Morphology and plasmonic properties of self-organized arrays of ...

3.1. GROWTH AND CHARACTERIZATION OF LIF SUBSTRATES 53corresponding Fourier spectral density is shown in the inset of fig. 3.4(a): the spectrumis predominantly elongated along the [1¯10] direction, indicating features of the real imagewith preferential [001] orientation, but it is also spread in the [001] direction, confirmingthe lack of long range order. Increasing the coverage, as in panels (b) and (c), themacrosteps get longer, merging with one another and eventually developing into a regularripple structure; correspondingly, the Fourier spectra become progressively sharper andconfined in the [1¯10] direction. After the deposition of ≈ 250 nm of LiF (panel (d)), theripple periodicity is well defined and the [001] macrosteps are fewμm long. At even highercoverages, the quality of the ripples does not change significantly, but several defects beginto proliferate, mainly in proximity of the polishing scratches, leading to the formation ofrandomly oriented nanocubes, similar to fig. 3.3(b).a.b.[001] [001]c. d.[001][001]Figure 3.5: AFM images of nanopatterned LiF(110) samples after the deposition of t LiF ≈250 nm of LiF, grown at different temperatures. Panel a: T = 250 ◦ C, Λ = (25±2) nm.Panel b: T = 300 ◦ C, Λ = (35±3) nm. Panel c: T = 350 ◦ C, Λ = (45±5) nm. Panel d:T = 400 ◦ C, Λ = (60±7) nm. Inset of the figures: Fourier spectra of the correspondingimages.The macrosteps periodicity Λ and, in general, the structure of the ripples are determinedby the adatoms mobility and diffusion length during the homoepitaxy, so theystrongly depend on the substrate temperature and the deposition rate. Having fixed therate at ≈ 1 nm/min, we focused on the study of the periodicity as a function of thetemperature T. In fig. 3.5 we report various AFM images measured on several samples