Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L10: Focus Session: Magnetic Impurities in Semiconductors |
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Sponsoring Units: DMP GMAG Chair: Jacek Furdyna, University of Notre Dame Room: LACC 153B |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L10.00001: Driving Mn into GaAs with an STM: Probing a Mn Acceptor Anthony Richardella, Dale Kitchen, Ali Yazdani Using a low temperature scanning tunneling microscope (STM), we can induce an individual Mn adatom to substitute for a Ga atom in the GaAs (110) surface. The Mn atom occupies the Ga site while the Ga atom comes to the surface as a weakly bound adatom. Using STM manipulation the Ga adatom is moved away, leaving a Mn in its substitution position in the (110) surface layer. A Mn atom, in this configuration, gives rise to a strong in-gap level as probed by spatially-resolved STM spectroscopy measurements. The Mn-induced in-gap state has an unusual highly anisotropic spatial structure, which shares some characteristic features with other p-type dopants. The influence of tip-induced band bending on the spectroscopic measurements will be discussed. This talk will provide background for the talk following by Kitchen, \textit{et al}. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L10.00002: Controlling Interactions between Mn Acceptors in the GaAs (110) Surface Dale Kitchen, Anthony Richardella, Ali Yazdani Low temperature scanning tunneling microscopy (STM) is used to control the substitution of individual Mn adatoms into Ga sites in the GaAs (110) surface. In a Ga site, a Mn atom gives rise to a strong in-gap level with highly anisotropic character as probed by spatially-resolved STM spectroscopy measurements. Modifications to this in-gap resonance can occur when two Mn acceptors interact. The interaction of Mn acceptors depends upon both orientation as well as spacing, leading to strong bonding/antibonding-like states under certain configurations. Such measurements of interacting pairs of Mn can potentially provide information on their spin orientation. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L10.00003: Mechanism for electric field driven single spin manipulation of a Mn dopant in GaAs Jian-Ming Tang, Jeremy Levy, Michael E. Flatt\'e We show that the spin orientation of a $J=1$ (Mn + hole) complex in GaAs can be manipulated using only electrical control fields. The spin degeneracy of the compound spin can be split by a dc electric field due to inversion symmetry breaking. The resonances, corresponding to transitions between the split levels, can be driven by an ac electric field. As the bound hole has an anisotropic shape that depends on the compound spin orientation, we propose that the resonance of a single spin can be detected in the tunneling current with scanning tunneling microscopy. The visibility of the resonance is high, as the total (not spin-resolved) local density of states can change as much as 90\% for sites near the Mn dopant as the compound spin orientation is changed. This work is supported by ARO MURI DAAD19-01-1-0541 and DARPA QuIST DAAD-19-01-1-0650. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L10.00004: Spatial structure of single and interacting Mn acceptors in GaAs Invited Speaker: Ferromagnetic semiconductors such as Ga$_{1-x}$Mn$_{x}$As are receiving a lot of attention at the moment because of their application in spintronic devices. However, despite intense study of deep acceptors in III-V semiconductors such as Mn$_{Ga}$, little information has been obtained on their electronic properties at the atomic scale. Yet the spatial shape of the Mn acceptor state will influence the hole-mediated Mn-Mn coupling and thus all of the magnetic properties of ferromagnetic semiconductors such as Ga$_{1-x}$Mn$_{x}$As. This study presents an experimental and theoretical description of the spatial symmetry of the Mn acceptor wave-function in GaAs. We present measurements of the spatial mapping of the anisotropic wavefunction of a hole localized at a Mn acceptor. To achieve this, we have used the STM tip not only to image the Mn acceptor but also to manipulate its charge state A$^{0}$/A$^{-}$ at room temperature. Within an envelope function effective mass model (EFM) the anisotropy in the acceptor wave-function can be traced to the influence of the cubic symmetry of the GaAs crystal which selects specific d-states that mix into the ground state due to the spin-orbit interaction in the valence band. Comparison with calculations based on a tight-binding model (TBM) for the Mn acceptor structure supports this conclusion. Using the same experimental and theoretical approach we furthermore explored the interaction between Mn acceptors directly by analyzing close Mn-Mn pairs, which were separated by less than 2 nm. We will discuss some implications of these results for Mn delta-doped layers grown on differently oriented growth surfaces. