Understanding of Capping Effects 331
Previous reports have given details of the inhomogeneous temperature distribution at the surface of tips made of different materials, such as silicon (Koelling et al., 2011), GaSb (Muller et al., 2012) and MgO (Seol et al., 2016), which induces the asymmetry in the evaporation behavior between the two sides. Afterfurther APTanalysistocollect another~106 atoms, Rlaser = 68nmwas still 17-nm larger thanRshadow =
51nm.The evaporation field of a material is inversely proportional to its radius of curvature (under the assumption that the tip has been subjected to field evaporation), so to compare the field difference across the apex of the tip during analysis, we calculated Rlaser/ Rshadow; it dropped from 1.55 down to 1.33 during the analysis. This result suggests that the difference in the field strength between the laser side and the shadow side can be reduced as APT measurement progresses. This effect can be interpreted by uniform laser-induced heat distribution on the tip surface (Vella et al., 2011).
Ni-Capped LAO Tips
To avoid the laser-driven uneven temperature distribution at the tip surface, the tip was capped with a 20-nm thick Ni layer. Contour maps showing the 2D distribution of time of flight in a selected volume without capping (Fig. 2a) and with Ni capping (Fig. 2b) demonstrate that the greater heating on the laser- illuminated side can be avoided by the introduction of a Ni-capping layer with high thermal diffusivity. Comparison
reduced the differences in time of flight between laser and shadow sides. This reduction is due to the homogeneous heat distribution in the entire surface regions, which can be achieved because the thermal conductivity is higher in the metal-capping layer than in the bulk oxides. Therefore, the metal-capping layer strongly affects the path of laser-induced heat during laser-pulsed APT. The overall sequences of field evaporation of Ni-capped
LAO were observed using stepwise analysis of APT with TEM, as shown in Figure 3. The TEM images were taken after APT data sets of about 0.5 million atoms had been collected. After collecting ~106 atoms (stage 2), Rlaser was ~32nm and Rshadow was ~27 nm. Hence, the difference was ~5 nm, and Rlaser/Rshadow was ~1.19, both of which are smaller than for uncapped LAO tips (Fig. 1). This result
of La3+ peaks in mass spectra acquired from the incident laser and shadow side of an uncapped tip (Fig. 2c) demonstrated asignificant peak shift and delayed evaporation on the shadow side, whereas the peak shift of the La3+ peaks from Ni-capped tips is almost negligible (Fig. 2d). In general, due to strong laser–matter interaction, ions evaporated sooner on the incident laser side than on the shadow side. However, on the shadow side, transverse heat propagation is needed, so thermal activation is delayed by a few nanoseconds; this delay results in a long thermal tail in the mass spectrum, and deteriorates the mass resolution. The Ni-capping layer on the LAO tips significantly
Figure 2. Contour maps of selected volumes in LaAlO3 samples (a) without capping layer, and (b) with Ni-capping layer. c: Mass spectra of La3+ peaks emitted from uncapped oxide, and (d) mass spectra of La3+ peaks emitted from Ni- capped oxide from three different regions. Time of flight (TOF) of La3+ (blue, short TOF; red, long TOF). Arrow—laser direction.
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