Hard X-ray photoemission spectroscopy

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 601 (2009) 32–47

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

Hard X-ray photoemission spectroscopy Keisuke Kobayashi NIMS Beamline Station at SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 619-5198, Japan

a r t i c l e in f o

a b s t r a c t

Available online 6 January 2009

Except in the very early stage of the development of X-ray photoemission spectroscopy (XPS) by Kai Siegbahn and his coworkers, the excitation sources for XPS studies have predominantly been the Al Ka and Mg Ka emission lines. The advent of synchrotron radiation sources opened up the possibility of tuning the excitation photon energy with much higher throughputs for photoemission spectroscopy, however the excitation energy range was limited to the vacuum ultra violet and soft X-ray regions. Over the past 5–6 years, bulk-sensitive hard X-ray photoemission spectroscopy using high-brilliance high-flux X-rays from third generation synchrotron radiation facilities has been developed. This article reviews the history of HXPES covering the period from Kai Siegbahn and his coworkers’ pioneering works to the present, and describes the fundamental aspects, instrumentation, applications to solid state physics, applied physics, materials science, and industrial applications of HXPES. Finally, several challenging new developments which have been conducted at SPring-8 by collaborations among several groups are introduced. & 2008 Elsevier B.V. All rights reserved.

Keywords: Hard X-ray photoemission Electronic structure Chemical bonding state Undulator X-rays

1. Introduction Since the pioneering works of Kai Siegbahn and his co-workers [1,2], X-ray photoemission spectroscopy has grown to be one of the most universal and powerful tools available for the investigation of chemical states and electronic structures of materials. Today it is widely used in laboratories and factories for scientific investigations as well as for industrial purposes such as failure analysis and product-line monitoring. This is due to the development of commercially available sophisticated hardware using Al Ka and Mg Ka excitation sources and sophisticated analysis software. The use of synchrotron radiation introduces the ability to tune the excitation photon energy over a wide range, and thirdgeneration synchrotron radiation facilities offer opportunities for high-resolution, high-throughput photoemission spectroscopy. However, the use of synchrotron radiation was for a long time limited to the VUV/SX energy region. The probing depth of photoemission spectroscopy was thus limited to be less than a few nm, and surface sensitivity, good or bad, was one of the characteristics of the method. The first report of high-resolution hard X-ray photoemission experiments (HXPES) using a thirdgeneration undulator X-ray source was published in 2003 [3]. (In this article, I use the abbreviation HXPES, but in the literature there are many other forms, including HX-PES, HXPES, HEPES, HAXPES and HIKE. The abbreviation HXPES ties in well with SXPES, the commonly used abbreviation for soft X-ray photo-

E-mail address: [email protected] 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.12.188

emission spectroscopy.) Several groups began HXPES activities in Japan as well as in Europe at almost the same time. In this article, I would like to describe the evolution of the utilization and technical aspects of HXPES following the ‘‘1st International Workshop on HAXPES’’ held at the ESRF in Grenoble, France from September 11–12, 2003. The purpose of this article is to offer an overview of HXPES activities and present the potential of this promising and versatile method for those who are not yet involved, but have an interest.

2. Historical of HXPES development Kai Siegbahn and co-workers are credited with the first hard X-ray photoemission experiments [2]. Successful examples of Mg 1s, Mg 2p, and O 1s core spectra were reported using Cu Ka and Cu Kb radiation. As the Cr Kb and Cu Kb wavelengths are very close to 1/4 and 1/6 of the Al Ka wavelength, respectively, Al/Cr or Al/Cu double X-ray sources using bent quartz crystals to monochromatize and focus the X-rays were developed (for example, SCIENTA ESCA300). However, due to insufficient intensity of the Cu Kb excitation, these sources have not been widely used for practical applications. The first high resolution hard X-ray photoemission spectroscopy experiments using synchrotron radiation were performed by Lindau et al. in 1974 [4], who used X-rays from a bending magnet at SPEAR (Stanford Synchrotron Radiation Laboratory) to measure the intrinsic linewidth of the Au 4f core levels with high energy resolution. The observed peak count rate was very low,

ARTICLE IN PRESS K. Kobayashi / Nuclear Instruments and Methods in Physics Research A 601 (2009) 32–47

have measured the attenuation of the Al 1s photoemission line intensity for various GaAs overlayer thickness in GaAs/AlAs/GaAs hetero epitaxial samples [19]. They found that the attenuation length can be described as 0.85  E1/2, where E represents the electron kinetic energy. If we define the information depth as the thickness of the layer from which 90% of the photoelectron signal originates, this result gives a value of 16 nm for electrons of 6 keV kinetic energy in GaAs. Experimental determinations of the attenuation lengths for electrons of 4–6 keV kinetic energy in wedge-shaped overlayers of Co, Cu, Ge, and Gd2O3 on Si substrates have also been reported [31] by observing the attenuation of Si 1s HXPES spectra. As an illustration of the large information depth characteristics of HXPES, we show in Fig. 1 the Si 1s spectra of NiGe/SiO2(12 nm)/ Si substrate structures at 8 keV excitations, showing the feasibility of buried layer measurements. Si 1s signal from the SiO2 layer buried by the 15 nm NiGe overlayer is clearly observed. The substrate signal is also recognized with sufficient intensity, even under a total overlayer thickness of 27 nm. These large information depths enable us to neglect intrinsic as well as extrinsic contributions from surface layers of thicknesses of a few nm. Thus HXPES opens unique possibilities for detecting bulk electronic structures, and for PES observations of laboratoryprepared thin films without surface cleaning prior to the measurements. It also opens up the possibility of site-specific bulk-sensitive XPS when combined with the X-ray standing wave technique. This method was successfully applied in pioneering works with medium energy resolution using 2–5 keV excitations [32–35]. However to the author’s knowledge no high-resolution HXPES standing-wave work has been reported to date. The large information depth feature of HXPES also tempts us to apply HXPES to the investigation of various kinds of layers and interfaces buried to depths on the 10s of nanometers scale, since the probing depth for non-destructive depth profiling is much larger than that of conventional PES. Depth profiling can be realized by measuring core level intensities as functions of take off angle (TOA) [36] or photoelectron kinetic energies [37]. An HXPES version of a method for selectively studying buried interfaces using PES combined with X-ray standing waves (SWs) generated above a multi-layer (ML) mirror with a wedge-profile sample configuration [38] is expected to be promising for studies of electronic structures as well as chemical bonding states in multilayered samples. A precise description of this method is given by C. Fadley elsewhere in this volume. An alternative method for probing buried interface profiles has recently been demonstrated which makes use of the chemical shifts of target core levels due to alloying at the interface [39]. The bulk-sensitivity of HXPES is realized by overcoming the problem of weak signal intensities due to the rapid decrease in photoionization cross section with increasing photon energy

Photoelectrons Hard X-ray hν= 7935 eV

d

NiGe

3. Fundamental aspects of HXPES One of the most advantageous features of HXPES compared to conventional photoemission spectroscopy is its potential for bulk sensitive measurements. The probing depths of PES are determined by the inelastic mean free paths (IMFP) of the electrons within the solid. These IMFP values are known to show minima at around 50–100 eV of a few tenths of a nanometer. Dallera et al.

