Raman spectroscopy of self-assembled InAs quantum dots in wide-bandgap matrices of AlAs and aluminium oxide
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E13.8.1
Raman spectroscopy of self-assembled InAs quantum dots in wide-bandgap matrices of AlAs and aluminium oxide D. A. Tenne, A. G. Milekhin, A. K. Bakarov, O. R. Bajutova, V. A. Haisler, A. I. Toropov, S. Schulze,1 and D. R. T. Zahn1 Institute of Semiconductor Physics, pr. Lavrenteva 13, 630090 Novosibirsk, Russia 1 Institut für Physik, Technische Universität Chemnitz, D-09107 Chemnitz, Germany ABSTRACT Vibrational properties of self-assembled InAs quantum dots (QD’s) embedded in AlAs and aluminium oxide were studied by Raman spectroscopy. The InAs/AlAs QD structures were grown by molecular beam epitaxy on GaAs (001) substrates. The following main features of the phonon spectra of InAs/AlAs QD nanostructures were observed: 1) asymmetric lines of QD LO phonons affected by strain, confinement and size inhomogeneity of QD’s; 2) confined phonons of InAs wetting layer (WL); 3) two bands of interface phonons in the AlAs frequency region, attributed to modes associated with the planar interface WL/AlAs matrix and the threedimensional QD/matrix interface; 4) doublets of folded acoustic phonons caused by periodicity in the multilayer QD structures. The influence of growth temperature, varied from 420 to 550°C, on the morphology of QD’s was investigated. QD’s grown at 420°C are found to have the smallest size. Increasing the temperature up to 480°C leads to the formation of larger InAs islands and improved size homogeneity. Further temperature elevation (above 500°C) causes partial re-evaporation of InAs leading to a decrease of QD size and density, and, finally, their complete disappearance. InAs QD’s embedded in aluminium oxide were fabricated by selective oxidation of the AlAs matrix in self-assembled InAs/AlAs QD’s. Micro-Raman spectroscopy data show that depending on oxidation conditions (humidity, temperature) InAs QD’s in an oxide matrix can be even more strained than before oxidation, or become fully relaxed. At the boundaries of oxidized/non-oxidized areas the presence of amorphous and crystalline As clusters is evident. INTRODUCTION Semiconductor quantum dots (QD’s) obtained by self-organized growth in highly strained heteroepitaxial systems have been intensively studied during the last decade (for a review see e.g. Refs. [1,2]). The most intensively investigated material system is InAs or InGaAs QD’s in a GaAs matrix. Recently InAs QD’s embedded in wide bandgap matrices of AlAs and AlGaAs have received increasing attention [3-6]. The higher barriers of the AlAs matrix produce shifts of the InAs QD bandgap energy towards the visible range as well as improved temperature stability of light-emitting devices. Most studies of self-assembled QD’s so far have been focused on their structural and electronic properties. Much less research attention has been paid to the phonon properties of QD structures. We present Raman studies of phonons in self-assembled InAs QD’s in an AlAs matrix. Our results show both strain and confinement effects on the optical phonon spectra of InAs/AlAs QD’s. Also we report the first Raman st
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