Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V15: Focus Session: Spins in Semiconductors - Spin Currents III |
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Sponsoring Units: GMAG DMP FIAP Chair: Ian Appelbaum, University of Maryland Room: D171 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V15.00001: Temperature Dependent Spin Transport in Silicon Controlled by an Electrostatic Gate Jing Li, Ian Appelbaum Long-distance ($\sim $500$\mu $m) lateral spin polarized electron transport in undoped silicon under the control of an electrostatic gate is studied from 40K to 120K. The temperature dependence of average spin polarization, transport time, and spin dephasing during coherent precession can be largely attributed to reduction of finite spin lifetime at higher temperatures. Measurements on devices with different transport lengths are shown to modify the effect of electrostatic gating. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V15.00002: Electrical detection of spin accumulation at 500K at FM/SiO2/Si(001) contacts via the Hanle effect Connie H. Li, Olaf M. J. van 't Erve, Eugene A. Imhoff, Berry T. Jonker We demonstrate the electrical detection of spin accumulation in Si (doped n-type 3E18 and 3E19/cm3) via injection from a ferromagnetic contact through a SiO2 tunnel barrier formed by plasma oxidation. The injection of spin-polarized carriers produces a net spin accumulation described by the splitting of the spin-dependent electrochemical potential, and is detected as a voltage. The decrease of this voltage with increasing out-of-plane magnetic field due to spin dephasing, i.e., Hanle precession of the electron spin, is observed at temperatures up to 500K. Lorentzian fits to the Hanle curves yield a spin lifetime of 100 and 320ps for the high and lower doped Si. The direct correlation between spin lifetime and carrier concentration in the Si, and that the magnitude of the Hanle signal agrees well with that expected from theory [1], provide clear evidence that the spin accumulation indeed occurs in the Si and not interface states. These results demonstrate that spin accumulation in Si can be a viable basis for spin-based devices. Supported by ONR. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V15.00003: Quantifying Electron Spin Polarization from Polarized EL in Si spin-LEDs G. Kioseoglou, P. Li, H. Dery, A.T. Hanbicki, C.H. Li, O.M.J. van 't Erve, P.E. Thompson, B.T. Jonker We analyze the circular polarization (P$_{circ}$) of the electroluminescence (EL) from Si-based spin-LEDs using a recent theory [1] which provides a quantitative relation between the polarization of phonon-assisted optical transitions measured in the EL, and the electron spin polarization electrically injected from Fe/Al2O3 and Fe/SiO2 tunnel barrier contacts [2,3]. EL spectra include features due to transverse acoustic (TA) and transverse optical (TO) phonon-mediated recombination occurring in the p-doped (p$\sim$10$^{19}$cm-3) substrate. P$_{circ}$ of 3.5\% is typical for the TA at 5K, and is systematically higher than that of the TO by a factor $\sim$1.7, consistent with theory. The maximum polarization predicted for the TA is 13\% for recombination of 100\% polarized electrons in p-type Si (10$^{19}$cm-3). Thus the measured P$_{circ}$ 3.5\% corresponds to an electron spin polarization (P$_{spin}$) of 27\% produced by electrical injection from our tunnel barrier contacts. A similar analysis applied to the TO phonon at 80K yields P$_{spin}$ of 25\%. Thus the theory enables quantitative interpretation of optical polarization in indirect gap semiconductors, facilitating future studies of spin injection. [1] P. Li and H. Dery, Phys. Rev. Lett. 105, 037204 (2010). [2] B.T. Jonker, et al., Nature Physics 3, 542 (2007). [3] C.H. Li, et al, Appl. Phys. Lett. 95, 172102 (2009). [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V15.00004: Electrical spin injection to Germanium using a single crystalline Fe/MgO/Ge tunneling junction Yi Zhou, Li-Te Chang, Wei Han, Faxian Xiu, Minsheng Wang, Michael Oehme, Joerg Schulze, Alexandros Shailos, Roland Kawakami, Kang Wang Germanium has long been predicted a superior candidate for spintronics with enhanced spin lifetime and transport length due to low spin--orbit interaction and lattice inversion symmetry. One of the critical challenges, however, is to electrically create spin accumulation in otherwise non-magnetic Ge. In this work, we report electrical spin injection to bulk n-type Ge using a single crystalline Fe/MgO/Ge tunneling junction. The spin lifetime and diffusion length are extracted from both 3-terminal Hanle measurement and non-local spin valve measurement. The spin relaxation mechanism in n-type Ge has also been explicitly analyzed from the bias and temperature dependence of the spin relaxation rate. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V15.00005: Spin accumulation in Fe/MgO/Si heterostructures A.T. Hanbicki, O.M.J. van 't Erve, S.