Chapter 2

In order to achieve significant advances in future microelectronic devices in comparison to present technology, their operation principles will have to be enhanced or even modified. Spintronics is a rapidly developing technology promising to benefit from the spin properties of electrons. Utilizing spin opens great opportunities to reduce the device power consumption in future electronic circuits. A number of potential spintronic devices have already been proposed [10922]. Significant efforts are focused on developing models to study the properties of future devices through simulations [26].

Silicon is the primary material for microelectronics. The long spin life time in silicon is a consequence of the weak intrinsic spin-orbit coupling in the conduction band and the spatial inversion symmetry of the lattice resulting in an absence of the Dresselhaus effective spin-orbit interaction [5670]. In addition, silicon is composed of nuclei with predominantly zero magnetic moment. A long spin transport distance of conduction electrons in silicon has already been demonstrated experimentally [48]. Spin propagation at such distances combined with a possibility of injecting spin at room temperature [21] or even elevated temperature [69] in silicon makes the fabrication of spin-based switching devices quite plausible in the upcoming future [4952]. However, the relatively large spin relaxation experimentally observed in electrically-gated lateral-channel silicon structures [56] might become an obstacle for realizing spin driven devices [70], and a deeper understanding of the fundamental spin relaxation mechanisms in silicon MOSFETs is urgently needed [107].

In this work 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 scattering in thin silicon films is investigated. An efficient approach allowing to analyze surface roughness and phonon induced spin and momentum relaxation in thin silicon films is developed.

A kp based method [9111] suitable to describe the electron subband structure in the presence of strain is generalized to include the spin degree of freedom [70]. In contrast to [70], the effective 4×4 Hamiltonian considers only the relevant [001] oriented valleys with spin degree included, which produces the low-energy unprimed subband ladder. Within this model the unprimed subbands in the unstrained (001) film are degenerate, without spin-orbit effects included. An accurate inclusion of the spin-orbit interaction results in a large mixing between the spin-up and spin-down states, resulting in spin hot spots along the [100] and [010] axes characterized by strong spin relaxation. These hot spots should be contrasted with the spin hot spots appearing in the bulk system along the same directions at the edge of the Brillouin zone [7017]. The origin of the hot spots in thin films lies in the unprimed subband degeneracy which effectively projects the bulk spin hot spots from the edge of the Brillouin zone to the center of the 2D Brillouin zone.

Shear strain lifts the degeneracy between the unprimed subbands [111]. The energy splitting between the otherwise equivalent unprimed subbands removes the origin of the spin hot spots in a confined electron system in silicon, which substantially improves the spin lifetime in gated silicon systems. Thus, shear strain applying to thin silicon films reduces the spin relaxation.

Spin field-effect transistors (SpinFETs) are promising candidates for future integrated microelectronic circuits. A SpinFET is composed of two ferromagnetic contacts (source and drain), which sandwich a semiconductor channel. Current modulation is achieved by electrically tuning the gate voltage dependent strength of the spin-orbit interaction in the semiconductor region. A study of the transport properties of the Datta–Das spin field-effect transistor is needed to make more reliable predictions on operation regime for such devices.