Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K12: Focus Session: Magnetic Thin Films and Narrow Gap Semiconductors |
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Sponsoring Units: DMP DCMP Chair: Juliet Correa, University of Muenster Room: Baltimore Convention Center 304 |
Tuesday, March 14, 2006 2:30PM - 3:06PM |
K12.00001: Subsurfactant Epitaxy for Ferromagnetic Semiconductors Invited Speaker: In this talk, I will discuss a discovery that is expected to have due impacts in two forefront areas of materials research. First, I will introduce ``subsurfactant epitaxy'' as a novel kinetic pathway toward the synthesis of non-equilibrium structures and materials [1, 2]. The discovery of subsurfactant growth is of broad fundamental- and paramount practical interest, particularly within the context of doping functional materials. Secondly, we have successfully applied this conceptual advance to alleviate a major bottleneck problem in spintronics, namely how to fabricate dilute magnetic semiconductors with ferromagnetic ordering temperatures that exceed room temperature by a comfortable margin while minimizing the structural disorder and inhomogeneities that are intrinsic to high doping levels. Here, we demonstrate that controlled Mn doping of Ge via subsurfactant epitaxy produces ferromagnetic ordering temperatures well above room temperature at doping levels that are at least 20 times lower than those typically invoked in other fabrication techniques. Detailed comparison between samples grown by conventional MBE [3-5], and samples grown via the subsurfactant procedure [2] indicates the vastly superior structure-property relationship of the latter. Subsurfactant epitaxy thus stands as a new and powerful avenue toward superior dopant control in dilute magnetic semiconductors and potentially other semiconductor applications that require doping levels far above the thermodynamic solubility limit. Work done in collaboration with M. Chisholm, L.C. Feldman, A.P. Li, J.R. Thompson, C. Zeng, and Z.Y. Zhang. Funded by NSF-DMR-0306239. ORNL is managed by UT-Battelle, LLC, for the U.S. DOE under constract No. DE-AC-5-00OR22725 [1] W. Zhu et al., Phys. Rev. Lett. 93, 126102 (2004). [2] C. Zeng et al., submitted [3] A.P. Li et al, Appl. Phys. Lett. 86, 152507 (2005) [4] A.P. Li et al., Phys. Rev. B 72, 195205 (2005) [5] Y.D. Park et al., Science 295, 651 (2002) [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K12.00002: Manganese silicide thin films on Si(001). Michael Krause, Andrew Stollenwerk, Vincent LaBella Theoretical considerations suggest that epitaxial MnSi thin films should order ferromagnetically on Si(001). This substantiates the hope that the Mn-Si system might be utilized as spin injectors through a Schottky barrier in future spintronics applications. Both thin and ultrathin films of manganese silicide on Si(001) have been investigated with different STM based techniques. In the ultrathin film regime three-dimensional manganese silicide islands form upon annealing. No closed and flat films have been found. With increasing annealing time the density of islands per surface area decreases while the average size of the remaining islands increases. The time dependence of the coarsening shows the typical characteristics of surface diffusion limited Ostwald Ripening. The activation energy for the clustering of Mn on Si(001) has been found to be 2.6 eV. Thicker (6 nm) continuous MnSi films were investigated using ballistic electron emission microscopy (BEEM) at 80 K to study the electrical properties. The BEEM spectra indicate a complex band structure at the MnSi/Si(001) interface. The MnSi overlayers show a high elastic scattering rate compared to Au/Si(001). [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K12.00003: Kinetic stabilization of Fe ultra-thin film on GaAs(100) grown at low temperature Jae-Min Lee, Se-Jung Oh, J.-Y. Kim, K.-J. Kim, S.-U. Yang, J.-S. Kim We grew ultra-thin Fe films on GaAs(100) at low temperature (around 130K) to suppress chemical reactions between Fe and substrate atoms (interface alloying and As- surface segregation). For various Fe film thickness ranging from 1 to 30$\AA$, we preformed photoemission experiment using synchrotron radiation to characterize the growth kinetics, and compared them with Fe films of similar thickness grown at room temperature. We confirmed that the interface alloying is obviously decreased at low temperature, and the surface segregation of As is blocked when Fe film is grown around 130K. As a result, more abrupt interface can be obtained in the low temperature growth. Furthermore, these Fe films grown at low temperature are found to be stable against As segregation even when the sample is annealed up to room temperature. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K12.00004: In-Situ Atomic-Scale Scanning Tunneling Microscopy Investigations of Cr on GaN Surfaces Arthur R. Smith, Muhammad Haider, Erdong Lu, Wenzhi Lin, Rong Yang, Costel Constantin, Hamad Al-Brithen Currently there is much interest in the magnetic properties of magnetically-doped nitride semiconductors. For purposes of doping GaN with Mn or Cr, it is important to investigate the action of these dopants on the clean surface under epitaxial growth conditions. In this study, we investigate the surface of GaN which has been doped with Cr during molecular beam epitaxial growth using in-situ scanning tunneling microscopy. In addition, we also investigate the surface of GaN which has been exposed to a fraction of monolayer of Cr deposited directly on the clean surface. For low concentrations, we find that the Cr can incorporate into the Ga lattice positions, participating in the known 3x3 and 6x6 reconstructions of GaN(000-1). For higher percentages of Cr, a novel Cr nanowire structure forms at the surface. Efforts are underway to study the magnetic properties of these surfaces. