Photoemission Spectroscopy of Germanium Surfaces, Interfaces, and Germanium-Silicon Systems.

High-energy electron diffraction and photoemission techniques providing controlled surface sensitivity were used to study several germanium surface and interface systems: the clean Ge(100)-(2 x 1) surface, the Ge(100)-Ag interface, and silicon-germanium systems. For the clean Ge(100)-(2 x 1) surface, both angle-integrated and angle-resolved measurements reveal a component of the Ge 3d lineshape corresponding to emission from surface atoms; analysis of this component yields information relating to the details of the (2 x 1) reconstruction and also a measure of the mean free path of electrons in Ge. The characteristics of the Ge(100)-Ag interface were determined, including the growth mechanism, chemical interaction, crystallographic relationships, and electronic properties. Ag was found to grow by submonolayer nucleation into metallic Ag(110) islands followed by three-dimensional growth. Photoemission spectroscopy from the Ge 3d core levels and also the valence bands indicates an abrupt interface with no substantial intermixing or chemical reaction, in contrast to previous reports. Implications for the structure of the (2 x 1) reconstruction are discussed. Ge-dissolving Si(111) surfaces displaying the (5 x 5) reconstruction were prepared by annealing Ge films deposited onto Si(111)-(7 x 7) substrates. Comparison of the lineshapes of the Si 2p core for the (7 x 7) and (5 x 5) surfaces revealed differences which can be interpreted as due to selective replacement of Si atoms by Ge atoms without any drastic change in the surface structure. The Ge 3d core lineshapes for both clean Ge(111)-c(2 x 8) and Si(111)-Ge(5 x 5) surfaces were also measured and compared. With thicker Ge deposits, strained layers of Ge(111) with a (7 x 7) reconstructed surface (instead of the natural c(2 x 8) reconstruction) were formed. Photoemission spectroscopy of the valence bands of these surfaces are compared to those of clean Si(111)-Ge(7 x7) and Ge(111)-c(2 x 8). Like Si(111)-(7 x 7), the Si(111)-Ge(5 x 5) and Ge(111)-(7 x 7) surfaces were found to be metallic, while the Ge(111) -c(2 x 8) surface is semiconducting.