-F. Cheng, R. Goswami, C.H. Li, G. Kioseoglou, P.E. Thompson, B.T. Jonker We report on spin injection experiments at Fe/MgO/Si interfaces using all electrical injection and detection. MgO is a promising magnetic tunnel junction material, and its incorporation with Si-based spintronics has only recently been reported in degenerately doped Si (n $\sim 10^{20} cm^{-3})$ [1]. We focus here on spin accumulation under the injecting contact for much lower n-doping levels by measuring the Hanle effect in a standard 3-terminal scheme where injection and detection are done using the same contact. The Fe/MgO spin injector was sputter deposited onto various n-doped Si bulk substrates using a variety of different substrate temperatures. The best tunnel barriers were obtained when the MgO was deposited at 70$^{\circ}$C and annealed $in-situ$ before Fe deposition. Fits to Hanle curves using the drift-diffusion model for Si samples with n=$4x10^{18} cm^{-3}$ yield spin lifetimes $\tau_{s}$ = 0.28 ns up to 30 K and a spin diffusion length L$_{s}$=$\sqrt{D\tau_{s}}$ of 0.65 $\mu$m (the diffusion constant D is obtained from the mobility assuming degenerate statistics). We determine the dependence on n, and comment on the potential differences between SOI and bulk Si wafer transport channels. [1] T. Sasaki, et al., Appl. Phys. Exp. 2 (2009). [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V15.00006: Electrical spin injection and detection in Si Yong Pu, Adrian Swartz, Jonas Beardsley, Vydia Bhallamudi, Chris Hammel, Roland Kawakami, Ezekiel Johnston-Halperin, Jon Pelz We report electrical spin injection from Fe into Si in a Fe/MgO/Si tunnel diode grown by molecular beam epitaxy. Incorporating the spin-degree of freedom into Si adds significant new functionality in a system with established utility. In addition, the use of spin as an intrinsically quantum mechanical degree of freedom may enable more speculative computing paradigms such as spin-based quantum computation. In this work, we investigate spin injection and spin detection and spin-related transport properties in Si. This work also lays the foundation for ongoing studies correlating structural, electronic and magnetic device properties with spin injection efficiency, spin transport mechanism and real-space imaging of spin transport. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V15.00007: Spin injection into Silicon using Al$_2$O$_3$, SiO$_2$ and MgO tunnel barriers Olaf van t Erve, Connie Li, Aubrey Hanbicki, George Kioseoglou, Berend Jonker We recently demonstrated injection of spin-polarized electrons from an Fe film into Si.\footnote{B. T. Jonker et al., Nature Phys. 3, 542 (2007), O.M.J. van 't Erve et al., App. Phys. Lett. 91, 212109, (2007)} The tunnel barrier is the key component in achieving a large spin accumulation in the semiconductor. Here we will compare three different tunnel barriers, Al$_2$O$_3$, SiO$_2$ and MgO, on highly doped Si using three terminal Hanle measurements. Hanle measurements give insight in the spin-accumulation directly underneath the spin injecting contact. We will compare temperature dependence and bias dependence as well as the tunnel barrier properties such as density of interface states based on I-V and C-V measurements. We will compare spin-injection properties, such as spin lifetimes and spin injection efficiency with the oxide/Silicon interface. A spin lifetime of 120ps was obtained for 3e19 n-doped Silicon for both the Al$_2$O$_3$ and SiO$_2$ tunnel barrier, indicating that the spin accumulation occurs in the Si rather than in surface states. Support by ONR. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V15.00008: Spin injection studies on thin film Fe/MgO/Si tunneling devices Jonas Beardsley, Yong Pu, Adrian Swartz, Vidya Bhallamudi, Roland Kawakami, Ezekiel Johnston-Halperin, Chris Hammel, Jon Pelz We report progress on the injection of spin polarized electrons into 35 nm thick Si films, using Fe/MgO injector/tunnel barrier structures grown by molecular beam epitaxy on SIMOX silicon-on-insulator substrates. The device requires heavy top-surface n-type doping of the Si film to produce a suitable tunnel barrier, accomplished by diffusion from a spin-on phosphorous-doped glass. Measurements indicate a roughly exponential doping profile with 7E20 per cubic cm at the top surface and a 2 nm decay length. Three terminal measurements showed evidence of spin injection similar to reports of Jansen et al. [1], while injection with a thinner MgO barrier shows more complicated behavior. On-going measurements and modeling will be discussed.\\[4pt] [1] R. Jansen et al.; Nature 462; 491 (2009) Funding for this research was provided by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (Award Number DMR-0820414). [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V15.00009: Room-temperature magnetocurrent in antiferromagnetically coupled Fe/Si/Fe Rashid Gareev, Maximilian Schmid, Johann Vancea, Christian Back, Reinert Schreiber, Daniel Buergler, Claus Schneider, Frank Stromberg, Heiko Wende Epitaxial Si-based ferromagnet/semiconductor structures demonstrate strong antiferromagnetic coupling (AFC) as well as resonant-type tunneling magnetoresistance, which vanishes at temperatures above T$\sim $50K [1]. Magnetoresistance effects in Fe/Si/Fe close to room temperature (RT) were not established yet. By using the ballistic electron magnetomicroscopy (BEMM) techniques, with its nanometer-scaled locality [2] we observed for the first time a spin-dependent ballistic magnetotransport in AFC structures. We found that the hot-electron collector current with energies above the Fe/GaAsP Schottky barrier reflects magnetization alignment and changes from I$_{cAP}\sim $50fA for antiparallel alignment to I$_{cP}\sim $150fA for the parallel one. Thus, the magnetocurrent [(I$_{cP}$-I$_{cAP})$/ I$_{cAP}$]*100{\%} is near 200 {\%} at RT. The measured BEMM hysteresis loops match nicely with the magnetic MOKE data.\\[0pt] [1]. R.R. Gareev, M.Weides, R. Schreiber, U. Poppe, Appl. Phys. Letts \textbf{88}, 172105 (2006); [2]. E. Heindl, J. Vancea, C.H. Back, Phys. Rev.\textbf{B75}, 073307 (2007). [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:24AM |
