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Electromigration is one of the most critical problems limiting the future scaling of integrated circuits (ICs).  As more transistors are packed into ever smaller volumes, the total length of interconnecting wire is increasing as the wire simultaneously becomes narrower and more vulnerable to failure. Surprisingly, while chip fabs are presently churning out microprocessors at the “5-nm process node” with correspondingly tiny interconnects, before recent work by a STROBE team no measurements of electromigration pressures had been reported at length scales smaller than 10 μm.

Using electron-beam lithography, the  team fabricates tiny aluminum nanowires on electron-transparent silicon nitride membranes. They then apply an electrical current density of 10⁸ A/cm²,  which is enough to drive electromigration, and map the nanowire’s density with parts-per-thousand precision using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).  Two separable effects appear.  First, thermal expansion decreases the wire’s density as Joule heating raises its temperature. Using aluminum’s known coefficient of thermal expansion, one can convert the density map into a temperature map with the same spatial resolution.  Second, via the electron “wind force” the electrical current simultaneously compresses one end of the wire while it tensions the other. Using aluminum’s known bulk modulus,  one can convert the density map into a pressure map.  The temperature effect is independent of the direction of the current, while the pressure effect changes sign with the current’s direction, which allows the two effects to be separated.  Thus the thermodynamic state variables T and P can be mapped with high spatial resolution everywhere inside the wire. The ability to simultaneously see the prompt thermal and electron-wind-force effects in nanoscale interconnects opens entirely new avenues for studying electromigration, a key technological problem in the microprocessors powering modern computing devices.

Over the last decade, course-based undergraduate research experiences (CUREs) have been recognized as a way to improve undergraduate science, technology, engineering, and mathematics education by engaging students in authentic research practices. One of these authentic practices is participating in teamwork and collaboration, which is increasingly considered to be an important component of undergraduate research experiences and laboratory classes. For example, the American Association of Physics Teachers Recommendations for the Undergraduate Physics Laboratory Curriculum suggest that one of the goals for students in physics labs should be to develop “interpersonal communication skills” through “teamwork and collaboration.” Teamwork can have tremendous benefits for students, including increased motivation, creativity, and reflection; however, it can also pose an array of new social and environmental challenges, such as differing styles of communication, levels of commitment, and understanding of concepts. It can also be difficult for lab course instructors to evaluate and assess. In this work, we study student teamwork in a large-enrollment physics CURE. The CURE was specifically designed to emphasize teamwork as a scientific practice. We use the two sources of data, the adaptive instrument for regulation of emotions questionnaire and students’ written memos to future researchers, to measure the students’ teamwork goals, challenges, self, co-, and socially shared regulations, and perceived goal attainment. We find that students overwhelmingly achieved their teamwork goals by overcoming obstacles using primarily socially shared regulatory strategies, and that the vast majority of students felt teamwork was an essential part of their research experience. We discuss implications for the design of future CUREs and lab courses and for lab instructors desiring to assess teamwork in their own courses.

2D lateral and vertical heterostructures have been actively studied for fundamental interest and practical applications. Although aberration-corrected electron microscopy and scanning probe microscopy have been used to characterize a wide range of 2D heterostructures, the 3D local atomic structure and crystal defects at the heterostructure interface have thus far defied any direct experimental determination. Now, a collaborative team from UCLA, Harvard University, MIT and UC Irvine demonstrates a correlative experimental and first principles method to determine the 3D atomic positions and crystal defects in a MoS₂-WSe₂ heterojunction with picometer prevision and capture the localized vibrational properties at the epitaxial interface. They observe various crystal defects, including vacancies, substitutional defects, bond distortion and atomic-scale ripples, and quantitatively characterize the 3D atomic displacements and full strain tensor across the heterointerface. The experimentally measured 3D atomic coordinates, representing a metastable state of the  heterojunction, are used as direct input to first principles calculations to reveal new phonon modes localized at the heterointerface, which are corroborated by spatially resolved electron energy-loss spectroscopy. In contrast, the phonon dispersion derived from the minimum energy state of the  heterojunction is absent of the local interface phonon modes, indicating the importance of using experimental 3D atomic coordinates as direct input to better predict the properties of heterointerfaces. Looking forward, it is expected that the ability to couple the 3D atomic structures and crystal defects with the properties of heterostructure interfaces will transform materials design and engineering across different disciplines.

Understanding and controlling the forces that drive the formation of symmetry-broken phases in quantum materials is a key challenge in condensed matter physics. The nature of the correlated interplay between charge, spin, and lattice degrees of freedom, however, often remains hidden in equilibrium studies where adiabatic tuning masks the causal ordering of rapid interactions. This motivates the use of ultrafast electron diffraction (UED) to capture structural dynamics on intrinsic time scales, for insight into the role of atomic-scale lattice distortions and vibrational excitations in driving, stabilizing and ultimately controlling emergent phases.

In a recent publication, a STROBE team carried out the first-ever ultrafast study of tantalum telluride (TaTe₂), yielding direct insight into its structural dynamics. The material exhibits unique periodic charge and lattice trimer order, which transitions from stripe-like chains into a (3×3) superstructure of trimer clusters at low temperatures. After cooling to 10 K, the thin crystalline films were optically excited and the structural dynamics was probed with the high-brightness electron bunches. Satellite peaks as well as sign changes in the complex diffraction patterns yield a fingerprint of the periodic order and structural transition. Our experiments captured the photo-induced melting of the trimer clusters in TaTe₂, evidencing an ultrafast phase transition into the stripe-like phase on a ~1.4 ps time scale. Subsequently, thermalization into a hot cluster superstructure occurred. Density-functional calculations indicate that the initial quench is triggered by intra-trimer Ta charge transfer, which destabilizes the clusters unlike CDW melting in other TaX₂ compounds.

Critical to this project were new methods and algorithms, enhanced microscopes and samples, advanced sample preparation as well as a unique high repetition rate ultrafast electron diffraction beamline utilized by the STROBE team from UC Berkeley, LBNL and UCLA