SiO2 Si sub.

12 nm

intensity (arb. units)

thus no further trials aiming at high resolution hard X-ray spectroscopy for practical studies were attempted until the first report of HXPES using undulator X-rays in 2003. Meanwhile, double crystal monochromators at bending magnet beamlines continued to be used for high energy photoemission spectroscopy with medium resolution for Auger and XPS studies [5–14]. A dedicated high energy XPS instrument was installed at a HASY LAB wiggler beamline in 1993 [15]. Combination of X-ray standing wave techniques with high energy PES were also tried by several groups [6,16,17]. Breakthroughs in the HXPES technique were made by several groups at the ESRF and also at SPring-8 independently in 2002–2004 [3,18–22,26]. At SPring-8, a JASRI-RIKEN collaboration succeeded in the first feasibility test using 6 keV X-ray excitation in 2002 at the RIKEN beamline BL29XU. In this collaboration, various user groups in the fields of Si-LSI, compound semiconductors, and spin electronics researches were involved from the beginning [3,18]. Collaboration with Hiroshima University Synchrotron Radiation Research Center (HiSOR) has also been included from the very early stages. At the ESRF, activities at ID32 had already begun [21], and HXPES instrumentation was installed in the VOLPE project at ID16 [20] at almost the same time. One of the most essential factors for the breakthrough in HXPES is the use of high brilliance undulator X-rays. The undulators at 3rd generation synchrotron radiation facilities such as the ESRF and SPring-8 provide photon fluxes of higher than 1011 photons/sec even after the reduction of bandwidths down to around 50 meV or less. The reduction of the bandwidths is realized by a channel-cut Si single crystal post monochromator, which is installed downstream of the beamline monochromator. Another improvement was the development of hardware for high kinetic energy electron analysis [20–26]. A total resolution of 60 meV has been achieved at 8 keV [27], but for practical purposes a total resolution of 200–250 meV is used. Channel-cut postmonochromators and high voltage analyzers have also successfully introduced to bending magnet beamlines at the ESRF [25], and BESSY II [28], where sufficient X-ray fluxes are available for core level spectroscopy. At SPring-8, HXPES R&D activities at BL47XU were began by the JASRI group in 2004. HXPES beamtime was partially opened for public use in the latter half of 2004, and the number of accepted proposals grew rapidly [29]. Because of the rapid increase in demand from industrial users, part of the beamtime at BL39XU was devoted to these studies. These industrial subjects have recently been transferred to BL46XU, where an experimental station exclusively for HXPES has been constructed. Meanwhile, a group from Osaka University began activities at the 20 m long undulator beamline BL19LXU in 2003. In 2006, the beamline station of the National Institute for Materials Science (NIMS) at SPring-8 introduced a HXPES experimental station to BL15XU, the NIMS contract beamline, in collaboration with groups from the Hiroshima University Synchrotron Radiation Research Center (HiSOR) and the Japan Atomic Energy Agency (JAEA). Consequently, five beamlines in total are currently used for HXPES activities at SPring-8. Outside SPring-8, an HXPES experimental station is going to be constructed at PETRA III [30].

33

Si 1s TOA=80º

Si-O

Si-Si

d 7 nm 12 nm 15 nm

1850

1846

1842

1838

1834

Binding energy (eV) Fig. 1. Si 1s spectra of NiGe(dnm)/SiO2(12 nm)/Si(1 0 0) substrate with NiGe overlayer thicknesses d of 7, 12, and 15 nm. The X-ray photon energy was 8 keV. Note that even in the thickest overlayer sample, the Si 1s peak of the substrate is clearly observed. (Unpublished data courtesy of H. Kondo, Nagoya Univ.)

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[40–42]. In general, the cross sections of s subshell ionisation decrease more slowly than those of the p and f subshells, as shown in Fig. 2. This strong photon energy dependence of the cross sections affects the HXPES valence band spectral shape. In order to understand how the HXPES valence band spectra are modified from the DOS, simulated spectral shapes for typical materials are compared with HXPES experimental results at 6 keV excitations in GaN, GaAs [43,44] in Fig. 3, and also in Ag [45] in Fig. 4. In these simulations, the calculated spectra were obtained by the weighted sums of partial DOSs. The HXPES valence band spectrum of GaN is mostly determined by the Ga 4s partial density of states, whereas that of GaAs rather resembles the total DOS, as shown in Fig. 3(a)–(d). In the Ag valence band spectrum at 7.91 keV excitation, the Ag 5s state manifests more strongly than in the spectrum at 1.48 keV excitation. The overall spectral shapes of the experimental results in Figs. 3 and 4, are consistent with expectations based on calculated photoionization cross sections, however the relative contributions from each partial state are found to be considerably depend on materials. Deviations from the calculated values of the cross sections were also observed for core levels in several other materials [26]. For the 4f rare earths, the 4f cross sections steeply decrease with increasing photon energy, as shown in Fig. 2. Crossovers with the 6s cross sections take place in the HXPES energy region, thus the appearances of the valence band spectra are expected to be strongly photon energy dependent in this region. The photoionization cross section for linear polarized X-ray excitation can be written as

GaAs EDC

GaAs DOS

3 Ga3d

As4s

Ga4s

2

Ga3d

1

Ga4p As4p

20

15

10

5

20

0

Binding Energy (eV)

15

10

5

0

Binding Energy (eV)

GaN DOS

b

GaN EDC

nb

nonbonding bonding Ga3d

dsi =dO ¼ ðsi =4pÞ½1 þ bP 2 ðcos yÞ þ ðg cos2 y þ dÞ sin y cos j. Here P2 (x) is the second order Legendre polynomial, y is the angle between the electric field and the photoelectron momentum, and j is the angle between the photon momentum and the plane passing through the electric field vector and the photoelectron momentum [42]. In photon energy regions lower than VUV and SX, the dipole approximation holds and only the first term needs to be taken into account. b is the asymmetry factor, and determines the angular distribution of photoelectron emission from a single atom in respect to the polarization vector.

Photoionization Cross Section (b)

1000

antibonding N2s Ga4s

2

Ga4p ab

1

N2p

20

15

10

5

Binding Energy (eV)

0

20

15

10

5

0

Binding Energy (eV)

Fig. 3. LDA calculated density of states (DOS) of (a) GaAs, and (c) GaN compared with HXPES spectra of (b) GaAs, and (d) GaN for 6 keV excitation. The experimental spectra are fitted using linear combinations of the calculated partial DOSs. Note that the GaAs HXPES spectrum rather resembles the overall DOS, but the GaN spectrum is strongly modified due to the small contributions from the N 2p partial DOS (Ref. [43]).

