The spectacular increase of computational speed and power of modern integrated circuits is supported by the continuing miniaturization of semiconductor devices. With scaling approaching its fundamental limits, however, the semiconductor industry is facing the challenge of introducing new innovative elements and engineering solutions and improving the performance of Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET). A promising alternative to the electron charge degree of freedom currently used in MOSFET switches and random access memories, is to take into account the spin of an electron. The electron spin possesses several exciting properties suitable for future devices. It is characterized by only two projections on a chosen axis – up or down, and it can change its orientation rapidly by utilizing an amazingly small amount of energy. Employing spin as an additional degree of freedom is promising for boosting the efficiency of future low-power nanoelectronic devices, with high potential for both memory and logic applications.
Silicon is the primary material for microelectronics. The long spin lifetime in silicon is a consequence of the weak spin-orbit interaction and the spatial inversion symmetry of the lattice. In addition, silicon is composed of nuclei with predominantly zero magnetic moment. A long spin transfer distance of conduction electrons has already been demonstrated experimentally. Spin propagation at such distances combined with a possibility of injecting spin in silicon at room temperature makes the fabrication of spin-based switching devices quite plausible in the upcoming future. However, the relatively large spin relaxation experimentally observed in electrically-gated lateral-channel silicon structures might become an obstacle in realizing spin driven devices, and a deeper understanding of the fundamental spin relaxation mechanisms in silicon is urgently needed.
We are investigating the influence of the intrinsic spin-orbit interaction on the subband structure, subband wave functions, and spin relaxation matrix elements due to the surface roughness and phonon scattering in MOSFETs. We are utilizing a k·p approach suitable to describe the electron subband structure in the presence of the spin-orbit interaction and strain. With the spin degree of freedom included, our effective 4x4 Hamiltonian considers only relevant [001] oriented valleys producing the low-energy unprimed subband ladders. Within this model the degeneracy between the unprimed subbands in the unstrained (001) channel is lifted due to the spin-orbit effects included. An accurate inclusion of the spin-orbit interaction results in significant mixing between the spin-up and spin-down states along the [010] and [100] directions. Shear strain lifts the degeneracy between the unprimed subbands, which results in improving the spin lifetime in gated silicon systems.