. applied

surface science

Applied Surface Science 70/71 (1993) 114-117

North-Holland

Surface segregation of NiZr

D.J. O’Connor a, H.J. Kang a, P. Pigram b, R.H. Roberts a and Songian He a a Department of Physics, University of Newcastle, Callaghan, NSW 2308, Australia b Department of Physical Chemistry, University of New South Wales, Kensington, NSW 2033, Australia

Received 27 August 1992; accepted for publication 18 November 1992

The preferentially sputtered surface of a clean polycrystalline NiZr sample has been studied with X-ray photoelectron

spectroscopy (XPS), Auger electron spectroscopy (AES) and low energy ion scattering (LEIS). The depth profile of the constituents

has been determined by AES measurements taken at two significantly different electron energies which have different escape

depths. Similarly the composition has been determined with XPS on both the clean surface and on surfaces contaminated with a

small amount of oxygen. The composition of the outermost layer has been determined by the use of LEIS at several incident

energies using Arf. In the LEIS analysis the effect of charge exchange has been estimated by a novel measurement of the charge

exchange parameters while simultaneously determining the relative concentrations of Ni and Zr. The results of the different

approaches and the complementary information obtained will be described. The composition of the clean annealed surface,

measured with AES only, will be contrasted with the surface concentration of the preferentially sputtered surface.

1. Introduction

The surface composition of alloys has been extensively studied to ascertain the equilibrium concentration of the constituents on an undis- turbed surface to test the models of segregation. In addition, many of the studies have included a measurement of the surface after ion bombard- ment to establish the nature and magnitude of the driving forces which lead to surface enrich- ment over and above the thermodynamic pro- cesses. This study leads to a better understanding of the surface binding of atoms in an alloy surface and the role of radiation induced segregation and radiation enhanced diffusion.

Ion bombardment changes to the surface com- position of alloys were first found by Gilliam [l] in a Cu,Au alloy. More systematic studies have been performed on CuNi alloys using principally AES [2]. With AES, the surface and bulk compo- sition can be determined from the energy depen- dent escape depth of the electrons. Most of these studies have concluded that the surface composi- tion under ion bombardment results from prefer-

ential sputtering which leads to an enrichment at the surface of the constituent atoms with the lower sputtering coefficient. The composition ex- ponentially approachs the bulk value. This model is supported by several theoretical studies based on a phenomenological steady-state equation for the surface concentration in terms of bulk con- centration and sputtering parameters.

Since the introduction of LEIS for surface composition analysis of the alloys it has become evident that some effects were masked by the sampling depth of AES under even the best of conditions. Wkile the best depth resolution of AES is S-10 A, LEIS can probe the outermost atomic layer. Thus a combination of AES and LEIS can sensitively probe the first few atomic layers of the surface which is the region in which the composition is changing. The LEIS results in combination with AES results has led to the recognition of the importance of radiation in- duced surface segregation (RI9 which controls mass transport from the second atomic layer to the first. This is contrasted to radiation enhanced diffusion (RED) which is responsible for the re-

0169-4332/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

D.J. O’Connor et al. / Surface segregation of NiZr 115

laxation of the composition dip generated by RIS to the full range of the altered layer region into the solid.

NiZr alloys have been studied because of their important and interesting structural, electronic and magnetic properties [31. This study of a poly- crystalline NiZr sample was initiated by a LEIS study which identified a strong surface composi- tion of Zr under 2-6 keV Ar+ bombardment. As that study involved not only measuring the scat- tered ion yield but also the charge exchange parameters it resulted in a determination of the surface concentration of the constituents on a surface which had experienced preferential sput- tering. However, there were no facilities to an- neal the sample in the LEIS study to return the composition to equilibrium. The study using AES and XPS was therefore initiated to confirm the findings of the preferentially sputtered surface and to extend the study to establish the composi- tion in the absence of ion bombardment.

In addition, the prediction of the bond break- ing model [4] is that the surface should be Zr rich but the tight binding model [5] of surface segrega- tion predicts a Ni rich surface.