Ce4f

100 10 Au5d 1 Ge4s Au6s

0.1

Si3s 0.01

Ce6s Cr4s Ge4p

Si3p

0.001

Cr3d 2

102

3 4 56

2

103

3 4 5 6

104

Photon Energy (eV) Fig. 2. Photon energy dependencies of the photoionization cross sections of s, p, d, and f valence electron states in Si, Ge, Cr, Au, and Ce. Data taken from Ref. [42].

Calculations show that b values are positive in the HXPES energy region in most cases, thus photoemission intensities from free atoms show maxima in the direction parallel to the polarization vector [41,42]. As the photon energy increases, the dipole approximation breaks and higher order terms become nonnegligible. Recently, Yoshikawa et al. have shown that the contribution from the quadratic term is certainly not trivial [46] in the photon energy region higher than 4 keV. Another important point to be mentioned here is the effect of photoelectron kick-back. In the photoemission process, the energies and momenta of all particles concerned must be conserved. The photon momentum is negligibly small in the HXPES energy region. However, the momentum that is transferred to the emitter atom from the out-going electron is not trivial for light atoms. The binding energy shifts towards higher energies and is estimated to be (m/M)EB where EB is the binding energy. For instance, this correction term is 0.3 eV for photoemission from the C 1s core level excited by a photons of 8 keV. Takata et al. [47] recently observed this photoelectron kick-back effect in C 1s

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Fig. 4. Results of curve fits using combinations of the calculated partial DOSs to experimental Ag valence band spectra for 8 keV (a) and 1.4 keV ( b) excitations (Ref. [45]).

photoemission spectra of highly oriented pyrolytic graphite (HOPG), varying the photon energy in the region of 500 eV–8 keV. The C 1s peak shows a binding energy shift and asymmetric broadening towards the higher binding energy side. The results were quantitatively simulated using a framework similar to the theory of the Mo¨ssbauer effect [48]. Recoil effects become more and more important as we go to higher energy excitation, especially for the lighter elements such as B, Be, and Li. Effects due to photoelectron kick-back have also been observed at the Fermi edge of Al by Takata et al. very recently [49].

4. Instrumentation Special care is needed to realise practical throughput despite the lack of signal due to the rapid decrease of photoionization cross sections with increasing photon energy. Already in the first HXPES test experiments at SPring-8’s BL29XU, the configuration of the experimental setup was carefully devised to maximise signal intensity [22,27]. The incident angles of the X-rays on the samples were kept near to the total reflection angles to make the penetration depths as close as possible to the escape depths of the photoelectrons. This results in the footprint of the X-ray beam on the sample surface being elongated in the horizontal direction. To accept as many electrons as possible, the entrance slit of the

analyzer should also be in horizontal direction. Because BL29XU uses a planar undulator, the electric filed vector of the X-rays is in the horizontal direction. Taking into account the fact that the asymmetry factor b for almost all subshells is positive in the energy range of interest, photoemission intensities show maxima in the direction of the electric filed vector [41,42]. Based on these considerations, we use a configuration in which glancing X-ray incidence and normal emission are simultaneously realized. The hemispherical analyzer used in the initial experiments used an electron lens of 5 times magnification in front of the entrance slit; enlarging the image of the X-ray spot on the entrance slit by the same factor of 5. In the first stages of our test experiments at BL29XU [3,22] and also at BL47XU, we did not use focusing mirrors for the X-rays, thus the collection efficiencies in the vertical direction were about 1/10 of the ideal case. The throughputs were thus improved by more than one order of magnitude following the introduction of focusing mirror systems [50] at both beamlines. As an example of a state-of-the-art experimental HXPES setup Fig. 5 shows the configuration of the experimental station at BL47XU constructed by E. Ikenaga. The bandwidths of the X-rays monochromatised by the Si 111 double crystal beamline monochromater are further reduced by a Si 111 channel cut post-monochromator. Photon energies of 6, 8, and 10 keV can be obtained using the 333, 444, and 555 reflections with intrinsic

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8.0x105 Au film, 300K BL47XU Intensity (cps)

6.0x105

Fermi Edge h =7.94keV E=228meV

4.0x105

2.0x105

0.0 7939.0

7939.5

7940.0

7940.5

7941.0

Kintic Energy (eV) Fig. 5. (a) State of the art HXPES configuration at BL47XU, constructed by E. Ikenaga in collaboration with T. Ishikawa’s group of RIKEN/SPring-8. The alignment of analyzer to the beamline is the same as that of BL29XU [22], (b) Au Fermi edge spectrum obtained for 8 keV X-ray excitation under practical measurement conditions. The total resolution as derived from edge shape analysis is 228 meV (figure courtesy of E. Ikenaga).

bandwidths of 50, 38, and 15 meV [51], respectively. An X-ray flux of 1011 photons/sec is available at 6 keV downstream of the postmonochromator. The X-ray spot size at the sample position is reduced to ca. 30 mm both vertically and horizontally by focusing mirrors. The long-term stability has been estimated by recording Au 4f spectra for about 10 days, as shown in Fig. 6. The drifts in binding energy and integrated intensity were within 75 meV and 71%, respectively. This extremely high stability is due to the high stability of the power supply electronics of the analyzer, and also the constant beam current due to the top-up operation of SPring8. The total energy resolution for practical use was estimated to be 228 meV by measuring the Fermi edge of Au (shown in the insert of Fig. 5). The highest resolution used was 55 meV at 8 KeV photon energy. For pursuing higher resolution, adoption of the post monochromator configurations illustrated by Ishikawa et al. [51] will be needed.

5. Applications The large information depth offered by HXPES offers much versatility of applications in various fields of solid state physics, applied physics, materials science, analytical science, industrial R&D and so on. Here I would like to summarize the recent activities of different HXPES groups, concentrating mainly on work carried out after the 1st HXPES workshop.

