P. H. Fuoss

In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Grazing-incidence small angle x-ray scattering studies of phase separation in hafnium silicate films

Grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution transmission electron microscopy (HRTEM) were used to investigate phase separation in hafnium silicate films after rapid thermal annealing between 700 and 1000 °C. 4-nm-thick Hf–silicate films with 80 and 40 mol % HfO2, respectively, were prepared by metalorganic vapor deposition. Films of the two compositions showed distinctly different phase-separated microstructures, consistent with two limiting cases of microstructural evolution: nucleation/growth and spinodal decomposition. Films with 40 mol % HfO2 phase separated in the amorphous by spinodal decomposition and exhibited a characteristic wavelength in the plane of the film. Decomposition with a wavelength of ∼3 nm could be detected at 800 °C. At 1000 °C the films rapidly demixed with a wavelength of 5 nm. In contrast, films with 80 mol % HfO2 phase separated by nucleation and growth of crystallites, and showed a more random microstructure. The factors determining specific film...

Imaging and alignment of nanoscale 180° stripe domains in ferroelectric thin films

Nanometer-period ferroelectric 180° stripe domains are observed in epitaxial PbTiO3 films using atomic force microscopy. Stripe domains can be aligned with surface step edges or in preferred crystallographic directions. A stripe alignment map as a function of temperature and film thickness is determined using synchrotron x-ray scattering. Pinning by step edges permits control of stripe domain morphology, as demonstrated by a film grown on a vicinal surface.

High contrast x-ray speckle from atomic-scale order in liquids and glasses

The availability of ultrafast pulses of coherent hard x rays from the Linac Coherent Light Source opens new opportunities for studies of atomic-scale dynamics in amorphous materials. Here, we show that single ultrafast coherent x-ray pulses can be used to observe the speckle contrast in the high-angle diffraction from liquid Ga and glassy Ni(2)Pd(2)P and B(2)O(3). We determine the thresholds above which the x-ray pulses disturb the atomic arrangements. Furthermore, high contrast speckle is observed in scattering patterns from the glasses integrated over many pulses, demonstrating that the source and optics are sufficiently stable for x-ray photon correlation spectroscopy studies of dynamics over a wide range of time scales.

Atomic Layer Engineering of Perovskite Oxides for Chemically Sharp Heterointerfaces

Atomic layer engineering enables fabrication of a chemically sharp oxide heterointerface. The interface formation and strain evolution during the initial growth of LaAlO(3) /SrTiO(3) heterostructures by pulsed laser deposition are investigated in search of a means for controlling the atomic-sharpness of the interface. This study shows that inserting a monolayer of LaAlO(3) grown at high oxygen pressure dramatically enhances interface abruptness.

In situ synchrotron x-ray characterization of ZnO atomic layer deposition.

The utility of in situ synchrotron x-ray scattering and fluorescence in gaining insight into the early stages of the atomic layer deposition process is demonstrated in this study of ZnO growth on Si. ZnO films are found to initially grow as islands, with the onset of coalescence occurring during the fourth growth cycle. The start of coalescence is accompanied by a small increase in surface roughness. After ten cycles of growth, the growth rate decreases from 4.2 to 3.0 A per cycle, with the growth following expected self-limiting behavior. The overall growth process is consistent with the model of Puurunen and Vandervorts for substrate-inhibited growth [R. L. Puurunen and W. Vandervorst, J. Appl. Phys. 96, 7686 (2004)].

In situ synchrotron x-ray studies of strain and composition evolution during metal-organic chemical vapor deposition of InGaN.

Composition and strain inhomogeneities strongly affect the optoelectronic properties of InGaN but their origin has been unclear. Here we report real-time x-ray reciprocal space mapping that reveals the development of strain and composition distributions during metal-organic chemical vapor deposition of InxGa1−xN on GaN. Strong, correlated inhomogeneities of the strain state and In fraction x arise during growth in a manner consistent with models for instabilities driven by strain relaxation.

Early-stage suppression of Cu (001) oxidation

In situ synchrotron x-ray studies of the early-stage oxidation behavior of Cu (001) reveal that for Cu2O nanoislands, the Cu–Cu2O equilibrium phase boundary is shifted to larger oxygen partial pressure (pO2) by many orders of magnitude relative to bulk Cu2O. Real-time scattering measurements find that an ordered surface structure appears with increasing pO2, followed by the nucleation of epitaxial Cu2O nanoislands. By adjusting the pO2, it is possible to reversibly grow or shrink these islands and accurately determine the equilibrium phase boundary. These observations provide insight into the general stability of oxide nanoclusters grown by various techniques.

Octahedral rotations in strained LaAlO3/SrTiO3 (001) heterostructures

Many complex oxides display an array of structural instabilities often tied to altered electronic behavior. For oxide heterostructures, several different interfacial effects can dramatically change the nature of these instabilities. Here, we investigate LaAlO3/SrTiO3 (001) heterostructures using synchrotron x-ray scattering. We find that when cooling from high temperature, LaAlO3 transforms from the Pm3¯m to the Imma phase due to strain. Furthermore, the first 4 unit cells of the film adjacent to the substrate exhibit a gradient in rotation angle that can couple with polar displacements in films thinner than that necessary for 2D electron gas formation.

Indium adsorption on GaN under metal-organic chemical vapor deposition conditions

Real-time synchrotron grazing-incidence x-ray fluorescence is employed to study indium adsorption on the GaN (0001) surface under typical process conditions for InGaN metal-organic chemical vapor deposition. An indium condensation boundary is mapped as a function of trimethylindium pressure, substrate temperature, and carrier gas composition. Below the condensation boundary, indium surface coverage reaches a maximum of ∼1∕4 ML. The addition of 8% H2 to the carrier gas is found to have a significant effect on both condensation and adsorption of indium.