Comparative investigation of quantum-dot-like localization centers in InGaN quantum well and quantum dot structures
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Comparative investigation of quantum-dot-like localization centers in InGaN quantum well and quantum dot structures K. Sebald, H. Lohmeyer, J. Gutowski, T. Yamaguchi, and D. Hommel Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, 28359 Bremen, Germany ABSTRACT We present comparative micro-photoluminescence measurements on InGaN/GaN quantum well and quantum dot samples. Single sharp emission lines were investigated for both kinds of samples as a function of temperature and excitation density. For the sharp emission lines of the quantum dots and the strong localization centers in the quantum well samples comparable experimental findings were obtained such as the independence of their spectral position of the excitation density and the observation of binding and antibinding multiexcitonic states giving clear evidence for the quantum dot nature of localization centers. 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 high quantum efficiency. Due to the large lattice mismatch between InN and GaN In aggregation occurs in InGaN leading to the formation of nanometer-scale clusters and hence respective potential fluctuations. Carriers generated in or injectied into the structures are trapped in the potential minima and recombine effectively. It is discussed that these strong localization centers or even quantum-dot like states, probably formed by In-rich InGaN islands [1, 2] or composition fluctuations [3], are the key to the efficient light emission of 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, having been measured in micro-photoluminescence experiments and, 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 and QD samples. The optical properties of the single sharp emission lines occurring in both sample types are characterized by temperature and excitation-density dependent measurements. The comparative investigation allows to gain insight into the nature of the strongly localizing centers in the samples. The purpose of this paper is to show that strongly localizing centers with quantum-dot like behavior are, under certain grwoth conditions, easily found in a variety of InGaN/GaN structures where just quantum well growth was intended and observable while systematic quantum-dot behavior is only found under very special circumstances in this material composition. However, the physica
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