5.1. Solid state physics Since Sekiyama et al. demonstrated that high energy high resolution SXPES can only reveal the bulk electronic states in CeRu2Si2 and CeRu2 [52], higher bulk-sensitivity in photoemission has been recognized to be truly necessary for the study of the physics of strongly correlated electron systems. The first bulksensitive HXPES study was performed by Sato et al. at on YbInCu4 at SPring-8’s BL29XU using 6 keV X-rays [53]. They found that the valence change as determined from the intensity ratio between the Yb2+ and Yb3+ components of the Yb 3d core levels showed a first-order-like jump, comparable to that determined from thermo-dynamical data, at the first-order valence transition at Tv=42 K. This result is very different from the earlier SXPES and VUVPES results, where the estimated valencies were smaller, and the jumps rounded. Very recently Moreschini et al. [54] performed a comparable investigation of several Kondo systems including YbAl3, YbInCu4, and YbCu2Si2 using high-resolution X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS), and HXPES. They found that HXPES using 6 keV excitation was still not sufficiently bulk-sensitive as compared to XAS and RIXS. The same argument was made by the Suga group for YbInCu4, who reduced the real bulk spectrum from the observed spectrum quantitatively by taking into account contributions from surface and subsurface layers [55]. For SmOs4Sb12, the coexistence of strongly mixed valence and heavy-fermion

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Au 4f BL47XU: hv =8keV

4f7/2

Intensity (arb. unit)

4f5/2

Au 4f7/2 peak position (Binding energy/eV)

K. Kobayashi / Nuclear Instruments and Methods in Physics Research A 601 (2009) 32–47

Au4f7/2peak drift

±5meV

0

After 235hour

50

100

150

200

250

Time (hour)

Au 4f7/2 Peak Area Deviation (%)

Au4f7/2peak area

fluctuation

0 Binding Energy (eV)

37

50

100

150

±1%

200

250

Time (hour)

Fig. 6. Long term stability data of HXPES measurements at BL47XU. (a) Comparison of the initial Au 4f spectra with one recorded after ten days, showing almost constant peak position as well as intensity, (b) Binding energy stability for Au 4f 7/2 over the 10 day period. The long term energy drift is within 75 meV, (c) Integral intensity fluctuation was within 71% during 10 days (unpublished data courtesy of E. Ikenaga).

character has recently been found by Yamazaki et al. by combining SXPES and HXPES observations [56]. Hole-doped R1xAxMnO3 (R stands for rare earths) manganese oxides of perovskite structure exhibit rich and complex phenomena due to the interplay among different degrees of freedom of such as spin, charge, orbital, and lattice. Among these materials, La1xSrxMnO3 (LSMO) exhibits the highest Curie temperature, at 360 K with x=0.4 (the optimum hole doping composition). The ferromagnetic phase is half-metallic in nature and shows colossal magneto-resistance. A bulk-sensitive HXPES study has been performed by Horiba et al. [57], who found a shoulder structure, becoming most prominent at x=0.4, on the low binding energy side of the main peak structure of Mn 2p, as shown in Fig. 7. This structure was confirmed to be smaller for surface sensitive PES, confirmed by changing the photon energy and take-off angle of the photoelectrons. The structure was assigned as a bulk ‘‘well screened’’ Mn 2p core spectrum, and successfully simulated using MnO6 (3d4) cluster model calculations with D4h symmetry. The study introduces new states, due to the doping-induced states developing into a metallic band at the Fermi level (labeled C in the inset to Fig. 7). The experimental spectra was fitted by varying two parameters: the charge transfer (CT) energy between Mn 3d and

the new C states (D*), and the hybridization between Mn 3d and the new C states (V*). All other parameters were fixed at the values determined by Taguchi and Altarelli [58]. The cluster calculations indicate a larger hybridization strength V* with the coherent states, or an increase in delocalization, for the ferromagnetic (FM) compositions (x=0.2 and 0.4) as compared to the antiferromagnetic (AFM) compositions (x=0 and 0.55). This is consistent with the known half-metallic ferromagnetism, which is stabilized with an increase in hybridization, for the manganites upon doping. The results also suggest an analogy to the Kondo coupling between f states and conduction band states with V*(EF)N(D(EF))1/2 [59], where D(EF) is the DOS at EF. The above results indicate that the ‘‘well screened’’ features in the core level spectra are expected to be useful probes for the electronic states at the EF, which determine the electronic as well as the magnetic characteristics of these classes of strongly correlated materials. The well screened feature becomes prominent with decrease in temperature below the metal-insulator phase transition. Tanaka et al. have investigated the temperature dependence of the well screened satellite of Mn 2p in strained epitaxial thin films of La0.85Ba0.15MnO3 (LBMO) on STO [60]. They found that the

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5.2. Applied physics and materials science

Fig. 7. Comparison between the cluster calculation and HXPES spectra of the Mn 2p core level. The inset shows a schematic diagram of the energy levels of the valence band (Ref. [57]).

integrated intensity of the well screened peak correlates with the temperature dependence of magnetization. The same type of well screened satellites were observed in V 1s, V 2p, and V 3s of V2O3 [61]. The bulk screening effect in La2xSrxCuO4 (LSCO) and Nd2xCexCuO4 (NCCO) [62], and V1.98Cr0.02O3 and Bi2212 [63] were also investigated by Taguchi et al. All the results mentioned above represent evidence that the screening effect is suppressed for the surface regions of a few nm for all the materials investigated. Very recently, Panaccione et al. have reported a precise analysis of surface-bulk screening based upon SXPES and HXPES investigations on NCCO [64]. They concluded that the difference between surface and bulk in this material is located within two unit cells. These results suggest that core level studies using bulk-sensitive HXPES offer a new means for the investigation of electronic structures in the class of materials with strong electron correlation. A clear correlation between coherent intensity at EF and the well screened/shake-down satellites of core levels has been verified by Panaccione in V2O3 [65]. A comprehensive summary of HXPES investigations on strongly correlated systems, including discussions on attenuation length, cross sections, well screened/shake down satellites and their correlations with coherent structures at EF, has been given by Panaccione et al. [66]. Significant enhancement of valence band intensity in La1.2Sr1.8Mn2O7 at high temperature (423 K) was reported by Offi et al. [67]. This phenomenon was interpreted as evidence of the localization of Mn 3d-derived charge. To complement the list of work on strong correlation physics using HXPES, I also list the papers of [68–72], and the very recent work of Taguchi et al. on NiO [73].

5.2.1. Si-ULSI related subjects, depth profiling The large probing depth of HXPES enables us to analyze the electronic and chemical states of solid materials 10–20 nm or more below the surface. Non-destructive depth profiling techniques using angle-resolved X-ray photoemission spectroscopy (ARXPS) have been developed, and widely used in the thin film analyses. The necessity for controlling the thickness and electronic properties of ultra-thin (1–2 nm) Si oxide in the Si-LSI process demanded the refining of this kind of technique, to allow for the precise determination of the profiles of SiO2/Si and SiON/Si interfaces using the maximum entropy method (MEM) for the analysis of take-off angle (TOA) dependencies of XPS spectra [36]. Demands for high speed Si-ULSI inevitably requires reduction in thickness or enlargement of the dielectric constant of gate insulators [74]. Instead of SiO2, for which the necessary thickness has already reached the physical limit, the introduction of the socalled high-k materials to the ULSI fabrication process are a matter of great urgency. High-k gate stacks present various difficulties to be overcome, such as the controls of interface reactions, fixed charges, and interface work-functions. Conventional XPS cannot be effectively applied to the investigation of high-k gate stacks because of insufficient probing depth. HXPES probing depths however match the typical thickness of the high-k gate insulators of 3–4 nm, making the Si substrate accessible through a metal gate electrode layer of 5–10 nm and insulator layers of 3–5 nm in total. The first ARHXPES application to depth profiling studies of high-k/interlayer/Si substrate structures were performed by Hattori et al. [75], who also subsequently investigated different kinds of high-k gate stacks [76,77]. The combination of ARHXPES with high-resolution Rutherford backscattering (HRRBS) has been shown to be promising for high-precision determination of the interface profile [75,78]. Some more examples should be mentioned as applications of HXPES to integrated circuit related R&D; the investigation of polySi gate electrode instability due to ion implantation and the annealing process [79], plasma doping of P and activation by annealing for the formation of shallow junctions [80,81], determination of SiO2/Si (100) interface dielectric constants [82], chemical state analysis of W/HfO2/GeON/Ge stacks [83], and oxide films on 4-H SiC epitaxial surfaces [84].

