heater. The deposition rate, i.e. the layer thickness of the Cu and Ag films, was carefully calibrated by the frequency shift of a crystal oscillator and by thermal desorption experiments performed before, during and after layer-growth experiments at the positron beam. The usual S-parameter and the positronium-fraction measurements (F-parameter) were used to characterize the defect structure in the layers [1]. These well-established techniques were completed by positron re-emission spectroscopy by a simple setup: The total count rate of positrons annihilating at the sample is recorded as a function of the voltage between sample and vacuum chamber, building up a retarding field. If the field strength is sufficiently high, re-emitted positrons will be pushed back to the sample and increase this count rate. This technique requires surfaces of negative workfunction for positrons, which is fulfilled in the case of Cu. The three methods are based on the effect that positrons can be trapped by lattice defects, such as vacancies, dislocations, vacancy agglomerates, and grain boundaries, as they diffuse to the surface. In this case, the number of positrons reaching the surface decreases and defect profiling is possible when the positron incident energy is varied in a defined way [3].

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of temperature and the diffusion length at room temperature was found to be L+ = 75 + 5 nm corresponding to a diffusion coefficient of D = 0.5 cm 2 S -1 "

4. Study of defects in epitaxial Cu and Ag layers Ag and Cu layers were coldly condensed at 130 K. No regular growth mode is obtained for a deposition at such low temperatures [4-6]. Thus, no LEED pattern can be obtained and it is assumed that there are large open-volume defects, such as big voids and cavities.

4.1. C~. o-condensed Ag layers on Ru(O001) Fig. 1 shows positron depth-scans of the S- and F-parameter for different annealing states of a 50 nm Ag/Ru(0001) layer that was deposited at 130 K. The first scans up to an annealing temperature of 200 K show a plateau of the S-parameter at a high level.

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3. Positron diffusion experiments on Ru(0001) The Ru single crystal was prepared by sputtering and subsequent annealing at 1550 K as a standard procedure before any experiment was performed. Thus, the sample should be in a defect-free state. This was confirmed by positron lifetime measurements at room temperature where a single-component lifetime spectrum was obtained, % = 105 _+ I ps. The positron diffusion was studied as a function

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C Hahn et al. /Applied Surface Science 116 (1997) 222-227

that all F-parameter curves taken for annealings below room temperature show almost the same steep decrease indicating a similar and short positron diffusion length, so that the defect concentration must be rather high and remains unchanged, while the S-parameter profiles show distinct changes. This indicates that strong structural changes must occur between 200 K and room temperature. Usually, depth scans of this type are analysed by programme packages like VEPFIT [7]. The diffusion length might be obtained and consequently, the positron trapping rate, and thus, the defect concentration could be determined. However, in case of cryo-condensed layers, it is assumed that large cavities form inner surfaces. Thus, the simple picture of positrons diffusing to the surface is not applicable any more and a more quantitative analysis of the data is impossible. Only annealings performed well above room temperature lead to depth scans that are comparable to those obtained for room-temperature deposition of Ag. These curves are smooth with a high back-diffusion probability, i.e. a large diffusion length. The number of defects detectable by positrons is drastically reduced during such annealings. In order to visualize the annealing behaviour in a more distinct way, we plotted the S-parameter for certain positron implantation depths as a function of annealing temperature. Fig. 2 shows that a main annealing stage is observed at about 250 K for all

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This indicates that positrons are trapped within the whole layer in open-volume defects. This plateau is not visible in the F-parameter, since this parameter is a real surface-sensitive one. This means that the defect structure of the film is not directly visible in the F-parameter. However, it is interesting to note

