Preparation of gallium-lithium co-doped silicon wafers as radiation hardened materials

This work aims to combine different approaches to produce crystalline silicon substrates featuring an improved radiation hardness: (i) p-type conductivity using gallium (Ga) as main dopant (ii) lithium (Li) co-doping (iii) thickness reduction (<100 µm). In particular, Li is known for its capability to mitigate recombination-active radiation-induced defects. However, the Li doping procedure raises important challenges, Li being in particular prone to surface accumulation. Furthermore, few studies focused on the Li properties in Ga-doped silicon. In this study, a Ga–Li co-doping protocol has been investigated, using beam line ion implantation and subsequent annealing steps. First results confirmed the incompatibility of this approach when Li is directly implanted into bare silicon wafers, due to both Li exo-diffusion and surface accumulation. To overcome these issues, different barrier layers were investigated (i.e. dielectric barrier layers and n+ phosphorus-diffused regions). The dielectric layers did not favor the Li bulk contamination, essentially due to Li accumulation within the dielectrics or at the dielectric/substrate interfaces. However, the n+ region was found to be an efficient barrier. It allowed to successfully produce 90 µm-thick p-type co-doped Ga–Li wafers with an electrically-active Li concentration of 4.7 × 1015 cm−3, value known for significant radiation hardness improvements.