5.2.2. Spintronics Spintronic devices are currently attracting much attention in the area of next-generation data storage and processing. The discovery of functional spintronics materials is quite important. The 3d transition metal oxides exhibit a rich variety of electrical and magnetic properties even at room temperature [60], and are regarded as candidates for applications in functional spintronic devices. The tunneling magneto-resistance (TMR) effect has been widely investigated for applications to detecting magnetic memory bits. HXPES can play a role in this research as a unique probe for studies of laboratory-prepared thin-films, multi-layers, and their buried interfaces. Wadachi et al. have recently observed the V 3d valence band state of layers of LaVO3 (LVO) buried underneath a LaAlO3 (LAO) layer [85]. They found that the Mott–Hubbard gap of LVO remained open at the interface. They also discussed the distribution of V3+ and V4+ in the layered structure based upon SXPES and HXPES core level measurements. Extensive studies on the valence band structures of half-metallic Heusler compounds, which are expected to be promising materials for TMR devices, have been conducted by Johanes Gutenberg University of Mainz group [86–88], who have also recently shown that HXPES can be

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Fig. 8. Comparison of the Co2MnSi valence band spectra with different thickness, 2 nm (a) and 20 nm (b) of the MgO interlayer with spectra from the bulk material (c) close to EF (hn =5.95 keV). Co2MnSi d states with different character are indicated by arrows. This result presents evidence that the valence band structure near the Fermi level in the MgO/Co2MnSi tunnel junctions are the same as those of bulk Co2MnSi (Ref. [89]).

5.2.3. Compound semiconductors and other functional materials The applications of valence band spectroscopy to advanced materials research was started at the very early stage of HXPES development at SPring-8. Examples of results are; [Ca24Al28O64]4+(e)4 as a solid-state electride (ionic material with electrons as anions) with a low workfunction [94], InGaZnO amorphous thin films for transparent electronic devices [95], GaMnN [18] and GaCrN [96] as materials for spintronics applications, InN [97,98] and ZnMgO thin films for electronic and optoelectronic devices [99], and superconducting B doped diamonds [100]. One of the most important features of HXPES for studies of these materials is that weak bulk-like occupied states in the region which spans the band-gap region up to above the conduction band minimum (CBM) are clearly detected in the laboratory-prepared samples without any surface treatment prior to the measurements. As a typical example, Fig. 9(a) and (b) show the evolution of the valence band and the occupied states above the valence band maximum (VBM) with O-incorporated InN [97,98]. The valence band structure is strongly modified correlating with the O 1s signal intensity. The intensity of the

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used to probe the valence band of a half-metallic Heusler alloy epitaxial layer beneath MgO (2 and 20 nm) and Al2O3 (1 nm) layers, as shown in Fig. 8 [89]. Even for MgO thicknesses of 20 nm, the Heusler alloy valence band is observed in the band gap window of MgO and Al2O3. The p-n junction is the most crucial key for developing a spinelectronics transistor. (Nd, Ce)MnO3 (NCMO) systems, which have been considered as promising materials that typify electrondoped manganites, have been investigated by Yanagida et al. [90]. The results demonstrated not only the suppression of Mn4+ (hole doping) but also the presence of a mixed valence state of Mn2+ and Mn3+ ( electron doping on the eg band.) within NCMO films. A HXPES investigation of the Fe3xMxO4 (M=Mn, Zn) solid solution system, which is one of the best candidate materials for ferromagnetic FET devices working at temperatures above room temperature, has been performed by Takaobushi et al. [91,92]. As designable and novel nano-materials for spintronic devices, transition metal substitutes titania nanosheets have been investigated by Osada et al. [93]. HXPES is being effectively used to study these classes of materials for the investigation of charge states of the magnetic ions and valence band structures.

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Fermi Energy (eV) Fig. 9. (a) Valence band HXPES spectra of InN with different O incorporations. The intensity of the structure at 1.5 eV increases whereas that of the structure at 5.2 eV decreases with increasing O 1 s intensity. (b) The valence band spectra near the EF are shown on an enlarged scale. A weak metallic band appears above the VBM. The separation between the VMB and the Fermi level, the positions of which are denoted by dashed lines, increases with increasing O 1 s intensity. (c) Fermi level positions and optical gap values evidently correlate, as shown in this figure. The increase of the optical gap in this material is explained by a rise of the Fermi level due to the occupation of conduction band states by carriers which are donated by the incorporated oxygen (Refs. [97,98]).

structure at 1.5 eV increases whereas that of the structure at 5.2 eV decreases with increasing O 1s intensity. The integral intensity of the occupied state above the VBM also increases with increasing O 1s intensity. The Fermi level position clearly correlates with the optical gap, as shown in Fig. 9(c). The optical gap and the carrier concentration were found to increase with the increase of the integrated occupied state intensity. These experimental results show that (i) oxygen atoms incorporated in InN act as donors, (ii) carrier occupation of conduction band pushes up the Fermi level position, resulting in an increased

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optical gap, and (iii) the incorporation of oxygen changes the valence band structure. These results also reveal the existence of in-gap states, which correlate with non-substituted O and with the electronic characteristics of the films. This shows that HXPES has potential applicability for the investigation and characterization of the transport properties of thin-film electronic materials. The results of HXPES characterization of InZnGaO amorphous films is shown as a typical example of this application in Fig. 10 [95]. The (GeTe)-(Sb2Te3) pseudo binary alloy system (GST) has been intensively studied for the research and development of optical and electronic rewritable memory devices. The amorphouscrystalline phase transition in this alloy system attracts much attention both in applied and basic research fields, however, no generally accepted understanding of the mechanism has yet been established. In order to elucidate the phase change mechanism, Kim et al. have conducted systematic HXPES investigations on the (GeTe)1x(Sb2Te3)x alloy system [101,102]. Fig. 11(a) shows the valence band spectra of crystalline phase GST. The dotted curve is the valence band spectrum of Sb. The doublet peaks at high binding energy and the band at lower binding energy are assigned as the lone-pair s-bands and the p-band, respectively. Sb, which has 5 valence electrons with s2-p3 configuration, takes a distorted cubic structure, a rhombohedral structure, dominantly sustained by p3 orbitals. GeTe also has 5 valence electrons per atom on average, and takes a rhombohedral structure. Evidently, the valence band structure resembles that of Sb, as seen in the same figure. The difference is that GeTe is a narrow-gap semiconductor, whereas Sb is a semimetal. All of the GST alloy’s valence bands show similar features to GeTe and Sb. Thus s-p hybridization is also weak in the GSTs.

