On the origin of the LEIS signal in TOF- and in ESA-LEIS

Nuclear Instruments and Methods in Physics Research B 267 (2009) 634–637

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

On the origin of the LEIS signal in TOF- and in ESA-LEIS S.N. Markin *, D. Primetzhofer, P. Bauer Institut für Experimentalphysik, Johannes Kepler Universität, 4040 Linz, Austria

a r t i c l e

i n f o

Article history: Received 30 September 2008 Received in revised form 5 November 2008 Available online 28 November 2008 PACS: 34.50.Dy 68.49.Sf 79.20.Rf

a b s t r a c t The ion fraction analysis of 4He+ ions backscattered from various faces of copper single crystals is performed by using time-of-flight (TOF) and electrostatic analyzer (ESA) low-energy ion scattering (LEIS) techniques. When an experiment that integrates over 2p azimuth (typical ESA-LEIS setup) is used, the yield of ions backscattered from the Cu(1 1 0) surface may be given by projectiles penetrated much deeper than just one or two monolayers. The threshold energy for reionization processes for 4He+ and Cu found earlier by TOF-LEIS is experimentally confirmed by ESA-LEIS. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Low-energy ion scattering Time-of-flight Electrostatic analyzer Single crystal Neutralization Reionization Cu(1 0 0) Ion fraction

1. Introduction Low-energy ion scattering (LEIS) is a well known analytical technique that is highly surface sensitive due to the strong neutralization probability of the probing ions [1]. When the fraction of ions that survived the surface collision without neutralization, P+, is well known, precise quantitative analysis of the surface composition by LEIS is possible. Study of P+ for different ion-target combinations is required to improve the physical understanding of the underlying charge exchange processes, which is indispensable for application of LEIS in quantitative surface analysis. When He+ ions are used as projectiles and metals as targets, Hagstrum’s neutralization model [2] holds and the ion fraction purely depends on the perpendicular component of the ion velocity, v\. In this case neutralization of ions is entirely due to Auger neutralization (AN) processes [3]. When the projectile energy exceeds a threshold value, Eth, various charge exchange mechanisms, e.g. collision induced neutralization and reionization (CIN and CIR, respectively), become possible. In this case the P+ is no longer a function of v\ only. Time-of-flight (TOF) and electrostatic analyzer (ESA) based techniques are the main representatives of the ‘‘LEIS family”. They * Corresponding author. E-mail address: [email protected] (S.N. Markin). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.11.022

utilize a similar projectile energy range (<10 keV). Although, different ion beam geometries induce distinctive information to be obtained from TOF- and ESA-experiments. In TOF-LEIS experiments both, neutrals and ions are measured. In ESA-LEIS only ions can be detected. Therefore, it is more straightforward to perform ion fraction analysis using TOF-LEIS. Also for analysis of surface structure and for determination of surface layer thickness, TOF-LEIS can easily be used. Due to the use of higher primary ion currents, data acquisition is faster in ESA-LEIS and higher sensitivity in surface concentration can be achieved. In this work experiments on backscattering of He+ ions from different low-index faces of Cu single crystals were performed with the aim to investigate the neutralization behavior of He+ by means of both ESA- and TOF-LEIS techniques and to reveal the information depth of the LEIS signal. TOF- and ESA-LEIS setups (described in [4] and [5], respectively) were used to operate purely in the AN regime and in the reionization regime where both AN and collision induced charge exchange are relevant.

2. Experimental Both, ESA- and TOF-LEIS experiments are available in our group at the Johannes Kepler University of Linz. In our ESA-LEIS setup MiniMobis, the ion beam of noble gas ions is incident

635

S.N. Markin et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 634–637

Fig. 1. A sketch demonstrating principal differences between ESA-LEIS (a) and TOF-LEIS (b) ion beam geometries. Direction of the incident ion beam is shown by an arrow and the projectiles backscattered into the detector solid angle are shown as dark areas. In (b) also the surface area ‘‘visible” by the detector is shown as a dashed ellipse.

perpendicular to the surface of a sample (a = 90°, see Fig. 1), irradiating a spot of 1 mm in diameter. Ions backscattered by an angle h = 136° are energy separated in a cylindrical mirror analyzer (CMA) with an azimuthal acceptance of 2p and detected by micro channel plates (MCP). The incident ion energy can be varied in the range from 0.2 to 4.5 keV. The sample is fixed on a carousel and only its horizontal and vertical position with respect to the ion beam can be varied. In our TOF-LEIS setup ACOLISSA, the chopped ion beam is directed to the target with an angle a with respect to the surface normal. The diameter of the beam is typically <1 mm so that the irradiated spot is fully ‘‘visible” to the detector. Projectiles backscattered at the angle h = 129° are charge separated in the drift tube and their time-of-flight is measured. The incident ion energy can be varied in the range from 0.6 to 10 keV. The sample position with respect to the ion beam can be changed in X, Y and Z directions. Also two rotations of the sample are possible, varying the angle of incidence (polar angle) by ±90° and the azimuth in a range of 110°. As samples Cu(1 0 0), Cu(1 1 0) and polycrystalline copper were used. The single crystalline samples were prepared by repetitive sputtering–annealing cycles, performed with 3 keV Ar+ ions and subsequent heating to 650 K. The polycrystalline samples were just sputtered. The purity of the samples and the crystal structure were checked by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), respectively.

