Strain-Induced Quenching of Optical Transitions in Capped Self-Assembled Quantum Dot Structures

Abstract

Strain-induced quenching of optical transitions has been found in capped self-assembled quantum dot structures. Light absorption at the E 1 and E 1 1 D 1 critical points of InSb islands buried in InP disappears for nominal InSb thicknesses lower than 10 monolayers as a consequence of the strain produced inside the islands by the cap layer. Certainly, this strain increases as the InSb deposition diminishes, changing the band lineup of the system from type-I to type-II and therefore drastically reducing the oscillator strengths of the island-related E 1 and E 1 1 D 1 transitions.[ S0031-9007(97) [2,3] of islands in self-assembled quantum dot (QD) structures, but also in the physical properties and, in particular, in the optical properties of such systems [4][5][6][7][8]. All previous studies considered only the strain-induced change of the energies of the island-related optical transitions, but strain also modifies the energies of the electronic states of the dots with respect to those of the matrix. In this paper we present a new effect related to the builtin strain characteristic of self-assembled QD structures: The quenching of optical transitions produced by a straininduced modification of the band lineup of the system. InSb dots grown on top of InP and InSb dots grown deeply buried in InP are the objects of study of this paper. Different modulation spectroscopies were used for optical characterization and transmission electron microscopy (TEM) and Raman scattering for structural characterization.Uncapped (dots on top) and capped (dots deeply buried) samples were grown by molecular-beam epitaxy [9], under identical conditions, on semi-insulating (001) InP substrates. Growth details are reported elsewhere [10]. The nominal InSb thicknesses were 2.2, 3.2, 5, and 7 monolayers (ML) in the uncapped samples and 3.5, 5, 10, and 15 ML in the capped samples, in which the cap layer thickness was 150 ML. Doubtless due to the large lattice mismatch existing between InSb and InP ͑ഠ10.4%͒, the onset of the island growth mode was observed after growing only 1.2 ML of InSb. In consequence, well-developed dots were obtained in every sample, as TEM results presented below demonstrate. The typical island size was of the order of several tens of nanometers. Figure 1 shows photoreflectance spectra from uncapped [1(a)] and capped [1(b)] samples recorded at 80 K paying attention to the island-related E 1 and E 1 1 D 1 transitions, which are associated with the corresponding critical points of the dots. Measurements on both kinds of samples were carried out under identical conditions in order to obtain comparable results. As can be observed, in uncapped samples the studied transitions are perfectly detectable even for nominal InSb thicknesses as small as 3.2 ML (some signal was barely detected in the 2.2 ML sample). Strikingly, in capped samples the studied transitions are not detectable by any means for nominal InSb thicknesses lower than 10 ML (also, no signal was detected in the 3.5 ML sample). The less InSb grown the smaller the...

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