New device applications are based on self-assembly quantum dot formation on the pre-patterned semiconductor substrates.

Semiconductor self-assembled quantum dots (QDs) have emerged as one of the simplest subjects for exploring and exploiting the physics and device applications of charge carriers and excitons in the three-dimensional confinement regime. Nanoscale-sized surfaces in the form of mesas or ridges on patterned substrates offer opportunities, not only for creating large densities of QDs with great homogeneity, but also for novel thin-film growth-control phenomena during the formation of QDs on the surfaces of Si stripe and mesa structures. Si mesa structures have been demonstrated to be an excellent template for studying homoepitaxy and heteroepitaxy phenomena.

Real-Time STM Images (5000 x 5000 Å2) of a CVD growth sequence of Si on Si(100)-(2×1) at 490 °C. The evolution of the surface morphology during the growth shows that 2D epitaxial islands can grow larger in size and the island density is smaller. Ideal layer-by-layer growth does not occur. The second layer growth starts before the first layer completes growth. Further growth leads to increased roughness.
Employing a variable-temperature scanning probe microscope (VT-SPM) on the patterned substrates, the atomistic chemical vapor deposition (CVD) growth mechanism of the QDs can be observed in situ on the top terraces as well as on the sidewalls of pre-patterned structures. The VT-UHV AFM/STM system is ultra-high-vacuum compatible and allows chemical vapor deposition and molecular beam epitaxy of various materials. There fore, users are able to study the growth phenomena occurring on the limited surface areas in situ at an atomic scale. The physics of group IV semiconductor surfaces and thin-film growth of Si, Ge, and P on silicon and germanium surfaces has been studied. Recently, atomic resolved noncontact AFM images of ultra-thin oxide surfaces have been obtained, enabling the imaging in sequence: a) the patterned structures with the oxide layer, b) the same surface after partial and complete oxide removal at 800-1200 °C, and c) the growth of thin films and QDs with chemical vapor source all in situ.

The top terraces and the side walls of silicon have very different atomic structures and dangling bond densities. During CVD, the sticking coefficients of source molecules on the two kinds of surfaces are often quite different. Also, adatoms migrate at different rates on the two surfaces. If heteroepitaxy is involved, the two interfaces will exhibit different strain fields, also, owing to different surface reconstruction. All of these factors lead to different film deposition rates and QD formation density on the top terraces and sidewalls.

Many promising new device applications are based on self-assembly QD formation on the pre-patterned semiconductor substrates. Numerous groups are currently working on better control of QDs’ density, size homogeneity, and position ordering, yet the detailed, atomic-resolved, and in situ observation of the self-assembly growth is still lacking. This proposed research intends to fabricate patterned stripe and mesa structures with well-defined sidewalls. With the help of variable-temperature SPM on the patterned substrates, the atomistic CVD growth mechanism of QDs on the top terraces as well as on the sidewalls are to reveal the structure evolution of all surface areas on a substrate, and the very initial stage of QDs’ formation.

This work was done by Deng-Sung Lin of the Institute of Physics, Taiwan, for the Air Force Research Laboratory. AFRL-0127

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