A Study on the Optimization of the Recessed Silicon Germanium Junction Parameters of p-channel MOSFETs with Channels under Uniaxial Compressive Strain

Abstract

It has been shown that recessed SiGe source/drain technology enhances the performance of metal oxide semiconductor field effect transistors (MOSFETs), by providing enhanced channel mobility and reduced source/drain contact resistivity. The focus of this dissertation is to study the effect of the recessed SiGe junction parameters on the biaxial compressive strain in SiGe, and its impact on the bandgap and contact resistivity. Due to its smaller size, boron can partially compensate the compressive strain in SiGe. This behavior was modeled using the covalent radii of Si, Ge and B, to calculate the lattice parameter of the ternary SiGeB alloy. It was also shown using micro-Raman spectroscopy that Houghton's kinetic model accurately predicted the SiGeB critical thickness. Formation of NiSiGe on SiGe was also studied, and it was found that NiSiGe induced tensile strain in SiGe, thereby reducing the compressive strain in the junctions. The impact of the NiSiGe thickness on SiGe bandgap was also studied using p⁺(SiGe)-n(Si) diodes, and it was shown that increasing the NiSiGe thickness led to an increase in the bandgap, due to loss in compressive strain. It was also shown that the barrier height followed the SiGe bandgap, and hence increased with NiSiGe thickness. The impact of the SiGe bandgap and the barrier height on contact resistivity was studied using four-terminal Kelvin structures. It was shown that contact resistivity increased with SiGe thickness and NiSiGe thickness, due to reduced biaxial compressive strain. It was shown that with fully strained SiGe junctions, and a germanium concentration of x=0.28, a minimum contact resistivity of 2.5 X 10⁻⁸ Ohm-cm² could be obtained. While the experiments in this dissertation are limited to SiGe and NiSiGe contacts, the fundamental knowledge gained from this work is expected to have a much wider impact. Specifically, this thesis introduces strain as a new parameter in contact engineering because of its impact on the semiconductor band structure and the metal-semiconductor barrier height, regardless of the metal and semiconductor choices.