**4. Discussion**

### *4.1. Fabrication of the Photodetector*

In Figure 2b, a conductive LM thin film and a transparent gallium oxide film that is separated from the LM film are shown. All films were fabricated on a large PDMS substrate (5 × 5 cm). As shown in Figure 2d, the morphology of the conductive LM film was not smooth because LM was immediately oxidized and solidified when exposed to air after the peeling-off process. The thickness of the conductive LM film measured by AFM was approximately 600 nm. To measure the thickness of the exfoliated gallium oxide layer, the transparent film on the PDMS substrate was transferred onto a silicon substrate. The measured thickness of the exfoliated metal oxide film by AFM was 8.7 nm, as shown in Figure 2i, and the surface roughness (RMS roughness) of the exfoliated 2D Ga2O3 layer for a flat area was 2.254 nm, as shown in Figure 2j. It is assumed that the measured value of the gallium oxide layer is thicker than the known value (~3 nm) because there is further oxidation during the separation process. In this work, the separated transparent film was used as an active layer in a photodetector because the oxidized film based on the gallium oxide is highly photo-detective.
