Growth of graded doped Si/SiGe Heterostructure Tunnel FET
Tunnel FETs are the most promising ultra low power devices due to their potential of steeper subthreshold slopes and capability of using very low drive voltages. Switching is based on quantum mechanical band to band tunneling and no longer on thermal emission as in MOSFETs. We will present the epitaxial growth of novel, graded and highly doped SiGe/Si vertical heterostrucutures which possess a small band-gap and effective mass at the tunneling junction while Si is used on the drain side to minimize ambipolar behavior as will be shown with device simulations. The pseudomorphic heterostructures were grown using an AIXTRON RPCVD tool with showerhead technology. Different vertical heterostructures have been synthesized at low growth temperatures between 500°C and 600°C using Si2H6 and Ge2H6 as Si and Ge precursors, and B2H6 and PH3 as dopant sources. Graded Ge concentrations up to 55% in ultrathin 5-10 nm SiGe layers with and without additional graded B doping are aimed for source/channel tunnel junction. Several analysis methods, Reciprocal Space Mapping, RBS in the ion channeling mode and ToF-SIMS were used to fully characterize the grown layers. In addition, ToF-SIMS measurements provide high doping levels, 2-4x10E20 cm-3, and sharp doping profiles which allow short screening lengths, meaning excellent electrostatics. Sentaurus device simulations are used to optimize the structure parameters, e.g. doping profile, Ge concentration, for improved performance of the vertical TFETs.