# Carrier Dynamics in InGaN/GaN on the Basis of Different In Concentrations
## 2 Sample Structures and Experimental Procedures  
In this study, four different In\({}_{\text{x}}\)Ga\({}_{1-\text{x}}\)N/GaN samples were grown using metal organic chemical vapor phase deposition. We performed the following steps to grow sample SI27. First, we purified the sapphire substrate at 1800 \({}^{\circ}\)C, applied nitride to the surface, and grew a thin (2 \(\upmu\)m) GaN layer at 530 \({}^{\circ}\)C. Second, we grew a 200 \(\upmu\)m thick layer of In\({}_{0.27}\)Ga\({}_{0.73}\)N at 650 \({}^{\circ}\)C. Then, we improved the joining between the In\({}_{0.27}\)Ga\({}_{0.73}\)N and GaN layers at 580 \({}^{\circ}\)C. CSI27 is a GaN layer with a growth thickness of 120 nm and a growth temperature of 530 \({}^{\circ}\)C on SI27. For sample PIN27, the growth steps were as follows. First, we purified the sapphire substrate at 1800 \({}^{\circ}\)C, applied nitride to the surface, and grew a thin (1.5 \(\upmu\)m) GaN layer at 530 \({}^{\circ}\)C. Then, we grew doped Si at 550 \({}^{\circ}\)C to form an N-type GaN layer with a thickness of 500 nm. Next, we grew the In\({}_{0.27}\)Ga\({}_{0.73}\)N layer at 650 \({}^{\circ}\)C. Afterward, we improved the joining between the In\({}_{0.27}\)Ga\({}_{0.73}\)N and GaN layers at 580 \({}^{\circ}\)C. Eventually, we grew doped Mg at 550 \({}^{\circ}\)C to form a P-type GaN layer with a thickness of 120 nm. Finally, we improved the joining between the In\({}_{0.27}\)Ga\({}_{0.73}\)N and P-type GaN layers at 580 \({}^{\circ}\)C. The growth conditions of sample PIN 15 were similar to those of sample PIN27, except that the In concentration was changed to 15%. Figure 1 shows the structures of the four samples. Carrier dynamics in InGaN/GaN samples were investigated using picosecond FWM. Temperature-dependent PL measurement was carried out through exciting 325 nm light using 50 mW He-Cd laser. Carrier flow dynamics in InGaN/GaN samples were studied using picosecond FWM [23; 24; 25; 26; 27]. The set-up shown in Figure 2 was used to perform a TR-FWM experiment. Different materials were measured using varying wavelengths to excite light from the carrier. A mode-locked picosecond YAG:Nd laser (PL2143 type, Ekspla) or various resonant waves from an optical numerical generator (PG401, Ekspla) were used as light source. The pump wavelengths provided by the set-up were 532, 355, and 266 nm, and the pulse energies were 15, 10, and 5 mJ, respectively. The initial beam diameter measured 1-2 mm for grating recording. The numerical generator could be adjusted in the spectrum ranges of 420-680 and 740-2000 nm. The maximum pulse energy at 500 nm was approximately 1 mJ. In the present study, the first resonant wave of most experimental lasers at 1064 nm was used as a probe beam, which ensured the highest sensitivity of non-resonant refractive index modulation for free carriers. The 355 nm picosecond numerical generator was used as pump beam for recording instantaneous free carrier grating in InGaN layer and preventing light absorption in the GaN layer. Attenuation was controlled, and pulse energy of the beam was recorded by rotating a \(\uplambda\)/2 plate in front of the grand prism. The optical path difference of interfering beams can be adjusted using a short step-driven pump delay line to allow a delay time adjustment of up to 1.5 ns. Incident beam energy was measured using an energy detector, and spatial profile of the beam was controlled by a charge-coupled device line. Five measurements were performed with fixed probe beam delay at time \(\Delta\)t, and data were integrated at the intensity window required for I\({}_{0}\pm\Delta\)I\({}_{0}\) (usually, \(\Delta\)I\({}_{0}\)/I\({}_{0}<5\%\)) to measure the grating attenuation dynamics. The exposure characteristics under different levels of excitation energy were measured by collecting 500-1000 times of laser irradiation. LabVIEW data acquisition system was used to monitor experimental and data processing, and time showed the measured FWM characteristics.