Fabrication of n-type dopant-based devices such as the ‘single atom transistor’ and 2D Quantum Metamaterials uses H Depassivation Lithography to create patterns of reactive Si dangling bonds on a background of passive H-terminated Si. Phosphine is used as a precursor molecule for P dopant atoms adsorbing extremely selectively onto the bare Si atoms. Atomic Layer Deposition (ALD) using TiCl4 and water has also been demonstrated with these patterns, growing thin layers of TiO2 as a hard etch mask, enabling Si features down to 6 nm width to be fabricated via Reactive Ion Etching.<br/>In order to expand the utility of this atomic-precision patterning technology, we are interested in precursors for other species, particularly Gr. III metals, useful both as acceptor dopant precursors, and for ALD. In particular, here we are exploring common CVD alkyl dopant sources, including Trimethyl Al (TMAl) and Triethyl Al (TEAl). We have focused mainly on TEAl, as it is preferred over TMAl for lower carbon contamination of the grown films.<br/>We have conducted adsorption and selectivity experiments with TMAl and TEAl on bare and H-terminated Si(001) surfaces. TMAl is selective but leads to significant carbon incorporation. For TEAl, we find that the sticking coefficient at room temperature is low even on the bare Si(001) surface. While is no apparent adsorption on the H-terminated surface, we find that patterns drawn on the H-terminated surface after a TEAl dose will slowly fill with adsorbates, indicating the presence on the surface of mobile molecules. By performing the adsorption at elevated temperatures, we were able to reduce the quantity of these mobile molecules, and at the same time, the reaction of the bare surface to these molecules greatly increased. After adsorption, a high-temperature anneal to around 400°C was used to drive ethyl groups off the surface and incorporate the Al atoms. This recipe was used to create delta layers of Al. SIMS data of these delta layers show confinement of Al to the delta layer. However, the selectivity of the pattern relative to the background is still far too low, and the degree of carbon contamination is too high.<br/>Looking at the bigger picture, while further refinement of the adsorption and annealing process may improve these issues, inorganic precursors such as halides are likely to be preferred over metalorganic precursors, due to the strong Van der Waals interactions between the organic ligands and the background H termination.

Triethyl aluminium as a precursor for atomic-precision advanced manufacturing

ACS Spring Meeting 2021

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