Holes in Silicon Hold On to Their Spin

Abstract

New experiments show that holes in silicon can transport spin over surprisingly large distances, adding p-type semiconductors to the toolkit of materials for spintronics. In spintronic devices [1–3], digital information is repre-sented by the direction of the quantum-mechanical spin of the charge carriers. A spin pointing in one direction denotes a logical 0, whereas a spin pointing in the op-posite direction represents the logical 1. Spintronic de-vices are almost exclusively fabricated out of n-type semi-conductors (electrons carry the current) as opposed to p-type semiconductors (holes carry the current), which may seem surprising since both electrons and holes have spin. The reason is that holes have been assumed to be unable to preserve their spin polarization over distances longer than a few tens of nanometers. This perspective is changing, as several recent experiments have shown that hole spins in p-type silicon can be polarized and retain their polarization for a surprisingly long time. However, experiments that directly probe the spin of the holes as they travel through the material have been lacking. Eiji Shikoh at Osaka University, Japan, and colleagues have now performed such an experiment [4]. Writing in Physical Review Letters, Shikoh et al. use a new ap-proach to show that holes in p-type silicon can preserve spin-based information and transport it over distances much longer than previously thought. Taken together, the new work and the previous experiments support the view that spin transport is realistic in p-type semicon-ductors. This opens the door to developing spintronic devices and circuits that exploit the unique features of p-type semiconductors and their combination with n-type materials. One of the challenges for spintronics is that semicon-ductors are not ferromagnetic: in equilibrium, the spins randomly point in all directions. In order to study the behavior of spins in semiconductors and exploit them to perform computations, one has to give a population of spins a preferred orientation such that its average is nonzero. But as soon as the source of spin generation is turned off, the spin polarization decays back to zero. The