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The chemical vapor deposition (CVD) of Si and Ge on Si(0 0 1) substrates covered by 1 monolayer of Ga has been studied. Ga modifies surface reactivity and surface atom mobility. We show that islanding occurs during Ga-mediated CVD even in the absence of strain, allowing for the formation of islands without a wetting layer or with a thinner wetting layer. The morphology of the grown layer is highly dependent on kinetic parameters, i.e. growth temperature and growth pressure, that control Si and Ge deposition and Ga desorption. Si Ga-mediated CVD promotes the formation of self-assembled epitaxial Si islands exhibiting {1 1 3} facets. Ge islands grown by Ga-mediated CVD present reduced lateral sizes and an increased surface density compared to non Ga-mediated CVD. The growth morphology (particularly island size and density) can be understood by considering a qualitative model in which atoms are deposited only on sites whose dangling bonds are not passivated by Ga. We suggest that modification of surface reactivity during surfactant-mediated CVD can thus provide a new degree of freedom for quantum dot fabrication.
We investigate the fundamental mechanism by which self-assembled Ge islands can be nucleated at specific sites on Si(001) using ultra-low-dose focused ion beam (FIB) pre-patterning. Island nucleation is controlled by a nanotopography that forms after the implantation of Ga ions during subsequent thermal annealing of the substrate. This nanotopography evolves during the annealing stage, changing from a nanoscale annular depression associated with each focused ion beam spot to a nanoscale pit, and eventually disappearing (planarizing). The correspondence of Ge quantum dot nucleation sites to the focused ion beam features requires a growth surface upon which the nanotopography is preserved. A further key observation is that the Ge wetting layer thickness is reduced in patterned regions, allowing the formation of islands on the templated regions without nucleation elsewhere. These results provide routes to the greatly enhanced design and control of quantum dot distributions and dimensions.
We compare in-situ electron microscopy observations of Ge island growth on Si surfaces modulated by different techniques. Si(001) substrates were patterned either using a focused Ga ion beam or lithographically to produce similar feature sizes. In the case of the focused-ion-beam patterned substrates, chemical or strain effects caused by low Ga doses control the positions at which islands nucleate. Nucleation on topographically modulated substrates, with the pattern produced either by high-dose sputtering or lithographically, is controlled by the nature of the side walls of topographic features. The two patterning approaches have different benefits in creating arrays of islands for device applications.
We demonstrate that the nucleation sites of nanoscale, self-assembled Ge islands on Si(001) can be controlled by patterning the Si surface in situ with a focused ion beam. At low doses of 6000 Ga+ ions per <100 nm spot, the selective growth is achieved without modifying the initial surface topography. At larger doses, topographic effects produced by sputtering and redeposition control the selective nucleation sites. Islands grown on irradiated spots are smaller with higher aspect ratio than islands grown on clean Si(001), suggesting a strong surfactant effect of Ga.
We show that templating a Si surface with a focused beam of Si2+ or Si+ ions can create suitable nucleation sites for the subsequent growth of self-assembled Ge quantum dots by chemical vapor deposition. To determine the mechanism of patterning we use atomic force microscopy to show that, similar to Ga+ patterning, the formation of a surface pit is required to enable control over Ge quantum dot locations. We find that relatively high implantation doses are required to achieve patterning, and these doses lead to amorphization of the substrate. We assess the degree to which the substrate crystallinity can be recovered by subsequent processing. Using in situ transmission electron microscopy heating experiments we find that recrystallization is possible at the growth temperature of the Ge quantum dots, but defects remain that follow the pattern of the initial implantation. We discuss the formation mechanism of the defects and the benefits of using Si ions for patterning both defects and quantum dots on Si substrates.
Microstructural changes in silicon induced by patterning with focused ion beams of Ga, Si and Au
(2013)
We use focused beams of Ga(+), Au(+) and Si(++) ions to induce local microstructural changes in single crystal silicon. The ions were delivered as single spot pulses into thin Si membranes that could subsequently be imaged and annealed in situ in a transmission electron microscope. For each ion, the focused ion beam implantation created an array of amorphous regions in the crystalline membrane. Annealing causes solid phase epitaxial regrowth to take place, but we show that the resulting microstructure depends on the ion species. For Ga(+) and Au(+), precipitates remain after recrystallization, while for Si(++), dislocation loops form around the periphery of each implanted spot. We attribute these loops to defects formed during solid phase epitaxial regrowth, with controlled placement of the loops possible.
We analyze the nucleation of dislocations in silicon at spatially localized stress fields generated by silicon nitride pads having a high intrinsic stress. The nucleation and final configuration of the dislocations were studied using hot-stage transmission electron microscopy and were compared with dislocation simulations based on calculations of the stress fields around the pads. We find that the simulated configurations match well with the experimental data, and we show that the dislocation configuration can be controlled by the pad size.
We describe two new approaches to the patterning of Si(1 0 0) surfaces for controlled nucleation of heteroepitaxial Ge semiconductor clusters. In the first method, a Ga ' -focused ion beam in situ to the growth chamber is used to create local regions of enhanced Ga ' concentration and surface topography. It is shown that at low ion doses (Â/10 14 cm (2), implanted Ga causes local nucleation of Ge clusters upon the implanted region. At higher doses (]/10 15 cm (2), a complex surface topography localizes nucleation of Ge clusters. This approach can be used to seed complex patterns of Ge clusters with diameters of tens of nanometers and positional accuracy of sub-100 nm. The second method employs self-assembly of complex strain-stabilized ''quantum quadruplet'' and ''quantum fortress'' structures, whereby cooperative island nucleation around shallow strain-relieving pits is identified during Ge x Si 1(x /Si(1 0 0) heteroepitaxy. These configurations are kinetically limited structures that exist over a range of compositions, growth temperatures, and growth rates, but which are destabilized by strain relaxation (e.g. by the introduction of misfit dislocations) and by growth conditions which provide high adatom surface mobilities. Both methods have broad potential application to nanoelectronic device architectures. 2003 Elsevier Science B.V. All rights reserved.