Modulation-Acceptor Doped Silicon-Germanium-Alloys for Junctionless-Nanowire-Transistors (MAcDope-SiGe)

• Applicant: Professor Dr. Daniel Hiller
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564400083

Silicon-germanium (SiGe, Si_xGe_1-x) is a compound semiconductor that is used particularly for the channel of PMOS transistors in many electronic devices. In contrast to pure Si, SiGe has a significantly higher hole mobility, which enables both faster switching operation and higher currents through a transistor. However, the inevitably highly doped source/drain regions of field-effect transistors exhibit a much lower hole mobility due to conventional impurity doping. From a Drude-model perspective, the electrical conductivity decreases with decreasing mobility and the increased resistance causes energy losses via heat dissipation. With billions of transistors per chip in innumerable everyday devices, this energy loss sums up globally to a considerable extent. On the other hand, specific transistor concepts, such as the junctionless nanowire transistor, require both high charge carrier densities and high mobilities for their functionality, which cannot be achieved with impurity doping. Apart from this, the scaling of transistors towards even smaller dimensions in the single-digit nanometer range causes fundamental physical and technological obstacles for doping. In the project MAcDope-SiGe, we want to investigate a novel doping method that provides high charge carrier densities (holes) without compromising the high intrinsic hole mobility. Therefore, the concept of SiO2-modulation acceptor doping, recently successfully demonstrated for pure Si-nanowires, will be adopted for the compound semiconductor SiGe. Here, acceptor atoms are spatially separated from the doped region, i.e., they are not substitutionally incorporated into the SiGe lattice but into an adjacent SiO2 layer. Specific trivalent elements create in the SiO2 an unoccupied state located energetically below the SiGe valence band edge. If an electron from the SiGe relaxes into this state, a hole is created in the SiGe as a majority charge carrier (equivalent to p-type doping). With increasing Ge-concentration, the hole mobility also increases, however, at the same time the valence band edge energy shifts as well, which might turn out critical for the modulation acceptor doping. In a proof-of-concept experiment, we recently demonstrated that test devices made of ultra-thin Si0.67Ge0.33 layers can be successfully modulation-doped. Now, we want to use ultra-low-temperature molecular beam epitaxy (MBE) grown SiGe layers with increased Ge-concentrations - up to pure Ge - to investigate the physical limits of modulation acceptor doping in this material system. Toward the end of the project, we want to demonstrate a modulation-doped junctionless SiGe-nanowire transistor.

• DFG Programme: Research Grants
• International Connection: Australia, Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partners: Professor Dr. Moritz Brehm; Dr. Dirk König; Professor Dr.-Ing. Walter Michael Weber