Papers

AVS2007 Session MI-ThM: Magnetic Semiconductors I

Thursday, October 18, 2007 8:00 AM in Room 619

Thursday Morning

Start	Invited?	Item
8:00 AM		MI-ThM-1 DMS Ferromagnets: Extrapolating from (III,Mn)V Materials A.H. MacDonald (University of Texas at Austin); T. Jungwirth (Czech Academy of Sciences); J. Sinova (Texas A&M University); J. Kucera, J. Masek (Czech Academy of Sciences) The body of work on (III,Mn)V diluted magnetic semiconductors (DMSs) started during the 1990's achieved a good understanding of the origins of ferromagnetism in these materials, and of the relationship between magnetic properties and the materials science of growth and defects. From the fundamental point of view, (Ga,Mn)As and several other (III,Mn)V DMSs are now regarded as textbook examples of something which is rare, robust ferromagnets with dilute magnetic moments coupled by delocalized charge carriers. Both local moments and itinerant holes are provided by Mn, which makes the systems particularly favorable for realizing this unusual ordered state. Advances in growth and postgrowth-treatment techniques have played a central role in the field, often pushing the limits of dilute Mn-moment densities and the uniformity and purity of materials far beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds, material quality and magnetic properties are intimately connected. I will review¹ some of this progress and use it as a spring board to discuss magnetism in other semiconductors with dilute local moments.² ¹ Tomas Jungwirth et al. Rev. Mod. Phys. 78, 809 (2006). ² Work suported by the Department of Energy under Grant No. DE-FG03-02ER45958.
8:40 AM		MI-ThM-3 Onset of Nonlinear Transport and Two-Level Fluctuation through a Pinned Domain Wall in Patterned Lateral GaMnAs Constrictions S.W. Cho, H.K. Choi, J.S. Lee, T. Hwang, Y.D. Park (Seoul National University, Korea) We report on the electrical transport measurements across clean lateral geometrical constrictions in diluted magnetic semiconductor GaMnAs. Constrictions are realized by e-beam lithography to define SiO_x etch masks to pattern nanometer-sized constrictions without plasma etching processes. DC transport behavior across the nanoconstrictions changes from ohmic to non-ohmic below temperatures corresponding to epifilm T_C. The nonlinear IV characteristics fit well with adapted transport equation accounting for spin-flop processes across the domain wall, analogous to pn junctions.¹ Fits to theory also indicate the domain walls to be smooth and wide, inhospitable to tunneling transport, and supported by magnetoresistance behavior dominated by anisotropic magnetoresistance-like response similar to Giddings et al. observations.² Extending the ‘spin diode’ concept of Vignale and Flatté,³ we conduct a series of dynamic measurements and observe a distinct two-level behavior dependent on bias conditions similar to certain behaviors found in bipolar junctions such as shot noise and random telegraph noise. ¹G. Vignale and M.E. Flatté, PRL 89, 098302 (2002). ²A.D. Giddings et al., PRL 94, 127202 (2005). ³M.E. Flatté and G. Vignale, APL 78, 1273 (2001).
