Overview of Spintronics and Its place in the Semiconductor Industry Roadmap

Transcription

1 Overview of Spintronics and Its place in the Semiconductor Industry Roadmap Dmitri Nikonov Collaborators: George Bourianoff (Intel) David Awschalom, Wayne Lau (UCSB) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 1

2 Introduction Good to be home!!! 04/06/2004 DENikonov, Talk at Texas A&M PAGE 2

3 Questions What is scaling of transistors and why beyond CMOS devices are required? What is the field of spintronics? How to manipulate spins? How to inject and detect spins? Road to the future? * Disclaimer: no original material, subjective review of publications, very superficial, incomplete, not always quoted the pioneering publication 04/06/2004 DENikonov, Talk at Texas A&M PAGE 3

4 Moore s Law K Transistors 1,000,000 1 Billion Transistors 100,000 10,000 1, i386 Processor Pentium 4 Processor Pentium III Processor Pentium II Processor Pentium Pro Processor Pentium Processor i486 Processor No exponential is forever, but we can delay Forever /06/2004 DENikonov, Talk at Texas A&M PAGE 4

5 Scaling into Nanotechnology 10 Nominal feature size Micron 0.1 Gate Width 130nm 90nm 100 Nano- meter Nanotechnology 70nm 50nm /06/2004 DENikonov, Talk at Texas A&M PAGE 5

6 Fundamental Limits Are Near a E b E b a E bit = 2 2mw e 2 τ min = E bit E bit w k T B ln 2 w w = 1.5nm τ min = 40fs E bit = = J 17meV Zhirnov et al., Proc. IEEE 91, 1934 (2003). 04/06/2004 DENikonov, Talk at Texas A&M PAGE 6

7 Power Grows Exponentially 100W/cm 2 Like a hotplate!!! Presently computing scaling is mostly limited by power dissipation 04/06/2004 DENikonov, Talk at Texas A&M PAGE 7

8 Future Integrated Circuit * European Technology Roadmap for Nanoelectronics 04/06/2004 DENikonov, Talk at Texas A&M PAGE 8

9 Requirements to Devices Performance (esp. gain) CMOS architectural compatibility Operational reliability CMOS process compatibility Room temperature operation Energy efficiency Low sensitivity to parameters Scalability 04/06/2004 DENikonov, Talk at Texas A&M PAGE 9

10 International Technology Roadmap for Semiconductors Two numbers: Performance potential Risk 3 = better than CMOS 3 = solutions known 2 = comparable to CMOS 2 = concept feasible 1 = worse than CMOS 1 = no known solution 04/06/2004 DENikonov, Talk at Texas A&M PAGE 10

11 My Definition of Spintronics Spintronics IS Science and technology of manipulation of particle spins in coherent quantum states in non-equilibrium IS NOT (Collection of phenomena) (Spectroscopy) (Optical pumping) (Traditional magnetics) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 11

12 Atomic Spintronics Orbital I=1 Na Spin S=1/2 Electron J=1/2 = Electron J=1/2 Laser λ1 Laser λ2 Nuclear i=3/2 = Total F=2 Good example of spin manipulation in atoms. Coherent state created. Electromagnetically-induced transparency and Lasing without inversion is observed. E.S.Fry, X.Li, D.Nikonov, G.G.Padmabandu, M.O.Scully, A.V.Smith, F.K.Tittel, C.Wang, S.R.Wilkinson, S.- Y.Zhu, Phys. Rev. Lett. 70, 3235 (1993). 04/06/2004 DENikonov, Talk at Texas A&M PAGE 12

13 Metal Spintronics Fe Cr Fe AF current Fe Cr Fe AF no current Anti-ferromagnetic layer pins the magnetization of the bottom layer Low resistance if the two ferromagnetic layers are parallel, high resistance if anti-parallel Used to be current in plane, now current perpendicular to the plane configurations 04/06/2004 DENikonov, Talk at Texas A&M PAGE 13

14 Magnetoresistance E N(E) E f E N(E) Metallic energy states. In a ferromagnet a band with one spin is completely occupied and not conducting Cu Co Co Cu Co Co Cu Co No available states for electrons with the spin which is conducted Low Resistance High Resistance * Courtesy S. Wolf (DARPA) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 14

