Taisuke T. Sasaki

The increasing resistivity of Cu interconnects at reduced device dimensions requires alternative conductors with suppressed size effects of resistivity. Materials with highly anisotropic electronic transport are a promising solution. CoSn exhibits pseudo-one-dimensional transport along the c axis with low resistivity comparable to conventional metals, making it a promising candidate for next-generation interconnects. In this study, we investigate the epitaxial growth of CoSn thin films on sapphire substrates with controlled crystallographic orientations to align the low-resistivity c axis in the plane. We successfully grow single-crystalline CoSn(101¯0) and CoSn(112¯0) films on sapphire substrates using appropriate buffer layers. Although the (112¯0) plane has a higher atomic packing density and is thus expected to yield lower surface energy and smoother films based on an analogy with conventional metals, the two orientations show markedly different surface morphologies: CoSn(101¯0) forms comparatively smooth films, whereas CoSn(112¯0) exhibits pronounced {101¯0}-faceting and rough surfaces. First-principles calculations reveal that the (101¯0) surface has lower surface energy than the (112¯0) surface, consistent with the experimentally observed facet formation. This demonstrates that surface energetics in CoSn are governed not by atomic packing density but by anisotropic bonding associated with the kagome lattice. Electrical transport measurements reveal strong dependence of resistivity on crystallographic orientation, consistent with the bulk properties, with the lowest resistivity achieved along the c axis, down to 10.6 μΩ cm for 50 nm-thick CoSn(101¯0). This work establishes orientation-controlled epitaxy on 300 mm-wafer-compatible sapphire substrates as a viable strategy for integrating CoSn into next-generation interconnect architectures.

Ductility-dip cracking is a type of solid-state cracking with corresponding ductility loss that is associated with solidification processes in FCC metals and alloys, including Ni-based alloys, at temperatures of typically 0.5 – 0.7 of the alloy melting point. The prevalence of this type of cracking during the tensile deformation of additively manufactured Inconel 738LC at elevated temperature was investigated in this study. Dog bone specimens were machined from IN738LC cylindrical samples fabricated via laser powder bed fusion. Noticeable ductility reduction was observed under tensile loading at 800°C – 900°C. Additionally, it was discovered that extensive cracking at intra-columnar high angle grain boundaries occurred along with significant γʹ precipitation, following tensile loading at 900°C. Furthermore, formation of γʹ along the high angle intra-columnar grain boundaries was revealed via transmission electron microscopy. It is concluded that the presence of a thin, discontinuous layer of grain boundary γʹ causes a loss of cohesion, and therefore weakens the boundaries, leading to easier grain boundary sliding, and ultimately, cracking.

Magnetic Heusler alloys are highly attractive for spintronics; however, realizing their full potential requires high-temperature annealing, which is often incompatible with practical device fabrication. Excimer laser annealing (ELA) potentially addresses this temperature constraint by making use of the short annealing time and temperature gradient along the depth direction. We investigated the effect of ELA on Co2Mn0.5Fe0.5Ge half-metallic Heusler-alloy thin films and demonstrated that ELA successfully induces crystallization and B2 atomic ordering. Optimized ELA condition using low fluence with a high number of laser irradiations achieved reduced resistivity and negative anisotropic magnetoresistance, indicating improved atomic ordering and high spin polarization, while maintaining flatness of the films. These findings establish ELA as a viable annealing method for integrating high-performance Heusler alloys into the device structure with strict thermal budget. Moreover, ELA offers additional advantages such as enhanced throughput and selective area annealing, thereby broadening the scope of Heusler-alloy applications in spintronic devices.