Fig. 1. (a) The SEM image of the Al_0.3Ga_0.7N Sample; (b) the SEM image of the Al_0.5Ga_0.5N Sample; (c) the SEM image of the Al_0.7Ga_0.3N Sample. The AlGaN and AlN layers in (a), (b) and (c) are marked with red and yellow squares respectively, in addition with the thickness labeled. (d) From left to right, the curves show the absorption coefficients of Al_0.3Ga_0.7N, Al_0.5Ga_0.5N and Al_0.7Ga_0.3N samples, respectively. The absorption coefficients are measured by an ellipsometer and the band gaps are fitted out and labeled.

Fig. 2. The area with obvious phase separation phenomenon on the Al_0.5Ga_0.5N sample surface. (a) A typical photoluminescence spectrum of the area. (b) Profiles of the topography and the surface potential shown in plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). The profile of the surface potential is extracted from the mark 2 in panel (f). The width of the falling edge marked by red lines in the profile of the surface potential is about 19 nm. This value presents the typical spatial resolution of the single-pass Kelvin force probe microscopy we applied. (c) and (d) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.

Fig. 3. The area without phase separation phenomenon on the Al_0.5Ga_0.5N sample surface. (a) A typical photoluminescence spectrum of the area. (b) Profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). The profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.

Fig. 4. The area on the Al_0.3Ga_0.7N sample surface. (a) A typical photoluminescence spectrum of the area. (b) Profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). The profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) The topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.

Fig. 5. The area on the Al_0.7Ga_0.3N sample surface. (a) A typical photoluminescence spectrum of the area. (b) profiles of the topography and the surface potential shown in the plot 1 and 2, respectively. The profile of the topography is extracted from mark 1 in panel (e). the profile of the surface potential is extracted from the mark 2 in panel (f). (c) and (d) the topography image and the surface potential image, respectively, obtained at the same area with a scan size of 10 μm. (e) and (f) the topography image and the surface potential image, respectively, obtained at the same area with a scan size of 3 μm. The white lines marked by 1 in panel (e) and 2 in panel (f) are picked at the same position.