Xa

JLE

a

AA

KXHSPF

1

mhisms

sasstdttet

&

m

G

0d

Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267

Contents lists available at ScienceDirect

Journal of Electron Spectroscopy andRelated Phenomena

journa l homepage: www.e lsev ier .com/ locate /e lspec

-ray standing waves and hard X-ray photoelectron spectroscopyt the insertion device beamline ID32

örg Zegenhagen ∗, Blanka Detlefs, Tien-Lin Lee1, Sebastian Thiess2, Helena Isern,ucien Petit, Lionel André, Jérôme Roy, Yanyu Mi, Isabelle Joumard3

uropean Synchrotron Radiation Facility, 6 rue Jules Horowitz, F-38000 Grenoble, France

r t i c l e i n f o

rticle history:vailable online 30 September 2009

a b s t r a c t

Investigations on the geometrical structure, chemical composition and electronic properties of surfacesand interfaces are performed on beamline ID32 at the ESRF. It is a high resolution beamline covering

eywords:-ray standing wavesard X-ray photoelectron spectroscopyynchrotron radiationhotoelectric multipole contributionsourier analysis

the photon energy range 1.4–30 keV optimized for X-ray standing wave, surface X-ray diffraction, andhard X-ray photoelectron spectroscopy experiments. Fresnel zone plates and compound refractive lenssystems are used to focus the monochromatic beam. After outlining the principles of X-ray standingwave measurements the unique features of the ID32 beamline are described. Future developments arediscussed and finally some selected examples illustrating the advantages of combining the X-ray standingwave technique with hard X-ray photoelectron spectroscopy are presented.

. Introduction

The instrumentation of the beamline ID32 at the ESRF is opti-ized for the X-ray standing wave (XSW) technique [1,2] and

ard X-ray photoelectron spectroscopy (HAXPES). It is used fornvestigating the structure, chemistry and electronic properties ofurfaces, interfaces, and bulk of materials. For the XSW studies, theain emphasis is on experiments employing Bragg reflections from

ingle crystals.The XSW technique adds spatial resolution to photoelectron

pectroscopy (PES) by utilizing a stationary X-ray wavefield, i.e.n X-ray interference field, for the excitation instead of a tran-ient wave from just one photon beam. The X-ray intensity is thuspatially modulated and since the X-ray absorption is proportionalo the X-ray intensity, the intensity of the photoelectron emissionepends on the location of the wavefield maxima with respect to

he atoms. If the atoms are located at specific positions relative tohe Bragg planes, such as a surface adsorbate or dopant atoms forxample, information on their location can be obtained by scanninghe position of the wave field.

∗ Corresponding author.E-mail address: zegenhagen@esrf.fr (J. Zegenhagen).

1 Present address: Diamond Light Source Ltd., Diamond House, Harwell ScienceInnovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom.2 Present address: HASYLAB at DESY, Notkestrasse 85, D-22607 Hamburg, Ger-any.3 Present address: SPINTEC (CEA/CNRS), URA 2512, 17 rue des Martyrs, F-38054

renoble Cedex 9, France.

368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.elspec.2009.09.008

© 2009 Elsevier B.V. All rights reserved.

X-ray photoelectron spectroscopy (XPS) using hard X-rays waspioneered by Siegbahn using laboratory X-ray sources [3] and laterthe advantages of using synchrotron radiation (SR) was demon-strated by Pianetta and Lindau [4]. Nowadays more brilliant X-raysources are available which make hard X-ray photoelectron spec-troscopy a viable technique, where brilliance is defined as thenumber of photons per second per bandwidth per unit area andsolid angle. There are a number of advantages of using hard X-rays for PES. At high electron energy the information depth issignificantly larger permitting the properties of bulk and buriedinterfaces/objects to be determined [5]. Particularly important isthe fact that at higher X-ray energies it is possible to achieve higherenergy resolution in the primary X-ray beam. In fact, resolution inthe meV range, or better, can be achieved if required [6]. Last butnot least, if a high-energy photoelectron spectrometer is availablethen other techniques can be used in conjunction with PES. At theESRF, HAXPES is combined with surface X-ray diffraction at thebending magnet beamline BM29 [7] and with the XSW techniqueat the insertion device beamline ID32.

For the XSW technique higher X-ray energies are required forsmaller lattice plane spacing d or higher order m reflection byBragg’s law, i.e. 2d sin (�B) = m�. Here�B is the Bragg angle of thechosen reflection and � (=hc/E� ) is the wavelength. Higher orderreflection, i.e., smaller lattice plane spacing has to be used for XSW

measurements to achieve higher spatial resolution and/or for highresolution XSW imaging. With respect to the latter, it is possibleto create an image of an atomic distribution by performing severalXSW measurements utilizing a range of Bragg reflections. Thus, ifwe want to employ the XSW method for a high resolution Fourier

opy an

aieiicnar

rio�etsoaFrdrtdBbaf

npdwmdparrt

tdXeWtsrroPssorotrptottb

J. Zegenhagen et al. / Journal of Electron Spectrosc

nalysis of a system with an atomic species with low electron bind-ng energies, such as organic molecules, the kinetic energy of themitted photoelectrons will consequently increase with decreas-ng diffraction plane spacing. Even for heavier atoms we may benterested in analyzing shallow core levels, since they exhibit largerhemical shifts. Thus, exploitation of the XSW technique in combi-ation with PES needs a suitable high resolution electron analyzerllowing to detect electrons with kinetic energies up to the 10 keVange.

The requirements of XSW and HAXPES for the X-ray source areather similar. Both techniques benefit from the energy tunabil-ty of synchrotron radiation and in particular from the brilliancef 3rd generation sources which enable high-energy resolutionE/E = 10−6 with sufficient flux to be achieved. Preferably, the

nergy resolution of the primary X-ray beam should be adjustableo be able to optimize the photon flux and minimize the data acqui-ition time. For photoelectron spectroscopy the energy resolutionf the X-ray beam should be matched to the analyzer resolutionnd/or the intrinsic width of the spectral features to be detected.or the XSW technique, smaller lattice plane spacing d or highereflection orders m require higher energy resolution of the inci-ent beam to resolve the correspondingly narrower single crystalocking curves. However, unlike HAXPES, XSW using single crys-als also requires a certain degree of angular collimation; the beamivergence needs to be smaller than the angular width of theragg reflection. Just as for the energy resolution, the requirementsecome more stringent for smaller diffraction plane spacing. Thus,n efficient use of the XSW technique relies on the high brilliancerom 3rd generation SR sources.

