Influence of growth temperature on emission efficiency of InGaN/GaN multiple quantum wells
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Influence of growth temperature on emission efficiency of InGaN/GaN multiple quantum wells Fei Chen, A. N. Cartwright, Paul M. Sweeney, and M. C. Cheung Department of Electrical Engineering, University at Buffalo, Buffalo, NY 14260 Jeffrey S. Flynn, David Keogh ATMI Inc, 7 Commerce Drive, Danbury, CT 06810-4169 ABSTRACT A comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation. INTRODUCTION It is striking to realize that although InGaN has emerged in the last few years as one of the most important materials for short-wavelength optoelectronics the mechanisms of radiative recombination in InGaN QWs is still subject to debate in the literature. It has been recognized that under typical growth conditions there is a positive enthalpy for indium mixing in GaN.1 Moreover, electromicroscopy and cathodoluminescence of InGaN have demonstrated the existence of nanometer and micron scale regions of high indium concentration.2-4 It has been hypothesized that the ‘quantum dot’ like nanoscale regions of high indium concentration prevent the carrier capture from the non-radiative defects and improve efficiency of light-emitting diodes.5,6 However, the effects of growth conditions on indium phase segregation, which affect the carrier recombination dynamics and device emission efficiency, although acknowledged,1 remain unclear due to the complex nature of growth processes in III-N materials. Clearly, one of the most important parameters for growth of high quality InGaN thin films is growth temperature. In this article, we present femtosecond time-resolved and CW photoluminescence spectroscopy to compare emission from MOVPE grown InGaN-based MQWs deposited on HVPE GaN/Sapphire at three different growth temperatures but with otherwise identical conditions. Moreover, a phenomenological carrier recombination dy
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