Modulation-Acceptor Doping of SiO2 as Novel Doping Method for Silicon Nanowires (MADSiNano)

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Silicon (Si) nanowires with few nanometers in diameter play a key role in the further development of the microelectronic transistor technology, which is essential for virtually all electronic devices. Nanowires enable maximum gate control over the source-drain current via the gate-all-around field effect transistor (GAA-FET) architecture. For the electronic functionalization of nanowire transistors several concepts are available (p/n-junctions, tunnel-FETs, junctionless transistors). However, all them rely on the conductivity control by classical impurity doping (using e.g. phosphorus or boron). On the nanoscale, reliable and efficient doping is impeded by a multitude of physical and technological problems. These problems include e.g. the undesired diffusion of dopants, segregation or deactivation of dopants at interfaces, dielectric and quantum confinement that inhibit the ionization of an impurity as well as statistical problems when attempting to dope ultrasmall Si nanovolumes with an identical amount of a defined number of dopant atoms.In this project we want to pursue a theoretical concept to adopt modulation doping known from III-V semiconductors for Si. Modulation doping means that the parent dopant atoms are spatially separated from the volume that is to be doped by embedding them into an adjacent material with a higher bandgap. Density functional theory (DFT) calculations predicted that aluminum (Al) atoms in SiO2 have an unoccupied state below the Si valence band edge, which upon electron capture creates a hole in the Si as majority charge carrier. Thereby, the SiO2 embedding a Si nanostructure is doped, whereas the free majority charge carriers contribute to the conductivity of the Si. All the problems associated to direct Si doping are circumvented in that way. In extensive preliminary work we proved the existence of this Al-induced acceptor state and determined several fundamental properties. Now, we want to adopt this concept to Si nanowires to demonstrate a massive increase of the conductivity via modulation acceptor doping of the SiO2 shell and to fabricate and characterize functional nanowire transistor test devices. In addition to Al, DFT calculations suggest 4 other possible elements, which might represent SiO2 modulation acceptors for Si. In the project, these elements will be investigated in detail and processes will be developed to implement them into Si nanowire transistors to enable a comparison of the doping properties of the different modulation acceptor elements.