9:00 AM		MI-ThM-4 Intrinsic Vacancy Chalcogenides as Dilute Magnetic Semiconductors: Theoretical Investigation of TM-Doped Ga₂Se₃ I.N. Gatuna, F.S. Ohuchi, M.A. Olmstead (University of Washington) The quest to functionalize semiconductors with additional magnetic properties through synthesis of dilute magnetic semiconductors (DMS) has led to a deeperunderstanding of semiconductor physics and the development of new magneticmechanisms. However, most current DMS materials (e.g., Mn-doped GaAs) are magnetic only well below room temperature, and/or have only limited compatibility with existing silicon electronics. We have investigated transition metal (TM) doping of the intrinsic vacancy semiconductor Ga2Se3 to address both scientific and technical goals. The intrinsic vacancies of this III-VI, zinc-blende-based semiconductor open possibilities for self-compensation as well as supply highly anistropic and polarizable band edge states. Ga2Se3 is also closely lattice matched to Si and may be grown heteroepitaxially on Si with high quality interfaces. Our first principles computations of X:Ga2Se3 (X = Mn, V, Cr, concentrations 5% to 16%) reveal that X atoms hybridize with neighboring Se in the p-d hybridization typical of III-V and II-VI DMS materials. This hybridization spin-polarizes states near the Fermi level in these T =0 calculations, and lowers the energy of the Se lone-pair orbitals that neighbor vacancies, reducing their prominent role in determining the properties of intrinsic Ga2Se3. There are distinct differences between substitution on a vacancy or for a Ga. Anisotropic, hole-like conductivity is predicted when X is located in a Ga site, while for X situated in a vacancy, a half-metallic state with an isotropic conductivity appears likely. Our calculations suggest that Mn offers the best choice for the dopant, perhaps because its 3d5 electronic configuration offers a large (~ 0.5 eV) separation of spin up and spin down states near the Fermi level, reducing the metallic densities of states at the Fermi level for all doping concentrations. The large energy splitting suggests that doped Ga2Se3 may be a suitable material for spintronics applications athigher temperatures than these T = 0 initial calculations. This work was supported by NSF grant DMR 0605601, the Japan Science Promotion International Program, NIMS (Japan) - UW Joint Research Pact and NIMS (Japan) Internal Research Fund.
9:20 AM		MI-ThM-5 Heteroepitaxial Growth and Electronic Structure of Mn:Ga₂Se₃ Thin Films on Si(100):As: Exploration of a Candidate Dilute Magnetic Semiconductor T.C. Lovejoy, E.N. Yitamben (University of Washington); T. Ohta (Lawrence Berkeley National Laboratory); F.S. Ohuchi, M.A. Olmstead (University of Washington) Magnetic thin film semiconductors grown on nonmagnetic semiconductors (NMS) may provide a route to injection of spin polarized electrons into the NMS. The lack of magnetic materials with both a high Curie temperature and spin-preserving transport into electronic materials (e.g., silicon) is currently the primary obstacle to the development of useful spintronic devices. A relatively unexplored class of dilute magnetic semiconductor is transition metal doped III-VI semiconductors. Group III-VI semiconductors such as Ga₂Se₃ have intrinsic vacancies which lead to highly anisotropic growth, and which may lead to a high degree of magnetic anisotropy if the films can be made ferromagnetic through suitable doping. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and photoemission spectroscopy (PES) have shown the addition of small quantities of manganese, about one percent, has a pronounced effect on the growth morphology and electronic properties of Ga₂Se₃ on arsenic passivated Si(100). Co-deposition of Mn and GaSe on Si(100):As results in tall, highly anisotropic, rectangular, Mn-rich islands with edges parallel to the [011] and [0-11] substrate crystal directions. Deposition of a pure Ga₂Se₃ layer before the doped material results in a different and more laminar structure. While islands still form, these islands are much shorter and wider with seemingly random shapes subject to the constraint that every piece of the perimeter lies along the same two crystal directions. These two structures can be easily distinguished by their LEED patterns where the first case shows 1x1 spots with streak features characteristic of the undoped Ga₂Se₃ structure, whereas the second case transitions to clear 1x1 spots with no other features. The effect of small concentrations of Mn on the band structure of thin film Ga₂Se₃ is equally pronounced. The Mn doping adds a new peak in the valence band about 4.2eV below the Fermi level, an area where pure Ga₂Se₃ has a low density of states. This work was supported by NSF grant DMR 0605601. TCL acknowledges support from NSF/NCI IGERT DGE-0504573. Some of the research was pursued at the Advanced Light Source, which is supported by the DOE under contract DE-AC02-05CH11231.
