Figure 3: Homogeneous Ga valence state change through resistive switching of a-GaO_x device.

(a) Procedure to prepare the bare a-GaO_x surface after polarization to the HRS or LRS. After polarization, UV set epoxy resin was pasted on the TE, and the TE was removed by scratching off the epoxy resin. (b) Photoemission electron microscopy (PEEM) image in the Ga 3d binding energy region, measured at the border of the pristine surface (bright grey region with red square) and the bare surface of HR film under the +2 V-biased TE mark (diameter=200 μm) (dark grey region with blue square). The black line along the border is due to segregation of contaminants such as Pt and resin residues. Inset: XPS spectra were reconstructed from the PEEM images at the red point within the pristine surface area (red line) and at the blue point within the polarized surface area (blue line). (c) μ-XPS in the Ga 3d binding energy region measured on the pristine surface and at marks of the TE polarized at different voltages during the 2nd and 3rd sweep cycle. In the polarized films, the measured spectrum (red dots) can be described well by two peaks (black line) corresponding to Ga³⁺ (P₁) and Ga⁺ (P₂), that is, Ga is highly reduced. A couple of peaks corresponding to metallic Ga⁰ (P₃ and P₄) are involved only in the unpolarized, pristine film. Molar ratios of Ga³⁺/Ga⁺ are 0.16/0.84 at 2 V and 0.52/0.48 at −2 V in the 2nd sweep cycle and 0.19/0.81 at 2 V, 0.28/0.71 at −1.3 V and 0.50/0.50 at −2 V in the 3rd sweep cycle, as calculated by peak fitting analysis. (d) Near-Fermi edge spectra of a-GaO_x film: pristine (black), HRS (at +2 V) (red) and LRS (at −2 V) (blue). The density of states near the top of the valence band is variable by negative or positive biasing.