V15.00010: Theory of spin-dependent phonon-assisted optical transitions in Si and quantifying spin polarization in Si Invited Speaker: The spin polarization of conduction electrons in a direct-gap semiconductor is readily quantified by measuring the circular polarization of the recombination light luminescence. However, in silicon, owing to its indirect band-gap, such a direct connection between spin polarization and luminescence has been conspicuously absent. This missing link is established with a theory that provides concise relations between the degrees of spin polarization and measured circular polarization for each of the dominant phonon-assisted optical transitions [1]. This theory has two important applications. First, it allows one to determine in a parameter-free manner the spin polarization of electrons from the measured circular polarization of the luminescence. Second, it provides a means to extract the spin relaxation time or the spin injection efficiency across ferromagnet/silicon interfaces. In the first part of the talk, by invoking symmetry arguments, I will derive concise optical selection rules for each of the phonon-assisted optical transitions in unstrained bulk silicon. It will be shown that phonon symmetries play a key role in determining the circular polarization degrees of the various phonon-assisted luminescence peaks. In the second part, the optical selection rules will be used to analyze the polarized luminescence spectrum that is calculated by a comprehensive rigid-ion model for doped silicon. The analysis is used to elucidate results of recent spin injection experiments in silicon [2]. The effect of the (weak) spin-orbit coupling in silicon on the luminescence turns out to be unique due to the proximity of the split-off band to the heavy and light hole bands in unstrained bulk silicon (44 meV). This proximity gives rise to a fast reduction in the circular polarization degree of the luminescence in p- type silicon.\\[4pt] [1] P. Li and H. Dery, Phys. Rev. Lett. 105, 037204 (2010).\\[0pt] [2] B. T. Jonker et al., Nature Phys. 3, 542 (2007). [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V15.00011: Tunability of spin lifetimes in strained silicon and germanium Jian-Ming Tang, Brian T. Collins, Michael E. Flatte The spin lifetimes due to electron-phonon interactions in silicon and germanium are calculated using a $sp^3$ tight-binding model. Despite of the strong spin-orbit interaction in germanium, the spin lifetime in germanium is only about one order of magnitude shorter than what is in silicon. Near room temperature, the spin-flip scattering is dominated by the inter-valley $f$ processes in silicon and by the inter-valley $X$ processes in germanium. The inter-valley scattering processes can be suppressed by shifting the valley minima with strain. We show that the spin lifetimes can be enhanced by about an order of magnitude in both materials. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V15.00012: Theory of optical spin orientation in silicon JinLuo Cheng, J. Rioux, J. Fabian, J.E. Sipe Despite weak spin-orbit coupling and an indirect band gap, significant optical spin orientation is possible in silicon. We show this by performing full band-structure calculations of the phonon-assisted absorption of circularly polarized light in bulk silicon. At 4~K a maximum spin polarization of 25\% is found at the band edge; at room temperature the polarization is still $15\%$. We present the selection rules and give the contributions from the individual phonon branches, valence bands, and conduction band valleys. Dominant are the TO/LO phonon-assisted transitions from the heavy-hole to the conduction band. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V15.00013: Spin-Dependent Phonon-Assisted Optical Transitions in Germanium Dhara Trivedi, Pengke Li, Hanan Dery We study the circular polarization of the photoluminescence due to phonon-assisted indirect optical transitions in Germanium. The band structure is calculated by empirical pseudopotential method with the spin-orbit interaction. Phonon modes are obtained by the adiabatic bond charge model and the L - $\Gamma$ electron-phonon matrix elements are calculated within the rigid-ion approximation. We have used group theory extensively to account for all possible transitions. We quantify the circular polarization of various phonon-assisted optical transitions. [Preview Abstract] |
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