Aþ ¼ I0  ni 

dr þ  P  gþ  f; dX

ð1Þ

where I0 is the number of incident ions; ni is the density of surface atoms; dr/dX is the differential scattering cross section; P+ is the ion fraction; g+ is the analyzer detection efficiency; n is an instrumental factor (analyzer transmission function in case of ESA-LEIS). Most of the parameters are experimentally defined or can be calculated. Major problems are the determination of P+, ni and f. An experimental spectrum obtained by ESA-LEIS using 4He+ ions for an incident energy E0 = 2 keV and Cu(1 0 0) as a target is shown in Fig. 2(a) (open symbols). Here, the ion yield is plotted versus the energy of backscattered ions. A Gauss function (solid line) with the maximum at EGc fits very well to the high energy part of the spectrum. This is generally the case when multiple scattering virtually does not contribute to the ion yield and when neutralization is given only by AN [6,7]. The broadening of the spectrum in the low energy part can be explained by multiple scattering events or by electronic losses, i.e. electron–hole pair excitation by the projectile. Therefore, in order to exclude possible inelastic processes, the ion yield, A+, was taken as twice the area of the experimental spectrum in the energy range from EGc to E0. Thus, in the ESA-LEIS experiment the ion yield as defined above is a measure for the ion fraction if other parameters from Eq. (1) are well established. In case of TOF-LEIS and a measurement in double alignment geometry, the task to determine P+ is easier [8], since both backscattered ions and neutrals can be detected. In this case, P+ is given by:

3. Results and discussion In a common LEIS experiment, the yield of backscattered ions, A+, is obtained in the single scattering approximation via:

9 8 7

experiment Gaussian fit

2 keV4He+ Cu(100)

b 500 400

Aþ =gþ ; Aþ =gþ þ A0 =g0

2 keV4He+ Cu(100)

ð2Þ

A0

6

Counts

Ion yield, arb. units

a

Pþ ¼

5 4 3

200

A+

A+ / 2

2 1 0 1500

300

1520

1540

1560

100

E0

EGc

1580

1600

Scattered ion energy,eV

Background 1620

0

550

600

650

700

750

800

850

900

Channel

Fig. 2. Experimental spectra obtained by ESA-LEIS (a) and TOF-LEIS (b) setups by using 2 keV 4He+ ions and a Cu(1 0 0) surface.

636

S.N. Markin et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 634–637

Fig. 4. Azimuth scan over Cu(1 0 0) surface performed by TOF-LEIS using 6 keV 4He+ ions. The minima 0° and 50° correspond to the double alignment conditions.

1

4

+

He

→ Cu

v =1

P

+

c

0.1

where A0 is the signal of backscattered neutrals, which is defined in analogy to Eq. (1) as A0 = I0  ni  dr/dX  (1  P+)  g0  n. In Fig. 2(b) a TOF-LEIS spectrum is shown measured using 2 keV He+ ions and a Cu(1 0 0) single crystal in double alignment conditions, when direction of the incident and the outgoing beams does coincide with the main channelling directions of the crystal. This situation is well illustrated in Fig. 3, where the stereographic projection of fcc (1 0 0) crystal [9] is shown along with the experimental beam geometry for both ESA- and TOF-LEIS setups. It is obvious, that channelling of the incoming and blocking of the outgoing particles is possible only for TOF-LEIS. In this case, the number of contributing layers in a TOF-LEIS experiment can be limited to one or two, depending on the azimuth orientation. In these conditions, when subtracting the background as shown in Fig. 2(b), one can easily deduce the value of P+ directly from the areas of neutral and ion peaks, A0 and A+, respectively. It has been shown in [10] that the ion yield for 6 keV He+ backscattered from the Cu(1 0 0) surface strongly depends on the azimuth angle and can differ by a factor of three for double alignment and random geometry, respectively (see Fig. 4). This fact has been explained by deeper layers contributions to the ion yield. In our ESA-LEIS the signal of backscattered ions is integrated over an azimuth angle of 2p. Therefore, the number of contributing layers is unknown. ‘‘Equivalent” experimental conditions for both setups are only achievable when using polycrystalline targets, for which the ion yield is azimuthally isotropic. Note that typically the diameter of the ion beam is larger than the mean grain size by orders of magnitude. Therefore, P+ data obtained for polycrystalline Cu by ESA-LEIS were normalized in the AN regime by use of the corresponding TOF-LEIS data. In Fig. 5, P+ data measured for clean Cu(1 0 0), Cu(1 1 0) and Cu-poly surfaces are plotted in semi logarithmic plot versus the inverse perpendicular component of the ion velocity. In this case, a linear behavior of the data corresponds to the AN regime and the slope of the line determines the

v= c 1. 68

ESA-LEIS: Cu(110) Cu(100) Cu-poly

Fig. 3. Stereographic projection of fcc (1 0 0) surface [9] with indicated TOF-LEIS and ESA-LEIS beam geometry. The incident ion beam is depicted as a five-point star.

