Research Area: Magnetism in semiconductors

Research Area Summary:   This research focuses on semiconductors that are rendered magnetic by either intrinsic or extrinsic effects. 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.

FIG. 1.   Ground state structure and lowest energy spin configuration of a magnetic silicon surface, Si(553)-Au. Yellow atoms are Au, all others are Si. Each terrace contains a Au double row and a graphitic Si honeycomb chain (green) at the step edge. Every third Si (red, blue) at the step has a spin magnetic moment of one Bohr magneton (arrows) from the complete polarization of the electron occupying the dangling-bond orbital. The sign of the polarization alternates along the step. The periodicity along the step is tripled by a small downward displacement of the magnetic atoms. The periodicity along the two Au rows is doubled, with alternatingly rotated Au-Au bonds.

The idea of creating magnetism in a nonmagnetic material by manipulating its structure has long intrigued scientists. The hope of realizing this idea in silicon has been widely discussed for decades, but so far none of these speculations has held up under scrutiny.

Silicon provides a unique entry point for combining magnetoelectronics based on single spins with standard electronics technology. If a single-spin device can be built on a silicon wafer, input and output electronics can be directly integrated with the magnetic part of the device. This has been an obstacle for current spintronics approaches. For example, spin injection from a metal into silicon is very inefficient unless the metal/semiconductor interface is carefully optimized.

Our predictions have the advantage that nature itself guides, by a self-assembly process, the formation of long chains of polarized electron spins with atomically precise structural order. This integration of structural and magnetic order is crucial for future technologies based on single spins at the atomic level. The magnetic silicon surfaces, one of which is illustrated in Fig. 1, naturally form steps which are stabilized by chains of gold atoms (yellow). According to the our calculations, some of the silicon atoms at the step edges have unpaired electrons that are fully spin polarized and probably magnetically ordered at sufficiently low temperatures.

The atom chains on the Si(553)-Au surface were discovered in the group of Franz Himpsel at the University of Wisconsin-Madison. Several other groups worldwide have been investigating such "one-dimensional" silicon surfaces in recent years.

FIG. 2.   Theoretical scanning-tunneling microscopy topography showing the tip height at constant current for tunnelling into empty surface states of Si(553)-Au at bias voltage +0.5 V. Coexisting tripled and doubled periodicities are visible along the Si step edge and Au chain, respectively.

This theoretical work suggests several experiments, such as spin-polarized scanning tunneling microscopy (STM), to test their predictions directly. But there is already indirect experimental evidence to support the possibility of magnetism at silicon surfaces. Two research groups, at Yonsei University in Korea and at Oak Ridge National Laboratory in the US, have found that Si(553)-Au develops periodic "ripples" with two different periodicities at low temperatures. One ripple occurs along the silicon step edges with three times the normal periodicity, and the other along the gold chains with two times the normal periodicity. The theoretically predicted STM image, shown in Fig. 2, reproduces this pattern perfectly. Moreover, this pattern only emerges when magnetism is allowed in the calculation. When magnetism is "turned off" in the theory, the ripples completely vanish. Thus the observation of threefold and twofold ripples offers indirect - if preliminary - confirmation of magnetism.

Linear chains of spin-polarized atoms provide atomically perfect templates for the ultimate memory and logic, in which a single spin represents a bit. One potential application is a "spin shift register" recently proposed theoretically by Gerald Mahan, a theoretical physicist at Pennsylvania State University. Another application is the storage of information in single magnetic atoms. Our work also predicts that the magnitude, and even the sign, of the spin coupling can be changed by doping electrons or holes into surface states. The closely related Si(111)-Au surface can be electron-doped by adsorbates (for example, silicon adatoms) on the surface. By varying this adsorbate population one can perform band-structure engineering with extraordinary precision. The possibility of tuning surface magnetism on Si(553)-Au and its relatives using surface chemistry suggests a fascinating new research direction.

Further Reading:  

• Intrinsic magnetism at silicon surfaces  S.C. Erwin and F.J. Himpsel. Nature Communications (August 24, 2010)
  
• Structure and energetics of Si(111)5x2-Au  S.C. Erwin, I. Barke, and F.J. Himpsel. Physical Review B (October 2, 2009)
  
• Self-Doping of Gold Chains on Silicon: A New Structural Model for Si(111)5x2-Au  S.C. Erwin. Physical Review Letters (November 10, 2003)
  
• Theory of the "Honeycomb Chain-Channel" Reconstruction of M/Si(111)-(3x1)   S.C. Erwin and H.H. Weitering. Physical Review Letters (September 14, 1998)
  

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

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. 3.   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. 3). 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)