2. Experiment

The NiZr sample has a composition of 50% Zr and is polycrystalline. It was prepared by melting the constituents in an Ar arc using high purity Ar gettered with Zr. To ensure uniform composition it was turned and remelted several times and in the final melt it was cooled rapidly to encourage the development of a fine grain polycrystalline structure. The composition and uniformity have been checked by EDAX on an electron micro- scope and by X-ray diffraction.

The LEIS measurements were performed in a custom built system designed to measure charge exchange using LEIS, secondary ion mass spec- troscopy and time-of-flight (hereafter referred to as system I). The chamber was evacuated by a turbomolecular pump to 3 x lo-” mbar after bakeout. The sample was mounted at an angle of 60” to the axis of a goniometer whose axis of rotation was parallel to the ion beam 161. This

ensured that the incidence angle and the scatter- ing angle (90”) were kept constant, while chang- ing only the exit angle of the ions to the surface. By changing the angle of rotation of the target about the beam axis it is possible to determine the charge exchange rate for the exit path. For a heavy inert gas ion this portion of the trajectory is the dominant contributor to the neutralisation process. Although the incoming path cannot be ignored, it is of little consequence to the relative composition estimation because it is: (1) common to the trajectory of ions scattered off Ni and Zr; (2) at a higher angle to the surface than the exit path; (3) at a higher energy than the exit energy. For these reasons the relative concentration of the two constituents need only consider, to first order, the sensitivity factors from the cross sec- tions and the charge exchange rates determined as part of the analysis. Ar+ ion energies from 2 to 6 keV were used in this study.

The detailed AES and XPS analyses were per- formed on the scanning Auger microscope devel- oped at the University of Newcastle (system II). This is a composite system comprising a Leybold EQ51 electron gun, PHI-04-151 X-ray source and a hemispherical electrostatic analyser placed at 90” to the electron beam. The base pressure of this system was 1 X lo-*’ mbar. In this system the NiZr sample and reference samples of Ni and Zr were mounted on the one holder so that all three could be analysed under identical condi- tions. Further confirmation of the XPS results were obtained with a Kratos XSAM/AXIS 800 surface analysis system (system III). As the Kratos system is used for routine analysis its base pres- sure was not as low as the system II AES/XPS system and the results obtained from it identified the effect of adsorption on surface segregation in this alloy.

3. Results

3.1. Low energy ion scattering

The composition of the surface was deter- mined with 2, 4 and 6 keV Ar scattered off the surface after extensive ion bombardment yielding

116 D.J. O’Connor et al. / Surface segregation of NiZr

a preferentially sputtered surface. The relative concentrations were calculated from [7]

where I is the measured scattered ion intensity, u is the differential scattering cross section and P+ is the probability of being scattered in the ionised state. The cross section can be calculated from a knowledge of the interatomic potential and in this study both the Moliere and the universal potentials have been used but little difference was found between them once the relative cross section was determined. The parameter P+ must be measured as it is affected by both geometry and surface composition. To estimate the value of P+ the following expression is used;

P+a exp( -u,/ui cos y)

where u1 cos y is the perpendicular component of velocity for the projectile. From an angular scan (in r> 121 the parameter u,/ui can be meas- ured and an estimate of P+ made. From these measurements the surface composition for the NiZr alloy has been found to be (75 & 3)% Zr the surface layer.

3.2. Auger electron spectroscopy

To determine the near surface elemental com- position the low energy Auger peak of NiMVV (56 eV) and of Zr MNN (146 eV) were used. To determine the composition at greater depth the high energy Auger peak of Ni LMM (844 eV) was used. The high energy Zr LMM (1936 eV) was not used as its intensity was too low. The relative composition can be estimated [8] using

where Zi and Zio denotes the intensity of element i from the alloy and a pure standard, respectively. F is the matrix correction factor which includes the backscattering factor, inelastic mean free path and the density correction. The F value was taken from a program for AES analysis [9]. The intensity of the Auger signal was determined both by peak height after background subtraction from the N(E) spectra and from the peak-to-peak height of the dN(E)/dE spectra. The surface

