Micro-photoluminescence studies of excitonic and multiexcitonic states of quantum dot-like localization centers in InGaN
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Micro-photoluminescence studies of excitonic and multiexcitonic states of quantum dot-like localization centers in InGaN/GaN structures K. Sebald, H. Lohmeyer, J. Gutowski, S. Einfeldt, C. Roder, and D. Hommel Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, 28359 Bremen, Germany ABSTRACT We present micro-photoluminescence measurements on unstructured InGaN/GaN quantum well samples. Single sharp emission lines were observed and their optical properties were studied as a function of temperature and excitation density. The experimental findings such as the independence of their spectral position on the excitation density and the observation of binding and antibinding multiexcitonic states give clear evidence for the existence of strong localization centers in the InGaN quantum well, which exhibit the same characteristics as they are known for quantum dot structures. INTRODUCTION The development of InGaN/GaN quantum well (QW) lasers has to be considered as a milestone in optoelectronics. Although the crystalline quality is rather poor, these devices possess a quite high quantum efficiency. Due to the large lattice mismatch between InN and GaN In-rich aggregation occurs in InGaN leading to the formation of nanometer-scale clusters and hence potential fluctuations. Generated carriers are trapped in the localization states of potential minima and recombine effectively. It is discussed that these strong localization centers or even quantum dot like states, probably formed by In-rich islands [1, 2] or composition fluctuations [3], are the key to the efficient light emission in InGaN based devices. Thus in order to clearly understand the optical properties of an InGaN film, the nature of these localization centers needs to be more specifically analyzed. Recently, Schöming et al. [4] reported on the appearance of individual spectrally narrow emission lines emerging on the inhomogeneously broadened InGaN luminescence measured by micro-photoluminescence experiments, thus confirming the existence of individual localization centers. Beyond this the detailed optical investigation of InGaN/GaN quantum dot (QD) structures are in the focus of research driven by promising device applications [5 - 8]. In this paper we report on micro-photoluminescence (µ-PL) studies on InGaN/GaN QW samples. The identification of single sharp emission lines and their optical properties characterized by temperature and excitation density dependent measurements gives us the possibility to gain insight into the nature of the strong localization centers. EXPERIMENTAL DETAILS The samples were grown by metalorganic vapor phase epitaxy on sapphire (0001) substrates. The InGaN quantum well with a thickness of ~ 5 nm was deposited on a 2.3 µm thick GaN buffer layer at a temperature of 820°C and atmospheric pressure. The growth conditions were similar to those used for the growth of QWs in our laser-diodes [9]. The InGaN was overgrown
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