Strain-Induced Self-Assembly In Semiconductor Quantum-Dot Systems

Quantum dots have promising technological applications in electronicand photoelectric devices, such as semi-conductor laser, photo-detectors,cellular automata, single-electron transistors and solid state quantumcomputer et al. Strain-driven formation of nanoscale islands onlattice-mismatched layers in heteroepitaxy offers an attractive way foreffective fabrication of coherent quantum dots of large density. Normally,these islands are not well assembled in space and show large dispersionin size distribution. This disadvantage hinders the application ofquantum dots in some electronic and photoelectric devices. Recentexperiments showed that both the spatial ordering and size uniformitycould be greatly improved by growing multilayers of islands separatedwith spacer layer. The self-assembly of stacked islands is believed to be the result of thestrain fields on spacer layer surfaces induced by embedded islands.Previous studies mostly treated the embedded islands as point forcedipoles of zero dimensions. In reality, islands have finite spatial extentwith a linear size often comparable to the spacer layer thickness. Finitesize of islands must be considered in order to achieve a comprehensiveunderstanding of the self-assembly process. At the same time, surfaceislands also induce strain on the spacer layer surface and its influence isnot negligible in the most case. However, all existing models completelyneglect its influence. Based on the continuum theory of elasticity, wehave investigated the strain-induced self-assembly in semiconductorquantum-dot systems and obtained following results: 1. Based on the surface strain field induced by point force dipoles in asemi-infinite space, we have obtained analytical solutions for embeddedsheet and cuboid. 2. We have detailed analyzed the strain field distribution on the spacerlayer surface induced by embedded sheets. Our results further confirmedthat the strain fields depend critically on island geometry. Embeddedislands can be considered as point force dipoles only as spacer layerthickness much lager than islands size. 3. We have indicated that the underlying mechanism for the local-assembly of islands above embedded islands is due to the strain-inducedmodulation of nucleation centers instead of the dipole repulsiveinteraction between adjacent islands. The finite size effect of embeddedislands on strain distribution allows growth of a cluster of islands on topof an embedded island with or without symmetrical distributions bychoosing appropriate spacer layer thickness and growth temperature.Our finding opens a new avenue to growth of well-assembledquantum-dot molecules. 4. We have proposed a model to elucidate the self-assembly processof islands in multilayer heteroepitaxial growth. The strain field inducedby embedded islands controls the location of the surface islands whilethe compressive strain field induced by surface islands plays a crucialrole in controlling the size of islands. The model predicts that islands insuccessive layers not only form ordered columns but also show uniformdistributions of island size and spacing, in agreement with experimentalobservations.