Researchers from Intel Corporation and the University of Minnesota and the University of Wisconsin have shown that strained atom-scale films of pure ruthenium (Ru) metal exhibit ferromagnetism at room temperature, openning up the possibility of using the material to build novel magnetic random access memory (MRAM) devices. As per details recently published in Nature Communications (https://doi.org/10.1038/s41467-018-04512-1), Ru thin films with a thickness of 2.5, 6, or 12 nm, were grown on Al2O3 substrates cut along the (110) direction, that had been covered with a 20 nm Mo seed layer. The thin films were grown using a eight-target UHV sputtering system with base pressure of 8 × 10−8 Torr or lower, resulting in the controlled epitaxial growth of strained body-centered tetragonal phase Ru.
From first principles of materials engineering, there should be ways to use different templating materials for this graphoepitaxial process such that silicon-oxide could be used as the substrate instead of aluminum-oxide. If so, then this process could be run on 300mm silicon wafers in today’s leading commercial IC fabs.
The (001) tetragonal Ru plane does not lie perpendicular to the substrates which leads to a soft coercive field, however, if out-of-plane texturing can be achieved high coercivity Ru may be realized. The thickness dependence was also examined, and it was found that due to Ru relaxing into a non-ferromagnetic phase, the magnetization drops with increasing thickness. The 12 nm thick sample showed magnetization of about one-half that of the two thinner samples.
Last month in Nature Communications (doi:10.1038/ncomms5836) IBM researchers Jeehwan Kim, et al. published “Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene.” They show the ability to grow sheets of graphene on the surface of 100mm-diameter SiC wafers, the further abilitity to grow epitaxial single-crystalline films such as 2.5-μm-thick GaN on the graphene, the even greater ability to then transfer the grown GaN film to any arbitrary substrate, and the complete proof-of-manufacturing-concept of using this to make blue LEDs.
The figure above shows the basic process flow. The graphenized-SiC wafer can be re-used to grow additional transferrable epi layers. This could certainly lead to competition for the Leti/Soitec/ST “SmartCut” approach to layer-transfer using hydrogen implants into epi layers.
No mention is made of the kinetics of growing 100mm-diameter sheets of single-crystalline GaN on graphene. Supplemental information in the online article mentions 1 hour at 1250°C to cover the full wafer, but the thickness grown in that time is not mentioned. From first principles of materials engineering, they must either:
A) Go slow at first to avoid independent islands growing to form a multicrystalline layer, or
B) Initially grow a multicrystalline layer and then zone anneal (perhaps using a scanned laser) to transform it into a single-crystal.
In either case, we would expect that after just a few single-crystalline atomic layers had been either slowly grown or annealed, that a 2nd much-higher speed epi process would be used to grow the remain microns of material. More details can be seen in the EETimes write up.