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We present an overview of the National Institute of Standards and Technology beamline X24A at the National Synchrotron Light Source at Brookhaven National Lab and recent work performed at the facility. The beamline is equipped for HAXPES measurements, with an energy range from 2.1 to 6 keV with Si(1 1 1) crystals. Recent measurements performed at the beamline include non-destructive depth dependent variable kinetic energy measurements of dielectric and semiconductor films and interfaces for microelectronics applications, band alignment at buried interfaces, and the electronic structure of bulk-like materials. The design and operation of the current beamline will be discussed, as well as the future NIST beamline at NSLS II.

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide ${\mathrm{Sr}}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{VO}}_{3}$. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O $2p$ valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O $2p$ electronic structure as a function of the Sr content is dominated by $A$-site hybridization. Near the Fermi level, the correlated V $3d$ Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

The x‐ray standing wave technique has been used to determine the adatom positions on the Si(111) 7×7 and Si(111) √3×√3–In surfaces. By monitoring the intensity of the surface sensitive Si 1s photopeak as a function of photon energy around the Si(111) Bragg backreflection condition, we determine the Si adatoms of the clean 7×7 surface to reside 1.69±0.1 A above the extrapolated Si(111) bulk plane. In contrast, the In adatoms on the 1/3 monolayer √3×√3–In surface are found to remove the Si displacements and lie at a position of 2.11±0.05 A. High‐resolution Si 2p core‐level measurements support the T4 geometry for the In adatom location.

Triangulation from x‐ray standing wave applied to three Bragg planes, together with symmetry considerations, have established the geometrical structure of epitaxial Bi overlayers on GaAs (110). Our results are consistent with the epitaxial continued layer structure model. Our analysis is based on prior experimental results that indicate two equally populated Bi sites.

Through a unique combination of surface sensitive extended x‐ray absorption fine structure (EXAFS) and x‐ray standing waves (XSW), we have determined the pertinent structural parameters of the clean InP(110) surface reconstruction. We find a rotation angle of 27° between the P–In chains, no change of the first neighbor P–In bond length, and a small but measurable, ∼0.1 A, expansion of the average P–P second neighbor distance. The general application of the EXAFS and XSW techniques to the study of clean surfaces will be discussed.

The strain and relaxation of InAs and InGaAs films grown on GaAs(001) have been examined by the x‐ray standing wave and extended x‐ray absorption fine‐structure techniques. While 1 monolayer (ML) films of both InAs and InGaAs are found to be tetragonally distorted in accordance with the prediction of macroscopic‐elastic theory, thicker InAs films are found to collapse to their natural‐lattice constant past a critical thickness Tc, of ∼2 ML’s. By 8 ML’s, bond‐length strain is no longer evident, and a large degree of structural disorder is observed.

The x‐ray standing wave technique has been used to determine the geometric structure of the monolayer Sb/GaAs(110) interface. Using the backreflection diffraction geometry, we find the average perpendicular distance of Sb atoms from the (220), (400), and (111) diffracting planes to be 2.27±0.05, 1.60±0.05, and 3.00±0.1 A, respectively. On the basis of these data, the atomic coordinates of the overlayer have been quantitatively established. The results are in agreement with theoretical calculations for the epitaxial continued layer structure and inconsistent with other models of zigzag chains.

X-ray standing-wave and extended x-ray-absorption fine-structure measurements have determined the strain and bond distortions in a buried InAs monolayer grown epitaxially on GaAs(001). The In atoms are found to reside 1.64{plus_minus}0.03 A above the last-As plane of the GaAs substrate with an In-As bond length of 2.57{plus_minus}0.02 A. Relative to bulk InAs, this corresponds to an 8% expansion in the In-As planar distance perpendicular to the interface and a 0.05-A compression in the In-As bond length. This experiment indicates that macroscopic-elastic theory describes the distortions in InAs/GaAs(001) films even in the monolayer limit.