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C. Hahn et al. / Applied Surface Science 116 (1997) 222-227

implantation energies. The same stage was earlier reported for a study of photoemission of adsorbed Xe (PAX) [4]. No PAX signal was observed after Xe deposition to the surface of coldly condensed Ag films (upper half of Fig. 2). This result could only be understood assuming that Xe diffuses into cavities of the solid which must be connected with the surface. After a subsequent annealing treatment, the PAX signal reappeared and indicated that the cavities are closed or have disappeared. The S-parameter change AS/S---1.12 is rather large. It must be concluded that positron trapping occurs mainly to large voids at low temperatures. Since the positron diffusion length remains small even for room temperature ageing, it is clear that still a large number of defects is present. This means that the observed annealing stage in Fig. 2 is not the result of a complete disappearance of defects, but it is a sign of a structural change. Thus, the S-parameter obtained after 290 K annealing is a defect Sparameter, showing that the open-volume of the defect responsible for positron trapping in this annealing state must exhibit a much smaller open volume. Since monovacancies are thermally unstable at room temperature in Cu [8], it must be concluded that dislocation or grain boundaries are probably the dominating trapping centres at this temperature.

4.2. Co'o-condensed Cu layers on Ru(O001) Similar annealing studies were performed for Cu layers of 100 nm thickness that were deposited at 130 K on a clean Ru(0001) surface and on a Ru(0001) surface carrying a coverage of about one monolayer oxygen as well (Fig. 3). The annealing behaviour is more complex compared to the Ag layer (Fig. 1). A steep annealing stage is observed at 150 K, followed by a slight increase at 200 K and a further decrease at room temperature. Basically the same sequence is found for the C u / O / R u system. As a clear difference, the structural changes occur at distinctly lower temperatures. At a glance, this is very surprising, since about 500 layers of Cu are grown on top of one single monolayer of oxygen. However, it is well known that a small amount of oxygen changes the growth mode in the C u / R u system at T > 300 K [9]. This peculiarity is described with the term surfactant (surface active substance). The oxygen will not be

225

buried during layer growth but it will always 'swim' on top of the layer and will change the adsorption behaviour and the film morphology distinctly. Thus, a layer-by-layer growth can be obtained at 400 K, while usually a three-dimensional StranskiKrastanov growth-mode is observed without oxygen [9,10]. Although no regular layer growth is obtained at low temperature, our results show that a single oxygen layer influences the growth and the structure of a thick coldly condensed Cu film on a Ru(0001) substrate. In order to obtain more information on the morphology of the coldly deposited films, STM investigations are planned at similar layered structures.

5. Buried defect layers obtained by Cu layer growth on sputtered Ru(0001) substrates Slow positrons are sensitive to defects created during sputter with medium-energy ions [! 1,12]. Defect profiles in Ar + sputtered Mo and Ai were studied and absolute defect concentrations were determined. During similar experiments on Ru(0001), Cu(ll 1), Cu(100), and thick epitaxial Ag/Ru(0001) layers, we found a comparable behaviour for the back-diffusion probability to the surface as reported earlier. Thus, the results will not be presented in detail. Here, we would like to add only one more interesting detail: The surface S-parameter (S~ur), i.e. the S-parameter measured with the lowest incident energy, was found to decrease during sputtering. This value is presented in Table 1. Since it is normalized to its value for unsputtered surfaces, one can see that this value decreases normally during sputtering. Although it is usually thought that the S-parameter is increasing at the presence of open-volume defects. this is not surprising. The surface S-parameter is comparable to the defect S-parameter of a very large

Table 1 Surface S-parameter of sputtered surfaces, S~Pr. normalized to the surface S-parameter, S~ur, obtained for the same surfaces in a well annealed state. Sputtering was performed with 5 keV Ar + ions at an incident angle of 60 ° Surface S~uP~/ S~ur

