Electron Beam Induced Crystallisation in Iron (III) Fluoride
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EXPERIMENT Iron MI"I) Fluoride (>97% pure) was evaporated onto carbon covered copper grids. The crystallisation behaviour of three types of specimen was investigated: thermally evaporated 'dry' films, thermally evaporated 'wet' films and electron beam deposited 'dry' films. To produce the dry thermal films, FeF 3 was dehydrated (heated at 300"C for 6 hours) to remove water prior to evaporation, whereas for wet films, FeF 3 was evaporated without pre-heating. For the electron beam deposited films dehydrated FeF3 was used. After the evaporation, the specimen was immediately transferred to a vacuum bottle and placed in the microscope within 10 minutes of its preparation. Electron beam damage studies were carried out in a Philips CM30 transmission electron microscope operating at an accelerating voltage of 100 keV in conjunction with a Gatan 666 parallel electron energy loss spectrometer. The energy resolution was better than 1eV at an energy dispersion of 0.12 eV per channel. All the spectra were acquired in image mode with an acquisition time of 12.8 seconds. The condenser lens C2 was overfocused to give a uniformly illuminated area. The total beam current was measured using the drift tube of the spectrometer. The beam current was adjusted every -10 minutes using a picoammeter attached to the viewing screen and any decay in beam current was compensated for. Adjacent areas of specimen were irradiated for different times and diffraction patterns recorded. The diffraction patterns were obtained under Identical conditions to facilitate direct comparisons by measuring the Intensity of diffraction rings.
195 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
(a) As prepared (b) 1.1x106 Cm-2 (c) 3.3x100 Cm-z Figure 1: Diffraction patterns (a) showing the amorphous structure of dry thermal FeF 3 film, (b) and (c) show the appearance of FeF 2 and FeF 3 rings at intermediate and high doses. L•i%
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Dose
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11 210
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(b) (a) Figure 2: (a) Radially averaged diffraction patterns of the dry thermal film as a function of 6 2 electron dose increasing up to 3.3x10 Cm- at the same intervals as in figure 3a. The arrows pointing upwards identify the FeF3 reflections and the pattern from the highest dose is shown enlarged in (b) with the FeF 3 and FeF2 reflections identified. RESULTS AND DISCUSSION Selected Area Diffraction Patterns Figures l(a)-(c) show three electron diffraction patterns from a damage series of a dry thermally deposited FeF3 film. It can be seen that the material is amorphous when6 deposited and with increasing electron dose it begins to crystallise. For low doses (up to -1x10 Cn- 2) one set of microcrystalline diffraction rings appears with increasing intensity (Figure lb). For higher doses, a second set of rings also appear (Figure lc). The electron diffraction patterns were quantified by digitising and radially averaging to obtain the intensity as a function of scattering angle. Figure 2a shows th
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