Magnetic semiconductors reach room temperature

A variety of proposed devices rely on manipulation of electron spins. Examples range from electro-optical switches to qubits for quantum computing. Many of these devices pair a magnetic "spin-injector" -- a source of electrons with a desired spin -- with a conventional semiconductor material. However, ferromagnetic and semiconducting materials have different crystal structures, lattice parameters, and physical and chemical properties. The interface layer between the two tends to be a magnetic "dead zone. " Spin-injection efficiencies are too low for practical devices.

One alternative approach combines magnetic and semiconducting properties in a single material, known as a dilute magnetic semiconductor (DMS), by doping the semiconductor with magnetic ions. Theoretical calculations predict that doping III-V and II-VI semiconductors with 3-5 at% manganese (Mn) will lead to ferromagnetic ordering. Experiments have confirmed the effect in such compounds as GaMnAs, with magnetic transition temperature (the Curie temperature, T_c) near 100 K (273 K = 0°C = 32°F).

As previously reported, Stephen Pearton and co-workers at the University of Florida recently demonstrated ferromagnetic behavior in GaMnP at temperatures above 300 K. Their results are especially interesting because GaP has a lattice constant close to that of silicon. It might be possible to integrate GaMnP spin-injection layers with established silicon CMOS technologies.

The Florida group used standard molecular beam epitaxy methods to fabricate their samples, after first testing Mn dopant levels by ion implantation. Both x-ray diffraction and high-energy electron diffraction (RHEED) ruled out the presence of ferromagnetic MnP or Mn inclusions. The samples remained single-phased, even with Mn dopant levels as high as 5 at%.

The group's measured Curie temperatures exceed theoretical predictions by a wide margin, giving theorists valuable additional information. Optimizing the film morphology and composition are likely to lead to still higher transition temperatures.