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L10.00005: The onset of Mn monomer and dimer adsorptions on GaAs(110) Jue-Xian Cao, Xin-Gao Gong, Lu Yu, Ruqian Wu Using the density functional VASP and FLAPW approaches, we studied the onset of Mn adsorption on GaAs(110). Large unit cells were used to explore the limit of monomer and dimer adsorbates. We found strong interplay between the magnetization and adsorption geometry, including substitution of Mn on the surface Ga sites. For Mn dimers, the nearest Mn-Mn distance is 8.1 {\AA}, a result which agrees with recent STM observations. These results are explained from electronic structures and lay a basis for further understanding of the mechanism of growth and magnetic ordering in(Ga,Mn)As, a prototype dilute magnetic semiconductor for spintronics applications. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L10.00006: Luminescence properties of GaAs / AlGaAs quantum wells doped with Mn in the extreme dilution limit Mark White, Pierre Petroff Isolated Mn atoms in bulk GaAs have six degenerate spin states. This degeneracy will be lifted when a Mn impurity is located in a quantum well or quantum dot. AlGaAs / GaAs quantum wells have been grown via molecular beam epitaxy to investigate the optical properties of the included Mn impurities. Normal GaAs growth temperatures (590 C) are used except during low-temperature depositions (258 C) of sub-monolayer quantities of Mn. Drift and diffusion of Mn atoms through an AlGaAs barrier are exploited to dope the quantum well with Mn during the growth. Micro-photoluminescence imaging and spectroscopy indicates spatially localized emission centers consisting of multiple lines. The origin of these spectrally sharp lines will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L10.00007: Electronic states in Mn4+ ions in p-type GaN Bing Han, Mel Ulmer, Bruce Wessels There is interest in Mn doped GaN for high Tc ferromagnetic semiconductors. However, Mn in GaN forms deep levels in the band gap. In this study GaN was doped with both Mn and Mg to increase its p-type conductivity. The electronic states of manganese in $p$-type GaN were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra $d$-shell transition $^{4}T_{2}$(F)-$^{4}T_{1}$(F) of Mn$^{4+}$ ions$.$ PLE spectrum of the Mn$^{4+}$ [$^{4}T_{2}$(F)-$^{4}T_{1}$(F)] luminescence reveals intra center excitation processes via the excited states of Mn$^{4+}$ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell $^{4}T_{1}$(P)-$^{4}T_{1}$(F) and $^{4}A_{2}$(F)-$^{4}T_{1}$(F) transitions of Mn$^{4+}$, respectively. In addition to the intra shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn$^{4+/3+}$ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn$^{4+/3+}$ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using \textit{ab initio} calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in $p$-type GaN:Mn when the Fermi energy is less than 1.11 eV above the valence band maximum. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L10.00008: SIC-LSD study of transition metal valencies in oxide materials Leon Petit, Thomas Schulthess, Axel Svane, Anderson Janotti, Zdzislawa Szotek, Walter Temmerman The electronic and magnetic properties of transition metal (TM) oxide materials are largely determined by the degree of localization of the TM d-electrons. With the self-interaction corrected (SIC) local spin density (LSD) approximation, we are able to differentiate between various localization/delocalization scenarios based on total energy considerations, and thus to determine the ground state valency onfiguration from the global energy minimum. Using the SIC-LSD, we studied the valencies of TM (Co, Mn) impurities in ZnO. We find the position of the TM(0/+) donor level to be such that the TM$^{2+}$ configuration is energetically most favourable both in n-type ZnO, and in ZnO without additional codopants, whilst in p-type ZnO one additional d-electron prefers to delocalize with the resulting TM$^{3+}$ groundstate configuration. We furthermore investigated the possibility of ferromagnetic order in the corresponding groundstate scenarios. Work supported in part by the Defense Advanced Research Agency and by the Division of Materials Science and Engineering, US Department of Energy. The Oak Ridge National Laboratory is managed by UT-Battelle LLC for the Department of Energy under Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L10.00009: Diluted Magnetic Semiconductors based on Cr-doped InN Rekha Rajaram, A Ney, R.F.C. Farrow, J.S. Harris, Jr., S.S.P. Parkin Diluted magnetic semiconductors based on GaN have been widely studied following theoretical work that predicts room temperature ferromagnetism in these materials. However, there have been no reports yet of InN-based DMS materials. Here, we have studied the InN system doped with Cr and Mn. The films were grown on c-plane sapphire substrates by Plasma- Assisted MBE using a MOCVD-grown GaN buffer layer. X-Ray Diffraction rocking curve measurements confirmed single crystalline orientation with the FWHM of InN (0002) X-Ray rocking curve about 0.1\r{ }. PL measurements revealed a room-temperature bandgap of 0.8 eV and Hall measurements yield an n-type carrier concentration of about 5*10$^{19 }$cm$^{ -3}$ in both undoped InN as well as Cr-doped InN. While XRD measurements of the Mn-doped films reveal secondary phase formation, the Cr-doped films show no evidence of the formation of compounds such as CrN or Cr$_{2}$N. Magnetic measurements were made on the films using superconducting quantum interference device (SQUID) magnetometry. While the Cr-doped films show a clear magnetic hysteresis as well as remanence up to 300K, the Mn-doped films have less clear magnetic properties. Thus we present evidence of ferromagnetism in n-type, Cr-doped InN. