Spindynamics in Semiconductors


The computation power of modern PC's has rised exponentially during the decades after the invention of the first semiconductor transistor. However data processing using only electrical charges is facing fundamental restrictions as the device dimensions shrink to dimensions where quantum effects start to dominate. One possible alternative is to use the spin degree of electrons or holes as a future representation of information. This is exploited most simply in conventional magnetic hard drives. More recently, the strong dependence of the current through magnetic multilayers on external magnetic fields (GMR Effect, Nobel Price 2007 Grünberg/Fert) has led e.g. to the development of new magnetic read heads in hard drives as well as the research field of Magnetoelectronics.

In Semiconductors, the electron spin has not been in the focus of interest since semiconductors do not, like magnets, intrinsically possess a magnetic moment. On the other hand, an electron, once brought into the semiconductor loses its original spin orientation much slower than in a magnet. This makes semiconductors interesting for spin manipulation, a key ingredient for future "spin electronics" (short: Spintronics).

In this project, we investigate the average spin of conduction electrons (i.e. Spin Polarisation) in bulk GaAs. The lack of inversion symmetry, combined with spin orbit interaction leads to a decay of the spin polarisation on the timescale of pico- to nanoseconds. Depending on doping, bandstructure properties and temperature mainly three processes prevail: the Elliot-Yafet (EY), the D'Yakonov-Perel (DP) and the Bir-Aronov-Pikus (BAP) process.

For p-doped GaAs, the latter is the most relevant one. BAP is determined by electron-hole exchange scattering. For comparison, the bulk scattering rate was determined by Faraday measurements to be in the order of 60 ps. Our photoemission experiments showed a strong energy dependence of the polarization decay time, contradicting the results of boltzmann simulations for bulk GaAs. This discrepancy can be explained by the surface sensitivity of photoemission experiments: at the surface a downward band bending occurs. Thus, electrons relaxing to lower energies become more localised. On the other hand, holes are driven away from the surface by the electric field gradient. Thus the effective hole density for surface electrons is reduced and the BAP loses efficiency, leading to longer decay times (see figure).




These results emphasizes the importance of surface effects for the application of semiconductors in future spintronics devices. By using Spin-and Time-Resolved 2PPE, we are able to explain and maybe show ways to overcome many surface related difficulties concerning spin injection in composite device structures.