Phase formation sequence at metal-germanium interfaces in thin film systems.
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Date
1996
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Abstract
Germanium thin films are used in integrated circuit electronic devices. Interest in germanium is due to the high mobility of its electrons and holes. It can therefore be used among other things as a high speed complementary transistor as well as in other devices. It is also used for making Ohmic contacts in GaAs devices. In strained layer (Ge,Si)/Si heterostructures, the band structure can be modified, which leads to interesting electronic properties. This thesis concerns itself mainly with phase formation in metal-germanium systems.
Characterisation of samples after heating in a vacuum furnace was done by Rutherford Backscattering Spectrometry (RBS) and X-Ray Diffraction (XRD). Ti-Ge, Pd-Ge, Zr-Ge, Fe-Ge and Cr-Ge thin film systems were investigated. First phases found in these systems were Ti6Ge5l Pd2Ge, ZrGe, FeGe. and CruGe8 respectively. Subsequent phase formation was also investigated. Results obtained were compared to the predictions of the Effective Heat of Formation (EHF) model. Nucleation and phase skipping was also investigated. In germanides non-congruent phases with more negative effective heats of formation tend to form first in some systems (e.g. CoGe in the Co-Ge system). The two systems Ti-Ge and Ni-Ge were chosen for detailed investigation because they have non-congruent phases (viz. TiGe2 and NiGe) with more negative effective heats of formation. An attempt is made to nucleatethese phases as first phases. A statistical view to phase formation is also introduced. In this model phase formation is described using elementary probability theory. Probabilities for atoms to meet in correct ratios to form phases are derived and used to predict first phases. Fractional heats which are closely related to effective heats of formation are also introduced. The model also makes use of diffusion a! and rotational activation energies to describe stability of phases against breaking into constituent parts. These activation energies are not readily available, which is a disadvantage of this model. Many models of phase prediction have been proposed in the past. Those of particular note are the Walser-Bene model, kinetic model of Gosele and Tu, the Zhang and Ivey model as well as the EHF model of Pretorius. These models as well as the statistical model developed as part of this work are described and compared to each other. The EHF model is found to be the most successful of the lot. It can predict phase formation in silicides, germanides and metal-metal systems. It can also predict phase formation sequence, phase decomposition, and the effect of impurities on phase formation. The success of this model is due to it's direct use of thermodynamic data, such as heats of formation (AH°), and kinetics through the effective concentration of interacting species at the growth interface. It can also explain why different experiments produce different first phases in the same binary system at times.
Description
Dissertation presented in the partial fulfilment of the requirements for the Degree of Doctor of Philosophy to the Department of Physics University of Zululand, 1996.
Keywords
Metallurgy, Germanium