For obtaining sufficient spatial resolution in the XSW tech-ique, reasonably large reciprocal lattice vectors (small diffractionlane spacing) have to be used for the Fourier analysis. Thisetermines typical excitation energies for the XSW technique,hich may range up to 15 keV, which in turn determines theaximum kinetic energy of the electrons that has to be han-

led by the electron analyzer. Unfortunately the cross-sections forhoto-ionization decrease rapidly with increasing kinetic energynd this presents a challenge. Again, high brilliance synchrotronadiation with efficient beam focusing is very helpful in thisespect to achieve sufficient count rates and reasonable measuringimes.

In the following, we will first give a concise description ofhe XSW standing wave formalism with emphasis on structureetermination. In this respect, in particular when combining theSW technique with PES, attention should be paid to the pres-nce of multipole contributions to the photoelectric absorption.e will also present a brief overview of the main features of

he ID32 insertion device beamline. This will be followed byome examples of HAXPES measurements at ID32 and some firstesults using our new HAXPES system at ID32, which is cur-ently being commissioned. Then we will present two examplesf unique applications of the XSW technique in combination withES: (1) the structural analysis of chemical species on surfaces, i.e.,tructural analysis beyond elemental sensitivity and (2) element-pecific imaging, i.e. the Fourier analysis of complex adsorbatesn surfaces. In the first case, we briefly review the publishedesults on the XSW analysis of the room temperature structuref Sn on Ge(1 1 1). In the second case, we illustrate the poten-ial of the XSW imaging technique in combination with PES toeveal the cation structure of SrTiO3. Just a few Fourier com-onents, derived from seven XSW measurements are sufficient

o reconstruct the atomic structure of the perovskite becausef the phase sensitive and element-specific structure determina-ion using the XSW technique. Finally, future developments athe ID32 beamline regarding XSW and HAXPES will be outlinedriefly.

d Related Phenomena 178–179 (2010) 258–267 259

2. The XSW technique

X-ray standing waves can be best understood as a Fourier tech-nique and for structural analysis some analogies to diffractiontechniques can be recognized. Basically, in an XSW measurementthe standing wave field is used to photo excite (specific) atoms.Scanning the wavefield, by traversing the rocking curve of a crys-tal, photoelectrons, Auger electrons, or X-ray fluorescence arerecorded. By proper analysis of the shape of such signal(s), the XSWparameters coherent fraction F and coherent position P, which canbe related to the X-ray structure factor, are determined. However,some important differences from diffraction techniques should bestressed at this point. For instance, in principle the spatial coor-dinates of a single atom can be obtained using XSW, in contrastto a diffraction experiment. The reason is that the XSW techniqueprobes the correlation of the atom(s) with the X-ray interferencefield whereas diffraction techniques probe their autocorrelation.Furthermore, diffraction techniques suffer from the so-called phaseproblem. Only the amplitude of the structure factor can be obtaineddirectly and the phase information is a priori lost. Furthermore,element-specific information is difficult to obtain by diffraction –in particularly for lighter elements. In contrast, the XSW techniqueis phase sensitive, element specific, and, using PES, even sensitiveto the structure of chemical species.

The XSW technique is in essence an X-ray interference tech-nique. With the knowledge of the exact pattern and position of anX-ray interference field, the real space distribution of the atomsis analyzed. The photo absorption or subsequent decay channels(photoelectrons, Auger electrons, X-ray fluorescence, etc.) serveas marker signals when scanning the wavefield. When travers-ing a single crystal reflection curve, an enhancement of the photoabsorption signal indicates that the maxima of the wavefield arelocated at atomic positions. A decrease of the signal would indicatethat atomic positions coincide with the minima of the wavefieldintensity. This principle is based on the fact that the probability forphotoelectron excitation reflects the wavefield intensity at the cen-ter of the atom. It furthermore requires being able to manipulatethe X-ray interference field, i.e., change its position and to turn theinterference effect on and off. Frequently a planar wavefield is usedfor XSW made by the superposition of two coherent plane wavesand the atomic positions are analyzed on the length scale of thewavefield spacing dH. More correctly speaking, utilizing a planarX-ray wavefield with spacing dH for the XSW analysis of an atomicdistribution, we obtain a Fourier component of the atomic distri-bution corresponding to the wavefield spacing, i.e., the Hth Fouriercomponent with |H| = 1/dH with the vector H being normal to thewavefield planes [8].

2.1. X-ray standing wave

A planar X-ray wavefield is created by the superposition of twoplane X-ray waves

E0 = e0E0 exp[2�i(�0t − K0r)] (1)

and

EH = eHEH exp[2�i(�Ht − KHr)]. (2)

Here e0 and eH are polarization vectors and K0 and KH are theX-ray propagation vectors with

|K0| = |KH | = |K| = K = �−1 (3)

where � is the X-ray wavelength. Since E� = h̄ω = h� and c =��, X-ray energy E� and � are related via �E� = hc = 1.23984 keV nm. Thetwo waves are coherent with identical frequencies

�0 = �H = � with �|K| = c. (4)

260 J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267

Ff

p

E

w

R

i

K

i

I

atdsocsiwpa

2

ttt(

Y

F

Fig. 2. Principle of the XSW technique. Averaged over time, the E-field intensity ofeach of the two coherent plane waves is uniform but modulated in the region of theiroverlap. The probability of photoelectric absorption is proportional to the wavefield

−1 ′ ′′

ig. 1. Schematic snapshot of two traveling plane waves. The two coherent wavesorm a standing wave in the region of spatial overlap.

