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

54 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTSafter the deposition of ≈ 250 nm of LiF at different temperatures, within the range between250 ◦ C and 400 ◦ C. In all cases the ripple structure is clearly recognizable and quiteregular. The ripples are very elongated especially at the lowest temperatures (T = 250 ◦ Cfor panel (a) and T = 300 ◦ C for panel (b)), where they extend longer than the imagesize (1μm) and are very straight. This is confirmed by the corresponding 2D Fourierspectra, which rapidly decay in the [001] direction and are much broader in the [1¯10]. Athigher temperatures, T = 350 ◦ C for panel (c) and T = 400 ◦ C for panel (d), the ripplesare more widely spaced but also more irregular; the average length is in-between 500 nmand 1μm, and the grooves appear slightly distorted. Looking at the 2D FFT spectra, thecentral peaks are still mainly spread in the [1¯10] direction, confirming that the uniaxialshape of the ripple remains the dominant feature of the surfaces, however an increasingbroadening in the [001] direction is also observed, indicating a gradual degradation ofthe ripples with the temperature. The broadening of the 2D Fourier spectra in the [1¯10]direction decreases with the ripples periodicity, however this is not an effect of disorder;in fact, the Fourier spectrum intensity for an ideal sawtooth profile with period Λ decaysproportionally to 1/Λ 2 [188].From a statistical analysis of the AFM data, we could extract the average rippleperiodicity Λ along with its standard deviation; we report the results as a function ofT in fig. 3.6. Λ follows an almost linear trend with the temperature, with a rate ofapproximately 1 nm/5 ◦ C: at 250 ◦ C the average ripple spacing is (25±2) nm (fig. 3.5(a));increasingT, Λraisesto(35±3)nmat300 ◦ C(fig.3.5(b)), (45±5)nmat350 ◦ C(fig.3.5(c))and (60±7) nm at 400 ◦ C (fig. 3.5(d)); correspondingly, the standard deviation remainsas low as 3 nm below 300 ◦ C and increases up to ≈ 7 nm at a temperature of 400 ◦ C.60Ripples periodicity [nm]50403020250 300 350 400Substrate temperature [°C]Figure 3.6: Ripples periodicity as a function of the substrate temperature during the LiFhomoepitaxy. Dashed line: linear trend of ≈ 1 nm/5 ◦ C to guide the eye.We did not consider depositions at temperatures outside the 250 ◦ C÷400 ◦ C range.However, lowering the temperature the adatoms mobility decreases and so the faceting ofthe (110) surfaces is harder to occur; on the contrary, at high temperature we are on oneside limited by the sample holder capabilities, on the other side the high adatoms thermalenergy is expected to gradually destroy the ripple coherence.