Structure Analysis Using Time-of-Flight Momentum Microscopy with Hard X-rays: Status and Prospects

Abstract

X-ray photoelectron diffraction (XPD) has developed into a powerful technique for the structural analysis of solids. Extension of the technique into the hard-X-ray range (hXPD) gives access to true bulk information. Here we give a status report on hXPD experiments using a novel full-field imaging technique: Time-of-flight momentum microscopy (ToF-MM). A special variant of ToF-MM is capable of recording high kinetic energies (up to >7 keV) and enlarged kfields-of-view. We present applications that are specific for high kinetic energies. The strong site specificity of hXPD is exemplified for NbSe 2 , the cubic-to-tetragonal transition in SrTiO 3 and the zinc-blende structure in epitaxial GaAs films. Bloch-wave calculations show a very good agreement with experiment and reveal fingerprint-like signatures of emitter sites in host lattices. We show a dopant-site analysis in two semiconductors (Mn in GaAs and Te in Si). Hard-X-ray ARPES plus core-level hXPD enable eliminating the strong diffraction signature imprinted in HARPES maps.

Advanced characterization of organic–metal and organic–organic interfaces: from photoelectron spectroscopy data to energy-level diagrams

“…an abrupt change of VL at the organic-metal interface [9,18]. Noteworthy, the effects mentioned above impact the position of energy levels and further effects impact the line shape and intensities of UPS and XPS spectra, they include energy dispersion [27,57,58], orbital delocalization [59,60], hole-phonon coupling [61,62], photoelectron angular distribution [63][64][65], photoelectron diffraction [66,67], core-hole screening [68][69][70] and shake-up excitations [71,72]. Several tutorial-like review articles give guidelines how to analyze XPS data [73][74][75]; for UPS the reader is referred to these books [76,77].…”

Are high-energy photoemission final states free-electron-like?

“…With E k increasing into the hard-X-ray energy range, λ PE and thereby the number of coherently scattered waves increases. This forms sharp Kikuchi-like structures in the XPD angular distribution, re ecting the long-range atomic structure 62 . In any case, the XPD stays incoherent between the emitters.…”

“…Finally, we should point out that the coherent photoemission process underlying the ARPES experiment discussed above (as well as the orbital tomography) is fundamentally different to the essentially incoherent process of X-ray photoelectron diffraction (XPD) (see, for example, the reviews [60][61][62] . In the first case, all photoelectron emitters (atoms) throughout the crystal surface region within the depth λ PE are coherent -or entangled, in the modern quantum mechanics discourse -and emit a coherent photoelectron wavefield characterised by a well-defined k. The resulting ARPES intensity as a function of E k and θ bears sharp structures reflecting, through the momentum conservation, the k-resolved band structure of the valence states.…”

Spin- and time-resolved photoelectron spectroscopy and diffraction studies using time-of-flight momentum microscopes

“…In all cases studied so far (graphite, Si, Ge, Re, Mo, W, GaMnAs, TiTe 2 , MoTe 2 , NbSe 2 , Fe 3 O 4 , SrTiO 3 , YbRh 2 Si 2 , EuPd 2 Si 2 ) hXPD captured patterns with unprecedented richness in details; an overview is given in Ref. 239 Next to a detailed structural analysis, such diffractograms have practical aspects: They provide a metric in k-space and show distinguished directions in real space. The widths of Kikuchi bands correspond to multiples of reciprocal lattice vectors.…”