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Silicon Heteroepitaxy
The next step in the fabrication of the actual SSQC call for growth of a 200 Å thick layer of crystalline silicon over the phosphorus array. The important question at this point becomes: Can crystalline silicon be grown at temperatures low enough to prevent diffusion and disruption of the array. This step can be carried out with the sample in the STM making it easier to locate the phosphorus atoms and characterize the array afterwards. Currently, we are studying epitaxial deposition of a thin silicon layer at low temperatures (<230?C) using an effusion cell small enough to be integrated into the STM chamber. Click on the link above to learn more about this step.
In our initial studies, the freshly prepared Si(100) 2 x 1 surface was exposed to a beam of silicon atoms for different lengths of time in order to grow progressively higher coverages ranging from 0.08 monolayers (ML) to 0.5 ML and then finally a complete ML. Images of the different silicon coverages is shown in Figure 1.
The crystalline quality of the deposited material is evidenced by the presence of dimer rows running perpendicular to the substrate rows in each terrace clearly visible in the STM images at the submonolayer coverages. Although the completed monolayer appears to have a number of vacancies and adatoms, this surface has not been annealed. The persistence of vacancy related defects is, however, a concern. One might expect that these would be filled in during the silicon overgrowth but apparently they are not. They indicate that there is some driving force for defect formation that we do not yet understand. Their presence also implies that they will be very difficult to completely eliminate in the end product and should, therefore, be evaluated with respect to any detrimental effect they may have on the operation of the SSQC.
Our initial epitaxy results are promising since we may not need a low temperature post-anneal. Further, since hydrogen can act as a surfactant layer, we anticipate growing an even better quality silicon layer when the growth is carried out directly on the hydrogen passivated surface. This method has the added advantage of eliminating the high temperature hydrogen desorption step following fabrication of the phosphorus array as originally planned.
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Related Links
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Structure/Property Relations, MST-8 