Growth and Investigation of GaN/AlN Quantum Dots
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Growth and Investigation of GaN/AlN Quantum Dots Hadis Morkoç1, Michael A. Reshchikov1, Keith M. Jones1, Feng Yun1, Paolo Visconti1,2, Marshall I. Nathan1, and Richard J. Molnar3 1 Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 2 Also with Istituto per lo Studio di Nuovi Materiali per l’ Elettronica, CNR, 73100 Lecce, Italy 3 Lincoln Laboratory, Lexington, MA 02320, U.S.A. ABSTRACT We have fabricated GaN quantum dots (QDs) in AlN confined layer structures by molecular beam epitaxy. The size distribution and density of the QDs have been estimated from an atomic force microscopy study. Very high quantum efficiency of photoluminescence (PL) has been obtained in some samples with QDs. Compared to the GaN bulk samples, it increased by orders of magnitude. In some samples the quantum size effect dominated, resulting in the blue-shift of the QD related PL peak, whereas in the samples with larger dots a red-shift up to 0.8 eV has been observed, which is related to strong polarization effects. We have observed a blue-shift of the PL peak with excitation intensity in the samples with large dots due to screening effect. The temperature-induced quenching of PL occurs at higher temperatures compared to bulk GaN due to the confinement of nonequilibrium carriers in the QDs. An excited state has been observed in some samples. INTRODUCTION Quantum dots (QDs) in nitride based semiconductors are expected to improve characteristics of the visible-to-UV optical emitters which are currently produced by employing two- and threedimensional growth. The idea is that the layer or layers of quantum dots will decouple the active layers from the substrate or buffer layer and, thus reduce the density of extended and point defects. This application of QDs is novel. In the same vein, Gérard et al [1] applied dots for reducing the degradation of internal quantum yield. They pointed out that once the carriers are captured by QDs, they become strongly localized and their migration toward nonradiative recombination centers is made more difficult. In the absence of a surface topology driven process, the dots nucleate on dislocations that propagate to the surface of the layer. The key to high quality is then to obtain a dot density that is substantially larger than the dislocation density. The goal then should be to reduce the dislocation density below 108 cm-2 and increase the dot density above 1011 cm-2. This would simply mean that less than one out of every 1000 dots would contain extended defects. Growth of GaN self-assembled QDs on AlGaN, with the aid a sub- to monolayer Si layer, which are then covered by AlGaN has been first reported by Tanaka et al. [2]. Other approaches have been reported by Widmann et al. [3,4] and Damilano et al. [5] who used AlN wetting layers which provide a larger lattice mismatch to GaN than AlGaN, and in turn provides the impetus for a 3-D growth. In addition, the surface topology of AlN is smoother than that of AlGaN, which removes the surface features from being the nucleation sites for dots. Dots have bee
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