15 Spintronics for Storage When can one buy spintronics in a store? TODAY!!! Giant magnetoresistance (many layer of interchanging magnetization) how most of 10GB 100GB hard drives are made 1MB Magnetic-RAM chip made on 0.5um CMOS, <35ns access, magnetoresistance >40% * Finding names of the companies left to reader as a homework 04/06/2004 DENikonov, Talk at Texas A&M PAGE 15

16 Map of Semiconductor Spintronics Spin injection Tunneling barriers Switching of magnetization Spin manipulation Optical, magnetic, electrical Picosecond pulses Ferromagnetic semiconductors Band structure Steady-state Seemingly disconnected fields of research 04/06/2004 DENikonov, Talk at Texas A&M PAGE 16

17 H mag µ = GS Spin in Magnetic Field = µb i = gq G = 2m e e µ B = 2me ds G dt = S B gµ BSB i q e Precession in the magnetic field. g-factor depends on the material g=2 for electron in vacuum Hamiltonian Magnetic moment vs. spin Gyromagnetic ratio vs. g-factor Bohr magneton Bloch equation 04/06/2004 DENikonov, Talk at Texas A&M PAGE 17

18 Optics is Spintronics Best Friend -1/2 1/2 electrons E e Rel. squares of matrix elements k hh -3/2-1/2 1/2 3/2 light holes lh heavy holes Polarized light is the most natural way to create and detect spin polarization in semiconductor E-h pair 1/2 1/2 04/06/2004 DENikonov, Talk at Texas A&M PAGE 18

19 Faraday Effect 04/06/2004 DENikonov, Talk at Texas A&M PAGE 19

20 Time-Resolved Faraday Rotation Circ. polar. pump, linear polar. 45deg probe, dt=50 to 900 ps, 50um spot Crooker, Awschalom, Samarth, IEEE Select. Topics Quant. Electr. 1, 1082 (1995). 04/06/2004 DENikonov, Talk at Texas A&M PAGE 20

21 Faraday Results Interesting non-monotonic dependence of spin lifetime Kikkawa, Awschalom, Phys. Rev. Lett. 80, 4313 (1998). 04/06/2004 DENikonov, Talk at Texas A&M PAGE 21

22 Motion of Spins in a Semiconductor Spin coherence persists for 100 s of nanoseconds over 100 s of microns Largely insensitive to temperature 560 um Dx Inject spins at x time pump probe Detect spins at x + dx distance Kikkawa and Awschalom, Nature 397, 139 (1999) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 22

23 H = g µ Β S B G-factor Modulation Parabolic Quantum Well (PQW) Magnetic field g-factor depends on material composition Al composition is parabolically graded to allow translation of the whole wavefunction to higher Al content region g-factor in Al x Ga 1-x As E=0 g-factor 0.4 g-factor Al concentration x E>0 increase Weisbuch et al., Phys. Rev. B 15, 816 (1977) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 23

24 G-factor Modulation Results evaporated thin Ti/Au (front gate) T=5K, B=6T 100nm PQW (undoped) 0 3.0V AlGaAs Barrier LT-GaAs n-gaas (back gate) 1.15 µm V g Kerr Rotation (a.u.) V 1.4V 0.8V GaAs buffer/smoothing SL SI-GaAs substrate G. Salis et a., Nature 414, 619 (2001) 0.0V Time Delay (ps) g-factor is electrically tuned! 04/06/2004 DENikonov, Talk at Texas A&M PAGE 24

25 Spin without Inversion Symmetry lab frame E e - H H R D Rashba (1960) effect α ( y x = p ) xσ pyσ β ( x y p σ p σ ) = x y electron s frame e - B k Dresselhouse (1955) effect relativistic transformation E need not be real electric field, but can be quasi electric field occurring from asymmetries in crystal fields, band gap, spin-orbit splittings, etc Allows zero-magnetic field spin manipulation by electric fields by dragging in two orthogonal directions (through control of k ) Courtesy of S.Wolf (DARPA) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 25

26 Precession from Stress and Motion Kato, Myers, Gossard, Awschalom, Nature 427, 50 (2003) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 26

