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Angle calculations are presented for a new type of diffractometer suitable for surface X-ray diffraction. The new geometry results from combining the four-circle and the z-axis geometries and involves six circles. Instruments based on this concept can be operated in different diffraction geometries. Compared to the four-circle and z-axis geometries, a larger fraction of the reciprocal space perpendicular to the sample is available to experiments when all six circles are used. This results in an improved out-of-plane resolution in any X-ray structure study. In each geometry, either the angle of incidence or the outgoing angle of the X-rays can be chosen. At the same time, the surface normal can be aligned in the horizontal diffractometer plane, providing optimum resolution and sample illumination. All equations are given in closed form. The different modes of operation are compared and operation schemes are discussed. Journal of Applied Crystallography

Erbium surface segregation is observed during growth of Er-doped Si by molecular beam epitaxy on Si(100) at 600 °C. Once a critical Er surface areal density of 2×1014 Er/cm2 is reached, enhanced Er trapping is observed, possibly due to the formation of silicide precipitates. Er segregation on Si(100) is fully avoided when growth is performed in an oxygen background pressure of ∼10−10 mbar, due to the formation of Er-O complexes. No Er segregation is observed on Si(111), which is attributed to the formation of epitaxial Er3Si5 precipitates.

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