Improved electrical properties of shallow p(+)-n junction using selectively grown graded Si1-xGex epitaxial structure

Abstract

A strained boron-doped graded Si1-xGex structure consisting of layered Si1-xGex with increasing Ge mole fractions was studied as the solid diffusion source for forming a p(+)-n junction suitable for raised source/drain metal oxide semiconductor field effect transistor (MOSFET) application. The in situ doped graded Si1-xGex structure was selectively epitaxially grown (SEG) on the active region using ultrahigh vacuum chemical molecular epitaxy (UHVCME). Improved electrical characteristics, not possible with conventional uniform SEG Si1-xGex junction, are achieved with the graded SEG Si1-xGex junction. For the conventional uniform Si1-xGex junction, while a high Ge composition is necessary for reducing the specific contact resistance and the sheet resistance, such a high Ge composition, also results in a degraded ideality factor and a leaky junction. In contrast, a near-perfect SiGe/Si interface property is demonstrated with the graded Si1-xGex structure. Optimum specific contact resistance and the sheet resistance can be obtained by the graded Si1-xGex structure without suffering degraded junction leakage and ideality factor. These results indicate that the degree of misfit dislocation and the undesirable undercut effects could be effectively reduced by using the graded structure with its lower Ge composition close to the silicon substrate, while a low sheet resistance and specific contact resistance is preserved with its higher Ge composition at the top of the layered structure. (C) 2000 The Electrochemical Society. S1099-0062(00)08-111-6. All rights reserved.