Thus one can relate the two E-field vectors by amplitude andhase factors via

H =√RE0 exp(iϕ) (5)

ith

= IHI0

= |EH |2|E0|2 . (6)

Furthermore, the propagation direction of the two plane wavess related by

H = K0 + H (7)

Thus, it is straightforward to show that for the total wavefieldntensity (normalized to the incident intensity I0) one obtains

H = 1 + R+ 2√R cos(ϕ − 2�Hr) = 1 + R+ 2

√R cos

(ϕ − 2�

�d

dH

).

(8)

Here, r is the distance with respect to a chosen reference pointnd thus �d is the projection of this distance along H, i.e. normalo the wavefield planes. The phase ϕ between the two plane wavesepends of course on the origin chosen for r [9]. When using atanding wave created by single crystal Bragg diffraction, the originf r corresponds to the origin chosen for the structure factor of therystal which produces the XSW. The wavefield spacing dH corre-ponds in this case exactly to the chosen diffraction plane spacing,.e., dH = d/m and H = [h k l] denotes the chosen diffraction vector

ith |H| = H = 1/dH with the Miller indices (h k l) of the diffractionlanes. The two coherent traveling waves and their overlap regionre shown schematically in Fig. 1.

.2. XSW analysis in the dipole approximation

In the dipole approximation for photoelectric absorption, theransition matrix element and thus the probability of photoelec-ron emission is directly proportional to the wavefield strength athe position r of the center of the absorbing atom and thus thenormalized) photoelectron yield YH from a single atom is given by

H = 1 + R+ 2√R cos(ϕ − 2�Hr) = 1 + R+ 2

√R cos

(ϕ − 2�

�d

dH

).

(9)

The principle of the XSW method is shown schematically inig. 2.

intensity. Atoms located at the maxima (minima) of the wavefield intensity willshow strong (weak) photoelectron emission indicated by the red (blue) “atom”. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)

For an ensemble of atoms, the yield is simply the sum of theindividual yields, which is, again normalized, given by

YH = N−1N∑i=1

{1 + R+ 2√R cos(ϕ − 2�Hri)}

= 1 + R+ 2√RFH cos(ϕ − 2�PH) (10)

The sum of cosine function yields of course again a cosine func-tion, however with a specific phase PH and, when normalized, withan amplitude FH ≤ 1. It is straightforward to show [2,8] that the twoparameters FH and PH are the amplitude and phase, respectively, ofthe Hth Fourier coefficient GH of the (normalized) distribution (r)[8] of a particular element

GH =∫(r)exp(2�iHr)dr = FH exp(2�iPH); PH = Hr (11)

From these definitions it is clear that FH = 1 only if (r) = ı(r),i.e., all atoms of the particular element or species reside at oneparticular position with respect to the planes of the wavefield. Fur-thermore, we can restrict PH to the range 0–1 as well, i.e. 0 ≤ PH ≤ 1,since the yield YH(PH) = YH(PH + n) with integer n. This is simply anexpression of the fact that we can obtain positional informationonly on the length scale of the wavefield spacing and not beyond.The so-called coherent fraction FH and coherent position PH can bedirectly compared with the more familiar X-ray structure factor,defined by

FQ =n∑j=1

cjFjQ (12)

with∑n

i=1cj = 1 and

Nj∑

FjQ = N

j

i=1

(fjo + fj + ifj )exp(−iQri) (13)

Here, Q is a reciprocal lattice vector, the index j counts the differ-ent elements and f0, f′ and f′′ are the atomic form factor, dispersion

J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267 261

F(z

c2

G

wsXb

2e

bcrbasebt

Y

v

bitapoes

Y

mmttoe

Fig. 4. XSW photoelectron yield recorded from a SrTiO3(0 0 1) crystal using a (0 0 2)Bragg reflection under close to backscattering geometry (E� = 3.18 keV) taken from[13]. The data have been fitted with Eq. (10). The different Sr orbitals as well as theoxygen 1s line give different values for the “coherent fraction” f, which are all largerthan unity indicating that multipole contributions are present and that the modifiedEq. (12) should be used to describe the data. The values of the coherent positions alsodiffer from the value P = 0 expected from the structure of the crystal. The oxygen KLL

ig. 3. The photoelectron emission (Yp) characteristic for pure dipole absorptionsolid line) and for quadrupole contributions (dashed line). The k-vector is in the-direction and the X-ray wave is polarized in the plane of the drawing.

orrection, and absorption coefficient of a particular element. With�H = Q, the XSW “structure factor” GH can be identified as

H = FjQfjo + f ′

j+ f ′′

j

= FH exp(2�iPH) (14)

hen the element j is selected by spectroscopy. Thus, an XSW mea-urement directly determines the atomic distribution whereas an-ray diffraction experiment measures the electron density distri-ution.

.3. Multipole contributions to the photoelectric absorption andxtinction effects

For the electron yield to be expressed in the simple form giveny Eqs. (9) and (10), two conditions have to be fulfilled. The firstondition is that the emission must originate from the near surfaceegion of the crystal, so that extinction effects (see e.g. [10,11]) cane neglected. This is generally valid when elastic photoelectronsre detected during an XSW experiment because of their relativelyhort mean free path. If the signal comes from deeper in the bulk,.g. X-ray fluorescence or X-ray excited photoelectrons that haveeen inelastically scattered several times, a modified Eq. (10) haso be used:

H = e(E�, �){1 + R+ 2√RFH cos(ϕ − 2�PH)}. (15)

The function e(E� , �) ≤ 1 describes the extinction effect whicharies strongly over the range of Bragg reflection [11].