The x-ray standing-wave technique has been used to determine the geometrical structure of Sb on GaAs(110). Using the back reflection diffraction geometry from (220) and (400) planes, we find the average vertical distance of Sb atoms from the GaAs surface to be 2.27±0.05 A in agreement with theoretical calculations for the epitaxial continued layer structure. Other models of buckled zigzag chains are found inconsistent with our results. Additionally, our data indicate less than 10% disorder in the Sb overlayer

The x-ray standing-wave technique through detection of surface-sensitive Auger electrons has determined the surface reconstruction of clean InP(110). Using the back-reflection diffraction geometry from (220) planes, we find the perpendicular displacements of the surface P and In atoms to be +0.18\ifmmode\pm\else\textpm\fi{}0.1 and -0.48\ifmmode\pm\else\textpm\fi{}0.08 \AA{}, respectively, from their unrelaxed bulk positions. The use of low-energy x rays combined with a novel method of data analysis opens the general problem of surface reconstruction to study by this technique.

Based on the surface extended x-ray-absorption fine-structure (SEXAFS) absolute surface coordination numbers, Woicik et al. [Phys. Rev. B 43, 4331 (1991)] argued that Sb trimers adsorb on the Si(111) surface in a modified ``bridge'' configuration rotated by 60\ifmmode^\circ\else\textdegree\fi{} relative to the atop ``milk stool'' geometry proposed by Abukawa et al. [Surf. Sci. Lett. 201, 513 (1988)]. Using the accurate near-neighbor bond lengths as determined from SEXAFS, these models predict perpendicular distances of 1.76 and 2.60 \AA{} between the top Si atoms and the Sb layer, respectively. New back-reflection x-ray standing wave data find the Sb atoms to reside 2.53\ifmmode\pm\else\textpm\fi{}0.1 \AA{} above the Si surface, in clear support of the milk stool geometry. Polarization-dependent SEXAFS relative coordination numbers support the milk stool model as well.

Using a combination of x-ray standing waves and surface extended x-ray-absorption fine structure, we have determined the interlayer relaxation of a metal surface in the presence of an adsorbate. The Cu(001) surface was studied with Cl c(2\ifmmode\times\else\texttimes\fi{}2) and S p(2\ifmmode\times\else\texttimes\fi{}2) overlayers using a back-reflection diffraction geometry from (111) planes at 2.9 keV. For the Cl-covered surface we find a 0.07\ifmmode\pm\else\textpm\fi{}0.04 A\r{} outward relaxation while for the S-covered surface the relaxation is more difficult to determine because of possible substract reconstruction.

X-ray standing wave (XSW), x-ray diffraction, and photoluminescence (PL) measurements were used to assess the layer perfection and positions of 1 and 1/2 monolayer (ML) InAs quantum wells buried in GaAs(001). Local structure in the 1-ML films was evaluated using x-ray absorption fine structure (XAFS) measurements. Growth temperature effects were studied in a series of samples produced by metal organic vapor phase epitaxy (MOVPE) between 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The XSW coherent position of the In atoms decreases with increasing temperature in the 1-ML samples, and the optimal growth temperature is near $550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ as evidenced by the coherent position of $1.15\ifmmode\pm\else\textpm\fi{}0.02$ and the relatively high coherent fraction of $0.72\ifmmode\pm\else\textpm\fi{}0.08.$ With decreasing growth temperature the XSW measurements may suggest segregation of In atoms, which results in an incorporation of the In into multiple layers. The segregation appears to be reduced at the higher temperatures due to the favorable kinetic conditions created in the MOVPE environment. Low-temperature PL measurements indicate that the sharpest and most intense In-excitonic emission is obtained from the 1-ML sample grown at $530\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ For the $\frac{1}{2}$-ML samples, growth temperatures of 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ produce similar standing wave results, although the PL reveals the higher temperature sample to be of far superior quality, due to excessive carbon incorporation at $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ In-As bond-length distortions found in the XAFS measurements agree with a macroscopic elastic description of the pseudomorphic epitaxy.