.*

Substitution of Nb⁵⁺ for Ti⁴⁺ in SrTiO₃ (denoted as Nb:SrTiO₃) is an excellent electron doping method with reduced disorder, superior mobility, and higher T_c, compared with the electron doping by creating the oxygen defects denoted as SrTiO_3-δ. We applied the Nb doping to the two different ferroelectric materials, Sr_0.95Ba_0.05TiO₃ and Sr_0.985Ca_0.015TiO₃, confirmed, by the x-ray structural analyses, they turned into polar metals with broken centrosymmetry. We can define a particular carrier density n* for the polar metals. When the carrier density n < n*, the resistance shows an upturn below the temperature T_K. The values of T_K increase monotonically as the carrier density decreases. Interestingly, they coincide with the ferroelectric Curie temperature at the zero carrier density limit. Although the values of n* for Nb:Sr_0.985Ca_0.015TiO₃ and Nb:Sr_0.95Ba_0.05TiO₃ are different by one order of magnitude, both materials show typical superconducting domes with single peaks commonly located around 10²⁰ cm⁻³. Compared with the nonpolar matrix Nb:SrTiO₃, the values of T_c are enhanced and reach 0.75 K for Nb:Sr_0.95Ba_0.05TiO₃. However, the Tc enhancement was insignificant (less than 0.1 K) at around n*. The enhancement becomes much more prominent further inside the dilute carrier-density region, where the screening is less effective. These results suggest that centrosymmetry breaking, i.e., the ferroelectric nature, does not kill the superconductivity. Instead, it enhances the superconductivity directly, despite the absence of strong quantum fluctuations. Our results call for a reconsideration of the existing microscopic models of superconductivity in SrTiO₃.