Istituto dei materiali per l'elettronica ed il magnetismo     
Seravalli L., Trevisi G., Frigeri P. The 2D-3D growth transition in metamorphic InAs/InGaAs quantum dots. In: SemiconNano 2011 - Semicon Nano 2011 - International Workshop on Epitaxial Growth and Fundamental Properties of Semiconductor Nanostructures (Traunkirchen, Austria, 11th - 16th September 2011). Abstract, article n. 13. Institute of Semiconductor Physics, University of Linz, Austria, 2011.
Self-assembled metamorphic InAs/InGaAs quantum dots (QD) are actively investigated for the extension of light emission towards 1.55 Ám in GaAs-based devices [1-2] and for the control of QD spatial arrangement without patterning [3]. However, this system presents many relevant differences from the mainstream InAs/GaAs pseudomorphic one. While optical and structural properties have been studied [4], less attention has been devoted to the physical process of island nucleation and growth on metamorphic layers. We report on the MBE growth of InAs on InxGa1-xAs metamorphic buffers (MBs) (0.15 < x < 0.35), considering two regimes: standard Stranski-Krastanow (SK) growth for high- density QDs and sub-critical growth followed by post-growth annealing for low-density QDs, allowing single photon detection at long wavelengths [2]. For both growth regimes, we studied the effects of the composition of the overgrown InGaAs surface x and of the QD-MB mismatch f (controlled by the MB thickness and composition) on the QD nucleation mechanisms. The 2D-3D transition was monitored by studying the RHEED pattern evolution, while the morphology and density of 3D islands were studied by AFM. For SK growth, the critical thickness for 2D-3D transition increases with reducing f, in agreement with model calculations [5], while it decreases when x is increased and f is kept fixed (Fig.1(a)). Possible explanations for this last effect are: i) an increased atom surface diffusion on In-richer InGaAs surfaces or ii) an enhanced In composition in the wetting layer (WL), in the picture of island nucleation due to In segregation in WL [6]. QD sizes for structures with 3.0 ML InAs coverage do not show evident dependence on f and x, but when x is increased beyond 0.30 larger QDs form with lower densities (Fig.1(b,c)). For sub-critical growth on InGaAs MBs covered by a thin (5 nm) GaAs layer - thus with x = 0 and different f - we observed an increase of the coverage values resulting in low-density QDs when f is decreased (Fig.2 (a)), but with a different dependence compared to the SK QDs, indicating a possible different influence of the strain on the growth processes. We also present morphological properties of low density structures, highlighting the influence of InGaAs surface undulations on QD placement (Fig.2 (b,c)). In conclusion, data reported here confirm that metamorphic InAs/InGaAs QDs deserve in- depth studies, as they can provide additional design parameters and interesting possibilities to control QD properties [4]. Beside their relevance for devices emitting light at long wavelengths, they can be considered as a good system to test advanced theoretical models for nucleation and growth of islands, thanks to the possibility of separately and independently control the two fundamental parameters x and f.
Subject III-V semiconductors
quantum dots

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