Time Resolved Optical Studies of InGaN Layers Grown on LGO
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Time Resolved Optical Studies of InGaN Layers Grown on LGO Maurice Cheung, Gon Namkoong1, Madalina Furis, Fei Chen, Alexander. N. Cartwright, W. Alan Doolittle1 and April Brown2, University at Buffalo, State University of New York, Buffalo, NY 14260, U.S.A. 1 Geogia Institute of Technology, Atlanta, GA 2 Duke University, Durham, NC ABSTRACT Radiative recombination processes in bulk InGaN grown by molecular beam epitaxy (MBE) on lithium gallate (LGO or LiGaO2) substrates were investigated using microscopic PL and time-resolved photoluminescence (TRPL). The improved structural quality resulting from a better lattice match of the LGO substrate to III-V nitride materials simplifies these investigations because well-defined composition phases can be analyzed for both homogeneous and phased separated InGaN samples. Epilayers of InGaN intentionally grown with and without indium segregation were studied. X-ray diffraction measurements showed that the homogeneous epilayer was high quality In0.208Ga0.702N and the segregated epilayer exhibited peaks corresponding to both In0.289Ga0.711N and In0.443Ga0.557N indicating the presence of higher In concentration regions in this sample. Spatially resolved photoluminescence spectra confirm the existence of these regions. The photoluminescence intensity decay is non-exponential for both samples and a stretched exponential fit to the decay data confirms the existence of local potential fluctuations in which carriers are localized before recombination. INTRODUCTION InGaN-based emitters are widely used for commercial high-brightness light emitting diodes (LED) from UV to amber [1,2]. There have been numerous reports on the improvement of efficiency in these LED devices due to In-segregation [3-7]. The current theory suggests that the In-rich regions existing inside InGaN act as localization centers, which trap the carriers injected into the system. The probability of non-radiative recombination for a carrier localized in a potential fluctuation is significantly reduced [4,6] and the excitons formed in the vicinity of the localization centers have greater oscillator strength [6]. As a result, the optical efficiency of the LED is greatly improved. Most of the commercially available InGaN emitters consist of a multiple quantum well (MQW) heterostructures, which are characterized by emission efficiency that depends not only on the In-segregation but also on the built-in electric fields present as a result of the piezoelectric polarization [8-10]. Here, we have studied the changes induced by In-segregation in an InGaN epilayer grown by Molecular Beam Epitaxy (MBE) on a lithium gallate (LGO or LiGaO2) substrate. The crystal quality of GaN and InGaN grown on this substrate is significantly improved as a result of a better lattice match between the substrate and the nitride materials [11,12]. In this paper we present the results of confocal photoluminescence (CPL) and timeresolved photoluminescence (TRPL) measurements on two InGaN epilayers. X-ray and CPL confirm the existence of In-segr
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