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L10.00010: Influence of Mn Distribution on Ferromagnetism in Magnetic Semiconductor Mn$_x$Ge$_{1-x}$ A.P. Li, C. Zeng, L.C. Feldman, J.F. Wendelken, N. Rao, J. Shen, H.H. Weitering The ferromagnetism of Mn-doped Ge, grown with molecular beam epitaxy, is studied by controlling Mn distributions in the films via post-annealing and digital doping techniques. Randomly doped Mn$_{x}$Ge$_{1-x }$films exhibit a high concentration of Mn trapped at interstitial sites in Ge, and reveal two ferromagnetic transitions at $T_{C}$* and \textit{Tc}, respectively. A strong correlation between magnetic and transport properties is observed both at $T_{C}$* and $T_{C}$. Upon annealing as-grown films at a low-temperature, some interstitial Mn atoms are driven toward the surface of the film and even to the substitutional sites of Ge, as predicted by a theory and revealed by ion channeling and x-ray photoemission spectroscopy. This Mn redistribution leads to a large increase in ferromagnetism with both $T_{C}$* and $T_{C}$ shifting toward higher temperatures. Spatial control of Mn atoms along the growth direction is achieved in a Mn$_{x}$Ge$_{1-x}$/Ge digital heterostructure. Ferromagnetism enhancement is also observed in digital structures as compared to randomly doped material with same nominal $x$. The ferromagnetism variation is studied by changing undoped Ge spacer layer thickness and $x$ in doped Mn$_{x}$Ge$_{1-x}$ layer. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L10.00011: Above Room Temperature Ferromagnetism in Mn-implanted Si Martin Bolduc, Chaffra Awo-Affouda, Andy Stollenwerk, MengBing Huang, Frank Ramos, Vincent P. LaBella Utilizing the spin of the electron in semiconductor devices holds great potential to provide novel device structures. The integration of ferromagnetic materials into conventional semiconductors is necessary to achieve spintronic devices. Ion implantation is an attractive means for the fabrication of diluted magnetic semiconductors by integrating magnetic materials into existing CMOS electronic devices. We demonstrate that p-doped and n-doped Si crystals can be made ferromagnetic above room temperature through Mn-ion implantation. 300-keV Mn$^{+}$ ions were implanted at dose of (1-10)X10$^{15}$ cm$^{-2}$ reaching peak concentration of (0.1- 0.8) at.{\%} as measured through SIMS profiling. Ferromagnetic hysteresis loops were obtained using a SQUID magnetometer at temperature of (10-300) K, yielding a saturation magnetization of 0.1-0.3 emu/g-sample. The Curie temperature is found $>$400 K with carrier concentration dependence. The crystal structure has been investigated by RBS in the channeling mode and by TEM cross-section images analysis. In this study, we will report the effects of Mn concentration and post implantation annealing on the strength of the ferromagnetism and on the crystal composition. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L10.00012: Electronic structure and magnetic properties of transition-metal doped Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$ for diluted magnetic semiconductors Paul Larson, Walter Lambrecht The semiconducting tetradymite-structure materials Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$ serve as the basis for high-performance room-temperature thermoelectric devices. Recently, it was found that these materials act as diluted magnetic semiconductors (DMS) with T$_{c} \sim$ 10 K using a few percent doping of transition metal atoms ($T$ = Ti, V, Cr, Mn, Fe). Electronic structure calculations have been performed using the full-potential linear muffin-tin orbital (FP-LMTO) method to understand these materials magnetic properties. The $T$ atoms substitute at the much larger Bi/Sb sites which leads to isolated atomic-like states with very little crystal-field splitting and approximately 3+ valence. This leads to a high spin state with the magnetic moments essentially following Hund's rule. The position of the $T$ 3$d$ states in the band gap will be investigated by analysis of the density of states (DOS). The effects of lattice relaxation and the magnetic interaction of $T$ atoms in the unit cell will also be investigated. [Preview Abstract] |
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