Impact of the Liquid/Solid Interface on the Strain State of Si1-XGex Layers Processed By Nanosecond Laser Annealing

Abstract

Si1-xGex alloys took a major place in the microelectronic field because of their potential for MOSFETs devices. These last decades, the miniaturisation of these electronic devices allowed optimising their performances. However, this also highlighted new issues concerning the use of conventional Rapid Thermal Annealing for the fabrication processes of such materials. Nowadays, Nanosecond Laser Annealing (NLA) has attracted considerable attention thanks to its low thermal budget. It has been showed that NLA allows performing high temperature annealing localised at the surface, keeping the underlying material at low temperature, which may allow a diversification of device architectures such as 3D integration schemes. Moreover, this process is interesting for the formation of source/drain junctions as dopant concentrations higher than their solubility limit can be incorporated in Si or Ge. However, the impact of NLA on the structure of Si1-xGex materials is not fully understood, especially concerning the melting regime of NLA.Recently, we have investigated the structural evolution induced by NLA in 30 nm-thick strained Si1-xGex layers, elaborated by RPCVD [1]. These investigations have highlighted a strong correlation between the roughness exhibited by the interface between the melted and unmelted areas (referred to as liquid/solid (l/s) interface) and the strain state of the Si1-xGex layers. In particular, in the case of a flat l/s interface, the strain state is determined by the elastic energy stored in the Si1-xGex layer. In contrast, a rough l/s interface always leads to the layer relaxation.In the present study, we carried out a detailed investigation of the evolution of the l/s interface roughness as a function of several experimental parameters (Ge content, doping level, pulse duration). The objective is twofold: (i) to improve our understanding of the origin of the l/s interface roughness and its impact on the strain state of Si1-xGex layers and (ii) to identify the best process conditions to achieve fully strained layers after NLA in the melt regime.As a first step, B-doping has been used to modify the initial strain state of the Si1-xGex layers. Indeed, due to the low covalent radius of B atoms, the introduction of high B concentrations results in strain compensation. To achieve that, strained Si0.7Ge0.3 layers were doped with B atoms during grown. The Z-contrast observed on STEM-HAADF images allowed to quantify the l/s interface roughness and determine its evolution depending on the B-doping level. As shown in Fig.1, it has been evidenced that the flattening of the l/s interface at high B-doping allows to avoid the formation of strain relieving defects in the whole layer. Here, the B-doping is expected to reduce the elastic energy density stored in the Si1-xGex layer, preventing the formation of defects.As a second step, to understand the connection between the l/s interface roughness and the formation of defects, similar investigations have been made using 700 nm-thick fully relaxed Si1-xGex layers. Removing the initial strain may allow to decorrelate the different phenomena inducing the formation of a rough l/s interface. The firsts results evidenced a similar Ge redistribution as in strained Si1-xGex for melt depth up to 115 nm (Fig.2), while the l/s interface roughness was low regardless of the melt depth. However, at deeper melt depths, the structure of these layers is strongly modified. In particular, the roughness of the l/s interface induces lateral Ge segregation during resolidification, leading to pure Ge “walls” in the regrown layers. The origin of such laser-induced self-organisation will be discussed in terms of the elastic energy accumulated in these Si1-xGex layers during resolidification and compared to the physical models proposed in the litterature [2].As a last step, effects of NLA on strained Si1-xGex layers will be investigated for different laser pulse durations. Indeed, the formation of a rough l/s interface may also be linked to the nano-structuration of the surface during the so-called surface melt regime. Lowering the pulse duration is expected to minimize the size of these structures and avoid the strain relaxation of the Si1-xGex layers.In summary, the results obtained are expected to provide a better insight on the different phenomena occurring when a melt laser process is carried out on Si1-xGex layers and contribute to the optimisation of this annealing technique in view of its application in the fabrication of future nanoelectronics devices.Acknowledgements:This work was supported by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 871813 MUNDFAB.