Preferential sputtering and oxidation of NbTa single crystals studied by low-energy ion scattering

Vacuum/volume

47tnumber IO/pages 1193 to 1196/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00

Pergamon I’ll: SOO42-207X(96)00132-7

Preferential sputtering and oxidation of NbTa crystals studied by low-energy ion scattering V N Semenov,= V G Glebovsky” and H H Brongersma,b alnstitute of Solid State Physics, 142432 Chernogolovka, Moscow district, Russia; bfacu/ty of Physics, Eindhoven University of Technology, P 0 Box 573,560O MB Eindhoven,

single

The Netherlands

To study preferential sputtering and oxidation, three monocrystalline Nb,Ta, _,(1701 alloys lx = 0.25, 0.5, 0.75) as well as pure single crystals of Nb and Ta have been prepared by electron beam floating zone melting (EBFZMI and investigated by low-energy ion scattering (LEG). After sputter cleaning, LEIS showed the enrichment of tantalum at the surface of all alloys. After exposure to oxygen, linear relationships between the oxygen and both the niobium and tantalum signals demonstrate that matrix effects do not influence the LEIS signals for these systems. The oxygen coverages of the different alloys have been determined by calibration against the Ni(lOO)-0 c(2x 21 surface. Copyright 0 1996 Elsevier Science Ltd

Introduction

Low-energy ion scattering (LEIS) is a surface-analysis technique with an extremely high and selective sensitivity to the outermost atomic layer of the surface.’ When matrix effects are absent one can quantify the surface composition by calibration2 The interaction of oxygen with metal surfaces is of great importance in catalysis, corrosion, and growth processes. A series of Nb,Ta, ox (110) alloys has been studied with LEIS to obtain quantitative information about the top layer of the NbTa system before and after interaction with oxygen. Nb and Ta both crystallize in a bee lattice with the same lattice parameters. They also have similar chemical and physical properties. Therefore, the NbTa system can represent a continuous range of substitutional solid solutions.3 The interaction of oxygen with Nb and Ta has been studied by several investigators.“16 In most of these studies, surface analysis techniques (LEED and AES) having a larger probing depth than LEIS have been used. In this paper, the preliminary results of LEIS experiments on Nb,Ta, _x alloys and pure elements are presented. It will be shown that the Nb and Ta surface concentrations of these alloys and the oxygen coverage at the top layer of the surface can be quantified with LEAS. Experimental

The alloys of Nb and Ta were prepared by mixing the pure elements in the desired ratio by using the high-frequency levitation melting process. This method uses no crucibles and provides a homogeneous mixing of the elements in the liquid state.” The melt was cast in cylindrical water-cooled copper moulds to form NbTa rods. Single crystals of these alloys were grown by the electron beam floating zone technique, which provides

refining of the material and a uniform distribution of the elements in the bulk.‘* The rods underwent four liquid zone passings, at a growth rate of 2-3 mm min-‘, using specially prepared monocrystalline seeds. The single crystals were cut by the electrospark method and then polished mechanically and chemically to prepare cylindrical disks of the (110) index plane. Laue back-reflection was used to crystallographically orient the samples. The remaining impurities mainly consisted of hydrogen, carbon and oxygen. The maximum metallic impurity levels were of the order 10 ppm. Unfortunately, the Nb0.r5Ta025 and Nb,,,Ta, 5 samples could not be grown from the liquid state as single crystals. The crystal grain sizes of the polycrystalline rods of these alloys were increased by strain deformation followed by high-temperature annealing (up to the melting point of the alloys). Five types of sample with different bulk compositions (Nb, Nb,,5Ta,,2,, Nb, sTa,,,, Nb, 25Ta0.75, Ta) were prepared and used in the present study. Rutherford backscattering spectrometry (RBS) was used to verify the bulk composition. The LEIS experiments were carried out using the ion scattering apparatus MINI-MOBIS. Its basic set-up is similar to that of the NODUS machine, which has been described in more detail elsewhere.” The primary ions are generated in a Leybold ion source (type 1QE 12/38) and are directed perpendicularly onto the target. The ions scattered through 144” by the target atoms are energy analyzed by a kind of cylindrical mirror analyzer (Figure 1). In this special analyzer the selected ions are imaged on a ring detector which is coaxial with the primary beam. This allows one to mass analyze the ion beam before it reaches the target. The use of these very pure ion beams is important for obtaining very low backgrounds in the spectra. The nominal base pressure in the vessel is in the low lo-‘* mbar range and can be 1193