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K12.00005: Epitaxial Growth of Binary Ferromagnetic Mn$_{3-\delta}$Ga Thin Films on Wurtzite GaN(0001) and Investigation by Atomic Scale Scanning Tunneling Microscopy E.D. Lu, R. Yang, M.B. Haider, C. Constantin, A.R. Smith, J.W. Knepper, F.Y. Yang Due to increasing interest in developing new magneto-optical and magneto-electronic devices, and spin injection sources in spintronics applications, ferromagnetic (FM){$\backslash$MnxGay} is an attractive candidate system to explore. Here we report that binary FM {Mn$_{3-\delta}$Ga}(1.14$<$$\delta$$<$2.0) single crystalline thin films has been epitaxially grown on ({\it w})-GaN(0001)surfaces using molecular beam epitaxy. The face-centered tetragonal structure of {Mn$_{3-\delta}$Ga} thin films with CuAu-L1$_{0}$ type ordering has been determined $\textit{in situ}$ by both reflection high energy electron diffraction and atomic-scale scanning tunneling microscopy. The epitaxial relationship of the {Mn$_{3-\delta}$Ga} with GaN(0001) is (111)[1$\bar{1}$0]$_{MnGa}$ $\parallel$ (0001)[1$\bar{1}$00]$_{GaN}$ and (111)[11$\bar{2}$]$_{MnGa}$ $\parallel$ (0001)[11$\bar{2}$0]$_{GaN}$. The hysteresis loops of the thin films show magnetic anisotropy along \textit{in-plane} and \textit{out-of-plane} directions. It is found that Curie temperature and magnetic moments of the {Mn$_{3-\delta}$Ga} thin films are closely related to the Mn content, and the reconstruction changes from {1$\times$1} to {2$\times$2} by increasing the ratio of Mn to Ga during the growth. Thus, we have achieved reconstruction-control of magnetic properties. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K12.00006: Indium nitride growth on Si(100) by femtosecond pulsed laser deposition Mohamed Hafez, Hani Elsayed-Ali Deposition of indium nitride (InN) on Si(100) is performed under ultrahigh vacuum with an amplified Ti:sapphire laser (130 fs) operating at a wavelength of 800 nm. The objective is to grow single crystal InN films on Si(100). An intermediate layer of indium on Si(100)-(2$\times $1) is prepared to avoid surface nitridation and decrease the lattice mismatch between the InN and Si(100). Reflection high-energy electron diffraction (RHEED) is used in situ during the deposition to study the growth dynamics and the surface structure of the grown films. Growth of indium on Si(100)-(2$\times $1) by femtosecond pulsed laser deposition showed high-quality 2D films. The initial 2D indium layers on Si(100)-(2$\times $1) formed in the In-(2$\times $1) structure at room temperature and the In-(4$\times $3) structure at a substrate temperature of 653-693 K. Growth of InN on the In-(2$\times $1) and In-(4$\times $3) buffer layer is discussed. The morphology of the InN films is examined by ex situ atomic force microscopy (AFM) and scanning electron microscopy (SEM). [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K12.00007: Influence of temperature on mobility and carrier density of InN films Jagdish Thakur, V.M. Naik, R. Naik, D. Haddad, G.W. Auner, H. Lu, W.J. Schaff We have investigated temperature dependence of Hall mobility and carrier density for thin InN films with low and high carrier density grown by Molecular Beam Epitaxy (MBE) and Plasma Source Molecular Beam Epitaxy (PSMBE), respectively. At very low temperatures, a large concentration of carriers which are temperature independent is observed in both the low and high density films. However, the behavior of mobility for the low density film is different from that of the high density film particular for temperatures less than 300K. For the low density film, mobility increases with temperature and passes through a maximum around 250 K in contrast to temperature independent mobility observed for the high density film for T$<$300K. Theoretically, we investigated the temperature dependence behavior of the mobility using Born-scattering and found the presence of charged dislocation in the low density film. At higher temperatures the mobility is determined by the electrons scattering from the longitudinal-optical (LO) phonons. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K12.00008: Electrolyte-based capacitance voltage analysis of InN J.W. Ager III, K.M. Yu, R.E. Jones, D.M. Yamaguchi, S.X. Li, W. Walukiewicz, E.E. Haller, H. Lu, W.J. Schaff The electron affinity of InN, 5.8 eV, is larger than for any other known semiconductor. As a result, InN has a surface accumulation layer of electrons reflecting pinning of the Fermi level ca. 0.9 eV above the conduction band edge. In addition, all metals form an ohmic contract to InN, so that it is not possible to measure the space charge distribution using standard capacitance voltage (CV) measurements. We show that electrolyte solutions can be used to make blocking contacts to InN and that under reverse bias conditions, the surface accumulation layer and up to ca. 10 nm of the underlying InN can be depleted. Analysis of CV data obtained under these conditions using the Poisson equation allows net charge as a function of depth to be modeled. In n-type InN, good agreement with bulk Hall effect data is obtained when the depletion region is extended to $>$ 5 nm below the surface. In Mg-doped InN, modeling of the CV data produces evidence for ionized acceptors below a surface inversion layer. