CuP 2+x thin films with a broad range of x (positive and negative) could be deposited by reactive sputtering in a PH 3 /Ar atmosphere at room temperature, using either a Cu target, a Cu 3 P target, or both at the same time (see the Experimental Details and the x-axis in Fig. ). The main available parameters to tune x are the RF power on the targets and the PH 3 partial pressure (see Supporting Information). Decreasing the power led to higher P contents due to a more Penriched target surface and/or to a lower flux of Cu at the substrate, promoting phosphorization at the substrate. Higher PH 3 partial pressures can be achieved by increasing the total pressure or the PH 3 concentration in Ar. Because the PH 3 concentration was limited to below 5% in our setup, we had to employ a relatively high sputter pressure (2 Pa ≃ 15 mTorr) to obtain films of CuP 2 stoichiometry. The wide tunability of the P content in Cu-P films was also observed in our recently reported amorphous B-P films by reactive sputtering. This compositional flexibility is likely related to the ability of P to form homoelement bonds in the films and segregate as an elemental impurity.