V N Semenov

et a/: Nb-Ta

single

crystals

mien

also provides the slow removal of the oxygen at the surface by sputtering. This procedure is repeated to check the reproducibility. In this way, LEIS spectra of samples with different oxygen coverages could be obtained. Figure 3 shows the results of these experiments for the Nb, Nb,,Ta,,, and Ta targets. The Nb and Ta intensities decrease linearly with increasing of oxygen intensity. Small changes in time in the primary ion beam are corrected for by calibration against a clean copper surface. In this way, effects of the target composition on the secondary electron emission and, thus on the effective target current, are avoided.

t

E,

I

Discussion of results

Ef = Figure 1. General

f(Ei

,

?J

9

?E atom

3

muon )

scheme of LEIS experiment.

monitored by a quadropole mass spectrometer. The apparatus is equipped with a sputter ion source (Leybold type IQE 12/38) at a grazing angle of 15”. Oxygen adsorption and sputtering. The surfaces of the Nb,Ta, + alloys were cleaned by cycles of sputtering with argon ions at room temperature and annealing at 800 K. This temperature is too low to remove all defects. It was also not possible to remove all the oxygen in this way, but it was effective for the removal of surface contaminations such as C, N and H. To achieve atomically clean surfaces for these refractory metal alloys, annealing temperatures above 2000 K are required. These heating facilities are not yet available. Oxygen with a purity of 99.995% was admitted into the experimental vessel. A typical oxygen dose was 30-50 L (Langmuir), which is sufficiently high to obtain a saturation of the surface. Figure 2 shows a typical LEIS spectrum of a clean and an oxygen-covered NbTa alloy. By covering the Nb-Ta surface with oxygen, Nb and Ta are shielded and as a consequence the Nb and Ta intensities in LEIS decrease. We studied this effect more extensively by the following procedure. A sample is first saturated with oxygen. The oxygen in the experimental vessel is then pumped away, and LEIS spectra were measured with a 1.5 keV4 He+ primary ion beam. This ion beam

Quantification of niobium and tantalum at the surface. In Figure 3, the Nb and Ta signals are plotted as a function of the oxygen signal. The linear dependence proves that matrix effects are absent for these ion-atom combinations. The removal of oxygen by sputtering apparently does not influence the scattering process and the ion fraction of the neighbouring Nb and Ta atoms. Only more Nb and Ta atoms are exposed to the primary ion beam, corresponding to the increase of the Nb and Ta signal. This behaviour is very different from that in secondary ion mass spectrometry (SIMS), where the ion fraction of sputtered particles will change by many orders of magnitude by the presence of oxygen. To obtain the Nb and Ta signals for Nb,Ta, _ y alloys without oxygen, the lines in Figure 3 were extrapolated to zero oxygen coverage. When plotting the extrapolated Ta signals as a function of the corresponding extrapolated Nb signals, a linear relation is obtained, if no matrix effects are present in these LEIS

400000

50000

8, 0

2000

4000

6000

Oxygen

8000

10000 12000 14000 16000 11 00

signal [ counts I s ]

160000 14woo 120000

500

1000

IWO

2000

2500

3

O’.___~I__/_ i_._ Energy t evl 2. Typical LEIS spectra of a clean and oxygen-covered Nb, ,sTa,,25(l 10) surface. The primary energy of the ‘He+ probe ions is 3.0 keV, the target current is 40 nA. To reduce measuring time, the oxygen covered sample is only measured in the ranges of interest.

