Epitaxal growth of InGaN quantum dots grown by MOVPE: Effect of capping process on the structural and optical properties
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Epitaxial Growth of InGaN Quantum Dots Grown by MOVPE: Effect of Capping Process on Structural and Optical Properties Tomohiro Yamaguchi, Kathrin Sebald, Jürgen Gutowski, Stephan Figge and Detlef Hommel Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany.
ABSTRACT The surface morphology of thin InxGa1-xN layers in uncapped structures and the overgrowth using GaN or InyGa1-yN with a lower In content than the InxGa1-xN layer as the protection layer of the InxGa1-xN layer were investigated in addition to their dependence on growth temperature during the growth by metalorganic vapor phase epitaxy (MOVPE). It was necessary to decrease the growth temperature of the InxGa1-xN layer in order to realize homogeneous dots. In order to obtain a high luminescence efficiency, on the other hand, it was essential to increase the growth temperature of the protection layer. Sharp lines related to the localized electronic states, which are attributed to InGaN quantum dots, were observed by micro-photoluminescence (µ-PL) for the samples with the InxGa1-xN layers protected by the InyGa1-yN layers grown at 700~740°C.
INTRODUCTION InGaN quantum dots (QDs) are attracting considerable attention for the realization of laser diodes (LDs) with a predicted improvement in device performance [1]. The formation of InGaN QDs in an uncapped structure has been widely investigated using not only a normal StranskiKrastanov (S-K) growth mode [2-4] but also special growth techniques such as those employing an anti-surfactant effect [5] and a post-annealing effect [6]. For a device application the QDs must be embedded using a capping layer. It is well known, for other material systems such as Ge/Si and InAs/GaAs [7,8], that significant changes in the size, shape and density of QDs is caused by a capping process due to the intermixing of the materials between a QD layer and a capping layer and the transfer of a strain of QDs into the capping layer during the growth of the capping layer. In order to suppress these influences, the capping layer is usually grown at a temperature lower than that used in the bulk growth of the layer. In the case of the InGaN/GaN QD material system, it was also observed that the capping of a nominal 2 nm GaN layer resulted in the dissolution of most of the InGaN islands [9]. In addition, cubic GaN phases were easily incorporated in a GaN capping layer even if the capping layer was grown at a low temperature in order to suppress the dissolution [10]. The dynamics of capping growth is complex but important, as well as that of QD growth. In this study, the surface morphology of thin InxGa1-xN layers in uncapped structures and the overgrowth on the InxGa1-xN layer using GaN or InyGa1-yN with a lower In content than the InxGa1-xN layer were investigated in addition to their dependence on growth temperature during the growth by metalorganic vapor phase epitaxy (MOVPE), since the growth temperature is one of the key factors for both QD growth and capping growth, as is discuss
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