The second condition is that higher order multipole contri-utions to the photoelectric absorption can be disregarded. This

s in general not the case when detecting differential photoelec-ron yields, i.e. when detecting photoelectrons at discrete emissionngles. In case of pure dipole photoelectron emission, the emissionattern is symmetric with respect to a plane normal to the k-vectorf the X-rays. Higher order multipole contributions to the photo-lectric cross-section introduce a forward backward asymmetry, ashown in Fig. 3 and Eq. (10) has to be modified [12,13]:

H = 1 + SRR+ 2√RSIFH cos(ϕ − 2�PH + ) (16)

Because of the forward/backward asymmetry, XSW measure-ents at �B ≈�/2 are particularly sensitive to the presence of

ultipole contributions since the incoming and reflected beams

ravel in opposing directions. Multipole contributions manifesthemselves clearly when coherent fractions greater than one arebtained when fitting XSW photoelectron data with Eq. (10). Coher-nt fractions larger than unity are unphysical and indicate that the

Auger line was recorded which is practically unaffected by multipole contributions[12,13]. However, it should be noted that Auger lines can also be excited by theinelastic scattering background from deeper in the crystal by which the fitted P andf-values may be affected.

modified expression given by Eq. (16) has to be used to describe thedata. An example is shown in Fig. 4 where yield curves were fittedusing Eq. (10) and the “coherent fractions” are (i) larger than unityand (ii) different for photoelectrons emitted from different orbitals.Both results are unphysical since the coherent fraction should be

uniquely determined by the structure of the system.

Quadrupole contributions to the photoelectric cross-sectionshave been calculated by Trzhaskovskaya et al. [14]. The multipoleparameter SR can be determined experimentally by an XSW mea-surement from an incoherent layer of the element of interest as

2 copy and Related Phenomena 178–179 (2010) 258–267

degbflCbwwteab

3

iuvm2a(9oaeovoub

bd(asmfpedsctcaasrsftetmtpl

tUm

Fig. 5. (a) Schematic arrangement of the ID32 beamline. (b) Positions of the key

62 J. Zegenhagen et al. / Journal of Electron Spectros

emonstrated by Woodruff [15]. With the help of SR, the param-ter SI can be approximated, but not the phase . However, for aiven element and X-ray energy the parameters SR, SI and cane determined experimentally by measuring simultaneously X-rayuorescence and the photoelectron lines of a surface adsorbate.ompared to photoelectron emission, X-ray fluorescence is negligi-ly affected by multipole contributions [16] and in particular = 0,hich permits the real coherent position and the coherent fractionith very good approximation to be determined. This informa-

ion allows SR, SI and to be derived from a fit of Eq. (12) to thexperimental photoelectron data. However, care must be taken tossure that the detected fluorescence signal was not excited fromackground scattering from within the sample.

. The ID32 beamline

The ID32 beamline has been in operation since 1995 [17] ands installed on a high-� section of the ESRF storage ring. Threendulators are mounted in the 5 m long straight section and pro-ide a beam linearly polarized in the horizontal plane with aaximum photon flux (0.1% bandwidth, 30 m source distance,

00 mA electron current) of ≈1015 ph s−1 mm−2 at around 2.5 keVnd ≈1014 ph s−1 mm−2 at around 30 keV. The virtual source sizeFWHM) determined by the electron beam is 20 �m vertically and00 �m horizontally. Two of the undulators, with a 35 mm periodf the magnet structure, are used for energies between 2.5 keVnd 30 keV. The third undulator with period of 42 mm extends thenergy range of the beamline down to 1.4 keV (at the minimum gapf 11 mm). The divergence (FWHM) of the photon beam is 20 �radertically and 30 �rad horizontally. The beamline is designed forperation without any windows so the X-ray beam is traveling inltrahigh vacuum from the source to the sample. The layout of theeamline is shown in Fig. 5.

In the primary optics hutch (OH1) the first optical elementehind the high power front end slit system (ESRF standardesign) is a high-heat-load monochromator [18]. This instrument“pseudo-channel-cut” design) was installed in the summer of 2006t a distance of 30.6 m from the center of the straight section. Twoets of crystals are mounted on a common, high precision, stepperotor driven rotation axis (harmonic drive, 1.11 �rad resolution at

ull step size) coupled to a Heidenhain RPN886 angle encoder. Therimary crystals are placed between liquid nitrogen cooled heatxchangers which enable them to withstand a maximum powerensity of about 100 W mm−2 without noticeable distortion. Theecondary crystals, mounted between heat exchangers cooled byopper braids to LN2 temperature, are mounted on a slide to adjusthe distance D to the first crystals. The monochromator is typi-ally operated with the exit beam at a fixed height H, which isccomplished with the second crystals of the two sets tracking asfunction of Bragg angle� (H = 2D cos(�)). The orientation of the

econd crystals with respect to the first can be adjusted in two Eule-ian angles (“pitch” and “roll”) over a range of more than 1◦ usingtepper motors (1.8 �rad and 0.7 �rad resolution, respectively atull step size) and over a range of 0.34 mrad with 10 nrad resolu-ion by piezo elements. Using Si(1 1 1) monochromator crystals, thenergies attainable at fixed exit beam position range from 2.05 keVo 28 keV determined by the design parameters of the monochro-

ator. The energy resolution of the main monochromator usinghe Si(1 1 1) crystals�E/E is 1.3 × 10−4 up to about 13 keV at whichoint the resolution degrades since the source divergence becomes

arger than the width of the Si(1 1 1) rocking curve.A set of two-dimensional parabolic beryllium compound refrac-

ive lenses (CRL) [19] with a 0.8 mm aperture is installed in aHV chamber located two meters downstream from the mainonochromator. The lenses are contained in four packets of one,

components on the ID32 beamline. The distances are measured from the center ofthe 5 m straight section. Fresnel zone plates (FZP) mounted in a UHV chamber inEH2 focus the beam down to a few �m for energies above 4 keV.

two, four and eight lenses. With the help of a wobble stick and a highprecision flip-flop motion each of the four packets can be movedmanually in and out of the beam. The chamber can be accuratelyaligned with the beam by two orthogonal translations. As in-lineoptical elements, which do not deviate the beam, CRLs are veryconvenient, but they are dispersive, i.e. their focal length is energydependent. With the lenses placed at a distance of 32 m from thesource and 16 m to the sample in the first experimental hutch, ademagnification of the source by a factor of two is achieved for 15discrete energies in the range between 5 keV and 25 keV. However,the measured focus size is typically a factor of about two larger thanthe ideal value of 10 �m.

