Template for integrating functional oxides on silicon

Charles H. Ahn, Yale University, DMR 0705799

The integration of crystalline functional oxides (high dielectric constant, ferromagnetic, ferroelectric) on silicon is a key to the development of new platforms for future generations of microelectronics devices. Currently, the most promising route for growth of these crystalline oxides involves the deposition of a submonolayer of strontium metal as a template layer. The exact growth mechanism of this layer has not been fully understood until now.We have addressed this problem by combining advanced theory and experiment. We find that new atomic surface structures (labeled 2x3 and 1x2 in figures) arise at high temperatures as strontium reacts with the silicon surface, replacing the top layer of silicon atoms. Understanding these reaction dynamics will advance the quality and range of future crystalline oxide-silicon devices.

Electron diffraction experiment showing new surface structures emerging (2x3 and 1x2) as Sr is deposited on the Si surface at 650°C. The surface structures do not arise when the experiment is carried out at 25°C.

Theoretical calculations of stable strontium atoms (yellow) on silicon surfaces (blue and green).

Silicon

CrystalOxide

Silicon

CrystalOxide

Reiner et al., Phys. Rev. Lett. 101, 105503 (2008)

Characterization of atomically flat metal oxide surfaces using atomic force microscopy

Charles H. Ahn, Yale University, DMR 0705799

Summer research programs for undergraduates can provide an excellent opportunity to stimulate careers in physics. In this project the goal is to obtain atomically flat metal oxide surfaces, using atomic force microscopy (AFM) to characterize the surface morphology. AFM is particularly suited for this task, since it reveals the topography of surfaces at the nanoscale.

Atomic force micrographs show the presence of strontium oxide mounds on top of atomically flat terraces, with sizes and shapes that depend on heating and etching during surface processing. By systematically studying these surfaces, Anna Walker and co-workers found process conditions that yield the smoothest surfaces.

Undergraduate student Anna Walker (Covenant College) and graduate student Jason Hoffman (Yale) image the surface structure of oxide surfaces with AFM.

Surface morphology of synthetic tausonite (SrTiO3) single crystals. The white circular features are segregated strontium oxide mounds. The surface terraces (running from top left to bottom right) are separated by atomic steps. 1 μm

1 μm