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Register a new userPolarization dependent interface properties of ferroelectric Schottky barriers studied by soft X-ray absorption spectroscopy
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We applied soft X-ray absorption spectroscopy to study the Ti L-edge in ferroelectric capacitors using a modified total electron yield method. The inner photo currents and the X-ray absorption spectra were polarization state dependent. The results are explained on the basis of photo electric effects and the inner potential in the ferroelectric capacitors as a result of back-to-back Schottky barriers superimposed by the potential due to the depolarization field. In general, the presented method offers the opportunity to investigate the electronic structure of buried metal-insulator and metal-semiconductor interfaces in thin film devices. Corresponding author: [email protected]
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The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110) crystal surfaces is investigated by means of soft x-ray absorption spectroscopy, to complement earlier spin-polarized STM experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) Mn L23-edge spectra were measured in the temperature range from 8 to 300 K and compared to results of fully-relativistic ab initio calculations. We show that antiferromagnetic (AFM) helical and cycloidal spirals give rise to significantly different Mn L23-edge XLD signals, enabling thus to distinguish between them. It follows from our results that the magnetic ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based on temperature-dependent XAS, XLD and field-induced XMCD spectra we deduce that magnetic properties of the Mn monolayer on W(110) vary with temperature, but this variation lacks a clear indication of a phase transition in the investigated temperature range up to 300 K - even though a crossover exists around 170 K in the temperature dependence of XAS branching ratios and in XLD profiles.
We have measured the intricate temperature dependence of the Co L2,3 x-ray absorption spectra (2p-3d excitations) of CoO. To allow for accurate total electron yield measurements, the material has been grown in thin film form on a metallic substrate in order to avoid charging problems usually encountered during electron spectroscopic studies on bulk CoO samples. The changes in spectra due to temperature are in good agreement with detailed ligand-field calculations indicating that these changes are mostly due to thermal population of closely lying excited states, originating from degenerate t2g levels lifted by the spin-orbit coupling. Magnetic coupling in the ordered phase, modeled as a mean-field exchange field, mixes in excited states inducing a tetragonal charge density. The spin-orbit coupling induced splitting of the low energy states results in a non-trivial temperature dependence for the magnetic susceptibility.
We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500~AA) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature $T_mathrm{c}$ assumes a thickness-independent enhanced value of $geq$43~K as compared with that of bulk MnSi, where $T_mathrm{c} approx 29~{rm K}$. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111]-direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi --- except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.
The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallisation scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high temperature events or its behaviour when exposed to air. Here, a combined soft and hard X-ray photoelectron spectroscopy (XPS) characterisation approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behaviour of a 300~nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the precursor titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft X-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO$_2$, WO$_3$ and a WO$_3$ oxide coordinated with a titanium environment. This combinatorial characterisation approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.
The electronic structure of the magnetic semiconductor Ga$_{1-x}$Cr$_{x}$N and the effect of Si doping on it have been investigated by photoemission and soft x-ray absorption spectroscopy. We have confirmed that Cr in GaN is predominantly trivalent substituting for Ga, and that Cr 3$d$ states appear within the band gap of GaN just above the N 2$p$-derived valence-band maximum. As a result of Si doping, downward shifts of the core levels (except for Cr 2$p$) and the formation of new states near the Fermi level were observed, which we attribute to the upward chemical potential shift and the formation of a small amount of Cr$^{2+}$ species caused by the electron doping. Possibility of Cr-rich cluster growth by Si doping are discussed based on the spectroscopic and magnetization data.
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