Interplay between Thermodynamics and Kinetics in the Capping ofInAs/GaAs(001)Quantum Dots

Abstract

A microscopic picture for the GaAs overgrowth of self-organized quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.

InAs Quantum Dot Formation Studied at the Atomic Scale by Cross-sectional Scanning Tunnelling Microscopy

“…It is attributed to In-Ga intermixing. The presence of In-deficient corners in the present GaAsSbcapped QDs indicates that they are not necessarily created by a redistribution of material from the top of the island to the bottom during capping, as has been recently reported [74]. In the present case, the top of the dot is not dissolved during capping but the In-poor corners are still present, hence they must originate during an earlier stage of the QD formation process.…”

“…This result is confirmed by atomic force microscopy measurements on uncapped surface QDs, which are found to have a height of 8 ± 1 nm, in good agreement with the height of the GaAsSb-capped QDs. The base length of the strongly dissolved GaAs-capped QDs is smaller than that of the preserved GaAsSb-capped dots (20 vs 26 nm, respectively), indicating that the material redistribution during capping is not from the apex to the base of the dot [74], but occurs instead to the wetting layer (WL), as was also shown in section 5.2. This is in agreement with the observation of a significantly higher In content in the WL of GaAs-capped QDs (1.8MLs vs 0.8 MLs in GaAsSbcapped QDs), obtained by counting individual In atoms in the X-STM empty states images.…”

“…Nevertheless, once created, the QDs are subsequently capped, a step which is required for any device application. Although a lot of effort has been dedicated to understand the QD growth mechanism, there are relatively few studies focused on the effect of the capping process [12][13][14][15][16][17][18][19][20]. Some of these studies have already shown significant differences in size, shape and composition between uncapped and capped QDs.…”

Growth and capping of InAs/GaAs quantum dots investigated by x-ray Bragg-surface diffraction

“…1 Size, shape, and composition are the most important parameters determining the discrete energy levels of charge carriers inside the dots. 2 Minimization of strain during the initial stages of heteroepitaxial growth of lattice mismatched materials plays an essential role in QD spontaneous formation, 3,4 but their final parameters also depend on other growth conditions such as temperature and deposition rates. 5 In addition, device processing also requires embedded QDs on the semiconductor matrix, which is achieved by deposition of capping layers.…”

“…It is an even more complex heteroepitaxial process since faceting, segregation, intermixing, and strain-enhanced diffusion are other phenomena, besides strain release, that also take place at the cap layer/QDs interface and can drastically change the QDs morphology and composition. 4 To gain knowledge on the mechanisms ruling the atomic arrangements on these nanostructured systems, great efforts have been pulled out in the last few years. Surface probing techniques can provide information of the QDs when they are on top of the exposed surface [6][7][8][9][10][11] X-ray Bragg-surface diffraction ͑BSD͒ is a particular case of three-beam multiple diffraction where an extremely asymmetric reflection, diffracting nearly parallel to the macroscopic surface of the sample, is excited simultaneously with the symmetric Bragg reflection ͑Fig.…”