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Boron is potentially useful as a spacer in multilayer x-ray optics for wavelengths between the boron K-edge at 67 angstrom and approximately 125 angstrom. To investigate the usefulness of this element as a spacer, we have fabricated multilayers incorporating boron by alternately depositing several material pairs, including Ag/B, Pd/B, and B/Si. We are interested in the Ag/B system as a possible crystalline superlattice x-ray mirror. Unfortunately, the tendency of Ag to form islands has frustrated our efforts to date to grow both single crystal B overlayers and quality nonepitaxial multilayers. The Pd/B system shows promise as an amorphous multilayer system. Our work shows that the palladium reacts with the boron to form an amorphous, palladium-rich boride. However, calculations indicate that the ideal reflectivity of the reacted multilayers may be as high as 51% at 80 angstrom. In the B/Si system, boron functions as the absorber layer for wavelengths greater than approximately 125 angstrom, resulting in a narrow bandpass mirror. We present evidence that the multilayers formed are layered and indirect evidence that the interfaces formed are relatively sharp. All depositions for the present work were performed under ultrahigh vacuum conditions in a Perkin-Elmer 433-S Molecular Beam Epitaxy system. All of the materials were electron-beam deposited except palladium, which was evaporated from a high temperature Knudsen cell. These multilayers were characterized in situ by reflection high energy electron diffraction (RHEED), and ex situ by low-angle x-ray diffraction.
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