Influence of Si-Doping on Carrier Localization of MOCVD-Grown InGaN/GaN Multiple Quantum Wells
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increasing the Si doping concentration (n) reduces the Stokes shift of the InGaN emission. TRPL experiments show the 10 K recombination lifetime decreases with increasing n, from - 30 ns (for n < 1 x 1017 cm-3) to - 4 ns (for n = 3 x 1019 cm-3). In addition, it is shown that Si doping improves the structural and interface properties of the InGaN/GaN MQW samples from atomic force microscopy (AFM) images. From the results, we demonstrate that Si doping in the GaN barriers affects the interface quality of the InGaN/GaN MQW systems, and influences the optical properties. EXPERIMENT The InGaN/GaN MQWs used in this study were grown on c-plane sapphire films by metalorganic chemical vapor deposition, following the deposition of a 1.8-1am-thick GaN buffer layer. The structures consisted of 12 MQWs with 3-nm-thick In 0.2Ga 0.8N wells and 4.5-nm-thick GaN barriers, and a 100-nm-thick A10 .07 Ga0 .93 N capping layer. Trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum, and ammonia were used as the precursors and disilane was used as the n-type dopant. The growth temperatures of the GaN base layer, the superlattice (SL) region, and the AIGaN capping layer were 1050, 790, and 1040'C, respectively. The TMGa and TMIn fluxes during the SL growth were 5 and 14 ltmol/min, respectively, while the ammonia flow was held constant at 0.35 mol/min. In order to study the influence of Si doping in the GaN barriers, the disilane doping precursor flux was systematically varied from 0 to 4 nmol/min during GaN barrier growth. Accordingly, a doping concentration in the range of < 1 X 1017 to 3 x 1019 cm-3 was achieved for the different samples, which was determined by secondary ion mass spectroscopy and Hall measurements. To investigate the effect of Si doping on the GaN surface morphology, three reference 7-nm-thick GaN epilayers were also grown at 800 TC, which have different disilane flux rates of 0, 0.2, and 2 nmol/min during growth. The surface morphology of the reference samples were investigated using a Digital Instruments Nanoscope III AFM operated in tapping mode and the root-mean-square surface roughness was estimated by the AFM software. Details of the growth procedure and the other structural properties are given elsewhere [10,14]. PL and PLE spectra were measured using the 325 nm line of a 20 mW cw He-Cd laser and the quasi-monochromatic light from a xenon lamp dispersed by a 1/2 m monochromator, respectively. TRPL measurements were carried out using a picosecond pulsed laser system consisting of a cavity-dumped dye laser for sample excitation and a streak camera for detection. The output laser pulses from the dye laser had a duration of less than 5 ps and were frequency doubled into the UV spectral region by a nonlinear crystal. The overall time resolution of the system is better than 15 ps. The temperature of the sample in the experiments described above was varied from 10 to 300 K. RESULTS AND DISCUSSIONS Figure 1 shows 10 K PL (solid lines) and PLE (dashed lines) spectra of the main InGaNrelated PL emission peak
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