Epitaxial beryllium films and beryllium-based multilayer mirrors for soft x-rays.

Abstract

Epitaxial Be films and Be-based multilayers were fabricated and characterized for a variety of deposition conditions. Epitaxial Be films were grown for the first time on α-Al₂O₃ (0001), Si (111), Si (100), GaAs (111), and Ge (111) single-crystal substrates. The effects of the substrate material, substrate temperature, deposition rate, and post-annealing temperature on the crystalline quality of the epitaxial Be films were studied. All of the resultant epitaxial films exhibited the hexagonal close-packed (hcp) Be crystal structure. Epitaxial Be films which were grown on substrates with hexagonal lattices [α-Al₂O₃ (0001), Si (111), GaAs (111), and Ge (111)] invariably grew with their hexagonal lattices parallel to the substrate plane. In nearly all cases, the Be hcp lattice grew in the anticipated best-fit epitaxial relation with the substrate. The crystalline quality of the Be films improved significantly with increasing substrate temperature. However, Be films deposited at substrate temperatures ≥ 400°C were discontinuous, and many exhibited hexagonal-shaped crystallites. The deposition rate had no effect on crystalline quality over the range studied (3.1-18.5 Å/min). Annealing epitaxial Be films at temperatures ≤ 625°C after deposition improved crystalline quality only slightly. Annealing the films at 700°C resulted in the collapse of hexagonal grains in order to relieve stress within the film. The highest quality epitaxial Be films were deposited onto Si (111) substrates maintained at 300°C. The crystal quality was improved further by annealing the sample at 300°C for 30 minutes after deposition. These samples exhibited the first known (2 x 3) reconstruction of the Be (0001) surface. The design and fabrication of several Be-based multilayer mirrors for use at soft x-ray wavelengths (λ = 10-500 Å) was also investigated. Germanium and bismuth were selected as the absorber material candidates for the Be-based multilayers. The Ge/Be combination resulted in a multilayered structure. This multilayered growth combined with the epitaxial growth of Be lays the foundation for fabrication of single-crystal multilayer (superlattice) mirrors for soft x-rays.