Hard X-Ray Photoelectron Spectroscopy (from few eV up to 15keV electron kinetic energy)

1. INTRODUCTION

The photoemission and the Auger electron spectroscopies play a preponderant role in the study of the electronic properties of solids [1]. These techniques have been extensively used in the last 40 years and have reached a high degree of sophistication [1]. Overall, their application has been limited to the investigation of surface phenomena by using energies between 40 and 2000 eV for both the excitation sources and detected electrons. The low electron inelastic-mean-free-path (IMFP) and/or the effective attenuation length (EAL) [2] in the solid materials, at the energies used is responsible for their surface sensitivity. Particularly, buried interfaces are not accessible for the great majority of the techniques of surface physics. However, using hard X-rays as excitation source which has a macroscopic penetration depth in the materials, high kinetic energy photoelectrons can be produced [3]. Consequently, HAXPES benefits from the exceptionally large escape depth of high kinetic energy photoelectrons enabling the study of bulk and buried interfaces up to several tens of nanometres depth [4]. Its advantage over conventional XPS is based on the long mean free path of high kinetic energetic photoelectrons. The information depth can reach several tens of nanometres for 15keV electron kinetic energy. Using the advantage of tuneable X-ray radiation provided by the synchrotron the photoelectron kinetic energy, i.e. the information depth can be changed and consequently electronic and compositional depth profiles can be obtained. HAXPES is a powerful emerging technique for bulk compositional, chemical and electronic properties determination. HAXPES is a novel method for non destructive and bulk sensitive electronic and chemical characterization of solids. At SpLine, the Spanish CRG beamline at the European Synchrotron Radiation Facility (ESRF) we have developed a novel and exceptional set-up that combine HAXPES and X-ray diffraction XRD (surface and bulk X-ray diffraction and X-ray reflectivity). Both techniques can be operated simultaneously on the same sample using the same excitation source.

 

2. HAXPES-XRD set-up

 

The experimental set-up is placed on branch B of the Spanish CRG BM25 beamline SpLine [5] at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The Branch B is located on the hard edge of the bending magnet D25 device with a critical energy of 20.6 keV and a horizontal angular divergence of 2 mrad. Two Si (111) crystals placed at ~30 meters from the source serve as a double-crystal monochromator, which gives an energy resolution of ΔΕ/Ε=1.5x10-4. The horizontal focusing is achieved by the second monochromator crystal through a sagittal cylindrical bending. A cylindrical bent mirror placed after the monochromator is used to focus in the meridional plane. In all the energy range the beam spot size is around 300x100 μm2 in the horizontal and vertical direction, respectively. A first mirror, Rh coated, is located before the monochromator that cuts-off the higher-order-harmonics reducing the heat load on the monochromator. The beamline X-ray energy ranges between 5 keV and 45 keV  with a  X-ray flux of 1013 photons/s flux, that is well adapted for the HAXPES and XRD requirements.

 

Fig1

Figure 1. (a) SpLine branch B photon flux in photons/sec/0.1% bandwidth in dark the achievable energy range (5-45 keV). (b). a picture of the whole experimental HAXPES and XRD setup.

Figure 1(a) shows the beamline photon flux at 01% bandwidth obtained at the branch B with a storage ring current of 200 mA. The bending magnet D25 of the ESRF generates the X-rays provided by the SpLine beamline. A high-resolution channel-cut (Si(311), Si(333), and Si(400)) post-monochromator is located close to the vacuum chamber for cases where a better excitation source resolution in the HAXPES measurements is required. The experimental set-up includes a heavy 2S+3D diffractometer, a UHV chamber and an electrostatic analyzer [6, 7]. The UHV chamber has also MBE evaporation sources, an ion gun, an electron gun, a UV discharge lamp, a LEED optic, a sample heating and cooling device, leak valves and a load-look port. In figure 1(b) a picture of the UHV experimental set-up is displayed. In our set-up design we have chosen an analyzer with the wider angular acceptance, keeping in mind that a great effort has to be done in optimizing the transmission and the energy resolution. The analyzer is an electrostatic cylinder-sector (FOCUS HV CSA), with a compact geometry and high transmission due to second order focusing. The analyser is based on a cylinder sector with 90° deflection and 300mm slit-to-slit distance and an entrance lens with 50mm sample distance [7, 8]. This gives a very compact design of the analyser that is easily integrated into a multipurpose experiment with different techniques. The analyzer is capable to handle kinetic energies both up to 15 keV and down to a few eV with the same analyzer setup and power supply.

