New Atomically Smooth Silicon Pyramids Exhibit Unusual Magnetics Properties

Home appliances, mobile phones, cars and other day-to-day technologies have been completely transformed by ultrasmall integrated circuits.

Reliable fabrication of circuits in three dimensions is essential to further miniaturize electronics and achieve sophisticated functions. It is challenging to achieve ultrafine 3D shape control through etching into silicon since even atomic-scale damage affects device performance.

In a new study published in Crystal Growth & Design, scientists from the Nara Institute of Science and Technology (NAIST) have etched silicon to take the shape of atomically smooth pyramids. When these silicon pyramids were coated with a thin iron layer, magnetic properties that were only theoretical so far were achieved.

Ken Hattori, a NAIST researcher and senior author of the study, has published widely in the field of atomically controlled nanotechnology. The main aim of Hattori’s research is to enhance the functionality of silicon-based technology.

"Silicon is the workhorse of modern electronics because it can act as a semiconductor or an insulator, and it’s an abundant element. However, future technological advances require atomically smooth device fabrication in three dimensions."

Ken Hattori, Researcher, Nara Institute of Science and Technology

The fabrication of arrays of pyramid-shaped silicon nanostructures requires a combination of chemical etching and standard dry etching. Thus far, it has been highly difficult to prepare atomically smooth surfaces.

"Our ordered array of isosceles silicon pyramids were all the same size and had flat facet planes. We confirmed these findings by low-energy electron diffraction patterns and electron microscopy."

Aydar Irmikimov, Study Lead Author, Nara Institute of Science and Technology

The researchers deposited an ultrathin iron layer measuring 30 nm onto the silicon to induce strange magnetic properties. The atomic-level orientation of the pyramids governed the orientation and in turn the properties, of the overlaying iron.

"Epitaxial growth of iron enabled shape anisotropy of the nanofilm. The curve for the magnetization as a function of the magnetic field was rectangular-like shaped but with breaking points which were caused by asymmetric motion of magnetic vortex bound in pyramid apex."

Ken Hattori, Researcher, Nara Institute of Science and Technology

The team observed that the curve lacked breaking points in similar experiments carried out on planar iron-coated silicon. Although several other research teams have theoretically estimated the anomalous curve for pyramid shapes, the NAIST team is the first one to have demonstrated it in a real nanostructure.

“Our technology will enable fabrication of a circular magnetic array simply by tuning the shape of the substrate,” added Irmikimov. Incorporation of the new technology into advanced technologies like spintronics―that encode information by the spin, and not the electrical charge, of an electron―will substantially increase the working speed of 3D electronics.

Journal Reference:

Irmikimov, A., et al. (2021) Atomically Architected Silicon Pyramid Single-Crystalline Structure Supporting Epitaxial Material Growth and Characteristic Magnetism. Crystal Growth & Design. doi.org/10.1021/acs.cgd.0c01286.