AVS 56th International Symposium & Exhibition
    Surface Science Monday Sessions
       Session SS1+EM-MoA

Paper SS1+EM-MoA4
Formation and Structure of Alkaline Earth Template Layers for Oxide Epitaxy on Semiconductor (100) Surfaces

Monday, November 9, 2009, 3:00 pm, Room M

Session: Semiconductor Surfaces and Interfaces I: Ge and III-V's
Presenter: E.I. Altman, Yale University
Authors: B. Lukanov, Yale University
K. Garrity, Yale University
J. Reiner, Yale University
F.J. Walker, Yale University
C.H. Ahn, Yale University
S. Ismail-Beigi, Yale University
E.I. Altman, Yale University
Correspondent: Click to Email
The finding that alkaline earth titanates can be epitaxially grown on Si and Ge(100) surfaces with atomically abrupt interfaces without oxidizing the semiconductor has spurred research into exploiting these materials for high k gate dielectrics, and for integrating new functionality into semiconductor devices. Attempts to epitaxially grow other oxides with similarly abrupt interfaces have thus far failed. It has been suggested that the initial interfacial layer formed by the reaction between the alkaline earths and Si and Ge is key to the success achieved with the alkaline earth titanates; therefore, we have been studying the interaction Sr and Ba with Ge and Si(100) with scanning tunneling microscopy (STM), complemented by density functional theory (DFT). At elevated temperatures, both Sr and Ba cause massive restructuring, indicative of surface alloy formation. Initial deposition causes an apparent etching away of the substrate dimers leading to dimer chains and islands on the surface. For Sr on Ge(100), increasing the coverage leads to an apparent c(4x4) structure decorated by bright spots that order only locally; the density of these bright spots decreases with increasing Sr coverage. The apparent c(4x4) structure ultimately gives way to a (3x2) structure that is characterized by periodic arrays of islands and trenches when it completely covers the surface. Electron diffraction data shows that continuing to raise the coverage causes a structural transformation to a (2x1) structure. On the atomic scale, STM images of the apparent c(4x4) and (3x2) structures vary dramatically with imaging bias; in particular changing the polarity of the bias voltage causes a contrast reversal where bright features in filled state images appear dim in empty state images and vice versa. Based on the observed bias dependence for Sr adatoms deposited at 300 K and simulated images derived from DFT calculations, a model of the (3x2) structure was developed in which Sr atoms replace every third row of Ge atoms on the surface. The striking periodic island and trench morphology is then associated stress relief of the the (3x2) structure. The results clearly show that at elevated temperatures the template layer does not form through simple surface adsorption.