  Ionic liquid gating can markedly modulate the materials' carrier density so
as to induce metallization, superconductivity, and quantum phase transitions.
One of the main issues is whether the mechanism of ionic liquid gating is an
electrostatic field effect or an electrochemical effect, especially for oxide
materials. Recent observation of the suppression of the ionic liquid
gate-induced metallization in the presence of oxygen for oxide materials
suggests the electrochemical effect. However, in more general scenarios, the
role of oxygen in ionic liquid gating effect is still unclear. Here, we perform
the ionic liquid gating experiments on a non-oxide material: two-dimensional
ferromagnetic Cr2Ge2Te6. Our results demonstrate that despite the large
increase of the gate leakage current in the presence of oxygen, the oxygen does
not affect the ionic liquid gating effect (< 5 % difference), which suggests
the electrostatic field effect as the mechanism on non-oxide materials.
Moreover, our results show that the ionic liquid gating is more effective on
the modulation of the channel resistances compared to the back gating across
the 300 nm thick SiO2.
