Understanding of Capping Effects 333


Figure 5. Mass spectra of (a)La3+,(b)Al2+,(c)O2 +, and (d)O+ peaks for uncapped (black line), Ni-capped (blue line),


and Co-capped (red line) LaAlO3 tips. All peaks are normalized after background subtraction.


In this case the tip had Rlaser ~32nm and Rshadow ~31nm (Rlaser/ Rshadow ~1.03). This symmetry means that the Co-capped LAO is likely to be sufficiently thermally activated without confinement of photon absorption, unlike uncapped tips. Furthermore, the Co-capping layers surrounding the tip evaporated nearly concurrently with the core regions of the LAO oxide. This synchronicity means that the metal layers are sustained during the APT analysis and form a path to conduct laser-induced heat. The concurrent evaporation suggests that the evaporation field of LAO is similar to that of Co. This use of equivalence can simplify estimation of the exact evaporation field strengths of oxide materials, which are not known because the calculation is quite complex. The surface absorption of laser photons by oxide materials may be due to band bending caused by the strong electric field (Kelly et al., 2014; Silaeva et al., 2013), but the physical origin of this surface absorption is still unknown.


DISCUSSION


Ni- and Co-capping layers deposited on the LAOtips increase the degree of symmetry of the tip shape evolution and the uniformity of heating at the tip surface during APT analysis; as a result, the reliability of the APT data increases. The use of Ni- and Co-capping layers improved both the mass-resolving power and the signal-to-noise ratios of the APT data compared with when analyzing uncapped tips. Figure 5 displays the La3+, Al2+,O2


+,and O+ peaks in the mass spectrum, which were


the major peaks in the mass spectrum taken from the uncap- ped, Ni-capped, and Co-capped LAO tips used in this study.


ThemeasuredFWHMat themass spectra fromLa3+,Al2+,O2 +,


and O+ are listed in Table 1. These results confirmed that the Ni- and Co-capping layers promote the mass-resolving power of APT data. This beneficial effect of capping on APT data of LAO


tips clearly depends on the selection of capping materials. For instance, compared with Ni, the Co-capping layer increases the mass-resolving power in La3+,Al2+,O2


+, andO+


peaks. This increase is related to the thermal diffusivities of capping materials, because the thermal diffusivity of Co layer is ~0.26cm2/s, which is higher than that of Ni (~0.20 cm2/s) at 40K (Foley & Sawyer, 1964; Montague et al., 1979). However, these values do not represent the true value, because when the laser irradiates the tip surface, the temperature changes depending on the laser influence. In addition to mass-resolving power, the occurrence of multiple events is an important factor affecting the accuracy of compositional analysis. The difference in the percentage of multiple events was ~5% higher with uncapped than with


Table 1. Mass Resolution Obtained at full-width half-maximum from the La3+,Al2+,O2


+, and O+ Peaks After Background Subtraction.


La3+ Al2+


O2 +


O+


Uncapped 165.2


119.1 120.1 90.5


Ni capped 260.1


156.7 187.9 125.1


Co capped 308.3


178.3 209.7 177.4


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