Application of hard X-ray photoelectron spectroscopy to electronic structure measurements for various functional materials

https://doi.org/10.1016/j.elspec.2013.01.009Get rights and content

Abstract

The revolver undulator beamline BL15XU at SPring-8 has been constructed by National Institute for Materials Science (NIMS). We have started hard X-ray photoemission experiments for various functional materials to measure the bulk-sensitive and buried interface electronic structures at BL15XU of SPring-8 since 2006. In this paper we report the performance of the NIMS contract beamline for hard X-ray photoelectron spectroscopy (HAXPES) and recent HAXPES results for several functional materials. By utilizing the large probing depth of HAXPES, it is possible to measure bulk and buried interface electronic structures of solids. We also describe the perspectives on HAXPES at the NIMS contract beamline.

Highlights

► High-resolution hard X-ray photoemission has been developed at BL15XU of SPring-8. ► ZnO single crystal showed a polarity dependence on the valence band spectrum. ► Mn doped titania nanosheets showed the mixed-valence states for the doped Mn ions. ► The interface electronic structure of Pt/HfO2/Pt was studied under device operation. ► Bulk and interface valence band structures of MgO/Co2MnSi were similar each other.

Introduction

Photoemission spectroscopy (PES) is most powerful tool to investigate the electronic structures of solids [1], [2]. Due to the development of the energy resolution of electron analyzers and of high-intense light source for PES, the ultra-high energy resolution (<1 meV) in PES using vacuum ultraviolet (VUV) region has been realized [2]. However PES using the VUV and soft X-ray (SX) light sources often gives a surface-sensitive electronic structure, which differs from a bulk electronic structure, due to the short inelastic mean-free-path (IMFP) [3] of photoelectrons. For this reason, the electronic structure obtained by VUV- and SX- PES is often inconsistent with the physical properties obtained by macroscopic measurements due to the difference between the surface and bulk physical properties.

In the last decade, hard X-ray PES (HAXPES) [4], [5], [6] has been available in a hard X-ray undulator beamline at third generation synchrotron radiation facilities. High brilliance hard X-rays from the undulator and the optimized experimental geometry [4] enable ones to perform HAXPES measurements even in lower photo-ionization cross section for hard X-rays in comparison with VUV and SX light [7], [8], [9]. Development of electron analyzer in the high kinetic energy range also contributed to perform HAXPES measurements. The advantage of HAXPES is large probing depth in the electronic structure measurements due to the large IMFP for high kinetic energy photoelectrons [3]. Various new findings in the electronic structures of functional materials have been reported by means of HAXPES [10], [11], [12], [13], [14], [15], [16], [17].

In this paper, we report the recent works on various functional materials by means of HAXPES at the NIMS (National Institute for Materials Science) contract beamline, BL15XU [18], [19] of SPring-8. Needless to say, the performance of the beamline is significantly important to conduct the HAXPES experiments, because the photo-ionization cross-section is very low in the hard X-ray regime [7], [8], [9]. We also describe the performance of the NIMS contract beamline, in which one can perform the HAXPES using X-rays in the energy range between 2 and 10 keV [19].

Section snippets

Performance of the NIMS contract beamline for HAXPES

Fig. 1 shows the schematic diagram of the NIMS contract beamline, BL15XU of SPring-8. The revolver-type undulator [20], which consists of the helical and planar undulator planes, has been employed for the light source. The helical undulator provides the X-ray energy range between 0.5 and 6 keV, while the planar undulator provides the 4–36 keV X-rays. The X-rays are monochromatized by a Si 1 1 1 or 3 1 1 double crystal monochromator (DCM) in the energy range between 2 and 36 keV. Then the energy

Experimental

All the HAXPES experiments shown in this paper were done at the undulator beamline BL15XU [19] of SPring-8. The excitation photon energy was fixed at 5.95 keV using a Si 1 1 1 DCM and the 3 3 3 reflection of a Si CCM. The photoelectrons were analyzed and detected by a high-resolution hemispherical analyzer (VG Scienta R4000). The angle between the objective lens axis of the electron analyzer and photon propagation was fixed at 90°. In many cases, HAXPES measurements were performed in near normal

Wurtzite-type semiconductor ZnO

Over the last decade, ZnO has attracted much attention as a semiconductor material and has been extensively investigated for applications in ultraviolet-light-emitting diodes and lasers [21]. Since wurtzite-type semiconductors such as ZnO and GaN are polar semiconductors, ZnO-based junction properties have to be considered in the relation to the polarization. We have performed HAXPES measurements to examine the effect of crystalline polarity on the electronic structure of ZnO [22].

Fig. 4 shows

Summary and perspectives

In this paper, recent HAXPES results for various functional materials measured at BL15XU of SPring-8 have been shown briefly. For the polar semiconductors such as ZnO, there is a polarity dependence on the valence band spectrum. HAXPES measurements have a capability to reveal the electronic states of nanosheets, which have high potentiality to nanodevice applications over a wide area. HAXPES under device operation will plays a key role in developing the nanodevices such as CMOS, ReRAM, MRAM,

Acknowledgements

The author would like to thank Drs. K. Kobayashi, Y. Katsuya, Y. Yamashita, Y. Matsushita, M. Tanaka, and Mr. Ishimaru for their kind help to develop the NIMS contract beamline. The author appreciates Drs. Y. Takeda and Y. Saitoh of JAEA at SPring-8 and Profs. K. Shimada, H. Namatame, and M. Taniguchi of HiSOR, Hiroshima University for the development of HAXPES at BL15XU of SPring-8. The author also acknowledges the staff of JASRI and SPring-8, especially, Drs. T. Mochizuki and T. Takeuchi, and

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