9:40 AM		MI-ThM-6 Investigation of Cr:Ga2Se3 as a Candidate Dilute Magnetic Semiconductor for Silicon Based Applications E.N. Yitamben, T.C. Lovejoy, I.N. Gatuna, F.S. Ohuchi, M.A. Olmstead (University of Washington) The intrinsic vacancy semiconductor Ga2Se3, which may be grown epitaxially on Si, poses several interesting issues for the study of dilute magnetic semiconductors. Substitution of transition metal impurities may occur on either occupied or vacant cation sites in the defected zincblende lattice. For dopants with different valence from the host cation, this may result in self-compensation of donors and acceptors, while an isoelectronic impurity can either add electrons by inserting into a vacancy, or minimally disturb the band structure by replacing a Ga. To probe the interrelationship between magnetism and free carriers in this new class of dilute magnetic semiconductors, we have performed both theoretical and experimental investigations of Cr-doped Ga2Se3 grown epitaxially on Si(001):As. Scanning tunneling microscopy shows nucleation of anisotropic islands, with the area between islands similar to pure Ga2Se3. The size and shape of the islands is dependent both on Cr concentration and on whether or not a pure Ga2Se3 buffer layer is deposited first. Despite the similar intrinsic valence between Cr and Ga, addition of a few percent Cr to Ga2Se3 results in metallic bands with minimal dispersion and leads to significant changes of the Se local environment, as measured with high resolution photoemission spectroscopy. These results may indicate Cr substituting into a vacancy rather than replacing Ga, or possibly creating local areas of CrSe, which computations show to be half-metallic. At higher concentrations, X-ray absorption and photoemission show two distinct Cr environments. This work was supported byt NSF grant DMR 0605601. TCL acknowledges support from NSF/NCI IGERT DGE-0504573. Some of the research was pursued at the Advanced Light Source, which is supported by the DOE under contract DE-AC02-05CH11231.
10:00 AM		MI-ThM-7 Giant Excitonic Zeeman Splittings in Transition Metal Doped CdSe Quantum Dots P.I. Archer, D.R. Gamelin (University of Washington) We report the first direct observation of sp-d dopant-carrier exchange interactions in colloidal doped wurtzite CdSe nanocrystals. Doped diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by thermal decomposition of an inorganic precursor cluster in the presence of TMCl₂ (TM²⁺ = Mn²⁺ or Co²⁺) in hexadecylamine and were characterized by multiple spectroscopic and analytical techniques. Using magnetic circular dichroism spectroscopy, successful doping and the existence of giant excitonic Zeeman splittings in both Mn²⁺- and Co²⁺-doped wurtzite CdSe quantum dots are demonstrated unambiguously.
10:20 AM		MI-ThM-8 Size-Dependent Excited State Dynamics in Mn²⁺-Doped CdSe Quantum Dots R. Beaulac, P.I. Archer, V.A. Vlaskin, D.R. Gamelin (University of Washington) Colloidal Mn²⁺-doped II-VI quantum dots are interesting materials for the study of magnetic and luminescent phenomena in quantum confined semiconductor nanostructures. In recent years, several reports have described luminescence, absorption and magnetism of Mn²⁺-doped ZnS, CdS and ZnSe quantum dots. In general, the emission properties of these nano-scale materials behave much like their bulk counterparts, showing a size insensitive Mn²⁺ ligand-field emission with a long lifetime. In contrast, Mn²⁺-doped CdSe nanoparticles are expected to behave differently from bulk because of the possibility of size-tuning the band-gap energy from below to above the Mn²⁺ emitting level. For this reason, Mn²⁺-doped CdSe offers an interesting opportunity for fundamental studies of quantum confinement effects in doped semiconductors. Curiously, although photoluminescence spectra of self-assembled Mn²⁺ quantum dots prepared by vacuum deposition have been reported, the Mn²⁺ is either absent of only tentatively reported, even for high Mn²⁺ concentrations. Moreover, CdSe excitonic emission is observed despite the fact that the energy gap is greater than the Mn²⁺ excitation energy. We recently presented a new method for preparing colloidal doped CdSe quantum dots.¹ Importantly, these particules show a giant Zeeman splitting of their excitonic transitions, as is expected for diluted magnetic semiconductors. Here we will describe the temperature-dependent photoluminescence of these particles, which gives insight into the energy transfer dynamics in Mn²⁺-CdSe quantum dots. A kinetic model will be described that explains the paradoxical absence of Mn²⁺ emission in Mn²⁺-doped CdSe quantum dots reported previously. ¹ Archer, P. I.; Santangelo, S. A.; Gamelin, D. R., Nano. Lett., 7, 1037-1043 (2007).