.18× 5 10 m

v= c 1 .92

×1

05

/s

m/s

×1

05 m/ s

0.01 0.0

5.0x10

-6

1.0x10

-5

1.5x10

-5

2.0x10

-5

1/v ⊥, s/m Fig. 5. Experimentally obtained ion fraction of 4He+ measured by ESA-LEIS on Cu single crystals (symbols). As lines (solid, dashed and dash-dotted), characteristic velocities deduced from the P+ data measured by TOF-LEIS setup on different Cu surfaces (Cu(1 1 0), Cu(1 0 0) and Cu-poly, respectively) are shown [10].

characteristic velocity that is a measure for the neutralization efficiency [2]. In Fig. 5 also solid, dashed and dash-dotted lines are shown, demonstrating the characteristic velocities deduced from the P+ data experimentally obtained by TOF-LEIS for the Cu(1 1 0), Cu(1 0 0) and Cu-poly surfaces, respectively [10]. For the Cu(1 0 0) surface, which exhibits a rather high atomic density, the data measured by ESA-LEIS agree very well with the TOF-LEIS data in the AN regime. This assures that close packed surfaces can be measured with both setups in an equivalent way in spite of different ion beam geometries. Clearly, in this case deeper layer contributions do not play a significant role in the AN regime. It is obvious that the ESA-LEIS results for the most open surface, i.e. Cu(1 1 0), underestimate the TOF-LEIS results measured in double alignment conditions, with only the first layer being ‘‘visible”. The observed difference is due to the effective contribution of deeper layers to the neutralization in case of ESA-LEIS. At lower values of 1/v\, the deviation of the ln (P+) data from the linear dependence on the perpendicular velocity points to the fact that CIN and CIR processes start to contribute to the total neutralization. Our ESA-LEIS measurements confirm the energy threshold Eth for CIN and CIR processes found earlier by TOF-LEIS experiments [11].

S.N. Markin et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 634–637

It should be noted that the ESA-LEIS data demonstrate very clearly a decrease of ln (P+) and, therefore, can be used for precise determination of the threshold energy Eth for CIN and CIR. Furthermore, our setup MiniMobis can be operated down to very low energies (200 eV). On the other hand, TOF-LEIS has demonstrated better selectivity in restricting the information depth to the outermost atomic layer and permits measurements at higher energies (up to 10 keV). Therefore, these techniques are rather complementary which may open the possibility to gain more information by a combination of both techniques when exploring charge exchange in LEIS. Acknowledgements Support by the Austrian Science Fund FWF (Projects P16173N08 and P16469-N08) and by ÖAD (WTZ Project 2005/20) is gratefully acknowledged. Authors are also very thankful to H.H. Brongersma for donated ESA-LEIS equipment. Daniel Primetzhofer

637

acknowledges a DOC-fellowship of the Austrian Academy of Science. References [1] H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Rep. 62 (2007) 63. [2] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [3] R. Souda, M. Aono, Nucl. Instr. and Meth. B 15 (1986) 114. [4] M. Draxler, S.N. Markin, S.N. Ermolov, K. Schmid, C. Hesch, A. Poschacher, R. Gruber, M. Bergsmann, P. Bauer, Vacuum 73 (2004) 39. [5] G.C. Leerdam, H.H. Brongersma, Surf. Sci. 254 (1991) 153. [6] W. Heiland, U. Beitat, E. Taglauer, Phys. Rev. B 19 (1979) 1677. [7] E. Steinbauer, P. Bauer, J.P. Biersack, G. Bortels, Rad. Eff. Def. Solids 130 (1994) 77. [8] D. Primetzhofer, S.N. Markin, M. Draxler, R. Beikler, E. Taglauer, P. Bauer, Surf. Sci. 602 (2008) 2921. [9] Stereographic projections of fcc-surfaces, Daresbury Laboratories . [10] D. Primetzhofer, S.N. Markin, J.I. Juaristi, E. Taglauer, P. Bauer, Phys. Rev. Lett. 100 (2008) 213201. [11] M. Draxler, R. Gruber, H.H. Brongersma, P. Bauer, Phys. Rev. Lett. 89 (2002) 263201-1.