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K12.00009: Observation of Quantized Electron Accumulation States in InN(0001--) Leyla Colakerol, Hae-Kyung Jeong, Lukasz Plucinski, Shancai Wang, Alex DeMasi, Kevin Smith, Papo Chen , Theodore Moustakas We report a study of the surface and bulk electronic structure of InN(0001--) thin films using high resolution synchrotron radiation excited angle-resolved photoemission. The InN thin films were grown by plasma-assisted molecular beam epitaxy on $c$-plane sapphire. Samples were cleaned both by annealing in ultra-high vacuum (UHV) and by cycles of nitrogen ion bombardment followed by UHV annealing. We have observed a series of quantized states above the Fermi level (E$_{F})$, for a narrow range of excitation energies and band momenta. These states have a parabolic dispersion around the surface Brillouin zone center and are due to an electron accumulation layer in the conduction band. The number of states observed is a function of annealing temperature . We observe an increase in the number of states following sputtering, which is likely due to segregation of In metal to the surface. Up to four individual free electron states are observed. Supported in part by the NSF, the U.S. ARO, and by the U.S. AFOSR. Experiments were performed at the NSLS. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K12.00010: Electron Mobility of InN Rebecca Jones, Sonny Li, Eugene Haller, Kin Man Yu, Wladek Walukiewicz, Henricus van Genuchten, Joel Ager, Leon Hsu, Hai Lu, William Schaff We use irradiation with 2 MeV H$^{+}$ and He$^{+}$ ions to create donor-like point defects in InN films and thereby predictably control the free electron concentration and mobility over a large conductivity range. Calculations of theoretical electron mobilities coupled with experimental annealing studies (at temperatures up to 500$^{o}$C) suggest at least two types of donor-like defects are formed by the irradiation: singly-charged nitrogen vacancies and triply-charged, relaxed indium vacancies. Under annealing at these temperatures, the nitrogen vacancies appear stable, while the indium vacancies are removed. The annealing creates InN films with electron mobilities well above those of as-grown films at similar concentrations. We use these results to analyze the factors limiting the mobility in as-grown InN films. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K12.00011: Role and Effect of Native Defect Complexes in InN Xiangmei Duan, Cathy Stampfl We have carried out the first-principles density-functional theory calculations to investigate the structural and electronic properties and the formation energies of native point defect complexes in InN. We report an extensive and systematic study of possible configurations and different concentrations, focusing on the nitrogen- and indium-vacancies. Nitrogen vacancies prefer to be situated close to one another resulting in local metallic indium-rich regions or ``clustering''; while indium vacancies prefer to be separated, or to cluster together, where neighbouring under-coordinated N atoms spontaneously form N2 molecules. These defect structures induce marked changes in the states in the egion of the band gap, which may explain the wide variation in experimentally determined band-gaps (see e.g. [1]). \newline \newline [1] T. V. Shubina, et al. Phys. Stat. Sol. (a) 202, 377 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K12.00012: Photoinduced magnetic and electronic phenomena in organic magnetic semiconductor V(TCNE)$_{x\sim2}$ Jung-Woo Yoo, R. Shima Edelstein, A. J. Epstein, K. I. Pokhodnya, Joel S. Miller V(TCNE)$_{x\sim2}$ is a fully spin-polarized half- semiconductor, whose magnetic order exceeds room temperature ($T_ {c}$$>$350 K), and electronic transport follows hopping mechanism in the Coulomb energy split in $\pi^{\ast}$ band. Substantial decrease of magnetization by illuminating with light ($\lambda$=457.9nm) has been found at low temperature ($T$$<$100K). The photo-excited metastable state has a lifetime $>$10$^{6}$s at low temperatures and completely relaxes to the state before illumination after annealing upto 250K. Photoinduced ESR analysis indicates strong increase of magnetic anisotropy by light irradiation. We also report substantial increase of conductivity induced by illumination with light ($\lambda$=457.9nm). The temperature dependence of resistivity clearly indicates substantial decrease in activation energy for electronic hopping. The photoinduced effect is proposed to originate from structural changes triggered by $\pi$ $\rightarrow$ $\pi^{\ast}$ excitation in (TCNE) molecules, which leads modification of the orbital wavefunction resulting in changes of magnetic exchange energy $J$ and the activation energy $\Delta E$. $^{\ast}$Supported in part by AFOSR Grant No. F49620-03-1-0175 and DOE Grant No. DE-FG02-01ER45931 and DE-FG02-86ER45271 [Preview Abstract] |
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