2000

4000

6000

8000

10000

12000

Figure

1194

Oxygen signal [ counts

14000

16000

/s]

Figure 3. Peak intensities of Nb and Ta vs the oxygen peak intensity (a) the pure elements Nb and Ta and (b) the Nb, 5T+.5 alloy.

for

V N Semenov

et al: Nb-Ta single crystals

400000~

0

50000

100000

150000

200000

250000

300000

Niobium signal [countsIs ]

Bulk concentration [ at% ]

Figure 4. Peak intensities of Ta vs Nb for the Nb,Ta, -~ system without oxygen.

Figure 5. The surface concentration

experiments.20 Figure 4 shows that this prediction is fulfilled within experimental error. The deviations of about 15% in this linear relation between the Ta and Nb signal can be attributed to several factors:

for the ratio R of the sputteryield of Ta Y,,:

l

l

l

l

Positioning and focusing of the system has to be done for all samples separately. The signals are calibrated against a standard copper sample. The errors in both measurements are a few percent. The bulk material we used is of very high purity, but segregation and adsorption processes can increase the impurity level at the surface. A contamination like carbon, with a low sensitivity in LEIS, is difficult to detect. It is possible that the different samples have different level of impurity atoms at the surface. Due to sputtering and low annealing temperatures, the surfaces of the various samples will not be ideal (110) index plane. When the surface structure of a sample is changed, the Nb and Ta densities will become lower than that of the most density packed (110) surface. Determination of the peak intensity of Nb from a LEIS spectrum is not straightforward for the alloys, since the Nb peak is superimposed on the low-energy tail of the Ta peak. A special oven is under construction that will enable high-temperature (2000°C) annealing of the samples in the preparation chamber. This will reduce the experimental errors in future.

The surface composition for clean Nb,Ta, -_~alloys can be calculated from the linear curve in Figure 4, because the signals for the two elements are proportional to their surface concentrations. Measurement data are transferred to the linear curve by drawing a straight line through a measured point and the origin of the graph and taking the intersection with the linear curve. The Nb and Ta surface concentrations are obtained by dividing these transferred Nb and Ta signals by the signal of pure Nb and Ta, respectively. The surface composition of all samples, calculated in this way. are given in Figure 5. The surface is cleariy enriched in Ta. Since these alloys have very high melting points (269@3270 K) thermally activated surface segregation can be completely neglected at room temperature. Also, the very small heat of mixing of Nb and Ta makes chemical effects on the sputtering very unlikely. NbTa alloys are, therefore, an ideal case for studying the influence of a mass difference on preferential sputtering. According to the theory of Sigmund 2’ one would expect

(in at%). The Sigmund comparison.

model

of Nb vs the bulk concentration

for preferential

sputtering

is shown

for

Y,, of Nb with respect to that

R = I’m,/ Yr, = NN~INT~(MT~IMN~)~~(UT~IUN~)’

