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       Basics: Dilute Magnetic Semiconductors

Conventional electronics manipulate electronic charges. An intriguing alternative is the field of “spintronics”, wherein the manipulation of the electronic spin in semiconductor devices gives rise to new possibilities for a variety of applications, such as non-volatile memory, quantum computing and communication in the solid state, magneto-optical communication devices. A crucial element for the success of spintronics is to find a proper material that combines the desirable properties of ferromagnets and semiconductors. Diluted magnetic semiconductors (DMS) also referred to as semimagnetic semiconductors, are alloys whose lattices are made up in part of substitutional magnetic atoms. In contrast to magnetic semiconductors, DMS offer the possibility of studying magnetic phenomena in crystals with a simple band structure and excellent magneto-optical and transport properties. Existing techniques for semiconductor heterostructure formation enable incorporation of DMS layers into transistors, quantum wells and other electro-optical devices in which the spin splitting can also be tuned by the confinement energy and the size quantization. A key goal in this field is achieving room temperature ferromagnetism in the DMS. Recent work has demonstrated injection of spin-polarized electrons into the semiconductor and ferromagnetic order temperatures of 110K in III-V-based. The most challenging task for broad applications is to find magnetic or diluted magnetic semiconductors which would operate at room temperature. GaN and ZnO appear to be the most promising materials to have Curie temperature at and above room temperature, but the low solubility of transition metal impurities in GaN, incorporation of transition metal ions is a considerable challenge. Zinc oxide (ZnO), on the other hand, is a most promising DMS material. It is a wide bandgap (~3.3 eV) semiconductor, which has received increasing attention due to its broad applications and its many desirable material properties.


       Recent Highlights: Dilute Magnetic Semiconductors

Room Temperature Magnetism in Fe-Implanted ZnO nanopillars
Using the appropriate parameters during MOCVD growth results in the formation of single crystal ZnO nanopillars (~ 500 nm tall, 50 nm diameter) oriented with their c-axis parallel to the rod axis.  Ion implantation was used to dope these nanorods with Fe at the ~ 3% level.  As seen in the electron micrograph below, the implantation process leads to a pronounced sharpening of the nanopillars as well as the introduction of Fe impurities.  Room temperature (RT) magnetization measurements [M(H) curves] obtained before (upper panel) and after (lower panel) annealing (700C, 10 min.) show that RT ferromagnetism is present in both cases, but the saturation magnetization after annealing is only ~ 20% of the as-implanted value.

 

The lower panels show TEM images of an individual nanopillar before (a) and after (b) annealing, where the Fe/Zn ratio, determined by X-ray emission, is shown in false color.  Panels (c) and (d) show the indicated concentration cross sections.  It is clear that diffusion-related redistribution of the Fe ions is directly correlated to the ferromagnetic response of these nanoscale systems.  Systematic studies of transition metal diffusion in these important potential room temperature spintronic materials are in progress.

Read more: Room temperature ferromagnetism in Mn ion implanted epitaxial ZnO films
 D.H.Hill, D.A. Arena, R.A. Bartynski, P. Wu, G. Saraf, Y. Lu, Wielunski, R. Gateau, J. Dvorak, A. Moodenbaugh, and Y.K. Yeo, Physica Status Solidi A, 203, 3836 (2006)

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