3. EXPERIMENTAL RESULTS

 

Representative HAXPES 3s, 3p and 3d Au core level spectra (see Fig. 2a) and Cu and Au valence band (see Fig. 2b) obtained at the SpLine HAXPES-SXD set-up measured on a 21 nm thick Au film growth on a Cu polycrystalline sample. The spectra displayed on fig. 2 are representative photoemission data and were obtained with a photon energy of hn = 9 keV (Ekin = 5.40 - 6.90 keV), hn = 17 keV (Ekin = 13.40 - 14.90 keV), hn = 7.5 keV (Ekin = 7.36 - 7.50 keV) and hn = 15 keV (Ekin = 14.86 - 15.00 keV), respectively. The bottom (Fig. 2a) and the upper spectrum (Fig. 2b) are multiplied by a factor 3 and 14.26, correspondingly. Note the absolute and relative cross section differences in the spectra.

Fig2

 

Figure 2. (a) HAXPES Au 3s, 3p1/2, 3p3/2, 3d3/2 and 3d5/2 spectra recorded at 9 and 17 keV photon energy. (b) Cu and Au valence band spectra taken at 7.5 and 15 keV photon energy.

Using the advantage of tuneable X-ray radiation the photoelectron kinetic energy, i.e. the information depth can be changed. Figure 3 shows the experimentally obtained photoemission peak intensities as a function of their kinetic energy for the Au overlayer and the buried Cu substrate. The signal is normalized with the corresponding bulk photoemissions peak (scatter symbols).  The displayed data was obtained by measuring the 1s, 2s, 2p, 3s, 3p and 3d subshells from Cu and 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p and 5d subshells for Au for a photon energy range between 7.5 and 17 keV. The normalized signal is only a function of the EAL and of the atomic density N(z) functions of the corresponding measured element, i.e. Cu and Au [8, 9]. The compositional depth profile analysis has been obtained by fitting the measured data with general polynomial depth dependence. The fit results are shown in Figure 3 as continuous line that match with a uniform Au over layer of 21 nm and an inter layer diffusion region of 2.4 nm. The obtained composition depth profile is in very well agreement with equivalent results obtained from X-ray reflectivity measurements performed in the same system and set-up.

Fig3

Figure 3. Photoemission peak intensity dependence with kinetic energy. Scatter symbols experimental data. Continues line fit results.

4. CONCLUSIONS AND OUTLOOK

 

We have developed a novel outstanding tool for non-destructive characterization of bulk and buried interfaces that combine XRD and HAXPES. The implementation of HAXPES open a direct way to correlate surface and bulk properties and in combination with XRD is opens a new research field.

 

The developed set-up offers a unique opportunity to obtain, on a same sample and under identical experimental conditions, simultaneous information about the electronic properties, geometric structure and chemical composition of bulk, buried interfaces and surfaces. The exceptionally large escape depth of high kinetic energy photoelectrons increases the sampling depth up to several tens of nanometres. Even more, using the advantage of tuneable X-ray radiation provided by the synchrotron the photoelectron kinetic energy, and hence the information depth can be changed and consequently electronic and compositional depth profiles can be obtained in a non-destructive way. HAXPES is a powerful emerging technique for bulk compositional, chemical and electronic properties determination.

5. REFERENCES

[1] S. Hüfner, "Photoelectron Spectroscopy", Springer Verlag (1994), ISBN number 3-540-41802-4

[2] S. Tanuma, C. J. Powell, D. R. Penn, Surf. Interf. Anal. 25 25 (1997)

[3] J. Rubio-Zuazo and G.R. Castro,  Journal of Physics: Conferences Series, 100 012042  (2008)

[4] J. Rubio-Zuazo,  E. Martinez,  P. Batude,  L. Clavelier,  F. Soria,  A. Chabli and G.R. Castro.“Hard X-ray photoemission experiments on novel Ge-based metal gate / high-k stacks”, AIP Conference Proceedings-2007: International Conference on Frontiers of Characterization and Metrology for Nanoelectronics. pp. 329-333

