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The effect of carrier localization in InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes is investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements. PL results show that two peaks obtained by Gaussian fitting both relate to the emission from localized states. By fitting the TRPL lifetimes at various emission energies, two localization depths corresponding to the In-rich regions and quasi-MQWs regions are obtained. Using a model we proposed, we suggest that compositional fluctuations of In content and variation of well width are responsible for carrier localization in In-rich regions and quasi-MQWs regions, respectively.

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The optical properties of the perovskite-type NaTaO 3 synthesized from sol-gel ͑SG͒ and solid-state ͑SS͒ methods were studied by absorption and photoluminescence ͑PL͒ techniques. We observed significant PL emission at room temperature from the SS NaTaO 3 , while the emission was negligible from the SG NaTaO 3 . Variation of the PL intensity and peak position with temperature demonstrated that the recombination of the localized exciton Ta 4+ -O − in the regular TaO 6 octahedra was the origin of the luminescence in NaTaO 3 . The Ta-O-Ta bond angle affected the delocalization of the excitons and thus, the PL behaviors of ABO 3 perovskite structures, where A is a group of I-II elements and B is a transition metal, have attracted considerable attention because of their unusual magnetic, dielectric, and luminescence properties.1-3 Recently, the luminescence properties have been extensively studied in many ABO 3 perovskite structures ͑A = Sr, Ba, K, etc., B = Nb, Ti, Ta, etc.͒, and this has provided information on the photochemical properties of ABO 3 perovskites, such as photocatalysis, photoinduced electron-transfer dynamics, and excitation energy transport.2-5 These ABO 3 perovskites exhibit a broad emission band with a large Stokes shift at low temperatures, whereas the luminescence is usually quenched at high temperatures and it is hard to detect at room temperature. Several mechanisms for the luminescence properties of ABO 3 perovskites have been proposed. They include the recombination of electrons trapped on the donors with holes trapped on the acceptors, 6 radiative transitions within the BO 6 , 7 self-trapped excitons, 2,5 and charge-transfer vibronic excitons. 8,9 Moreover, Longo et al. have attributed the origin of the visible luminescence emission in perovskite-type compounds to the localized electronic level induced in the valence band by the symmetry. 10 The semiempirical quantum chemical method of intermediate neglect of differential overlap has been used by Grigorjeva et al. for investigating the radiative recombination of correlated ͑bound͒ self-trapped electron and hole polarons in the highly polarizable ABO 3 -type matrix.11 Orhan et al. have turned to the firstprinciples theory as an appropriate tool to analyze the mechanism of the luminescence behavior in disordered ABO 3 materials.12 However, a clear understanding of the luminescence behavior of ABO 3 materials is still needed.Among the different ABO 3 materials, NaTaO 3 presents a widely versatile structure, depending on growth techniques, and the structure would influence the optical properties of NaTaO 3 .13-15 Wiegel et al. have reported the relationship between crystal structure and energy delocalization for perovskite-type alkali tantalates. 13 The excited energy is efficiently delocalized at a bond angle of TaϪOϪTa close to 180°, while the excited energy would be localized at bond angles deviating from 180°. The luminescence of ABO 3 materials at low temperatures has been explored by numerous studies. In contrast, only a limited number of...

Three dual-wavelength InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) with increasing indium content are grown by metal-organic chemical vapor deposition, which contain six periods of low-In-content MQWs and two periods of high-In-content MQWs. For the low-In-content MQWs of three studied samples, their internal quantum efficiency (IQE) shows a rising trend as the emission wavelength increases from 406 nm to 430 nm due to the suppression of carriers escape from the wells to the barriers. However, for the high-In-content MQWs, the sample IQE falls rapidly with a further increase of emission wavelength from 496 nm to 575 nm. Theoretical calculation reveals that the electron-hole wave function overlap in the high-In-content MQWs is reduced because of an increase in the internal polarization field as indium content is increased. In addition, time-resolved photoluminescence decay curves show that the carriers generated in the low-In-content MQWs can be effectively transferred to the high-In-content part through the reabsorption process. However, the transfer time gradually becomes longer as emission wavelength increases, which means a reduction of carrier transfer rate between the different indium content MQWs. Furthermore, nonradiative recombination is enhanced in the high-In-content MQWs with longer emission wavelength due to the decline of crystal quality. Therefore, the fast drop of IQE for high-In-content MQWs can be attributed to the increase of the internal polarization field, the decrease of carrier transfer efficiency, and the enhanced nonradiative recombination. This research has a certain guiding value for an understanding of the recombination mechanism in the InGaN/GaN MQWs and for achieving high quality multiple-wavelength LEDs with better performance.