The second optics hutch (OH2) is designed for further condi-tioning of the monochromatic beam. For diffraction experiments inthe first experimental hutch, a 450 mm long flat mirror with threedifferent stripes (SiO2, Ni, and Pd) can be inserted into the beampath, acting as a low-pass filter for higher harmonics, i.e. the reflec-tions of higher order m (m = 3, 4, 5,. . .) transmitted by the Si(1 1 1)monochromator. The mirror is mounted on a hexapod and is usuallyoperated with the monochromatic beam incident at a fixed angleof 0.1◦.

For high-energy resolution experiments (especially high res-olution HAXPES and XSW using higher order reflections) twohigh-precision post-monochromator (PM) axes, mounted in twoUHV chambers, have been commissioned recently (cf. Fig. 6). Eachaxis is equipped with several channel-cut crystals (CCCs) which

can be moved with high precision in and out of the beam. Therotation is driven by a stepper motor with sub-�rad resolutionand the channel-cut crystals are mounted on a torsion bearingdriven by a piezo crystal. The rotation is measured using a Hei-

J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267 263

F romaa witht

ddfifidcvtapatgatdw1artat

dmeammtWnetwwbt

ipvieet

ig. 6. (a) The two UHV chambers containing the two axes of the ID32 post-monochxis, mounted on a kinematical support for adjustment in height and lateral positionranslation as indicated.

enhain RPN886 encoder with 50 nrad resolution. Mounted in aispersive arrangement (+/−, −/+) and using the same CCC designor both axes and reflections with the same diffraction plane spac-ng, the second axis corrects for the beam-offset introduced by therst axis. However, for maximum intensity throughput, the sourceivergence needs to be smaller than the angular acceptance of thehosen reflection. Thus, high-energy resolution can be achieved atariable energy when both axes are in use, but this may reducehe photon flux. When the output beam position is not an issuend the excitation energy can be freely chosen, a single axis of theost-monochromator and just one CCC can be used. However, tochieve the desired energy resolution, the angular acceptance ofhe selected PM CCC should not exceed the photon beam diver-ence from the undulators. At present, sets of Si(1 1 1), Si(2 2 0)nd Si(3 1 1) crystals are available that can be translated into posi-ion. Further channel-cut crystal, for specific applications are underesign. The incident photon energy bandpass can be varied over aide range. For instance at 9.9 keV it can be varied from 1.3 eV to

5 meV by using either the Si(1 1 1) main monochromator or usingSi(5 5 5) CCC. A beam intensity monitor (measuring the photocur-

ent) mounted on a linear translation is used for the alignment ofhe CCC of the first PM. A beam stop can be moved into place tobsorb the higher harmonics that pass through the first CCC usinghe same linear motion.

The first experimental hutch (EH1) houses a diffractometer [20]esigned for XSW and surface X-ray diffraction (SXRD) measure-ents. It can be used as a six-circle Kappa diffractometer for XSW

xperiments with fluorescence detection or for SXRD experimentss a so-called 2 + 2 + 2 six-circle instrument. The diffractometer isounted on a kinematical support for horizontal and vertical align-ent and can be tilted by ±1◦ to adjust the angle of incidence of

he beam on the sample for grazing incidence SXRD experiments.hen experiments are carried out in EH1, the UHV flight tube con-

ecting the vacuum line at the entrance with the vacuum line at thexit of the hutch is removed. The beam position is kept constant onhe sample on the diffractometer with an accuracy of about 1 �mith the help of gas filled beam position monitors, the signal ofhich is fed back to the piezo-element adjusting the pitch (vertical

eam movement) and roll (horizontal movement of the beam) ofhe second crystal of the monochromator in OH1.

The photoelectron spectroscopy measurements are performedn experimental hutch 2 (EH2) with the UHV flight tube in EH1 inlace which connects both UHV systems directly to the storage ring

acuum. The upstream UHV chamber (base pressure 10−10 mbar)s equipped with a PerkinElmer PHI model 10-360 hemisphericallectron analyzer fitted with a multi-channel plate detector with 16lectrodes and is used for XPS and XPS/XSW experiments with elec-ron kinetic energies of up to 4.8 keV. The UHV system is equipped

tor. (b) View of the interior of one of the chambers with the high resolution rotationrespect to the X-ray beam. Different channel-cut crystals can be selected by lateral

with a He-flow cryostat coupled with a Cu braid to the sampleholder permitting temperatures down to 30 K. Standard holders[21] can heat the samples up to 1000 ◦C and the VG OMNIAX manip-ulator provides x, y and z translations, rotation around the verticalmanipulator axis and rotation around the orthogonal axis. A sput-ter gun, an electron gun, rear-view low-energy electron diffraction(LEED) system, evaporators, and a residual gas analyzer are avail-able for sample preparation and characterization.

The second UHV chamber, located downstream is not yet fullyequipped at the time this report is written. The most prominentinstrument mounted on the mu-metal chamber is a PHOIBOS 225HV photoelectron analyzer which is able to detect photoelectronswith kinetic energies of up to 15 keV using a 2D delayline detector[22]. A high precision three-axis sample manipulator, a detectorarm, storage magazines and a surface preparation chamber areunder construction and a He-flow cryostat is being installed. Thechamber is equipped with rear-view LEED system, a He UV-lamp,a differentially pumped sputter gun, and a load lock.

