Optical Properties Of The Phase-separated InGaN/GaN Mutiple Quantumwells LED

InGaN/GaN multiple quantum wells (MQWs) structure is the preferred heterostructure for the fabrication of blue/green light emitting diodes (LEDs) and laser diodes (LDs). Since the InGaN bandgap ranges from0.7eV for InN to3.4eV for GaN, which cover the entire visible light district and expand to the scope of the ultraviolet. Despite rapid advances in growth technology, high indium InGaN is still difficult to achieve using Metal-organic Chemical Vapor Deposition (MOCVD). The low solubility of indium in GaN was calculated to be less than6%at800℃. The indium incorporation is strongly temperature dependent and indium separation often appears in the InGaN films. The researches of phase-separated InGaN are expected to be helpful to develop high-performance LEDs.In this paper, we report on the observation of strong phase separation in InGaN/GaN MQWs by means of TEM image, photoluminescence (PL) and time-resolved photoluminescence (TRPL). Two InGaN-related emission peaks observed in the full PL spectrum were assigned to the In-rich QDs (PD) and the InGaN matrix (PM). The temperature and excitation power dependences of these two emissions were examined to investigate the transfer and recombination mechanism of the carriers within these two phase-separated structures. In order to investigate the carrier dynamic characteristics, the carrier lifetime were also obtained by TRPL. The main research work and results are as follows:1. Temperature dependence of the PL spectra was studied in InGaN/GaN MQWs. The PM peak energy showed an S-shaped temperature-dependent behavior, which could be attributed to the potential inhomogeneity and localized character of the carrier recombination due to the slight composition fluctuation in the InGaN matrix. While the PD peak energy initially decreased and then increased with increasing temperature up to300K, indicating that the carriers within QDs relax to stronger localized states first and then are thermalized to higher levels with increasing temperature.2. The dependence of the PL spectra on various excitation power was also reported. With increasing excitation power in low excitation power range, an increase peak energy and an unchanged FWHM for both the emission peaks at6K, was due to the combined action of the Coulomb screening effect of the quantum confined Stark effect (QCSE) and the state-filling effect of localized states. While at300K in the low excitation power range, the nonradiative recombination process of the carriers in the matrix dominated the PM emission. In contrast, the carrier transferring from the shallower localized QDs to deeper localized QDs led to the redshift of the PD emission.3. From the time-resolved PL spectra, we obtained the carrier lifetime of both emissions. The results were consistent with the characters of the localized system.