Straightforward downscaling of the metal-oxide-semiconductor field-effect transistor (MOSFET), the workhorse of the electronics industry, is coming to an end as nanoscale dimensions have been reached. To reduce power dissipation of future integrated circuits (ICs), new device architectures and materials that support steep inverse subthreshold slopes and thereby low voltage operation are intensively investigated. A promising candidate to replace the MOSFET in future technology nodes is the vertical nanowire (NW) based tunnel-FET (TFET). Due to its built-in tunnel barrier, the TFET does not suffer from short-channel effects, and the subthreshold slope of TFETs is not limited to 60mV/dec, the physical limit of MOSFETs. Moreover, the NW geometry offers the most ideal structure from an electrostatic perspective with 3-dimensional stacking capability.
However, NWs also present other properties that make them attractive for advanced devices, such as the possibility of in-situ doping, core-shell structures, and not least the axial growth of non-lattice matched heterostructures. Axial NWs heterostructures allow energy band-edge engineering along the axis of the NW, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures not easily accessible in planar devices.
Interface abruptness of doped NWs and the heterojunctions in general imposes challenges in these structures and others for realizing high performance TFETs in p-i-n junctions or band gap engineered materials. Such properly designed silicide/germanide-semiconductor heterojunctions provide a sharp interface with band-edge control and will be investigated as possible tunnel barriers to reduce ambipolar transport behavior generally observed in TFETs.
In this research proposal we merge: (a) on the personal level four research groups out of France and Austria, as well as (b) on the scientific level two significant advances in the area of heterostructure NWs and tunnel FETs: the realization of compositionally and doping modulated Si/Ge axial NW heterostructures as well as silicide/germanide-semiconductor heterojunctions with lengths suitable for device fabrication. The synthesis techniques will be based on the vapor-liquid-solid (VLS) approach and the use of controlled phase conversion by diffusion as a vehicle for creating sharp interfaces within particular NW materials, respectively.
Three promising systems will be investigated within our research: axial NW heterostructures of (i) Si and Ge with a p-i-n doping structure (ii) NW heterostructures of Si, Ge and Si-Ge alloyed NWs (iii) heterojunctions of Si and Ge NWs of different doping levels and as a first choice the respective silicides and germanides of Pt, Ni, Cu and Co.
The resulting NW heterostructures will need a comprehensive structural characterization using e.g. transmission electron microscopy (TEM), X-Ray diffraction and confocal Raman microscopy so that the optimum synthesis parameters can be identified. The main focus of this project is the investigation of the correlation between the structures and electronic functionality, in order to enable novel electronic devices.
At the end of the project, proto-type TFETs will be realized and their electrical properties will be investigated by combining I(V), C(V) measurement at room and cryogenic temperature. The results will be correlated with the simulation performed to understand the physics of the devices.