4. HAXPES at ID32

Hard X-ray photoelectron spectroscopy (HAXPES) employingsynchrotron radiation in the keV range for the investigation ofvalence bands [23] and buried layers [24] is a field which is attract-ing increasing interest owing to the highly brilliant hard X-raysavailable at third generation synchrotron sources. Already the firstinternational HAXPES workshop [25] held at the ESRF in Grenoblein 2003 confirmed the growing interest in this technique. At ID32we explored several years ago PES up to 14.5 keV to demonstratethe feasibility of such studies and we experimentally determinedphotoemission cross-sections in the hard X-ray range for the firsttime [26,27]. No commercial electron analyzers for this energyrange were available at that time so we modified our PHI 10-360multi-channel hemispherical electron analyzer by adding a spe-cially designed retarding lens, as shown schematically in Fig. 7.With the sample biased up to 10 keV by a high stability powersupply, the basic operational range of the analyzer of 4.8 keV wasextended to 14.8 keV electron kinetic energy. Cross-sections for Auwere obtained on an absolute scale by comparison with well knowncross-sections of C 1s. For C the 1s (K-shell) cross-section can bemeasured to good accuracy simply by absorption experiments sincethe contribution of the L-shell is an order of magnitude smaller. Forthese experiments, the electron yield from shallow Au levels was

measured under exactly the same experimental conditions. It wasan important finding that some solid state cross-sections of Au weresignificantly different from the values calculated for atoms. In par-ticular the Au 6s cross-section turned out to be significantly largerthan those predicted by atomic calculations, as shown in Fig. 8.

264 J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267

Fig. 7. Schematics of the retarding lens mounted on the PerkinElmer PHI model10-360 hemispherical analyzer. Figure taken from Ref. [26].

Fva

Tdsttbtttpf

Bnf

5

5

bctscSS

ig. 8. Theoretical [31] and experimental cross-sections of Au 5d. The cross-sectionalues for the 6s conduction band at 5.95 keV were taken from the data of Takata etl. [32]. Figure taken from Ref. [26].

he same set-up was used to investigate the chemistry of semicon-uctor/metal/oxide stacks by angle-resolved X-ray photoelectronpectroscopy (AR-XPS) to obtain depth selective information abouthe buried interfaces of high-k dielectric thin films [28,29]. Fur-hermore, the high kinetic energy of the photoelectrons excitedy hard X-rays turned out to be extremely useful for disentanglinghe contributions from extrinsic and intrinsic inelastic scattering tohe photoelectrons emitted from carbon [30]. An intrinsic inelas-ic scattering process is, for example, plasmon excitation while thehotoelectron traverses the absorbing atom which can still inter-ere with the photo absorption process.

With the help of the new HAXPES electron analyzer, the PHOI-OS 225 HV [33], electrons with kinetic energies up to 15 keV canow be detected without biasing the sample. A survey spectrum

rom a gold sample is shown in Fig. 9.

. XSW and HAXPES at ID32

.1. Chemically resolved surface structure of Sn on Ge(1 1 1)

Chemical-state specific structural information can be obtainedy combining XSW with PES and exploiting the so-called ‘chemi-al shifts’ in photoelectron binding energies. These can change dueo changes in the local potential caused by changes in chemical

urrounding of the corresponding atom. The potential of utilizinghemical shift for XSW investigation was first demonstrated byugiyama et al. [34] and a few years later by Jackson et al. [35,36].ince then chemical shift XSW was used to study a range of com-

Fig. 9. Survey spectrum from a gold sample recorded at 13 keV excitation energywith the PHOIBOS 225 HV at ID32.

plex molecular adsorbates on surfaces [37]. We used this approachrecently to solve the structure of the Ge(1 1 1) (

√3 × √

3)-Sn phaseat room temperature [38].

The structure of the room temperature Ge(1 1 1) (√

3 × √3)-Sn

phase has been under debate for quite some time. The most impor-tant question was whether the Sn adatoms form a flat layer, assuggested by scanning tunneling microscopy images, or fluctuatevertically between two positions. This was suggested by core-levelphotoemission, which revealed that the Sn 4d line shape containedtwo chemical components [39] with a ratio of 1:2 corresponding toSn adatoms with a filled and an empty surface state. We decided tosettle this controversy by exploring the Sn adatom structure behindthe 4d core-level shift for the Ge(1 1 1) (

√3 × √

3)-Sn phase by mea-suring the photoelectrons excited by an X-ray standing wavefield.As shown in Fig. 10, the Sn 3d and Sn 4d photoelectron peaks containtwo components with an intensity ratio of two to one. In the XSWmeasurement, both peaks are recorded while scanning through theGe(1 1 1) rocking curve by changing the energy of the monochro-matic beam. From�E = −0.13 eV to 1.35 eV, the XSW moves inwardby half of the (1 1 1) lattice spacing. Here�E is the deviation fromthe Bragg energy, i.e. the energy where the Bragg reflection wouldoccur according to the simple Bragg equation, i.e. ignoring thatthe refractive index for X-rays is less than unity.. It is obvious thatthe ratio of the two components changes when scanning over theGe(1 1 1) rocking curve (cf. Fig. 10). This is a clear indication thatthe Sn species associated with the two chemically shifted compo-nents are located at different heights above the Ge(1 1 1) surfaceand are reached by the maxima of the XSW at different points onthe Ge(1 1 1) rocking curve. This can be seen more clearly on theright hand side of Fig. 10 where the integrated intensities of bothcomponents of the Sn 3d and 4d peaks are shown as a functionof the position on the Ge(1 1 1) rocking curve. By fitting Eq. (10)to the yield curves, we could immediately associate the 400 meVbinding-energy shift with a 23 pm vertical separation of the twoSn positions, linking directly the chemical states of the adatomsto their positions [38]. Interestingly, the stronger components ofthe Sn 3d and 4d line, which appear on the lower binding-energyside and account for 2/3 of the Sn adatoms, are associated with thehigher Sn position. The weaker components with higher binding

energy account for 1/3 of the Sn atoms. They can be associated witha more empty valence state character of 1/3 of the Sn atoms whichreside 23 pm closer to the surface. This is shown schematically inFig. 11. Thus, we could demonstrate that the lower binding-energy

J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267 265

F ed ars froms to the( Ref. [

ctm

5

Fapc

ig. 10. Left: XSW-modulated and spectra recorded at six incident energies centerpecies, i.e. (a) Sn 4d and (b) Sn 3d5/2. When traversing the Ge(1 1 1) rocking curvepacing. All spectra are normalized to their peak heights. Right: best fits of Eq. (10)b) 3d. (c) Measured and calculated Ge(1 1 1) reflectivity curves. Figures taken from

omponent is associated with the higher Sn atoms, which is in con-rast to previous assignments based mostly on scanning tunneling

icroscopy images and less straightforward arguments [40,41].