Table 1

The measured surface and subsurface composition of NiZr

using a range of analysis with different depth sensitivities for a

surface which has been subjected to preferentially sputtering

2-6 keV Arf

Tech-

nique

LEIS

AES

AES

AES

AES

XPS

XPS

XPS

XPS

Peak

(eV)

Ni (MVV) 56

Zr (MNN) 146

Ni (LMM) 844

Zr (MNN) 146

Ni 2ps,,

Zr3ds,, Ni 3p

Zr3ds,2

Mean free

path [lo] (&

2

4.8

5.7

13

5.7

8

20

17

20

Ni (at%) Zr (at%)

25+3 75+3

35+5

65+5 55+5

45+5

55+2

45+2

50+2

50+2

composition obtained from the most surface sen- sitive low energy AES is that the Zr composition is 65%. A more complete list of estimated com- positions are listed in table 1.

3.3. X-ray photoelectron spectroscopy

XPS has been used to determine the composi- tion to a depth of a few nanometres. In the XPS quantification the Ni2p,,,, Zr 3d,,, and Ni3p peaks were used and the concentrations were estimated from the N(E) spectra using the F values of Seah [81. As shown in table 1 the com- position obtained from Ni2p,,, and Zr3d,,, re- sults in an estimate of 45% Zr which is in agree- ment with the NiLMM and Zr MNN Auger re- sults. This agreement is reasonable as the average inelastic mean free path of these signals are com- parable.

3.4. Oxidation

Although carefully controlled oxygen adsorp- tion studies are yet to be made, the role of oxygen can be seen in the change in surface composition during the initial cleaning of the sample in system III. After a 10 minutes of ion etching (from exposure to atmosphere) when the principal form of oxygen present is as ZrO, (0 : Zr = 2 : l), the Ni : Zr ratio was 12 : 88 indicat- ing a strong oxygen induced segregation of the

D.J. O’Connor et al. / Surface segregation of NiZr 117

Zr. After 30 minutes ion etching (and longer periods) the composition was Ni : Zr = 23 : 77 with a decrease in the oxygen signal. Under the best conditions in this system oxygen content of the surface layer reached 0 : Zr = 3 : 97 with no fur- ther change to the Ni : Zr ratio. This leads to the conclusion that the presence of oxygen on the surface enhances the segregation of Zr.

3.5. Annealing

It is not possible to completely understand the properties of the surface unless the composition of the undisturbed surface is measured and com- pared to expectations. To obtain that information the Ni : Zr ratio was monitored with AES during annealing after sputter etching. Under this treat- ment the surface composition of Zr increased from 65% to 82% at 500 K. Annealing to higher temperatures did not cause any further increase in the surface Zr concentration. Upon recom- mencement of ion etching the surface composi- tion reverted back to a value of 65% Zr after the removal of just a few monolayers. This observa- tion supports the conclusion that most of the composition change occurs with a decay length into the solid of l-2 atomic layers.

4. Discussion and conclusion

The clean annealed surface of NiZr has a surface concentration of 82% Zr and this layer is only two atomic layers thick. The influence of oxygen has been studied during cleaning from exposure to atmosphere and shows a gradual change from a ZrO, enriched layer to one typical of a clean sputtered surface after cleaning.

From table 3 it is evident that the surface of NiZr is enriched in Zr under the influence of

preferential sputtering with a low energy Ar+ beam. LEIS shows that there is a large concentra- tion of Zr on the surface, however, from the low energy AES results it would seem that this en- hancement has a decay length of only one or two atomic layers. According to Monte Carlo calcula- tions the sputtering yield ratio for Ni to Zr is 1.8 over the range of Ar+ energies used in this study. This results in a prediction of 65% for the Zr composition which is too low to explain the ob- servations.

Current models for surface segregation, radia- tion induced segregation, radiation enhanced dif- fusion and preferential sputtering can explain the observation for this NiZr alloy if all are consid- ered together. Further work is underway with a heating stage on the LEIS system to monitor the surface layer composition as the samples are an- nealed to 800 K.

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