Impurity doping of silicon (Si) nano-volumes (NVs) as currently used in very large scale integration (VLSI) faces serious challenges at miniaturization efforts below the 14 nm technology node. Dopant out-diffusion, local density fluctuations and inactivation by clustering are major issues for Si field effect transistors (FETs). Self-purification and a massive increase in ionization energy cause doping to fail at Si nanocrystals (NCs) which show quantum confinement [1].

In analogy to the concept of modulation doping [2], I demonstrate a direct modulation doping method for silicon [3] using 1 mono layer (ML) Al in SiO₂. By relocating the dopants from silicon to silicon dioxide, mentioned Si nanoscale doping problems are circumvented. In addition, the method provides excellent passivated hole selective tunnelling contacts as required for high-efficiency Si solar cells.

Picking B as example, I will show in theory and experiment that the choice of modulation acceptor is not trivial, rendering knowledge of acceptors in bulk Si inapplicable to SiO₂ acceptor modulation doping. A screening of Grp. IIIA and IIIB elements by DFT shows that several alternatives to Al exist, offering a tuning of hole-selective contacts for Si solar cells and tunnel-FETs. I will elucidate atomistic and quantum-chemical parameters which decide over mentioned elements to be a modulation acceptor in SiO₂.

Ideally, n- or p-type conductivity in VLSI-Si requires just energy offsets of lowest unoccupied states (LUS) and of highest occupied states (HOS) between different regions of the same VLSI-Si volume. As a result, doping would be eliminated altogether.

I will show in theory (analytics [4], DFT) and experiment (long-term synchrotron UPS, TEM, XPS, ERDA, PL, SIMS) that a few MLs of SiO₂ (vs. Si₃N₄) achieve such an energy offset [5]. The induced n- (vs. p-) type behaviour in VLSI-Si can potentially be big enough to allow for band-to-band tunnelling (BTBT) for realizing extremely small and fast tunnel-FETs.

[1]  Sci. Rep. 5, 09702 (2015)                             [4]  in draft (2018)

[2]  Appl. Phys. Lett. 33, 665 (1978)                    [5]  Adv. Mater. Interfaces 1, 201400359 (2014)

[3]  Sci. Rep. 7, 46703 (2017)                             [6]  Beilstein J. Nanotech, under review (2018)