Simulation of Spin Transport, Diffusion and Injection into Semiconductors

Public demand for increasingly faster and smaller electronic devices, such as computers, requires that more and even smaller transistors are packed on every chip. This has led to the birth of nanotechnology and, more recently of the nanotechnology field called 'spintronics'. Here not only the charge, but also the spin -- another fundamental property of electrons and holes -- is used to design device functionalities. Among the potential benefits of spintronics devices is the possibility of computers in which the same unit is used for computation and storage, of lower power consumption, of miniaturisation, and more generally the possibility of designing conceptually new devices which mix old functionalities with completely new ones.The first commercial application of a spintronics-related effect was the read heads for magnetic hard disk drives by IBM in the 90s, which increased the storage on a disk drive to tens of gigabytes and have since then secured a market of billions of dollars. It is clear that every development in this fascinating field is important to the specialists as well as of the general public. The basis of spintronics is understanding the spin dynamics. Unfortunately key issues such as how to inject a current of spins in a semiconductor, how to sustain it across the interfaces of the different materials forming the devices, which materials/nanostructures are best and what lengths a current of spin can travel in a specific material are still open questions. This project aims to master the principles underlying the spin dynamics, with particular attention to applications such as nanocircuits and their components. My objectives are to fully understand spin transport, diffusion and injection into semiconductors. These properties are fundamental for developing semiconductor and hybrid (metal/semiconductor) spintronics devices. The problem of electrical spin injection into semiconductors (fundamental when thinking of an electronic circuit) is still open, as well as the understanding of the role played by many-body interactions in spin transport and dynamics. In this context the ability of performing computer simulations of these phenomena is of paramount importance. Simulations can in fact be thought as ideal experiments, which allow a systematic analysis of the system response even for conditions (e.g. extreme temperatures or defect-free or exotic materials) which are too difficult or expensive to reproduce in the lab. I plan to develop computer simulation codes using two powerful techniques, non-equilibrium Monte-Carlo and spin-dependent drift-diffusion equation techniques and apply them to spin-transport in bulk and through semiconductor heterostructures, to the study of many-body interaction effects, and, ultimately, to device components.In order to do so, I will need to develop and adapt the simulation techniques themselves to the specific problems as well as to develop the underlying spin-transport theory, for example in relation to many-body effects such as the spin Coulomb drag which I predicted in 2000 and was observed experimentally in 2005.In the second part of the project, I plan to combine non-equilibrium Monte-Carlo techniques with the atomistic model: the merger of these two techniques will allow me to analyse the effects of magnetic impurities and nanostructures with an accuracy never reached so far. My aims are to improve our understanding of spin-dynamics (especially many-body interaction effects), to solve the puzzle of electrical spin-injection into semiconductors and ultimately to design basic elements for spintronics circuitry components.