330 Chang-Min Kwak et al.
only few studies have been conducted to reveal the field eva- poration mechanism when using a metallic-capping layer. Therefore, the aim of this study was to improve the
mass-resolving power of laser-assisted APT for bulk LaAlO3 (LAO) and to elucidate the related capping effects on the laser-assisted evaporation of oxide tips. We selected bulk LAO because it is used extensively in spintronic and micro- electronic applications as a representative insulator film with high dielectric constant and wide bandgap (Kim et al., 2016). Another specific challenge in this study lies in observing the tip shape evolution occurring at the tip surface before, during, and after APT analysis. This was done using corre- lative and interrupted APT analysis with transmission electron microscopy (TEM) observation (termed the “step- wise APT with TEM”)onthe same tip(Fasthetal.,1967; Krakauer & Seidman, 1992; Sha et al., 2008; Shariq et al., 2009; Haley et al., 2011; Devaraj et al., 2013; Diercks et al., 2013; Schreiber et al., 2013; Lee et al., 2014). This work provides the basis for understanding the interactions of ultrafast laser illumination with an oxide tip, and provides a method to determine the evaporation field of oxide materials.
EXPERIMENTAL
Undoped LAO was used; it is a high-k dielectric bulk oxide. The needle-shaped samples for APT were prepared using a dual-beam focused ion beam (FIB; Helios Nanolab, FEI Co., Hillsboro, Oregon, USA) lift-out method (Miller et al., 2007; Thompson et al., 2007). The milling was performed step- by-step until the sample thickness became <100 nm. Subse- quently, low-energy milling at 2 kV was used to minimize Ga-ion damage at the tip surface. Next, Ni- or Co-capping material was deposited onto the FIB-processed tips by radio frequency plasma sputtering at room temperature. In this study, three samples were considered; uncapped LAO, 20-nm thick Ni-capped LAO, and 20-nm thick Co-capped LAO. The sputtering conditions were precisely controlled to reduce the effects of geometrical parameters such as shank angle and capping thickness, which govern the evaporation field strength of the APT tip. However, the surface morphologies of the Ni-capping layer were irregular compared with the Co-capping layer. This difference is due to the sputtering deposition
process and the agglomeration tendency of Ni atoms (Choi et al., 2000). We expect that the surface morphology has less influence with respect to the APT data set compared with thermal conductivity of capping layers. To determine both the thickness of the capping layers surrounding the tip apex and the tip shape evolution to ensure the evaporation sequence at the tip surface, the samples were observed using TEM (JEOL Electron Microscope; JEM-2100F, JEOL Ltd., Akishima, Tokyo, Japan). The APT analyses were performed using a laser-assisted wide-angle tomographic atom probe (LAWATAP; AMETEK Inc., CAMECA SAS, Gennevilliers Cedex, France). The APT measurements were performed in an ultrahigh vacuum (1×10−8 Pa) using a 343nm ultraviolet laser pulse (100 μm spot diameter, 400 fs pulse, 100 kHz pulse repetition rate) (Kodzuka et al., 2011) and the base temperature was ~40K. The evaporation rate was fixed in the range of 0.003–0.007 atoms/pulse. Each data set contained ~2 ×106 atoms. In themass spectrum of the metallic-capped LAO, peaks at 46.3 and 69.5Da were identified as La ions. Several other peaks could be assigned to oxygen-related molecular ions, O2
+, or to molecular ions such as LaO+,
LaO2+ , and AlO+. Peaks detected at 13.5 and 27Da were assigned to Al ions. The raw data were reconstructed by IVAS software using shank angles and tip radii measured by a TEM microscope operated at 200 kV.
RESULTS
Uncapped LAO Tips To examine the overall evaporation sequences during APT analysis, the change in surface curvature of the tips (Fig. 1) during the process was observed using high-resolution TEM. The uncapped LAO tips were flattened on the laser-illuminated side (Fig. 1b, arrow), as a result of uneven evaporation. The radius of curvature was 17-nm larger on the laser side (Rlaser = 48nm)thanonthe shadow side (Rshadow=31nm) after APT analysis of 1×106 atoms (Fig. 1b). The difference occurs because atoms at the surface of the tip evaporated primarily on the laser side. This result confirms the presence of laser-induced local heating at the surface of tips, and has been termed the “shadow effect” (Sha et al., 2008).
Figure 1. Transmission electron microscopy images of uncapped LaAlO3 sample (a) before analysis, and of the morphological evolution of the tip apex after collecting about (b) 1.0 million atoms, (c) 2.0 million atoms. Dashed lines—the radius of curvature on laser side (red) and shadow side (black). Arrow—laser direction.
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