Carrier Dynamics of Abnormal Temperature-Dependent Emission Shift in MOCVD-Grown InGaN Epilayers and InGaN/GaN Quantum W
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In this study, we report a systematic photoluminescence (PL) study of InGaN epilayers and InGaN/GaN MQWs as a function of temperature by means of PL, PL excitation (PLE), and timeresolved PL (TRPL) spectroscopy. As the temperature is increased, the peak energy position of the InGaN-related PL emission (EPL) exhibits an S-shaped behavior (redshift-blueshift-redshift). In the case of the InGaN epilayer, EPL decreases in the temperature range of 10 - 50 K, increases for 50 - 110 K, and decreases again for 110 - 300 K with increasing temperature. For the InGaN/GaN MQWs, EPL decreases from 10 - 70 K, increases from 70 - 150 K, then decreases again from 150 - 300 K. This temperature-induced S-shaped PL shift is strongly affected by the change in carrier dynamics with increasing temperature for both the InGaN epilayer and the InGaN/GaN MQWs. EXPERIMENT The InGaN epilayers and the InGaN/GaN MQWs used in this study were grown on c-plane sapphire films by metalorganic chemical vapor deposition (MOCVD), following the deposition of a 1.8-lm-thick GaN buffer layer. For the InGaN epilayers, a 100-nm-thick InGaN layer was capped with a 50-nm-thick GaN layer. The MQW structures consisted of 12 MQWs with 3-nmthick InGaN wells and 4.5-nm-thick GaN barriers, with a 100-nm-thick A10.07Ga0 .9 3N capping layer. The growth temperatures of the GaN base layer, the MQW regions, and the AlGaN capping layer were 1050, 790, and 1040 TC, respectively. The In content of the InGaN layers was estimated to be about 18 % for both the epilayer and the MQWs by means of high-resolution xray diffraction measurements. We observed optically pumped stimulated emission from the MQW sample with a low threshold density (< 60 kW/cm2 ) at room temperature. Details of the growth procedure and results of other structural and optical properties were reported elsewhere [12-15]. Additionally, the influence of Si doping in the GaN barriers of the MQWs on the optical properties is also given elsewhere [7]. PL spectra were measured as a function of temperature ranging from 10 to 300 K using the 325 nm line of a 20 mW cw He-Cd laser. PLE spectra were measured using the quasi-monochromatic light from a xenon lamp dispersed by a 1/2 m monochromator. TRPL measurements were carried out using a picosecond pulsed laser system consisting of a cavity-dumped dye laser synchronously pumped by a frequency-doubled modelocked Nd:YAG laser for sample excitation and a streak camera for detection. The overall time resolution of the system is better than 15 ps. RESULTS AND DISCUSSIONS Figure 1 shows 10 K PL and PLE spectra of the InGaN-related emission with a peak energy of - 2.99 and - 2.76 eV for (a) the InGaN epilayer and (b) the InGaN/GaN MQWs, respectively. A large Stokes shift of the InGaN emission between the PL peak energy and the band-edge obtained from the PLE spectra is clearly observed, which is mainly due to crystal imperfections such as In alloy fluctuations and/or interface roughness. We note that the observed Stokes shift for the MQWs is much larger than that of the epi
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