Electronic Structure Simulation of Thin Silicon Layers: Impact of Orientation, Confinement, and Strain

Fully Depleted Silicon on Insulator transistors (FDSOI) are a promising approach for further scaling. The device features a fully depleted body which is isolated by an insulator box. This introduces better electrostatics, lower leakage current and thus better channel control. The device performance is heavily influenced by the orientation, confinement and strain in the ultra-thin body. In this work the electronic structure of ultra-thin silicon layers is investigated using Density Functional Theory (DFT). The simulation parameters for the model system were calibrated to reproduce the experimental band gap of bulk silicon. This ensures that the model describes the electronic structure of ultra-thin silicon layers accurately. Our study demonstrates the impact of confinement, orientation and strain on material dependent transport properties and their influence on the device performance. For this purpose our results will be used as an input for device simulations using Synopsys Sentaurus TCAD.
We find that the band gap of the silicon layer increases with decreasing slab thickness which is a clear indication of quantum confinement. From the simulation, the band gap for the {100} confinement is found to be higher than {110} and {111} scenarios. Band gap is one of the factors which influence the intrinsic carrier in the semiconductor and thereby the transport. Another important factor for the transport is lattice strain. Strain is a useful method for modulating band structures. One good example is the transformation of direct band gap in {100} confined silicon slab to indirect band gap with 2 % biaxial compression. In our presentation we will discuss the influence of the effective mass as well. Furthermore, the strain dependence of the electronic structure and its impact on device properties is analyzed systematically.