PHYSICS OF SELF-ASSEMBLED Ge/Si ISLANDS (Ge quantum dots)
The interest in the study of self-assembled semiconductor dots stems from the possibility of using these materials as active layers in the fabrication of new mesoscopic devices. To this purpose it is necessary to achieve a good control of the size and the shape of the islands, in particular it is important to reduce the island dimensions in order to increase the confinement and the charging effects in these systems. We have investigated the influence of the growth parameter (growth temperature, growth rate, and amount of the deposited material) on the size, size distribution and areal density of self-assembled Ge islands grown by Ultra-High-Vacuum Chemical Vapour Deposition technique on Si(100) substrates. We have obtained a good control of the island density on a wide range of deposition temperatures by tuning the growth rate by means of the flow and the pressure of the reacting gases. Furthermore, we found that the deposition temperature play an important role in defining the typical sizes of the islands. We explained this effect as due to a temperature-promoted Si-Ge intermixing that reduces the effective epilayer-substrate lattice mismatch and, consequently, the existing strain field. For the first time, we evidenced  the existence of scaling laws linking the island sizes to their actual composition and strain.

The self-assembly offers the possibility of obtaining nanometer-sized islands without the use of expensive nano-lithography processes. The random nature of the island formation leads to a non-uniformity and a non-predictability in their spatial distribution. On the other hand, in order to use these structures into integrated devices, an accurate control over their spatial positioning is required. To solve this problem, we have proposed a bottom-up approach to the ordering of self-assembled island exploiting the strain modulation at a silicon surface arising from a self-ordered stack of buried Ge islands. We have demonstrated that islands can be placed in arrays having  a short- as well as a long-range order. The effect of the buried island strain field intensity and of the island-island interaction, have been taken in to account in the description of the island growth dynamics on such "strain templates".

 

 

First we will show that, by capping randomly-positioned, self-assembled Ge islands with Si at 750°C , lateral ordering can be achieved through a lateral displacement of the islands themselves. The Si-Ge intermixing  occurring during the island exposure to the silane flux, induces an island-shape transformation from domes to truncated pyramids with larger basis following a reverse Stranski-Krastanov (SK) dynamics. By means of Atomic Force Microscopy we observed that, while this enlargement proceeds, the edges of different islands  come close together and the islands arrange spontaneously in a well-ordered square lattice aligned along the “soft” [100]-[010] crystalline directions. Upon changing the deposition parameters, we have experimentally verified that an increased ordering can be obtained by enhancing the elastic repulsion through a higher average Ge content in to the islands and a higher island density. This behavior indicates that the island-island elastic strain  repulsion between neighboring islands is the main driving force for the island lateral motion and the consequent ordering process. We observe that  the island ordering is improved as the Si capping, thus the Ge-Si intermixing,  proceeds . This evidences an increased island-island strain interaction and, as a consequence that  the effect of island enlargement overcomes those of the observed  island aspect ratio and Ge content decrease. These picture has been confirmed by means of theoretical modeling and calculation. Atomistic simulations and continuum elasticity-theory calculations show that the strain interaction between two closely-spaced islands induces a net flux of Ge and Si atoms  from the facets close to the facing island  in to the more distant facets. This effect is enhanced by the SiGe intermixing and the consequent island enlargement. This mass transport results into the  lateral displacement of  the whole island during the capping process. If the displacing island approaches another one, a new driving force causing an inverse flow of Si and Ge atoms can cause the end of the motion. The final result is an arrangement of the islands in a square lattice oriented along the “softer” [100]-[010] directions.