Progress in Growth and Physics of Nitride-Based Quantum Dots
GaN-based semiconductors have received great attention because of various potential device applications including short wavelength laser diodes (LDs), light emitting diodes (LEDs), photodetectors, and electron devices [1 ]. Short wavelength LED are used f
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Introduction
GaN-based semiconductors have received great attention because of various potential device applications including short wavelength laser diodes (LDs), light emitting diodes (LEDs), photodetectors, and electron devices [1]. Short wavelength LED are used for full-color flat display panes and illumination systems. The blue/violet LDs will be for ultra-high density optical data storage. In addition, GaN electron devices for application to high frequency and high power oscillators are now intensively investigated. GaN and related materials (i.e., InAIGaN alloy) have various unique characteristics, such as wide bandgap energy (1.8-6.2eV), possible large band discontinuity in heterostructures, strong polarization effects (~ 2 MVcm -1) due to piezoelectric field effects and spontaneous polarization effects, large binding energy of excitons (> 50meV), high saturation velocity (~ 2.7 x lOcm 4 s- 1 ), and high breakdown field (~ 2 X 106 Vern -1). In spite of some pioneering work on growth of GaN in 1960s, there had been various difficulties for obtaining epitaxial layers of high quality. Although Ab03 (sapphire) was one of candidates for the substrate, there exists a lattice mismatch as large as 16% between GaN and c-face Ab03. In 1986, Amano and Akasaki succeeded in demonstrating high-quality GaN films using an AIN buffer layer grown at low temperature on Al z0 3 substrates [2]. This had great impact on the development of GaN growth. This was followed by Nakamura's work which demonstrated the growth of low-temperature GaN buffer layers. Recently, high-quality GaN substrates have been reported, but there are still lots of issues including donor impurities [3]. Another critical issue was the difficulty to obtain p-type nitride semiconductors. It was also the team of Amano and Akasaki which first demonstrated p-type GaN by Mg-doped GaN treated with low-energy electron beam irradiation [4]. After that, Nakamura succeeded in developing an easier way to obtain p-type GaN by using a thermal annealing technique [5,6]. In addition, he demonstrated high-quality InGaN films which have strong band edge emission as well as high-quality InGaN/GaN quantum wells [7]. These results led to room temperature operation of blue light emitting InGaN LEDs [8] and LDs [9]. The high efficiency of photoluminescence (PL) in InGaN /GaN M. Grundmann (ed.), Nano-Optoelectronics © Springer-Verlag Berlin Heidelberg 2002
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Yasuhiko Arakawa
quantum wells is attributed local confinement of carriers owing to In-content fluctuations which result in lateral potential fluctuations [10,11]. As a natural consequence of these successful results, GaN-based quantum dots have been also studied. There are two main methods which have been investigated for the growth of GaN-based quantum dots (QDs), which is similar to other material systems such as InAs QDs. One is the self-assembling growth technique and the other is the selective growth technique. In the self-assembled growth, at the initial stage of the research on GaN dots, 'anti-surfactants'
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