Growing Perfectly Uniform Quantum Dots and Quantum Cellular Automata

 
 
niform and regular quantum dot arrays with precisely controlled positions and sizes may serve as a template for the next generation of nanoelectronic devices; this promise has motivated many researchers to use self-assembled heteroepitaxial growth. Numerous theoretical, modeling and experimental

works have shown that unguided self-assembled growth of quantum dots usually fails to realize perfectly ordered dot arrays. Recently, more effort has been shifted to use guided self-assembly through pre-patterning.

So far, several pre-patterning procedures have been reported. It is expected that through pre-patterning, uniform and regular quantum dots can be achieved. In reality, quantum dots have been found to nucleate at different positions even if an ordered pre-patterned substrate is used, often failing to produce the one-to-one relation. Hence, reliably and reproducibly achieving ordered quantum dot arrays through surface pre-patterning is still an unsolved problem.

We have performed systematic three dimensional computer simulations to determine the growth windows for achieving ordered quantum dot or ring arrays through pre-patterning substrate surfaces. In the modeled thin film/substrate system, the pre-patterned substrate surface is considered to be fixed. A thin transition layer with a linearly varied mismatch strain covering the pre-patterned substrate surface is introduced to model the wetting effect. Here the focus is on the morphological evolution of the film surface atop the wetting layer through diffusion and deposition.
Our simulations show that for isotropic thin film systems, concave patterning in a squared array may lead to the formation of ordered dots, whereas convex patterning in a squared array may initially lead to the formation of ordered quantum rings, and then a transition into quantum dots with further growth. The evolution of the surface chemical potential during growth explains the formation of the ordered surface structures. However, for anisotropic thin film systems, novel surface structures, such as ordered quantum dot arrays (Figure 1) and quantum-dot automata arrays, fortress-enclosed quantumdot automata arrays (Figure 2), can be obtained by controlling the pre-patterning pitch distance and growth rate. In addition, the quantum dot (ring) density, according to the present study, can be significantly higher than the current experimentally achieved dot densities. For example, for the Ge/Si system, the dot density is predicted to be as high as 400 dots/ μm2. However, the current experimentally achieved dot density is only roughly 4~20 dots/μm2. Hence, there is still a large potential and challenge to increase the dot (ring) density. It is expected that the present work will provide a new guideline for controlling the formation and self-assembly of novel surface structures.


Contact person

Assoc Prof YW Zhang
Tel: 6516 4877, Fax: 6776 3604
E-mail: msezyw@nus.edu.sg
 
 


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