We report the first demonstration of active boron (B) doping concentration $\left(N_{A}\right)$ higher than $2.50 \times 10^{21} \mathrm{~cm}^{-3}$ in high Ge content (> 65%) Si1-x Gex using In-situ growth technique with a low temperature below 500 °C. We achieve excellent uniformities of Si1-x Gex thickness and resistivity across the entire 300 mm wafer and obtain an ultra-low as-deposited specific contact resistivity $\left(\rho_{c}\right)$ down to $2.9 \pm 2.8 \times 10^{-10} \Omega-\mathrm{cm}^{2}$ on the sample with a high average active doping concentration $\left(N_{A}\right)$ of $2.80 \times 10^{21} \mathrm{~cm}^{-3}$ and Ge composition of 65%. We further detail the progression of the selective growth of such $\mathrm{Si}_{1 \cdot x} \mathrm{Ge}_{x}$ film on advanced 3D structures. Using metal $/ \mathrm{Si}_{1-x} \mathrm{Ge}_{x}$ ladder TLM (LTLM) structures, we investigate the contact properties from room temperature to cryogenic temperatures as low as $5 \mathrm{~K}$, disclosing for the first time the insignificant change of $\rho_{c}$ at such ultra-low $\rho_{c}$ regimes.