(Invited) Highly Doped Si1-XGex Epitaxy in View of S/D Applications

Abstract

Modern CMOS devices are a fertile ground for innovation, as scaling and conceptual evolutions drive regular updates in process and material specifications. Requirements are usually set by device modelling, which defines the performance objectives for a new technology. Simulations recently carried out assuming fin [1] and nanosheet [2] field effect transistor geometries have for instance highlighted limitations due to the growing importance of contact resistance in devices with reduced dimensions. To meet the objectives, source/drain (S/D) contact regions receive a specific attention and aggressive targets are set to enable higher-performing generations of devices. Within this framework, contact resistivities (rc) lower than 1E-9 W.cm2 are specified for sub 5 nm technology nodes. Reaching this objective for n and p-type S/D is a challenging task, both from the perspectives of obtaining the right materials and of reliably measuring low rc values [3]. Several material parameters are known to affect the final contact resistivity. Novel epitaxial growth schemes are considered to maximize the active doping concentration to narrow the Schottky barrier at the metal - S/D interface. Additionally, the composition of and strain relaxation in the S/D layer affect band alignment and must be considered.This contribution focuses on S/D epitaxial layers grown and in-situ doped by chemical vapor deposition, and on the properties of fabricated Ti / p-Si1-xGex contacts. For a given Ge concentration, the Ti / Si1-xGex:B contact resistivity is minimized for an optimized range of Si1-xGe1-x:B layer thickness, and increases as the layer relaxes [4]. With increasing Ge content, the rise in contact resistivity begins for lower thicknesses. Observed trends are nevertheless similar (Fig. 1). Please note that the targeted boron concentration is constant within each series of samples but differs for different Ge concentrations. The variations in resistivity and contact resistivity as seen for different series are assigned to the different boron concentrations rather than the variation in Ge concentration. Indeed, lower resistivities and contact resistivities are measured for higher active boron concentrations.The correlation between the electrical properties and layer thickness reported in figure 1 confirms that preserving strain in the Si1-xGex:B S/D material is important. However, the increase in S/D layer and contact resistivities with increasing layer thickness is not caused by the (small) increase in electrical band gap. Instead, B incorporation during epitaxial growth is affected by the initiation of layer relaxation and decreases as the material relaxes (R) as can be concluded from Fig. 2. This figure shows the boron concentration as function of depth for two partially relaxed Si1-xGex with 25% and 65% Ge, respectively. A reduction in B incorporation when the S/D material thickness exceeds the critical thickness for strain relaxation (tc) is observed for the whole range of Ge concentrations. This phenomenon is supported by density functional theory results which indicate a reduced propensity of boron to occupy substitutional lattice sites in relaxed Si1-xGex.