Crystalline phase (GeTe)1-x(Sb2Te3)x Te 6s Sb 5s Ge 4s Sb2Te3 x=1/3 (225) x=1/4 x=1/5 x=1/9 x=1/17 x=1/3 x=1/43 x=0 (GeTe)

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Fig. 11. (a) HXPES valence band spectra of Sb, GeTe and GSTs. (hn=7.94 keV). (b) Comparison of amorphous and crystalline phase HXPES valence band spectra (Refs. [101,102]).

A comparison between amorphous and crystalline valence band spectra is shown in Fig. 11(b). Astonishingly, both the phases show very similar spectra. The resemblance becomes stronger as the composition x increases. This result is in clear contrast to the case of amorphous Si, where a drastic change in valence band structure from that of crystalline Si takes place due to the inevitably introduced breaks of the rigid sp3 bonding. One of the interesting features of the amorphous spectra is the appearance of a clear band-gap between the cation s-band and the p-band. The lone pair s-bands exhibit very weak dispersions, and thus should reflect local randomness in the line shape. In case of GeTe, the s-band line-widths are obviously broader in the amorphous phase. In GST225 (x=1/3), they are almost the same in the crystalline and amorphous phases. This apparently shows that local randomness in the crystalline phase is already as heavy as that in the amorphous phase. The unusual resemblance of the electronic structures between the amorphous and crystalline phases is considered to be an essential key to the fast reversible phase change in GST. A very recent DFT calculation of the DOS by Akola and Jones [103], reproduced the overall spectral features and also the appearance of the bandgap mentioned above, as shown in Fig. 12. 5.3. Industrial applications Here I would like to introduce three typical examples of HXPES applications to industrial problems. Nakai et al. of Toshiba Ltd. performed, for the first time, a HXPES investigation of the influence of interface layers on the chemical and electronic states of a phase-change GeTe-Bi2Te3 compound (GBT) recording material [104]. The samples used have the same film structure as re-writable HD DVD media suitable for fast over-write, and are

Fig. 10. (a) HXPES valence band spectra films and (b) a magnified view around the bandgap of amorphous-IGZO films, deposited on silica glass substrates at room temperature (RT) using pulsed laser deposition (PLD) with a KrF excimer laser (l=248 nm) at an oxygen pressure of 1.0 Pa to control the electron density Ne at 1019 cm3. Films deposited at a pulsed laser power of 2 J cm2 have a lower Hall mobility (mHall) of 2.5 cm2 V1 s1 (hereafter, denoted low-quality [LQ] films), while those deposited at 9 J cm2 have a larger Hall mobility of 15 cm2 V1 s1 (highquality [HQ] films). Thermal annealing was carried out at 400 1C for 0.5 h in vacuum (103 Pa) to improve film quality and carrier transport. Note the correlation between film quality and in-gap state density (Ref. [95]).

(i) ZnS–SiO2/IF/GeBiTe/IF/ZnS–SiO2/Ag alloy/PC substrate, and (ii) ZnS–SiO2/GeBiTe/ZnS–SiO2/Ag alloy/PC substrate. The ‘IF’ in (i) represents interlayers located above and below the GBT recording layer. The GBT valence band spectra near the VBM were observed through the bandgap of a ZnS-SiO2 overlayer. Nakai et al. showed that by closely examining the Ge and Te peaks, the bonding state of the elements in the amorphous state of the

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the composition and chemical state of alloy elements below the oxide layer. (ii) It was confirmed by the application of ARHXPS that Cu segregates at the oxide–metal interface of Ni-based alloys in the metal dusting corrosion environment. This Cu segregation is considered to increase corrosion resistance against metal dusting.

6. State of the art and future developments In this Section 1 would like to introduce several challenging developments conducted by the NIMS and JASRI groups. 6.1. SPIN-HAXPES and MCD-HXPES

Fig. 12. (a) DFT calculated valence band DOS of crystalline and amorphous (GeTe)1x(Sb2Te3)x with x=0.3 (GST225). (b) Difference spectrum between the amorphous and crystalline phase spectra. Note the appearance of a clear gap between the p-band and Ge–Sb s bands in the amorphous phase. This is consistent with the HXPES observations of Fig. 11(b) (Ref. [103]).

phase-change recording film with interface layers was found to be closer to that of the crystalline state than the amorphous film without the interface layers. The DOS of the valence band for the crystalline GBT recording film was almost the same as that of the amorphous state for the sample with interface layers on both sides of the recording film. They postulate that these findings are closely related to a factor allowing high-speed crystallization. Lithium nickel oxide (LiNiO2) based materials are promising candidates for use as positive electrodes for high-power Li-ion cells, one of the keys to the development of hybrid automobiles due to their high power-to-energy ratio. The characteristics of the interfaces between the electrolyte and the electrodes are very important in the case of the positive electrode surfaces, because their role in the mechanism of power degradation in high-power Li-ion cells is significant. HXPES studies were performed by Shikano et al. on positive electrodes used in high-power batteries [105]. Li2CO3, hydrocarbons, ROCO2Li, polycarbonate-type compounds, and LiF were observed on the positive electrode surfaces, indicating that HXPES is a powerful tool for the investigation of the degradation process of the positive electrodes in the Li- ion cells. Metal dusting, a type of corrosion resulting from the catastrophic carburization or graphitization of steels and alloys in carbonaceous atmospheres, is a prominent cause of corrosion damage for high temperature materials used in ammonia, methanol, and synthesis gas plants. Metal dusting is often encountered when steels and alloys are exposed to CO-containing synthesis gas environments at 723–973 K. A Sumitomo Metal Industry group has performed HXPES investigations of corrosion damage in Ni-based alloys containing Cu, which are known to show good resistance to metal dusting [106]. The main results they obtained are summarized as follows: (i) The large information depth of HXPES was useful for the non-destructive analysis of

