PL excitation density dependence. (a, b) Integrated intensities of the PL spectra versus excitation density at different temperatures for (a) sample S650 and (b) sample S850. Different symbols indicate the measurement temperatures, 15 K (squares), 70 K (circles), 130 K (upward triangles), 170 K (downward triangles), and 210 K (diamonds). The value of P0 is 70 W/cm2. The dashed lines are the fit of the experimental data with the equation. (c) Dependence of α on temperature for samples S650 (full diamonds), S700 (triangles), S750 (open diamonds), S800 (circles) and S850 (squares).