X-ray photoelectron spectroscopy (XPS) is the most widely used surface analysis technique for materials characterisation. Hard X-ray photoelectron spectroscopy (HAXPES) generally refers to excitation sources with photon energy greater than 2100 eV. Standard X-ray sources for lab. based instruments are Al Kα or Mg Kα which provide photons with 1486.6 eV and 1254.6 eV energy respectively. The spectrometer measures the kinetic energy (KE) of photoelectrons ejected from a material under the irradiation of X-rays, allowing the binding energy (BE) of the photoelectrons to be calculated using the following equation: KE = hν – BE – Φ Equation 1 Where hν is the photon energy and Φ is the work function of the spectrometer. An implication of this equation is that the highest BE core level electron that can be ejected from a material is dependent on the energy of the photon source used. The surface sensitivity of XPS is derived from the attenuation of the photoelectrons as they travel through a material. Only photoelectrons that are elastically scattered, and therefore have not lost energy, contribute to the photoelectron peak. Those that lose some energy but still escape to the potential of the surface contribute to the inelastically scattered photoelectron background. With conventional lab. based excitation sources the information depth from which the photoelectrons are detected is up to 10 nm, although it is noted that this is dependent on their KE. The photon energy of Ag Lα is 2984.3 eV. The greater photon energy allows the excitation of additional, higher binding energy core lines for a number of elements as detailed in the Table. In addition to accessing a greater number of core levels, a higher photon energy also results in a decrease in attenuation length of higher KE photoelectrons [1], or expressed in another way, an increase in information depth. By way of an example, the inelastic mean free path (IMFP) of C 1s (BE 285 eV) using Al Kα X-rays is 3.14 nm at KE 1201 eV, but for Ag Lα the IMFP is increased to 5.89 nm at KE 2699 eV [2]. A practical application of the Ag Lα excitation source is shifting Auger electron transitions which interfere with core level photoemission peaks excited by Al Kα. This is particularly useful in the analysis of 3d transition metals and semiconductor elements, such as Ga and Ge, where the LMM Auger features overlap with the regions of interest when using either Al Kα or Mg Kα excitation sources. Similarly broad MNN Auger features obscure the Al Kα excited spectra of the 4f lanthanides. By using a higher energy photon source, these Auger transitions are shifted by approximately BE 1500 eV. HAXPES provides electronic and chemical information from the uppermost 20 nm of a material. Greater insight into surface properties can be gained by complementary surface analysis techniques.