.2. X-ray standing wave imaging

X-ray standing wave measurements yield element-specificourier components of atomic distributions according to Eqs. (10)nd (11). Hence, provided that a sufficient number of Fourier com-onents are measured, it should be possible to reconstruct the

orresponding atomic distribution [42]:

(r) =∑

H

{1 + 2 cos[2�i(PH − Hr)]} (17)

Fig. 11. Schematic side view of Sn atoms on Ge(1 1 1).

ound the Ge(1 1 1) reflection and best fits with the two components of the two Sn�E = −0.13 eV to �E = 1.35 eV, the XSW moves inward by half of the (1 1 1) latticenormalized peak intensities I1, I2, and I1 + I2 for the two Sn core levels: (a) 4d and

38].

It should be noted that long-range structure cannot be obtainedbecause the XSW imaging analysis is limited to the Fourier com-ponents corresponding to allowed substrate reflections. In caseswhere the XSW is generated by Bragg reflection length scalesbeyond the unit cell of the crystal are not accessible.

We applied the XSW imaging technique to study the early stagesof growth of YBa2Cu3O7 on SrTiO3(0 0 1) [43,44]. An ultra thinfilm of YBa2Cu3O7 was deposited on SrTiO3(0 0 1) (see Fig. 12)by pulsed laser deposition as described in an earlier publication[45]. The sample was then transferred under vacuum into the

XSW/XPS chamber at the ID32 beamline. Seven SrTiO3 reflections(see Fig. 13) were then excited in backscattering geometry andseveral photoelectron lines (see Fig. 14) were recorded by the PHIphotoelectron analyzer. For each element and each reflection, Eq.

Fig. 12. The SrTiO3 unit cell; Sr, Ti, and O atoms are indicated by the large (green),small (red), and medium-sized (blue) spheres, respectively. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthe article.)

266 J. Zegenhagen et al. / Journal of Electron Spectroscopy and Related Phenomena 178–179 (2010) 258–267

Fig. 13. The seven diffraction planes used for the XSW analysis of SrTiO3 and YBa2Cu3O7 on SrTiO3(0 0 1) [43,44].

F iO3 ree

(peidFYpYp

5

mftHefita

10 meV at 10 keV electron kinetic energy. To perform XSW imagingroutinely in combination with PES requires a high precision, UHVthree-circle diffractometer, which is presently under constructionfor the HAXPES UHV system at ID32.

ig. 14. XSW/XPS results for a 0.5 nm film of YBa2Cu3O7 on SrTiO3(0 0 1). The SrTnergy around the seven reflections, which are indicated schematically on top.

10) was fitted to the data which yielded the amplitude and thehase of one Fourier coefficient for each reflection and element. Byxploiting the fourfold rotational crystal symmetry, this resultedn a total of 29 Fourier components and the result of the ren-ering according to Eq. (13) for Sr and Ti is shown in Fig. 15.or more details, the reader is referred to [43]. The structures of, Ba, Cu, and O can be reconstructed in the same way. A com-lete account of the analysis of the distribution and structure ofBa2Cu3O7 on SrTiO3(0 0 1) will be published in a forthcomingaper [44].

.3. Conclusions and outlook

In summary, HAXPES and XSW benefit from the brilliance ofodern synchrotron X-ray sources and have similar requirements

or the beamline optics. Combining HAXPES with XSW adds struc-ural resolution to the strengths of photoelectron spectroscopy.owever, multipole contributions to the photoemission matrix

lements must be taken into account in order to achieve theull accuracy in the structure determination by XSW. It will bemportant in the future to systematically determine multipole con-ributions experimentally, especially for excitation from higherngular momentum orbitals (l > 1).

flectivity curves and the photoelectron emission lines are plotted as a function of

Exploiting chemical shifts systematically in combination withthe XSW technique and thus identifying the structure of chemicalspecies, requires a HAXPES electron spectrometer with high-energyresolution. The instrument which is currently being commissionedat ID32 and is designed to provide an energy resolution down to

Fig. 15. Rendered image of the Sr and Ti positions with respect to the SrTiO3 unitcell reconstructed from the results of seven XSW measurements shown in Fig. 14.

opy an

A

bUFi

R

[

[[[

[

[[[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[[[[

[

[

[

[

[[41] R. Pérez, J. Ortega, F. Flores, Phys. Rev. Lett. 86 (2001) 4891.[42] L. Cheng, P. Fenter, M.J. Bedzyk, N.C. Sturchio, Phys. Rev. Lett. 90 (2003) 255503.

J. Zegenhagen et al. / Journal of Electron Spectrosc

cknowledgments

The HAXPES analyzer and UHV system are installed at the ID32eamline within the framework of a collaboration contract with theniversity Hamburg (Robert Johnson) and DESY (Wolfgang Drube).inancial support by the German BMBF by the contract 05KS4GU3/1s gratefully acknowledged.

eferences

[1] B.W. Batterman, Phys. Rev. Lett. 22 (1969) 703.[2] J. Zegenhagen, Surf. Sci. Rep. 18 (1993) 199.[3] See e.g.: K. Siegbahn, Nucl. Instrum. Meth. Phys. Res. A 547 (2005) 1, and refer-

ences therein.[4] P. Pianetta, I. Lindau, Nature 250 (1974) 214.[5] See e.g.: C. Dallera, L. Braicovich, L. Duo, A. Palenzona, G. Panaccione, G. Paoli-

celli, B.C.C. Cowie, J. Zegenhagen, Nucl. Instrum. Meth. Phys. Res. A 547 (2005)113, and references therein.

[6] See e.g.: T. Ishikawa, K. Tamasaku, M. Yabashi, Nucl. Instrum. Meth. A 547, andreferences therein.

