Spectroscopy of Intraband Electron Confinement in Self-Assembled GaN/AlN Quantum Dots
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Spectroscopy of Intraband Electron Confinement in Self-Assembled GaN/AlN Quantum Dots Ana Helman1, Khalid Moumanis1, Maria Tchernycheva1, Alain Lusson1, Francois Julien1, Benjamin Damilano2, Nicolas Grandjean2, Jean Massies2, Christophe Adelmann3, Frederic Fossard3, Daniel Le Si Dang3, Bruno Daudin3 1 Action OptoGaN, Institut d'Electronique Fondamentale, UMR8622 CNRS, Bât.220 Université Paris-Sud, 91405 Orsay, France 2 Centre de Recherche sur l'Hétéroepitaxie et ses Applications, CNRS, rue B. Gregory, Sophia Antipolis, 06560 Valbonne, France 3 CEA/CNRS Research Group "Nanophysique et Semiconducturs", DRFMC/SPMM, CEA/Grenoble, 17 rue des Martyrs, 38054 Grenoble, France ABSTRACT We present a detailed analysis of inter- and intraband transitions in GaN/AlN self-organized quantum dots grown on sapphire, silicon (111) and 6H-SiC substrates. Quantum dot samples have been characterized by means of transmission electron microscopy, photoluminescence and photo-induced absorption spectroscopy. Interlevel transitions in the conduction band are observed in the 0.52-0.98 eV energy range, thus covering the telecommunication band. The s-pz absorption is peaked at 0.8 eV (0.52 eV) for samples with dot height of 1.5 nm (6 nm). Calculations show that in bigger dots the transition energy is governed by the value of the internal field. INTRODUCTION Group III nitride materials are attracting much interest due to applications to the blue and ultraviolet light emitting devices [1]. Recently, they gained new interest for the realization of unipolar devices operating at telecommunication wavelengths i.e. in the 1.3-1.55 µm spectral region thanks to the giant conduction band offset offered by their heterostructures (~ 1.8 eV for GaN/AlN). Intersubband absorptions have already been observed in GaN/AlN and GaN/AlGaN quantum wells (QWs) at wavelengths as short as 1.06 µm [2,3]. In addition, the very short absorption recovery time (150-400 fs) opens prospects for the realization of ultrafast devices such as optical switches and modulators operating in the Tbit/s regime [4,5]. A GaN/AlN QW based photodetector with peak responsivity at 1.9 µm has also been reported [6]. Progress in epitaxial growth of nitride materials allows the fabrication of QDs with an excellent control of the size dispersion [7, 8]. In recent works we have reported intraband absorptions in self-organized GaN/AlN quantum dots (QDs) showing in-plane (TE) as well as caxis (TM) polarized transitions [9,10]. In this work, we further investigate the effects of QDs size on the intraband transition energies. We show that in big dots the transitions energy is governed by the value of the internal field, which can be tuned by adjusting the respective thickness of the GaN and AlN layers. In smaller dots the electronic structure is essentially dictated by the confinement along the c-axis, related to the dot height, which can be controlled during growth [11,12]. We report for the first time an s-pz interlevel absorption at 1.55 µm in small dots.
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