Research Area: Magnetic semiconductors

Research Area Summary:   This research focuses on dilute magnetic semiconductors, a new class of materials in which magnetic dopants are used to create ferromagnets from normal semiconductors such as GaAs. These materials, which offer the advantages of semiconductors combined with the non-volatile properties of magnetic materials, are the materials foundation for future "spintronics" technologies.

In 2001, Jonker and coworkers at NRL developed the first ferromagnetic semiconductor based on an elemental host, Mn-doped germanium. To help understand the origins of ferromagnetism in this material, we used density-functional theory to compute the effective coupling strength between Mn spins as a function of their separation (see "Further Reading" below). Interestingly, this coupling is antiferromagnetic for nearest-neighbor Mn atoms, but ferromagnetic for most separations greater than this. To calculate the Curie temperature (as a function of Mn concentration), we then used a simple percolation theory based on our computed coupling strengths. The predicted Curie temperature increases approximately linearly with Mn concentration, in agreement with experiment, but the absolute temperatures are too large by roughly a factor 4-5.

FIG. 1.   Potential energy surface for Mn adsorption on GaAs(001), plotted in a plane normal to the surface and containing the As surface dimer. The minimum energy adsorption site is the subsurface interstitial site labeled i; the corresponding surface geometry is shown (light gray for As, dark gray for Ga, yellow for Mn). Typical adsorption pathways funnel Mn adatoms into the interstitial site (heavy curves) or a cave site c (light curves). Inset: Binding energy of a Mn adatom centered on the As-dimer; for comparison, results are also shown for a Ga adatom. When additional As is deposited, the metastable substitutional site, s, becomes more favorable and leads to partial incorporation of substitutional Mn.

The reason for this overestimate is two-fold. First, the local-density approximation to density-functional theory generally overestimates spin coupling strengths by similar factors. Second, the actual materials (both Mn-doped Ge and Mn-doped GaAs) are found to be strongly compensated: that is, their measured hole concentrations are much lower than expected based on the measured Mn concentrations. Recently we proposed that the source of this compensation is Mn in interstitial sites (rather than substitutional sites). We have shown that even though the substitutional site is strongly preferred in the bulk crystal, during the growth process Mn atoms can follow a very low-energy pathway starting from the gas phase and come to rest at a subsurface interstitial site (see Fig. 1). In the bulk crystal environment interstitial Mn is an electron donor, with each interstitial Mn compensating two substitutional Mn.

We are also conducting theoretical research on other types of magnetic semiconductor systems. Our current projects include: a computational search for promising new ferromagnetic semiconductors in the chalcopyrite family; a first-principles investigation of the structure of Fe/GaAs interfaces; a study of Co-doped TiO₂, a relatively new dilute magnetic semiconductor with Curie temperatures reported to be 700 K or higher; studies of magnetic impurities in diamond; and a theoretical assessment of spin injection from the magnetic semiconductor CdCr₂Se₄ into semiconductors such as Si and GaAs.

Further Reading:  

• Thermodynamics of Carrier-Mediated Ferromagnetism in Semiconductors  A.G. Petukhov, I. Zutic, and S.C. Erwin. Physical Review Letters (December 21, 2007)
  
• Spin Injection and Detection in Silicon  I. Zutic and S.C. Erwin. Physical Review Letters (July 14, 2006)
  
• Determination of Interface Atomic Structure and Its Impact on Spin Transport Using Z-Contrast Microscopy and Density-Functional Theory.    T.J. Zega, A.T. Hanbicki, S.C. Erwin, I. Zutic, G. Kioseoglou, C.H. Li, B.T. Jonker, and R.M. Stroud. Physical Review Letters (May 16, 2006)
• Tailoring ferromagnetic chalcopyrites  S.C. Erwin and I. Zutic. Nature Materials (June 6, 2004) [see also this News and Views article by S. Picozzi]
  
• Cross-sectional STM of Mn-doped GaAs: Theory and experiment  J.M. Sullivan, G.I. Boishin, L.J. Whitman, A.T. Hanbicki, B.T. Jonker, and S.C. Erwin. Physical Review B (December 23, 2003)
  
• Predicted absence of ferromagnetism in manganese-doped diamond  S.C. Erwin and C.S. Hellberg. Physical Review B (December 30, 2003)
  
• Electrical spin injection and transport in semiconductor spintronic devices    B.T. Jonker, S.C. Erwin, A. Petrou, and A.G. Petukhov. MRS Bulletin (October 10, 2003)
  
• Theory of dopants and defects in Co-doped TiO2 anatase   J.M. Sullivan and S.C. Erwin. Physical Review B (April 18, 2003)
  
• Self-Compensation in Manganese-Doped Ferromagnetic Semiconductors   S.C. Erwin and A.G. Petukhov. Physical Review Letters (November 25, 2002)
  
• First-principles study of nucleation, growth, and interface structure of Fe/GaAs  S.C. Erwin, S.-H. Lee, and M. Scheffler. Physical Review B (May 22, 2002)
  
• A Group-IV Ferromagnetic Semiconductor: Mn(x)Ge(1-x)  PDF (109 kB)  Y.D. Park, A. Hanbicki, S.C. Erwin, C.S. Hellberg, J.M. Sullivan, J.E. Mattson, T.F. Ambrose, A. Wilson, G. Spanos, and B.T. Jonker. Science (January 25, 2002)
  

Point of contact:   Steven.Erwin@nrl.navy.mil (Privacy Advisory)