The substitution of halogen dopants on the chalcogenide site is a common n-type doping strategy in TE materials e.g., I doping of PbTe, 62 I-doping of Bi 2 Te 3 , Br-doping of SnSe, and halogen doping to realize n-type BiCuSeO. However, as the defect energetics (and, therefore, the carrier concentrations) are dependent on the thermodynamic equilibrium of the growth conditions, we emphasize the need to dope Y 2 Te 3 under Y-rich conditions. In practice, precise control of the thermodynamic equilibrium (set by the elemental chemical potentials) can be achieved through careful phase boundary mapping experiments, wherein the competing phases of a material are detected as impurity phases to gauge the thermodynamic of the material.