Optical characterization of hierarchically self-assembled GaAs/AlGaAs quantum dots
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Optical characterization of hierarchically self-assembled GaAs/AlGaAs quantum dots F.Marabelli1, A.Rastelli2, O.G. Schmidt2, G.Beaurin1, M.Geddo3, G.Guizzetti1 1 CNR-INFM Pavia and University of Pavia, via Bassi 6, I-27100 Pavia, Italy 2 Max-Plank-Institut fur Festkorperforschung, Heisembergstr.1, D-70569 Stuttgart, Germany 3 CNR-INFM Pavia and University of Parma, v.delle Scienze 7, I-43100 Parma, Italy ABSTRACT Recently, a simple method has been developed to obtain inverted GaAs/AlGaAs quantum dots (QDs) below a quantum well (QW) via multi-step (hierarchical) self-assembly [1]. Here we report on the optical characterization of a series of structures GaAs QDs with different size. The study is performed by means of reflectance (R) down to T=15 K, photo- and thermo-reflectance (T>80 K) and by spectroscopic ellipsometry (at RT). Such measurements allow us to check the thickness and composition of the barrier layers and to complete the study of the electronic states involved in the emission properties of QDs and QW levels. QW states were well identified and their energies and shift with the size parameters well agree with the photoluminescence (PL) data. The identification of QD states seems less straightforward in R measurements and deserves further investigation.
INTRODUCTION Unstrained GaAs QDs embedded in an AlGaAs matrix are attractive both for fundamental studies and applications. In fact, heterostructures based on GaAs/AlGaAs have been extensively studied and are characterized by an almost perfect match of lattice parameters and by sharp interfaces. This renders it easier to predict and interpret the properties of the QDs [1] with respect to strained Stranski-Krastanow QDs, where intermixing is usually significant (see e.g. [2]). Moreover, structures with light emission in the near infrared and visible range, where sensitive photo-detectors are available, can be produced and this makes GaAs QDs particularly suitable for coherent optical studies.
EXPERIMENTAL Self-assembled QDs of GaAs on AlGaAs were grown in a solid-source molecular beam epitaxy (MBE) system equipped with an in situ AsBr3 etching unit [1, 3]. Semi-insulating GaAs(001) substrates were outgased and deoxidized prior to the growth of a 340-nm thick GaAs buffer. This is followed by the growth of nominal 1.8 monolayers (ML) of InAs at a very low rate of 0.01 ML/s and a relatively high substrate temperature Ts=500°C. With these parameters, InAs islands with an average height of 11 nm and a narrow size distribution are obtained [4]. Islands are then overgrown with 10 nm of GaAs and etched for nominal 5 nm by AsBr3 gas. The etching process results in the formation of nanoholes about 5-6 nm deep and 70 nm wide at the positions occupied by the buried InAs islands. The InAs islands are completely removed during etching [3], while a thin In(Ga)As wetting layer remains. The nanoholes are overgrown with a
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Nominal values (samples A-F) AR A B C D E F Nominal values (samples GR-G) GR G
Al2O3 thick. (nm) 3.3 3.2 3.2 2.8 3.1 3.2 4.6
Cap layer
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