Comparison of Conventional Impurity Doping with Modulation Doping of Silicon Nanostructures (invited talk)

Silicon nanowires (Si NWs) or nanosheets are the building blocks for future transistors, but suffer from severe difficulties with efficient impurity doping due to a plethora of physical and technological effects occurring when device dimensions are reduced to a few nanometers (dielectric and quantum confinement, self-purification, interaction with surface states, statistics of small numbers, etc.). Especially junctionless nanowire transistors require high conductivities which is often enabled by high impurity doping concentrations consequently leading to a severe degradation in the carrier mobilities and on-currents due to increased scattering. Thus, implicating the need for higher operation voltages leading to an increase in electrical energy dissipation. Therefore, alternative doping methods are desirable that separate doping-induced free carriers from their parent dopant atoms.
Here, we present a novel doping concept for Si NWs analogous to the modulation doping approach of III-V semiconductors. Based on results from density functional theory (DFT) calculations, we use Al-doped SiO2 shells around the Si NWs, which contain unoccupied Al-induced acceptor states with an energy level located slightly below the Si valence band edge. These states can capture electrons from the Si, creating free holes as majority charge carriers in the silicon [1-5]. Experimentally, the Al-doping of SiO2 is realized by atomic layer deposition (ALD) to form (sub-)monolayers of AlOx sandwiched between an ultrathin tunnel-SiO2 and a capping-SiO2 layer.
We demonstrate that Si NWs surrounded by a SiO2:Al-shell have more than 3 orders of magnitude lower electrical resistances as compared to NWs with an intrinsic, undoped SiO2-shell [6]. Moreover, it will be shown that modulation-doping, in addition to lowering the channel resistance, effectively reduces the Schottky barrier height of Si NWs with nickel silicide contacts enabling nearly ohmic contact behavior with a very low contact resistivity [7].
[1] D. König et al., Sci. Rep. 7, 46703 (2017)
[2] D. Hiller et al., ACS Appl. Mater. Interfaces 10, 30495 (2018)
[3] D. König et al., Phys. Rev. Appl. 10, 054034 (2018)
[4] D. Hiller et al., J. Appl. Phys. 125, 015301 (2019)
[5] D. Hiller et al., J. Phys. D Appl. Phys. 54, 275304 (2021)
[6] I. Ratschinski et al., Phys. Status Solidi A 2300068 (2023)
[7] S.Nagarajan et al., “Evaluation of Schottky barrier height at Silicide/Silicon interface of a Silicon Nanowire
with Modulation Acceptor Doped Dielectric Shell”, Device Research Conference (DRC) (2023)