[7] J. Rubio-Zuazo, G. Castro, Rev. Adv. Mater. Sci. 15 (2007) 79.[8] N. Hertel, G. Materlik, J. Zegenhagen, Z. Phys. B 58 (1985) 199.[9] M.J. Bedzyk, G. Materlik, Phys. Rev. B 32 (1985) 6456.10] Max v. Laue, Röntgenstrahl-Interferenzen (Akademische Verlagsgesellschaft,

Beccker & ErlerKolm.-Ges., Leipzig, 1941); Röntgenstrahl-Interferenzen(Akademische Verlagsgesellschaft, Frankfurt, 1960).

11] J. Zegenhagen, G. Materlik, W. Uelhoff, J. X-ray Sci. Technol. 2 (1990) 214.12] I.A. Vartanyants, J. Zegenhagen, Solid State Commun. 113 (1999) 299.13] I.A. Vartanyants, T.-L. Lee, S. Thiess, J. Zegenhagen, Nucl. Instrum. Meth. Phys.

Res. A 547 (2005) 196.14] M.B. Trzhaskovskaya, V.K. Nikulin, V.I. Nefedov, V.G. Yarzhemsky, At. Data Nucl.

Data Tables 92 (2006) 245.15] D.P. Woodruff, Nucl. Instrum. Meth. Phys. Res. A 547 (2005) 187.16] I.A. Vartanyants, J. Zegenhagen, Nuovo Cimento 19D (1997) 617.17] F. Comin, R. De Martino, K. Fezzaa, V. Formoso, L. Ortega, R. Signorato, J. Phys.

IV 7 (1997), C2/343.18] CINEL Strumenti Scientifici s.r.l., via dell’Artigianato, 14-14/A, 35010 Vigonza

(Padova), Italy, http://www.cinel.com.19] e.g.: A. Snigirev, V. Kohn, I. Snigireva, B. Lengeler, Nature 384 (1996) 49;

A. Snigirev, V. Kohn, I. Snigireva, A. Souvorov, B. Lengeler, Appl. Opt. 37 (1998)653.

20] HUBER Diffraktionstechnik GmbH & Co. KG, Sommerstrasse 4, D-83253 Rimst-ing, Germany, http://www.xhuber.de.

[[[

d Related Phenomena 178–179 (2010) 258–267 267

21] VG-Scienta Ltd., Maunsell Road, Castleham Industrial Estate, Hastings, EastSussex, TN38 9NN, United Kingdom, http://www.vgscienta.com/.

22] Surface Concept GmbH, Staudingerweg 7, D-55128 Mainz, Germany,http://www.surface-concept.com/.

23] K. Kobayashi, M. Yabashi, Y. Takata, T. Tokushima, S. Shin, K. Tamasaku, D. Miwa,T. Ishikawa, H. Nohira, T. Hattori, Y. Sugita, O. Nakatsuka, A. Sakai, S. Zaima, Appl.Phys. Lett. 83 (2003) 1005.

24] C. Dallera, L. Duo, L. Braicovich, G. Panaccione, G. Paolicelli, B.C.C. Cowie, J.Zegenhagen, Appl. Phys. Lett. 85 (2004) 4532.

25] J. Zegenhagen, C. Kunz (Eds.), Proc. of the Workshop on Hard X-ray Photo-electron Spectroscopy, Grenoble 2003; Nucl. Instrum. Meth. Phys. Res. A 547(2005).

26] S. Thiess, C. Kunz, B.C.C. Cowie, T.-L. Lee, M. Renier, J. Zegenhagen, Solid StateCommun. 132 (2004) 589.

27] C. Kunz, S. Thiess, B.C.C. Cowie, T.-L. Lee, J. Zegenhagen, Nucl. Instrum. Meth.Phys. Res. A 547 (2005) 73.

28] E. Martinez, O. Renault, L. Clavelier, C. Le Royer, V. Cosnier, J.-M. Hartmann, J.Zegenhagen, T.-L. Lee, Y. Campidelli, V. Loup, S. Minoret, J. Vac. Sci. Technol. B25 (2007) 86.

29] L. Libralesso, T.-L. Lee, J. Zegenhagen, Appl. Phys. Lett. 90 (2007) 222905.30] C. Kunz, B.C.C. Cowie, W. Drube, T.-L. Lee, S. Thiess, C. Wild, J. Zegenhagen, J.

Electron Spectrosc. Relat. Phenom. 173 (2009) 29.31] J.H. Scofield, Lawrence Livermore National Laboratory Report UCRL-51326,

1973.32] Y. Takata, et al., Appl. Phys. Lett. 84 (2004) 4310.33] SPECS GmbH, Voltastrasse 5, D-13355 Berlin, Germany, http://www.specs.de.34] M. Sugiyama, S. Maeyama, S. Heun, M. Oshima, Phys. Rev. B 51 (1995) 14778.35] G.J. Jackson, J. Lüdecke, D.P. Woodruff, A.S.Y. Chan, N.K. Singh, J. McCombie, R.G.

Jones, B.C.C. Cowie, V. Formoso, Surf. Sci. 441 (1999) 515.36] G.J. Jackson, D.P. Woodruff, R.G. Jones, N.K. Singh, A.S.Y. Chan, B.C.C. Cowie, V.

Formoso, Phys. Rev. Lett. 84 (2000) 119.37] See e.g.: A. Hauschild, K. Karki, B.C.C. Cowie, M. Rohlfing, F.S. Tautz, M.

Sokolowski, Phys. Rev. Lett. 94 (2005) 036106.38] T.-L. Lee, S. Warren, B.C.C. Cowie, J. Zegenhagen, Phys. Rev. Lett. 96 (2006)

046103.39] J. Avila, A. Mascaraque, E.G. Michel, M.C. Asensio, G. LeLay, J. Ortega, R. Pérez,

F. Flores, Phys. Rev. Lett. 82 (1999) 442.40] R.R. Stumpf, J.M. Carpinelli, H.H. Weitering, Phys. Rev. B 59 (1999) 15779.

43] S. Thiess, PhD thesis, University Hamburg, 2007.44] S. Thiess, T.L. Lee, F. Bottin, J. Zegenhagen, in press.45] T. Haage, J. Zegenhagen, H.-U. Habermeier, M. Cardona, Phys. Rev. Lett. 80

(1998) 4225.