Comparison Study of Structural and Optical Properties of In x Ga 1-x N/GaN Quantum Wells with Different in Compositions
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Through HRXRD analysis, we found that samples with higher In composition have a larger full width at half-maximum (FWHM) of superlattice (SL) peaks, indicating rougher interfaces. However, these samples have lower RT SE threshold densities and lower nonradiative recombination rates, as determined by SE and TRPL experiments. We attribute the lower RT SE threshold densities of the higher In composition samples to the suppression of nonradiative recombination, due to the incorporation of In. EXPERIMENTAL DETAILS The set of InGaN/GaN MQW samples used in this study were grown on c-plane sapphire substrates by metalorganic chemical vapor deposition [7]. The samples were nominally identical, apart from deliberate variations in the In composition of the InGaN well layer. The samples consisted of (i) a 2.5-µm-thick GaN buffer layer doped with Si at 3 × 1018 cm-3, (ii) a five-period SL of 3-nm-thick undoped InGaN wells and 7-nm-thick GaN barriers doped with Si at ~5 × 1018 cm-3 to improve the interface properties [8], and (iii) a 100-nm-thick GaN capping layer to prevent surface recombination. During the SL growth, the trimethylgallium and ammonia fluxes were held constant at 2.2 µmol/min and 0.32 mol/min. To obtain samples with different In compositions in the InGaN wells, trimethylindium (TMIn) fluxes of 13, 26, and 39 µmol/min were used for the different samples, while the InGaN well growth time was kept constant. To evaluate the interface quality, the MQW average In composition, and the SL period, the samples were analyzed with HRXRD. PL and PLE experiments were performed using quasimonochromatic light dispersed by a ½ m monochromator from a xenon lamp. To examine the relevance of these MQWs to device applications, optically pumped SE experiments were performed at RT in the side-pumping geometry. The SE experimental details are reported elsewhere [9]. To check the temperature-dependent optical efficiencies of the MQWs, PL spectra were obtained as a function of temperature from 10 to 300 K using the 325 nm line of a cw He-Cd laser. Carrier lifetimes were measured by TRPL, using a streak camera for detection and a tunable picosecond pulsed laser system as an excitation source [8]. DISCUSSION Figure 1 (a) shows the HRXRD diffraction pattern for the (0002) reflection from the five-period InxGa1-xN/GaN MQWs with different In compositions. The FWHM of SL-1 and SL-2 peaks are plotted as a function of In composition in Fig. 1 (b). The strongest peaks are from the GaN layers. SL satellite peaks are marked as SL-1, SL-2, and SL1. The zero-order SL peaks (SL0) appear as a low angle shoulder on the GaN peaks. All spectra clearly show higher-order SL diffraction peaks indicating good layer periodicity. Best fitting of the spectra in Fig. 1 (a) yields In compositions of 8.8%, 12.0%, and 13.3% for the samples with InGaN well layers grown with TMIn fluxes of 13, 26, and 39 µmol/min, respectively [8]. As shown in Fig. 1 (a) and (b), with increasing In composition, the FWHM of the higher-order SL satellite peaks broadens. This
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