Physics 904: Experimental Surface Physics

Experiment Protocol No. 1

Transition Metal X-ray Photoelectron Spectroscopy

References

• Briggs & Seah (TP156 S95 P73 1983): Chapter 2 (Instrumentation); Chap. 3: (Spectral Interpretation)
• Woodruff & Delchar Chapter 3.1-3.2: XPS
• ESCAlab operating instructions: Chap. 5 (X-ray source) Chap. 7 (Analyzer)
• VGS5000 Users Manual (software): Chap. 2 (Intro to VGS5000); Chap. 3. (Set up an experiment)
• A. E. Hughes and C. C. Phillips, "An experimental and theoretical study of the transmission function of a commercial hemispherical electron energy analyzer," Surface and Interface Analysis 4, 225 (1982).
• E. M. Purcell, "The focussing of charged particles by a spherical condenser," Phys. Rev. 34, 818 (1938).

Goals

UHV sample preparation techniques for foils. Familiarization with experiment-description software, and beginning data analysis software. Use of twin-anode and monochromatic x-ray source. Surface preparation by wide-area sputter etching. Differentiation of Auger and XPS spectral lines. Differentiation of surface and bulk impurities. Instrumental and intrinsic contributions to photoemission line-shapes.

Protocol

1. Sample preparation
   You will be examining the chemical composition of the outermost atomic layers of the material. Removal of gross surface contaminants, and possible thick oxide layers, is important. Use a standard UHV organic wash cycle (acetone-ethanol-distilled H2O). If a thick oxide layer is suspected (discoloration may be a guide here), use a dilute acid rinse followed by distilled H2O (consult instructor). Mount the sample on a sample stub by spot-welding.
2. Native-surface (as received) XPS
   • Survey scan of 0-1100 eV binding energy, once with Mg Ka and once with Al Ka.
   • Detailed scans of transition metal peaks, C, O and any other contaminants. "Detailed" means one or two peaks per region only. This is typically a width of less than 150 eV but more than 10 eV.
   • Identify all spectral features (peaks) as being due to core-level photoemission or Auger-electron emission, and identify the chemical origin of the feature. Label the peaks on your detailed spectra.
   • Estimate the thickness of the contamination layer by comparing the intensity of substrate emission at normal-emission and grazing-exit angle emission, for two widely spaced kinetic energies. You may use a "standard" value for the electron mean-free-path at each of these kinetic energies.
3. Argon-ion sputter etch surface
   • The total sputter time may be as high as 30 minutes to remove the surface impurities, depending on sputter conditions and sample preparation. Check the sample surface composition periodically using Al Ka XPS to determine the end-point of this sputter operation.
   • Identify all bulk impurities which remain after sputter etching the surface region.
4. Bulk photoelectron core-level line-widths
   • Select a strong, narrow transition-metal core-level for a detailed scan using monochromatic Al Ka. Measure the core-level line-shape for at least two widely-spaced settings of analyzer pass-energy (say 20 eV and 100 eV).
   • You now have three measurements (one non-monochromatic and two monochromatic) of the same peak. Use these measurements to remove the instrumental contributions to the line-width, and determine the intrinsic line-width of the photoelectron peak.
5. Fermi-level position and core-level binding energies
   • Determine the binding-energy position of the Fermi-level E_f using monochromatic radiation (should be ideally zero, but practically it is shifted a small amount), and use this to
   • measure the main transition-metal core-level binding energies with an accuracy of +/- 0.1 eV.