-2m,

where NNb and NTa are the atomic concentrations (number of atoms per volume), MNb and MTa the atomic masses and U,, and Uri,, the surface binding energies of Nb and Ta, respectively.The exponent m, which is nowadays assumed to be about l/6, is a parameter characteristic of the interaction potential. The ratio of the surface binding energies of Ta and Nb is estimated to be equal to the ratio of the enthalpies of evaporation of these elements (1 .09).22 Since sputtering is largely restricted to atoms from the outermost layer, this preferential sputtering of Nb should lead to an enrichment in Ta in the top layer by a factor of 1.3. As is shown in Figure 5, the observed enrichment is even larger than the prediction based on preferential sputtering. Since the present setup did not allow for removal of the oxygen by heating, it is likely that an oxygen induced segregation combined with preferential sputtering is responsible for the observed effect. In future the contribution of these two effects will be separated. Quantification of oxygen. The quantification of the oxygen signal can be done by calibration against the Ni(lOO)-Oc(2 x 2) surface. It is known that this very stable structure, which is obtained with saturating the surface with oxygen, has an oxygen coverage of half a monolayer,23 and corresponds to 8 x lOI oxygen atoms per 1 cme2. For calibrations the oxygen adsorption on Ni(lOO) is investigated with LEIS under the same conditions as those in the NbTa experiments and the results are given in Figure 6, using again the clean copper signal for normalisation. In Figure 6 we observe that the maximum oxygen coverage on Ni( 100) corresponding with an atomic oxygen density of 8 x lOI atoms cm-* gives an oxygen signal in LEIS of 7.3 x lOi counts s-‘. The linear decrease of the Ni signal with increasing oxygen signal demonstrates again the absence of matrix effects. Through this calibration, quantification of the maximum oxygen densities on the NbTa samples is possible. Dividing the oxygen density by the metal density, which is 13.0 x lOI and 12.9 x lOI atoms cmm2 for the Nb(ll0) and Ta( 1 IO) monocrystalline faces, respectively, gives the oxygen coverage. Table 1 presents the results. In the literature, the oxide growth on Ta(l10) and Nb( 110) 1195

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et a/: Nb-Ta single crystals oxygen coverages on pure Nb and Ta single crystals, investigations are in progress.

further

Conclusions

Oxygen signal [counts

Is1

Figure 6. Peak intensity of Ni vs the oxygen peak intensity for the Ni(lOO) surface. Different oxygen coverages are obtained by sputtering (open

marks) and by monitoring the oxygen exposure (filled marks).

Low-energy ion scattering is very useful to obtain quantitative compositional information about the outermost layer of the surface. It is demonstrated that there are no matrix effects. The Nb,Ta, I-alloys differ from those in the bulk. A Ta enrichment at the surface is found for all alloys, indicating preferential Nb sputtering. This is in a reasonable agreement with theory. The oxygen coverages on Nb,Ta, ~, alloys after oxygen exposure have been determined with an accuracy of about 15% through a calibration with the maximum oxygen coverage of the known system of Ni( 100) - Oc(2 x 2). The maximum atomic surface density of oxygen is determined to be 13 x IO” atoms crne2 on Ta( 110) and 18 x 10” atoms cm-* on Nb( 1 lo), corresponding to an oxygen coverage of 1.0 and 1.4, respectively. The maximum oxygen coverages of the alloys increase with the Nb content. References

Table 1. Quantification of the surface single crystals with LEIS Surface composition

composition

of Nb,Ta,_x

Maximum oxygen density (lOI at.cm-*)

Maximum oxygen/metal ratio 1.41 1.33 1.17 1.10 1.00

Sample

%Nb

%Ta

Nb Nbo ,~Ta0 25 Nb, ,TaO 5 Nb, 25Ta0 75 Ta

100 65 31 9

-

1.82

35 69 91 100

1.I2 1.51 1.43 1.30

(110)

is described by the formation of TaO( 111)5 and NbO(l1 I),‘” respectively. The oxygen/metal ratio of 1.O that we obtained with LEIS for the Ta(ll0) single crystal is in agreement with these investigations. For the Nb( 110) single crystal, however, we find an oxygen/metal ratio of 1.4 which clearly exceeds the value for Ta. The higher oxygen saturation coverage on Nb is also clearly reflected in a better shielding of the Nb, as compared to Ta. The oxygen coverages and the shielding of the metal atoms in the NbTa alloys increase with increasing Nb content. For niobium it is expected that the surface contains a larger amount of oxygen than in the tantalum surface. Hu et a/.” reported on the presence of at least two kinds of niobium oxides (NbO and Nb,O,, detected with XPS) on the Nb(ll0) surface exposed to 3000 L oxygen. Haas et al.” mentioned that the oxygen solubility in Nb is larger than in Ta (4.5% and 3%, respectively). Also a difference in the structure between the niobium and tantalum oxide at the surface can introduce differences in the exposed oxygen density on the Nb or Ta surfaces. To solve the interpretation of these different

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