The e-See project aims at studying electric phenomena (charges, electric fields, electrostatic potential) in semiconducting nanostructures in a quantitative way, with the ultimate goal of detecting a single deterministically controlled charge with nm scale spatial resolution using electrical in-situ transmission electron microscopy (TEM). Since many devices we use in everyday life work because of their electrical properties, and single charges already affect device performance, it becomes relevant to have a tool to measure and control a single charge.
Main results:
(i) We believe to have observed Hall Effect at nm length scale, which may be a tool to identify both fixed as well as mobile charges in a material. This novel kind of characterization may help improving devices, yet needs to be further developed first. We observed the Hall effect in a nanowire (nw) containing a p-n junction, due to the applied electric bias as well as the magnetic field present in the TEM, allowing us to see both an electric field along the nanowire axis, due to the pn junction, as well as an electric field perpendicular to the nanowire axis, due to the free charges of different sign on both sides of the junction. We are now optimizing our TEM for further experiments, with the idea of actively using the objective lens as an in-situ tool to generate a magnetic field for Hall effect, with the idea of performing a very similar experiment with in-situ biasing with and without the magnetic field of the objective lens.
(ii) We believe to have shown single charge detection, of an electron on a Vanadium dopant atom introduced in WSe2, by combining Center of Mass as well as electron ptychography and comparing the resulting potential maps with DFT simulations [https://hal.science/hal-04688082]. We may now apply this technique to 2D materials relevant for tomorrows transistors, or to semiconductor transistors at low temperature.
(iii) We have been able to manipulate many charges using in-situ biasing. Combining in-situ biasing and finite element modelling we obtained quantitative information about the doping concentration as well as the junction abruptness [https://hal.science/hal-03881003]. We believe this is the first TEM based study demonstrating quantitative data on the junction interface profile. This kind of analysis may help building better p-n junctions with improved doping profiles.
(iv) We have optimized the propagation of Al in group 4 nanowire materials (Si and Ge) to fabricate novel quantum dot materials with tuneable electrical properties [https://hal.archives-ouvertes.fr/hal-03348045].
We demonstrated that we can understand and control the exchange reaction between Al and Ge [https://doi.org/10.1021/acs.nanolett.8b05171] which is a model system for our project, and is promising as a platform for quantum mechanical phenomena.
Then, by carefully tuning all aspect of this reaction and using in-situ Joule heating by in-situ biasing, we were able to fabricate a Ge quantum dot of deterministic size of only 7 nm in width between mono-crystalline Al contacts [https://hal.archives-ouvertes.fr/hal-02452338] exhibiting an atomically abrupt interface between metal and semiconductor [https://doi.org/10.1021/acsnano.9b06809]. This kind of structure may feature tomorrows quantum computers.
(v) In the Al/SixGe1-x alloy nanowire/metal couple [https://hal.archives-ouvertes.fr/hal-02465092] we observed a thermally assisted partially reversible thermal diffusion process.
The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1-x part, forming an axial Al/Si/SixGe1-x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the SixGe1-x alloy nanowire while maintaining the rod-like geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si based technology. Moreover, we observed that electrical properties of these structures can be tuned from a single hole quantum dot to the Josephson effect [https://hal.archives-ouvertes.fr/hal-03348045]. These achievements pave the way to use this system at low temperature in the TEM.
We have done a Highlight on this work [https://e-see.neel.cnrs.fr/2021/05/25/highlight-on-reversible-diffusion/].
(vi) We have made progress on developing a helium temperature cryogenic biasing TEM sample holder WP1. We have adapted the design of a holder and biasing chip, we have done several cooling tests. The holder will have one contact for high frequency experiments. We are finalizing the design of contacting the chip.
(vii) During the project, we optimized our silicon nitride on silicon membrane-chip design. After repeated requests from the community, we have published the fabrication protocol of our membrane-chips, including a review of the studies we performed on these membranes-chips [https://hal.archives-ouvertes.fr/hal-02464411]. We are still further optimizing the chip design to avoid long lived charges (see technical report).