Cu(100) 0.910

Cu(111) 0.982

Ag/Ru(0001 ) 0.950

Ru(0001 ) 1.040

C. Hahn et al. /Applied Surface Science 116 (1997) 222-227

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void. Thus, a maximum increase must be expected. When sputter defects roughen the surface, an increasing number of core annihilations will occur leading to a smaller S-parameter value. The only exception in our data is Ru(0001). Although the defect layer after sputtering is very thin compared to the positron diffusion length in defect-free materials, the measured effects are rather large. This is due to the fact that this layer is located at the surface where defect profiling by positrons is most sensitive. However, the question comes up whether positrons may detect defect layers of similar width that are buried under thick layers of nominally defect-free material. To clarify this question, the Ru surface was sputtered with 5 keV Ar ÷ ions at room temperature and a 32 nm Cu layer was grown on top of this disturbed surface. Fig. 4 shows depth scans of the S- and F-parameter as well as the re-emission intensity versus sample bias voltage for differently treated C u / R u layers. The depth scans of the layer structure grown on annealed Ru do not show any significant difference after a 500 K annealing (filled squares and open circles). Only the re-emission intensity differs for 1 keV positron energy. Fig. 5 shows the re-emission intensity at - 5 V sample bias in a schematically

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drawn cross-sectional image of the layer structure. Here, the implantation profiles calculated as Makhovians [1] for Cu were added to get an visual impression on the depth distribution of positrons after implantation. It is obvious that in case of the 1 keV implantation all positrons start their diffusion within the Cu overlayer. Since the re-emission intensity increases, it must be concluded that the diffusion length in the Cu layer increases too. Consequently,

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C. Hahn et al. /Applied Surface Science 116 (1997) 222-227

one must assume that the defect concentration in the Cu layer is decreased during annealing. After growing the Cu layer on the sputtered Ru substrate, the depth scans as well as the re-emission intensity changes drastically. Hence, the thin buried defect-rich layer of a thickness of about 1 to 2 nm on top of the Ru host crystal can be clearly detected by slow positrons. When positrons are implanted behind the interface, they will hardly reach the surface by diffusion through the defect-rich layer, as they do in case of an unsputtered Ru substrate. This can be seen by comparing the S- and F-parameter for an implantation energy in the range between 3 and 10 keV. In agreement to these findings, the re-emission intensity for deeply implanted positrons is also much smaller compared to the unsputtered case. An annealing at 500 K does not change the re-emission intensity very much (Fig. 5). This is not surprising since a 1000 K annealing is necessary to recover the properties of a sputtered Ru surface (results not shown). However, it is impossible to apply such high temperatures to a C u / R u structure, since the Cu layer will undergo irreversible changes and, finally, will be evaporated. An exception is again observed for the 1 keV implantation of positrons. The re-emission is as high as obtained for Cu on annealed Ru. This means that the Cu layer does not contain a high defect concentration when it is grown on a sputtered Ru substrate. Surprisingly, the re-emission intensity drops down by a factor of 2.5 when the layer structure is annealed. Thus, it must be concluded that defects from the defect-rich Cu-Ru interface diffuse towards the surface through the formerly defect-pure Cu overlayer. Again, STM experiments are planned to get information on the structure morphology of the Cu layers grown on a sputtered Ru surface.

6. Conclusions Open volume-defects were detected in cryo-condensed Ag and Cu layers grown at 130 K on Ru(0001). A strong decrease of the S-parameter was observed during annealing below room temperature. It was attributed to a structural change of the domi-

227

nating positron trap from large voids to defects exhibiting a smaller open volume, such as dislocations or grain boundaries. Only an annealing treatment well above room temperature led to similar results that were obtained for high-temperature grown Cu and Ag layers. Buried defect-rich layers were prepared by depositing Cu on a sputtered Ru surface. Although this defect layer is small compared to the Cu overlayer thickness and to the positron diffusion length, the layer can easily be detected by slow positrons. The Cu layer does not show a high defect concentration after epitaxial growth. However, the defect concentration increases during moderate annealings at 500 K. This indicates that defects from the defect-rich Cu-Ru interface are detected during their diffusion towards the surface.

Acknowledgements The work was financially supported by the Volkswagenstiftung.

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