The direct measurement of spin-dependent electronic states can be realized using spin-resolved PES [107]. The problem with this method is that the efficiency of electron spin detectors is low. This is especially serious in the HXPES energy region, where the photoionization cross sections are much smaller than those in the VUV and SX regions. Nevertheless SPIN-HXPES is a challenging target as a bulk-sensitive tool for the observation of the spinresolved DOS. The development of a multi-channel spin-detector and a wide acceptance-angle analyzer are the keys to realizing practical throughput. This challenge is being undertaken under the framework of a collaboration between the Johannes Gutenberg University of Mainz, JASRI/SPring-8, and NIMS [108]. An alternative method for obtaining both electronic and magnetic state information of magnetically ordered materials is PES combined with magnetic circular dichroism (MCD-PES) [109–112]. The extension of this method to the hard X-ray region, MCD-HXPES, offers the following advantages: (i) bulk-sensitive electronic and magnetic states can be obtained without elaborate surface treatment, (ii) core-level MCD-HXPES offers elementspecific measurements at a fixed photon energy, and (iii) the corelevels of most elements are accessible, because the binding energies of the core-levels of interest are lower than 2000 eV. Ueda et al. [113] have very recently reported a MCD-HXPES experiment on Fe3xZnxO4 (x=0, and 0.5) solid solution systems [91,92] using a diamond phase retarder for the generation of circular polarized X-rays from the p-polarized undulator X-rays at BL39XU of SPring-8. The degree of circular polarization was 0.95 at 7.94 keV. Fig. 13 shows the Fe 2p core-level HX-PES spectra, measured for the magnetization parallel (IP) and anti-parallel (IAP) to the photon spin, of a 10 nm-thick Fe3O4 thin film. The Fe 2p core-level MCD, which is defined as the intensity difference

Fig. 13. Fe 2p core-level HXPES and MCD spectra of a 10 nm-thick Fe3O4 thin film (Ref. [113]).

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between the IP and IAP spectra, is shown at the bottom of the figure. The Fe 2p3/2 and 2p1/2 main peaks correspond to the Fe3+ component, while the shoulder structures at the lower binding energy side of the main peaks correspond to the Fe2+ components. The Fe 2p core-level MCD spectrum shows a positive peak at EB710 eV and also a strong negative peak at EB708 eV, around the shoulder structure of the Fe 2p3/2 photoemission spectra. The sign of the MCD in the Fe 2p1/2 region is opposite to that in the Fe 2p3/2 region. The structure of Fe3O4 can be rewritten as; ½Fe3 þ ð#: 5mB ÞAO½Fe3þ þ ð": 5mB ÞFe2 þ ð": 4mB ÞB O3 , where A and B denote the sites of metallic ions in the normal spinel structure, and arrows denote the directions of the magnetic moments, which are also designated beside the arrows. The net magnetic moment of the Fe3+ ions vanishes due to the compensation between the A and B sites, and only the magnetic moment of the Fe2+ ions at the B site remains. This is consistent with the observation of MCD signal only for the Fe2+ ions in Fe3O4, as shown in Fig. 13. An application of MCD-HXPES to the valence band is also expected to be useful for investigations of the electronic and magnetic states. The valence band MCD-HXPES can be comparable with spin-resolved band calculations, because it is possible to calculate the valence band MCD-HXPES under the dipole approximation. Another application of MCD-HXPES is to probe the depth information for both electronic and magnetic states. The TOA dependence of MCD-HXPES enables ones to probe the depth information.

between the two mirrors is 150 mm, and the working distance of the K-B system is 100 mm to the focus point. An incident slit of 300 mm  300 mm is located 10.65 m upstream from the center of MV. To ensure stability of the K-B system, it is mounted on a rigid base separate from the HXPES system. The alignment of parameters such as the glancing angle and the perpendicularity between the two mirrors was achieved using a mirror manipulator with a high degree of accuracy [10]. Behind the K-B system, a wire scan system with a piezo-actuated translation stage was placed at the focal position. The X-ray intensity is monitored using an ionization chamber located behind the wire scan system. Figs. 15(a) and (b) show the measured intensity profiles (solid squares) and their derivative profiles (open circles) for horizontal focusing and vertical focusing. The solid curves are fits to the derivative profiles using Gaussian functions. The FWHMs of the fitted curves indicate that a beam size of 1.10 mm (H)  0.98 mm (V) was achieved. Figs. 16(a) and (b) show the Fermi edge spectra and 4f core level spectra of Au measured using the K-B mirrors, and Figs. 16(c) and (d) show the same spectra recorded without using the mirrors. By fitting the Fermi edge spectra (solid line) the energy resolution is shown to be reduced to 217 meV by using K-B mirrors, 20 meV better than without the K-B mirrors (Fig. 16(c)). Comparing the Au 4f spectra with and without the K-B mirror system, the intensity is reduced by about a factor of 30 by the K-B mirrors (Fig. 16(b) and (d)). The dominant factor for this intensity loss is that the K-B mirror system used in this experiment spills X-rays considerably due to the small numerical aperture of the mirrors (0.7 mm), which makes precise alignment difficult.

6.2. HXPES with a micro/nano focused beam Focusing of the X-ray beam is one of the crucial points for high resolution and high throughput in HXPES, as has already been mentioned. Yang et al. have achieved a spot size of 1.1 mm  0.97 mm by combining high precision K-B mirrors [114] with a HXPES spectroscopy system for the first time. The HXPES spectra of gold film around the Fermi edge indicate an improvement in total energy resolution by about 20 meV using the K-B system. The experiments were performed at the undulator beamline BL47XU of SPring-8. Fig. 14 shows the optical configuration of the K-B mirrors system, installed downstream of the Si channel-cut postmonochromator. Both the vertical and horizontal focusing mirrors (MV and MH) are elliptical mirrors, with shape errors of about 2 nm and lengths of 100 mm. The silicon mirror surfaces were coated with Pt film, resulting in a reflectivity of about 89.5%. The focal length is 258 mm for MV and 154 mm for MH. The glancing angle is 4.06 mrad for MV and 3.6 mrad for MH. The distance

Fig. 15. Intensity profile and differential intensity profile for (a) horizontal and (b) vertical focusing (Ref. [114]).

Fig. 14. Optical configuration of the K-B mirrors (Ref. [114]).

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Fig. 16. HXPES spectra of Au EF (a) with K-B mirrors and (b) without K-B mirrors, and HXPES spectra of Au 4f (c) with K-B mirrors and (d) without K-B mirrors (hn=7.94 keV) (Ref. [114]).

Another factor is the air path introduced upstream of the HXPES chamber of about 2 m which is necessary for the K-B mirror and associated alignment instruments. This reduces the beam flux by absorption and scattering by about a factor of 0.5. The reflectivity of each mirror is 89.5%, resulting in a total flux loss of about 20%. In future the flux loss problem will be addressed by using larger mirrors (200 mm long), and by purging the air from the optical path using He gas.

