Phase-Dependent Functionality and Defect-Induced Magnetism in Monolayer SnTe Polymorphs.

Monolayer SnTe offers an unusual opportunity for functional materials design because its electronic and magnetic behavior can be tuned through two coupled control parameters: crystal phase and local defect chemistry. Here, first-principles calculations establish how these complementary design knobs govern the intrinsic and defect-driven properties of four SnTe polymorphs. Among the considered monolayers, cubic SnTe is identified as the ground-state phase, closely followed by γ-SnTe, while hexagonal and β'-SnTe are less favorable. This phase hierarchy is accompanied by strongly distinct electronic and transport regimes, with the HSE+SOC band gap spanning 0.35-1.84 eV, ultrahigh hole mobility emerging in cubic SnTe, strongly anisotropic electron transport appearing in hexagonal SnTe, and anisotropic hole transport found in β'-SnTe. Guided by this phase-stability landscape, defect engineering is then examined in cubic and γ-SnTe through substitutional doping and native vacancies. A clear phase-dependent defect response is revealed: dopants drive cubic SnTe toward effective metallicity, whereas γ-SnTe supports more selective outcomes, ranging from metallic to semiconducting behavior depending on dopant species. In particular, Mn is the most favorable magnetic dopant in both phases, and Mn-doped γ-SnTe combines strong spin polarization with a retained direct band gap, highlighting it as a promising magnetic semiconducting state. Together, these results show that phase selection defines the intrinsic band gap and transport landscape of monolayer SnTe, while defect engineering provides additional control over metallicity, magnetism, and carrier polarity. Monolayer SnTe therefore emerges as a tunable platform for infrared optoelectronic and spin-functional applications.