27 Ferromagnetic Semiconductors S=5/2 Mechanism: exchange part of the Coulomb interaction between localized Mn magnetic moments and free holes MnGaAs demo at 150K Also MnGe ferromagnetism at 110K demo by B.Jonker et al. (NRL) first group IV 04/06/2004 DENikonov, Talk at Texas A&M PAGE 27

28 Towards Room Temperature Dietl, Ohno, Matsukara, Phys. Rev. B 63, (2001) MnGe 350K (2003) Univ. of Minn. GaMnN 400K (2001) NCSU MnZnO 450K (2003) Sweden CrZnTe - 300K (2003) Japan Requires verification. Also doubts free carrier magnetism or impurity band. 04/06/2004 DENikonov, Talk at Texas A&M PAGE 28

29 Spin Injection FM metal Semiconductor Naïve approach: let electrons pass from a ferromagnetic to a semiconductor Hammer et al., Phys. Rev. Lett. 83, 203 (1999); Gardelis et al., Phys. Rev. B 60, 7764 (1999). Only 1% polarization. Does not work!!! 04/06/2004 DENikonov, Talk at Texas A&M PAGE 29

30 Conductivity Mismatch LF / σ 1/ Σ 2 L / σ N N Spin up LF / σ 1/ Σ 2 L / σ N N Spin down Ferromag Semicon Polarization Contact resistance γ r Σ Σ r σ σ Σ +Σ c = r + r + r Σ +Σ r + r + r σ + σ 4 Σ Σ c F = + F N c F N c r Preferred if contact resistance dominates, e.g. tunneling Based on Rashba, PRB 62, 16267(2000) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 30

31 Semiconductor Injector Ohno et al., Nature 402, 790 (1999). Optical detection by polarization of luminescence 04/06/2004 DENikonov, Talk at Texas A&M PAGE 31

32 Luminescence Tunneling Injection from Metal E F CoFe MgO e- Magnetic Field Ga As quantum well AlGaAs Collector AlGaAs A + p-gaas substrate EL Intensity (a.u) σ+ σ- H = 5 T H = 0 T n-i-p sample LH HH T = 100 K V = 1.8 V I = 120 µa V T MgO tunnel barrier n-i-p structure n ~ 5 x 1016 cm-3 Al0.08Ga0.92As/GaAs quantum well (QW) S. Parkin et al. (IBM) I C 60% SP H = -5 T Wavelength (nm) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 32

33 Optical Control of Currents Independent charge or spin currents A. Smirl (U. of Iowa) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 33

34 Electric Detection Scheme similar to GMR, or Spin-voltaic effect non-equilibrium spins are injected into a non-magnetic n-region; junction with a magnetic p-region; changes current (or voltage) through the junction Fabian, Zutic, Das Sarma, Phys. Rev. B 66, (2002) 04/06/2004 DENikonov, Talk at Texas A&M PAGE 34

35 Spintronic Device Synthesis Spin injection Tunneling barriers Switching of magnetization Spin manipulation Optical, magnetic, electrical Picosecond pulses Spin injection Spin switching Spin detection Ferromagnetic semiconductors Band structure Steady-state Need to combine all elements into a functional device; at room temperature 04/06/2004 DENikonov, Talk at Texas A&M PAGE 35

36 Requirements to Devices Performance (esp. gain) CMOS architectural compatibility Operational reliability CMOS process compatibility Room temperature operation Energy efficiency Low sensitivity to parameters Scalability 04/06/2004 DENikonov, Talk at Texas A&M PAGE 36

37 Spintronics Device of Choice And the winner IS??? 04/06/2004 DENikonov, Talk at Texas A&M PAGE 37

38 Answers CMOS transistors miraculously successful, soon to reach nanometer limit. Need complementary technologies with special advantages. Manipulate electron spins in semiconductors for computing [non-solid state is irrelevant, metal spintronics is done]. Optically created spins (urgently need electrical). Magnetic field, electric field, semiconductor nanostructures to manipulate. Tunneling to inject spins. Ferromagnetic semiconductors of utmost importance. Optical detection so far, urgently need electrical. Need a functional spintronic device, advantages compared to CMOS still in definition. 04/06/2004 DENikonov, Talk at Texas A&M PAGE 38