Polar materials have recently attracted significant interest due to their rich multifunctional properties. The chalcogenide polar semiconductor ${\mathrm{Cu}}_{1-x}{\mathrm{Mn}}_{1+y}{\mathrm{SiTe}}_3$ (Cu-deficient, $0.04\le x\le 0.26$; $0.03\le y\le 0.15$) is an emerging multiferroic system in which electric polarization is coupled to magnetization. However, its macroscopic ferroelectric polarization is strongly suppressed due to the presence of a high density of stacking faults. In this work, we demonstrate that these crystal defects, likely originating from nonstoichiometry, can be substantially reduced by increasing the Cu content. Cu-enriched samples, ${\mathrm{Cu}}_{1+x}{\mathrm{Mn}}_{1-y}{\mathrm{SiTe}}_3$ ($0.04\le x\le 0.3;0.13\le y\le 0.31$), crystallize in a noncentrosymmetric monoclinic structure (space group $\mathit{Pm}$) as the Cu-deficient counterpart but show a nearly stacking-fault-free phase, which is attributed to the emergence of an interstitial site. Consequently, the Cu-enriched samples show a pronounced enhancement of the second-harmonic generation (SHG) response compared to Cu-deficient compositions. Magnetically, the Cu-enriched crystals retain long-range antiferromagnetic order with a Néel temperature of $T_{\mathrm{N}}\sim 33$ K without a glassy state but manifest a distinct metamagnetic transition along the polar $b$ axis that is absent in the Cu-deficient compositions. Furthermore, the electronic ground state evolves from insulating to doped semiconducting behavior upon Cu enrichment. Together, these results establish this material system as a unique and versatile platform for elucidating the interplay among composition, crystal defects, and multifunctional properties, offering a route to design magnetic polar systems with tunable quantum functionalities.

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• Magnetism
• Magnetotransport
• Weak antilocalization
• Multiferroics
• Single crystal materials
• Neutron diffraction
• Optical second-harmonic generation