RRCAT: Raja Ramanna Centre for Advanced Technology, Indore

Installation and testing of PES beamline on Indus-1

S.M. Chaudhari, D.M. Phase, A.D. Wadikar

Inter University Consortium for DAE Facilities, University campus, Khandwa road, Indore.452 017.

Photoelelctron spectroscopy beamline described earlier has been erected on one of the bending magnet port of Indus-1, by adopting following alignment procedure. Initially an optical axis and dispersion plane to locate various beamline components, were determined using shadow method. For this light from storage ring at very low current was used. For dipole source by using a window at the end of the front-end and a plumb line which is hung up just downstream of window, the shadow of the plumb line indicating the tangent to the ring was determine at several distances from the source. Position of the plumb line and its shadow at various distances were marked on the floor. All optical components were then roughly positioned at the proper distance from the source and at the proper height along this marked line. A fine adjustment of beamline optical components such as premirror, entrance slit, grating monochromator, exit slit and post mirror was initially carried out using He-Ne laser. This alignment is repeated many times, by properly adjusting the distances of each optical element from the source and ground. After completing this alignment work the entire beamline was joined for evacuated and baked to 150⁰C to achieve the vacuum of the order of 2x10^-9 torr.

With 10 mA of electron storage current and after through checking for leakage of γ-ray radiations by radiation safety officer at various places around the beamline, a re-alignment of entire beamline was carried out using SR bam. Photo detector placed behind exit slit was used to detect photon beam. In order to maximize photon flux, each optical element in the beamline was properly adjusted by using precision X-Y-Z and tilt motion mechanism.

The resolution of the beamline was tested using Penning discharge type of source capable of producing intense spectral lines of He, Ar and N₂. Measured resolution as shown in figure.1 for entrance and exit slit widths of 200μ each is 950 at λ=304Å using a grating having 600 lines/mm. For the same size slits it is 650 at λ=584Å for grating having 200 lines/mm.

Fig. 1

After commissioning the beamline on Indus-1, the first photoelectron spectrum was recorded successfully on 9 November 2000 at 8.20 p.m. using indigenously developed angle integrated photoelectron spectrometer The spectrum was recorded with spectrometer slit with setting of 3mm at 30eV analyzer pass energy. The as-recorded spectra for Pt core level and Ag valence band are shown in Fig. 2a and 2b respectively. Improved spectra of Pt core level and valence band were subsequently recorded with lower background counts (Fig. 3a and 3b). The Pt core level spectrum show well resolved two spin-orbit split peaks corresponding to 4f_5/2 and 4f_7/2 levels at binding energy values 74.6 eV and 71.2 eV respectively. The spin-orbit splitting of 3.3 eV matches well with that reported in the literature. The measured FWHM, which is a measure of energy resolution, is 0.8 eV. Fig. 3b shows valence band spectrum of Pt foil and shows a two-peak structure corresponding to 5d_5/2 and 5d_3/2 at about 2 eV and 5 eV respectively.

Fig. 2 (a) Pt core level (b) Ag Valence band

Valence band and core level spectra have been recorded for Cu, Au and W thin films. Fig. 4 shows valence band photoelectron spectrum of 50Å thick Cu film. Essentially all features corresponding to Cu-3d valence band are clearly reproduced. The measured spectrum is compared with theoretical density of states.

Fig.4 Valence band spectrum of Cu along with theoretical density of states (dashed lines)

Figure 5 shows valence band spectrum of p-GaAs (100) sample recorded at 134eV photon energy keeping pass energy of 30eV. The recorded spectrum has a typical three-peak structure. Peak-I represents predominantly p-states, II-sp-hybridized states and III-mainly s-states. In addition, a peak due to O-2p valence band is also present. The energies of peaks I,III and III are 3.7 eV, 5.81 eV, and 11.67 eV respectively are in agreement with the reported values.

Fig.6 Core level spectrum of GaAS Both spectra are fitted with three components corresponding to 3d_5/23d_3/2 and oxide states

The core level spectrum of Ga and As 3d levels is shown in Fig.6. The two peaks at 18.4eV and 41.3 eV correspond to 3d core level of Ga and As respectively. Observed peaks are broad with FWHM 1.58 eV and 1.68 eV for Ga and As respectively. Each spectrum is fitted with three components. These components in the case of Ga correspond to 3d_5/2and 3d_3/2and its oxide with corresponding binding energy values of 18.52 eV, 18.96 eV and 19.52 eV. Similarly in the case of As three components corresponding to3d_5/2 ,3d_3/2and its oxide at binding energy value of40.48 eV, 41.15 eV, and 41.69 eV are fitted. The peak positions are in agreement with the values observed in earlier studies.

Figure-7 shows valence band spectrum from an oxide sample, viz. Sr₂Ru₄, which has been studied earlier in our laboratory. The spectrum, as expected can be recorded much faster than in an UPS or XPS laboratory experiment.

A number of photoelectron spectra recorded on bulk, metal film, semiconductor and oxide. Systems demonstrated that good quality data can be generated with present beamline.