SPring-8, the large synchrotron radiation facility

Date&Time: May 13, 2003, 14:00 - 15:30

Speaker: Dr. Wolfgang Braun
Affiliation: Paul-Drude Institute for Solid State Electronics

Abstract
　　The vacuum environment during molecular beam epitaxy (MBE) allows the use of a multitude of in situ characterization techniques. Among the available diffraction methods, reflection high-energy electron diffraction (RHEED) is probably the most widely used. Despite its advantage of being very surface sensitive, the strong electron-matter interaction in RHEED leads to multiple scattering effects that dramatically complicate a quantitative analysis.
　　X-ray diffraction data, on the other hand, can in most cases be quantitatively analyzed using kinematical (single scattering) theory. In addition, the variation of the incidence or detection angles in surface diffraction geometries allows a variation of the sampling depth. In this way, one can study either surface morphology, surface structure, thin films, or interfaces. This offers a variability not possible with RHEED. X-ray scattering measurements require high incident fluxes, especially if temporal resolution during epitaxy is desired. It is therefore necessary to use synchrotron x-ray sources.
　　We have designed and constructed a dedicated beamline at the synchrotron BESSY II in Berlin that combines in situ x-ray diffraction in the 6-12 keV wavelength range with a state-of-the-art MBE system. During the initial phase of the project, we have studied the homoepitaxy of β(2x4)-reconstructed GaAs(001), and MnAs/GaAs(001) heteroepitaxy and the phase transitions in the resulting films.
　　On GaAs(001), the intensities and reflection profiles can be acquired on integer and fractional order rods, allowing a separate analysis of the terrace or island dynamics and the surface reconstruction domains. To study the surface reconstruction, we deposit one monolayer and then study the coarsening kinetics of the 4 equivalent reconstruction domains (order along the 2x direction is weak) with stationary surface morphology. We measure an increase of the domain correlation length l ∝ t (0.42+/-0.05) in the half-order reflections and l ∝ t(0.22+/-0.05) in the quarter-order reflections, with exponents that somewhat smaller than expected from Lifshitz-Allen-Cahn theory. The fraction of the higher energy double-kink domain boundaries increases as the logarithm of time. To study the island kinetics, we deposit 0.5 GaAs monolayers and follow the time evolution of the diffraction profiles. We measure a linear increase of the island/terrace correlation length, in contradiction to the established models for 2D spinodal decomposition. The linear peak intensity recovery is observed at all coverages, allowing us to develop a model for the slow and fast recovery processes following growth termination. We find that the fast recovery process is due to a mutual annihilation of island and pits in the lower and higher layers, followed by coarsening of the remaining two-layer system during the slow recovery phase.
　　MnAs films on GaAs(001) undergo several structural phase transitions during the post-growth quenching to room temperature. The intermediate structures are only accessible in an in situ experiment. We find that the MnAs at growth conditions optimized for spin injection structures is strained during deposition. At the transition to the ferromagnetic state (βMnAs toαMnAs), a strain-mediated phase coexistence leads to the formation of stripes of alternating phases in the film.