Geometry and doping control in silicon nanowires (SiNWs) are both crucial aspects in fabricating three-dimensional (3D) radial junction thin film solar cells, while the coupling between them remains a peculiar aspect to be better understood. In this work, we focus on the geometry evolution and the doping effects realized in tin-catalyzed SiNWs grown via a plasma-enhanced vapor-liquid-solid procedure by using different diborane (B2H6) dopant flows. It is shown that with the increase of B2H6 flow rate from 0.3 to 2.1 SCCM, the radial growth of SiNWs is greatly accelerated by more than 30%, while the length is shortened to 50%. This can be related to the enhanced chemisorption probability of SiHx radicals, with the addition of B2H6, on the SiNW sidewall during silane (SiH4) plasma deposition in PECVD system, which leads to easier nucleation directly on the sidewalls and faster radial expansion of the SiNWs. A trade-off has to be sought between seeking a strong light trapping and ensuring a sufficient doping for high-quality PIN junction with the increase of B2H6 doping flow. These new understandings lay a critical basis for understanding and searching for an optimal growth control for constructing high-performance 3D radial junction thin-film solar cells.