High energy photoelectron diffraction: model calculations and future possibilities

We discuss the theoretical modelling of x-ray photoelectron diffraction (XPD) with hard x-ray excitation at up to 20 keV, using the dynamical theory of electron diffraction to illustrate the characteristic aspects of diffraction patterns resulting from such localized emission sources in a multi-layer crystal. We show via dynamical calculations for diamond, Si, and Fe that the dynamical theory well predicts available current data for lower energies around 1 keV, and that the patterns for energies above about 1 keV are dominated by Kikuchi bands which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations to results from a cluster model that is more often used to describe lower-energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.Comment: 29 pages, 13 figure

Real-time observation of the dry oxidation of the Si(100) surface with ambient pressure x-ray photoelectron spectroscopy

We have applied ambient-pressure x-ray photoelectron spectroscopy with Si 2p chemical shifts to study the real-time dry oxidation of Si(100), using pressures in the range of 0.01-1 Torr and temperatures of 300-530 ??C, and examining the oxide thickness range from 0 to ???25 A. The oxidation rate is initially very high (with rates of up to ???225 Ah) and then, after a certain initial thickness of the oxide in the range of 6-22 A is formed, decreases to a slow state (with rates of ???1.5-4.0 Ah). Neither the rapid nor the slow regime is explained by the standard Deal-Grove model for Si oxidation.open171

In situ observation of wet oxidation kinetics on Si(100) via ambient pressure x-ray photoemission spectroscopy

e initial stages of wet thermal oxidation of Si (100) - (2??1) have been investigated by in situ ambient pressure x-ray photoemission spectroscopy, including chemical-state resolution via Si 2p core-level spectra. Real-time growth rates of silicon dioxide have been monitored at 100 mTorr of water vapor. This pressure is considerably higher than in any prior study using x-ray photoemission spectroscopy. Substrate temperatures have been varied between 250 and 500 ??C. Above a temperature of ???400 ??C, two distinct regimes, a rapid and a quasisaturated one, are identified, and growth rates show a strong temperature dependence which cannot be explained by the conventional Deal-Grove model.open7

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.Comment: http://www.pnas.org/content/early/2014/04/10/1405435111.abstrac

In-Situ observation of wet oxidation kinetics on Si (100) via ambient pressure x-ray photoemission spectroscopy

The initial stages of wet thermal oxidation of Si(100)-(2x1) have been investigated by in-situ ambient pressure x-ray photoemission spectroscopy (APXPS), including chemical-state resolution via Si 2p core-level spectra. Real-time growth rates of silicon dioxide have been monitored at 100 mTorr of water vapor. This pressure is considerably higher than in any prior study using XPS. Substrate temperatures have been varied between 250 and 500 C. Above a temperature of {approx} 400 C, two distinct regimes, a rapid and a quasi-saturated one, are identified and growth rates show a strong temperature dependence which cannot be explained by the conventional Deal-Grove model

Interface properties and built-in potential profile of a LaCrO3_3/SrTiO3_3 superlattice determined by standing-wave excited photoemission spectroscopy

LaCrO$_3$ (LCO) / SrTiO$_3$ (STO) heterojunctions are intriguing due to a polar discontinuity along (001), two distinct and controllable interface structures [(LaO)$^+$/(TiO$_2$)$^0$ and (SrO)$^0$/(CrO$_2$)$^-$], and interface-induced polarization. In this study, we have used soft- and hard x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a quantitative determination of the elemental depth profiles and interface properties, band alignments, and the depth distribution of the interface-induced built-in potentials in the two constituent oxides. We observe an alternating charged interface configuration: a positively charged (LaO)$^+$/(TiO$_2$)$^0$ intermediate layer at the LCO$_\textbf{top}$/STO$_\textbf{bottom}$ interface and a negatively charged (SrO)$^0$/(CrO$_2$)$^-$ intermediate layer at the STO$_\textbf{top}$/LCO$_\textbf{bottom}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO-contributed and STO- contributed matrix-element-weighted density of states (MEWDOSs) from the valence band (VB) spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the deconvoluted MEWDOS shifts with a change in probing profile, the alternating charge- induced built-in potentials are observed in both constituent oxides. Finally, using a two-step simulation approach involving first core-level binding energy shifts and then valence-band modeling, the built-in potential gradients across the SL are resolved in detail and represented by the depth distribution of VB edges.Comment: Main text: 29 pages, 5 figures; Supplementary Information: 20 pages, 10 figure