Spin Electronics: High Temperature Ferromagnetic Semiconductor Materials and Devices

Northwestern University, Evanston IL

Investigators

Abstract

This grant has been co-funded by the Division of Electrical and Communications Systems, and the Division of Chemical and Transport Systems in the Engineering Directorate. This proposal was received in response to the Spin Electronics for the 21st century initiative, Program Solicitation NSF 02-036. The proposal focuses on diluted magnetic semiconductors (DMS), which are promising materials for future electronic and opto-electronic devices that utilize both the charge and the spin of electrons. These materials have potential uses in spin valves, non-volatile magnetic random access memories, quantum computation and other spin polarized transport and optical devices. Much of the recent work has focused on the III-V DMS which have shown excellent low temperature ferromagnetic properties. DMS substitutional III-V alloys with Curie temperatures of 105 K have been realized Their utilization in thin film heterostuctures, however, has been limited because of difficulties in preparing materials with both a high Curie temperature and magnetization. Nevertheless, recent advances in the epitaxy of ferromagnetic semiconductors indicate that III-V materials with high Curie temperatures should be realizable. Furthermore other semiconductor systems including the pseudo III-V compounds II-IV-V2 chalcopyrites have been recently shown to have high Curie temperatures. The proposed work builds upon our recent demonstration of InMnAs alloys with transition temperatures in excess of 300 K prepared by metal-organic vapor phase epitaxy (MOVPE) as well as our discovery of high Tc II-IV-V2 compounds. MOVPE enables the preparation of III-V DMS with high magnetization not possible by bulk techniques. Specific systems to be investigated include: InMnAs, and II-IV-V2 compounds ZnMnP2 and MnGeP2. While these semiconductors already show considerable promise, the nature of the magnetic species is not well understood. In the proposed program DMS crystals, thin films and quantum structures will be deposited with a range of magnetic ion concentrations to optimize both the Curie temperature and magnetization. The relationship between the atomic structure and magnetic properties will be determined and compared to theory. The highly accurate fully linearized augmented plane wave (FLAPW) method will be used for these calculations. Of specific interest is the extent to which magnetic ion clustering occurs and their role in the formation of alloys with high Curie temperatures. Experimental techniques to be used included high resolution transmission electron microscopy, extended x-ray fine structure analysis (EXAFS), temperature and field dependent magnetization measurements, Hall effect and magneto-optical measurements. Several thin film spintronic electronic and opto-electronic heterostructures will be fabricated and their spin dependent properties measured. The program will involve the training of both doctoral graduate and undergraduate students in magnetic semiconductor materials and device research. Collaborations between Sandia National Laboratories and Northwestern. University will be further developed. International collaboration with U. of Ulsan, South Korea will be undertaken.