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Our principal simulation methods are based on density functional theory (DFT), which provides a first-principles model of the electronic and atomic structure of solids, surfaces, and molecules. Using this theory we can predict the atom positions and relative energetics of molecules adsorbed on surfaces and identify stable configurations. Via the Tersoff-Hamann approximation we can generate simulated STM images to be compared with those measured experimentally. Transition state calculations allow the estimation of reaction rates; such calculations help with the interpretation of temperature-dependent transitions observed in STM experiments. The method of kinetic Monte Carlo (KMC) is used to aggregate our accumulated knowledge of the elementary chemical transitions into dynamical models that can describe phenomena at time- and length-scales relevant to fabrication. Some of the specific areas of research are discussed below.
Co-doping strategies for bipolar nanoelectronic device fabrication
One research aim is to resolve the adsorption chemistry of other dopant sources such as boron in collaboration with scanning tunnelling microscopy (STM) experiments conducted at UNSW. The objective of this work is to provide design-input towards new STM-lithographic protocols for p- and n-type dopant placement.
This built on our previous successes in fully elucidating the phosphine chemistry on Si(001) [see Fig. 1] that underpins the bottom-up fabrication of Si:P devices. We are looking into complementary chemistries to afford a similarly selective placement of p-type dopants such as boron.
Another research aim is to look at the chemical processes of silicon growth on the Si(001) surface. The burial of deposited Si:P nanostructures under silicon in particular requires mild conditions for silicon overgrowth so as to preserve the placement of phosphorus atoms. In this regard, thermal overgrowth using silanes may offer advantages and will seek to describe the chemical dynamic of silicon overgrowth using DFT calculations and kinetic Monte Carlo (KMC) simulations.
The electronic structure of low-dimensionally doped nanostructures
We have refined our methodology to describe the electronic structure of low-dimensionally doped silicon, using phosphorus δ-doped silicon [see Fig. 2] as our initial testing ground. We are now using this methodology and look into more complicated structures. Specific objectives are phosphorus dopant wires in silicon, interacting stacks of phosphorus δ-layers, and the effects of lattice defects on the electronic structure of highly δ-doped silicon. Our electronic structure calculations provide valuable input and validation for Si:P device-scale model, such as those based on effective mass theory.