6.3. 3D chemical analysis and laboratory HXPES As mentioned above, HXPES development has been very successful at third generation synchrotron radiation facilities. This tempts us to develop advanced instrumentation for expanding further applications of HXPES. One of the targets to challenge is the three-dimensional analysis of chemical states in advanced materials and devices by TOA dependence measurements of core level spectra using micro-nano focused X-ray excitation. Nanoscale thin layers and their stacks, the thicknesses of which match well the probing depths of HXPES, play essential roles to bear the functionalities in these advanced materials and devices. Another attractive target for us is the development of a laboratory HXPES system with practical throughput. If this is realized, it is expected to be very effective for offering the opportunities of HXPES applications to users widespread in various fields of science and technology. Combining the above two challenges, we began in 2006 a project for the development of a HXPES spectrometer for 3D chemical state analysis under the framework of a SENTAN project of the Japan Science and Technology Agency (JST). The main purpose of the project consists of two parts. (1) the development of a system which utilizes a micro-nano focused

X-ray beam at a SPring-8 hard X-ray undulator beamline, and (2) the development of a laboratory system using a focused Cr Ka monochromatic source. The development of a wide-acceptance objective lens with angle-resolution capability is a key to both these targets. Daimon and Matsuda have designed an objective lens to fulfill these specifications using an ellipsoidal mesh electrode in the first stage of the lens [115,116]. The most crucial point in this objective lens development is the high-precision fabrication of the mesh electrode. The specifications we need for the objective lens are as follows: 7351–451 acceptance angle, 771–91 exit angle, a spot size at the exit focusing point of 0.5 mm or less, 11 angular resolution, and maximum operating voltage at electrodes of 8–10 keV. Fig. 17(a) shows the results of a simulation of the objective lens. An ellipsoidal metal mesh is adopted in the first stage electrode of the lens. We have manufactured several prototypes and tested them using a focused electron gun with minimum spot size of 2 mm. The coincidence between the lens parameter sets which were obtained experimentally by minimizing the focus spot size and those obtained by simulation was excellent. An example of the observed focused spot image and its profile are shown in Fig. 17(b) and (c). As for the beamline-based focused X-ray sources, the K-B mirror system described in the preceding section will be applied. For the laboratory system, we have developed a CrKa (5.4 keV) focused X-ray source. The water cooled Cr target is excited by a focused electron beam, and the emitted X-rays are monochromatized and focused on to the sample surface using a bent crystal monochromator with a 300 mm Roland circle. The X-ray spot is variable from 10 mm (1.25 W) to 200 mm (50 W), and TOA dependencies of HXPES spectra are measurable without

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Fig. 17. Structure and electron trajectory simulation of the wide acceptance objective lens. (b) Focused spot at the exit of the wide acceptance objective lens. (c) Crosssection profile of the focused spot. (unpublished. H. Matsuda, M. Kobata, H. Daimon, and K. Kobayashi.)

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rotating the sample. This makes possible the 2D mapping of depth profiles by scanning the focused X-ray spot on the sample surface. Here some of the preliminary results are introduced to prove the feasibility of this laboratory HXPES system which is still under development. Fig. 18 shows a comparison of Au 3d spectra recorded using a VG Scienta R4000 10 kV analyzer with and without the new objective lens. The experimental conditions were as follows; beam size 200 mm, TOA 871, sample current 29 nA, pass energy 200 V, analyzer entrance slit 4 m. The acquisition time for the Au 3d spectrum with the objective lens was 16 min. Enhancement of the throughput was confirmed to be a factor of 7 at the present stage. The angular acceptance and angular resolution were estimated using a test sample to generate photoelectrons from a thin linear region equipped with a multislit hemi-cylinder for collimation. As shown in Fig. 19, an acceptance angle of7351 and resolution of less than 11 was established. Considering that there still remains room for further improvements in the objective lens and its matching with the analyzer, we believe that this system shows considerable promise as a laboratory HAXPES instrument with practical throughput.

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Fig. 19. The angular resolution of the wide acceptance objective lens was tested using a test device composed of a Au plate, in front of which is mounted a combination of a long slit and a cylindrical multi-slit. (a) Image recorded Au 3d5/2 peak on the 2D detector in the angular mode of the VG Scienta 10 kV analyzer. (b) Profile of the image along the line indicated in (a). A 7351 acceptance angle with angular resolution better than 11 is evident in this result. (Period of the multi-slit is 2.81) (M. Kobata, I. Iwai, H. Yamazui, H. Takahashi, M. Kodama, A. Tanaka, M. Suzuki, H. Matsuda, H. Daimon and K. Kobayashi.)

7. Conclusion High-resolution HXPES using synchrotron X-rays is a very powerful and versatile tool for investigations in a wide range of fields throughout solid state physics, applied physics, materials science and chemistry, device R&D and analytical science and technology. HXPES is not only complementary to conventional PES, but also opens up possibilities in new fields which have not been accessible to conventional PES. To further widen the fields of application to increase the user base, a laboratory HXPES system with practical throughput is being developed. This will help to expand HXPES utilization even further.

The author is grateful to E. Ikenaga for his kindness in offering unpublished data obtained during his zealous work at BL47XU. The development of the 3D chemical analysis technique which is introduced in the final part of this article is supported by a JST ‘‘Sentan’’ project, with the involvement of M. Kobata and H. Iwai of NIMS, H. Matsuda and H. Daimon of NAIST, E. Ikenaga, K. Yang, M. Machida, and J.-Y.Son of JASRI, and M. Suzuki, H. Yamazui, H. Takahashi, M. Kodama, and A. Tanaka of ULVAC PHI, who are thanked for the unpublished data presented in this article on this topic. Critical readings of the manuscript by J. Harries and S. Ueda are much appreciated. References

Acknowledgements Rapid growth in HXPES research fields as described in the text is the fruit of stimulating competitions and collaborations among pioneering groups, that is, ID 32, VOLPE project (ID16), and Spanish beamline (BM16) groups at ESRF, JASRI (BL4XU, and BL46XU), RIKEN (BL29XU), Osaka University (RIKEN beamline at BI19LXU), and NIMS (BL15XU) groups at SPring-8, and also HIKE group at BESSY II (KMC-1). The progress of HXPES activities are also supported by cooperation in pioneering groups with analyzer makers such as VG SCIENTA AB, SPECS GmbH, FOCUS GmbH, MB Science AB, and Physical Electronics. At SPring-8, constructions of X-ray optics in all the HXPES beamlines are due to collaborations with T. Ishikawa and his co-workers. The prototype of the experimental configuration was mostly established during the early stage of the test experiments by Y. Takata, whose contributions during the raise of HXPES at BL29XU were inevitable. Collaborations among JASRI, RIKEN, HiSOR, and JAEA were also inevitable, resulting in realizing HXPES experimental stations opened for public use only 1–2 years after the success of the first test experiment. Above all, active users’ contributions were the most essential factors of the rapid progress of HXPES applications to wide spreading basic, applied, and industrial research fields. The author is grateful to E. Ikenaga, J.J. Kim, M. Kobata, and S. Ueda for their devoting works during the incubation period of HXPES public activities at BL4XU. The author is also thankful to Y. Takeda, Y. Saito, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, E. Ikenaga, S. Ueda, and H. Yoshikawa, for their efforts during the construction period of HXPES station at BL15XU.

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