Transistor Contacts in the Making: Live Atomic Scale Dynamics

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Researchers at the Integrated Electronics and Biointerfaces Laboratory (IEBL) at the University of California San Diego gaze at atoms as they move in specific order to control their movement and to make better contacts to the ever-shrinking transistor switches. IEBLers have pioneered the metallurgy of forming metal contacts to nanoscale semiconductor channels for materials relevant to current and emergent transistors that are only a few atoms across. They devised a variety of fabrication processes to combine precisely controlled nanochannels with in-situ TEM (Transmission Electron Microscopy) heating platforms to record the dynamics of metal-semiconductor solid-state reactions. In recent work published in Nano Letters and in the journal Small, they reported imaging the dynamics of these reactions in the cross-section of a nanowire, and the details of their complex phase change mechanism for the first time.

“Nanoscale transistors have entered into an age of relentless shrinking of dimensions to a few atomic layers in both length and cross-sectional width and their performance have been greatly hindered with the increase of their electrical contact series resistance. We are developing strategies to control the atomic ordering at these contact interfaces and to improve their fabrication and further enhance their performance as they shrink down to a few atoms across,” said Shadi Dayeh, electrical engineering professor at UC San Diego.

Both articles were first authored by Ph.D. student Renjie Chen, one of Prof. Dayeh’s Ph.D. trainees who conducted the experiments. Renjie utilized the InGaAs (Indium Gallium Arsenide) high electron mobility channel and first devised approaches to minimize the formation of intermixing layers that naturally occur due to latent heat exchange when vapor Ni deposits on InGaAs. Then he adjusted the reaction temperatures to observe with in-situ TEM the layer-by-layer growth and its anisotropic nature due to different density of atoms in different crystal orientations. This phase transformation, which usually occurs in flat facets has evolved into stepped edges, exposing the terminations of {111} planes that eventually form a {111} rhombohedral central crystalline region. By capturing the reaction dynamics in nearly cylindrical cross-sections, Renjie was able to create a model that would fit the experimental data and predict their growth rates and phase transformation.

“Renjie’s studies at such unprecedented clarity of atomic processes of the reaction dynamics in the cross-section of a nanowire sheds light on the early stages of the metal-semiconductor reactions that will help not only fabricate but also model the thermodynamic and electrical properties of these contacts,” said Dayeh.

Along the nanowire length, Renjie reported in Small the mechanism of their crystalline phase transformation from a ternary (a material system with three elements i.e. InGaAs) cubic crystal to quaternary (a material system with four elements i.e. Ni-InGaAs) hexagonal crystal. He observed unconventional atomic-layer movements due to large interfacial stresses during this phase transformation and provided a precise model that describes the atomic arrangements at the interface between the two phases.

“Credited to Renjie’s focus, scientific depth, and experimental talent, this is yet another milestone in the reaction dynamics with compound semiconductor nanochannels,” said Dayeh.

Moving forward, the team will tinker these interfacial atomic layers to result in reduced contact resistances. They collaborated with Sandia scientists Katherine Jungjohann, William Mook and John Nogan to establish the experimental results reported in their work. Dayeh and his students use the facilities at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Basic Science user facility, that provides access to top-line equipment under a user proposal system.

“We are grateful for CINT colleagues, staff, and management to grant us access to the CINT facilities to perform these experiments and to continue our nearly one decade-long collaboration in electronic materials science. We are also grateful for the support of the Division of Materials Research at the National Science Foundation under program manager Tania Paskova, and formerly Haiyan Wang and Charles Ying,” said Dayeh.

The work was supported by the National Science Foundation under the following awards: DMR-1503595 and ECCS-1351980.

About Sandia National Laboratories

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, C.A., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.