Schottky Barrier Height of Nickel Silicide Contacts Formed on $\hbox{Si}_{1 - x}\hbox{C}_{x}$ Epitaxial Layers

Embedded Si_{1 -} xCx source/drain junctions are currently considered to achieve electron mobility enhancement in nMOSFETs by inducing uniaxial tensile strain in the channel region. To utilize the mobility advantage of this technology, it is imperative to form low-resistivity contacts to Si_{1 -} xCx alloys. In this letter, the electron and hole barrier heights at the NiSi/Si_{1 -} xCx interface were measured up to a carbon concentration of 1.2%. The results indicate that the NiSi Fermi level moves away from the valence-band edge with increasing carbon concentration such that the hole barrier height increases by 68 meV in spite of the upward movement of the valence band. Within the same carbon concentration range, the electron barrier height decreases by as much as 170 meV, which is significant considering the exponential dependence of contact resistivity on barrier height.

I. Introduction

It is well established that the electron mobility in nMOSFETs can be enhanced by applying uniaxial tensile stress to the channel. The desired stress levels can be achieved by forming embedded epitaxial source/drain junctions, which provide a smaller lattice constant than the Si substrate [1]–[4]. In order to take full advantage of the strain-induced mobility enhancement, the MOSFET parasitic series resistance must be limited to a small fraction of the channel resistance. The contact resistance, which emerges as the most critical component of this resistance, is an exponential function of the metal–semiconductor barrier height and the doping density at the interface. Since the doping density is limited by the solid solubility of dopant atoms, the barrier height remains as the only parameter that can be engineered to reduce the contact resistivity.