Another long-term project undertaken by the MPI-MF group has been the investigation of the interfacial structure of Schottky contacts using XRR and GID. The material system chosen was a liquid metal in contact with silicon. At the interface, the electronic structure of a Schottky contact exhibits band bending. The high-density Fermi liquid should limit electronic screening effects within the liquid metal to ~1Å (due to the metallic screening length). Theoretical investigations of a solid wall in contact with a liquid predict a density oscillation that corresponded to atomic layering in the liquid that decays as the distance into the liquid increases. The primary technique for this project has been to use XRR to measure the density profile normal to the surface. The initial measurement of liquid-Pb in contact with silicon produced a surprising result, a high densification in the Pb over a nanometer length scale. To confirm this unexpected results, two liquid metals (either Pb or In) were measured, as well as, either undoped or n-type doped silicon. Two surface orientations, (100) and (111), were also measured. The general trend remains the same for the different systems. The systems can be fit with a two-layer model, the first with an increased density (+30%) layer followed by a decreased density layer over a 30Å distance. As a control check, the semiconductor was replaced with an insulator material. Liquid-indium/sapphire (0001) was measured and found not to have this densification effect. The experimental data and fit curves are shown in Figure 30. These results point to an electronic effect in the silicon and create a more complicated picture of the interfacial structure. One possible explanation is that the silicon draws charge from the liquid metal, thereby creating ionic-like behaviour in the metal near the interface, and thus causes a densification due to smaller ionic radii. This project has demonstrated how a new technique using high-energy x-rays has allowed for new materials system to be measured and creating a challenge to our current understanding of physics.

H. Reichert, M. Denk, J. Okasinski, V. Honkimäki, and H. Dosch - Giant Metal Compression at Liquid-Solid (Pb-Si, In-Si) Schottky Junctions - Phys. Rev. Lett. 98, 116101 (2007).