[5] G.R. Castro, J.Synchrotron Rad 5, 657 (1998)

[6] J. Rubio-Zuazo and G.R. Castro, Nuclear Instruments and Methods in Physics research, Section A, 547 64 (2005)

[7] M .Escher, M. Merkel, J. Rubio-Zuazo and G.R. Castro, "An energy analyser for Hard X-ray Photoelectron Spectroscopy”, Proceedings of Recent Trends in Charged Particle Optics and Surface Physics Instrumentation, Brno 2006, Czech Republic

[8] J. Rubio-Zuazo and G.R. Castro “Hard X-Ray Photoelectron Spectroscopy and X-Ray Surface Diffraction station”. Pico, Vol 10. No2 (2006)

[9] J. Rubio-Zuazo and G.R. Castro, Reviews on Advanced Materials Science, 15 79 (2007)

 

 

 

 

 

 

Surface X-ray Diffraction

 

The experimental set-up installed at the second focusing point of Branch B is devoted to the wide used X-Ray Photoelectron Spectroscopy (XPS) and Grazing Incidence Diffraction techniques. Both tools will be operated either simultaneously or independently to each other. This offers a unique opportunity to obtain, on a sample and under identical experimental condition, electronic, geometrical and chemical information.

 

SXD-HEXPS

 

The robust 2S+3D diffractometer with its main axis vertical support loads up to 1000 kg. A UHV chamber with a High energy photo-emission spectrometer is available for combined studies with X-Ray diffraction.

 

F1

                   Figure 1. Side viewof the experimental set-up

 

 

Figure 1 shows a side view of the experimental set-up. The UHV system consists on a cylindrical vessel with 27 flanges, 19 of them pointing to the sample position, three turbomolecular pumps, two dry pumps for the turbomolecular’s pre-vacuum, an ionic pump and a titanium sublimation pump surrounded by a cryogenic panel. The sample surface is mounted vertically. The X-rays enter and leave the vacuum system through Be-windows, welded onto the stainless-steel chamber, which are strong enough to hold vacuum. A 200° in-plane access and a 50° out-of-plane access are allowed by the exit Be-window. A wide portion of the available reciprocal space is therefore achievable. The electrostatic analyser is mounted on a motorised table in order to scan the sample surface without moving the sample position. In that way, the counting rate is maximized without disturbing the diffraction experiment. it could be seen that the electron analyser does not obstruct the scattering solid angle allowed by the exit Be-window. Even more, the sample-lens distance is fixed to 50 mm so that the previous solid angle is left free. Figure 2 shows a picture of the set-up from the opposite side where  

 F2

                                  Figure 2. Side-view seen in the direction of the X-rays

 

The whole set-up is mounted on a 2+3 diffractometer in horizontal geometry from which it can be detached without breaking the vacuum, allowing other users to use the diffractometer. The sample moves on the diffractometer coupled into the vacuum by a rotating feedthrough. The feedthrough is composed of a differentially pumped rotating seal and a bellow that enables the adjustment of the vessel respect to the diffractometer. The diffractometer rotates the UHV chamber without moving the sample (requisite imposed by the GID experiment) so high and low kinetic energy photoelectrons could be measured in an optimal configuration. A specially designed mini-LEED mounted on a 63 mm flange is incorporated on the UHV system so that pre-characterisation of the ordered surface could be done. An ion bombardment gun, evaporation and gas leakage sources are also mounted on the UHV chamber that allows the possibility of doing X-ray diffraction and XPS experiments during growth deposition. Helium and nitrogen cooling system are incorporated so that low temperature experiments could also be performed. The experimental set-up is located at the experimental hutch B at ~57 meters from the source, i.e., at ~27 meters from the monochromator. Such a large distance imposes the necessity of high stability at the monochromator. A slight movement of the monochromator will be greatly enhanced at the sample position. However, as the sample-monochromator distance is nearly the same as the monochromator-source distance, the source beam size is recovered at the sample position. Even more, the divergence of the beam at the sample position is reduced due to the large distance from the monochromator upgrading the experimental conditions for the diffraction experiment.

 

F3

                                                       Figure 3. Set-up