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Published on November 21, 2007

Author: Octavio

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Carrier Dynamics Using Ultrafast Lasers; Electrons, Holes, and Spin Carriers :  Carrier Dynamics Using Ultrafast Lasers; Electrons, Holes, and Spin Carriers Mikel Barry April 23, 2004 Outline:  Outline Motivation Electron and hole carrier dynamics in semiconductor heterostructures - Electric field-induced second harmonic (EFISH) generation - Internal photoemission (IPE) Spin carrier dynamics - Photoluminescence - Magnetic field-induced second harmonic (MFISH) generation Conclusion Carrier, and particularly spin carrier, dynamics are at the forefront of interest in science and technology. Not only does their transport characterization hold the key to future technology, but it is an important and intriguing direction in physics. :  Carrier, and particularly spin carrier, dynamics are at the forefront of interest in science and technology. Not only does their transport characterization hold the key to future technology, but it is an important and intriguing direction in physics. Problems for Moore’s Law:  Problems for Moore’s Law dielectric Gate metal n+ n+ p silicon Source Drain V D >0 V G >0 I D Metal Oxide Semiconductor (MOS) Device The silicon dioxide dielectric thickness can only be reduced so much without current leakage. Moore’s law says: Device densities will double every eighteen months. A Possible Solution is High-K Dielectrics!:  A Possible Solution is High-K Dielectrics! We need a way to nondestructively characterize both SiO2 and its possible replacements. Second Harmonic Generation is Contactless!:  What is Second Harmonic Generation? Lack of Inversion Symmetry e.g. Electric Field at Interface I 2 (t)  |(2) + (3) EDC (t) |2 (I  )2 Contactless technique. Time-dependent SHG can give information on injection recombination through tunneling trap densities and lifetimes. Second Harmonic Generation is Contactless! SiO2 Si Experimental Schematic for Studying STO:  Experimental Schematic for Studying STO Time Dependent SHG as a Function of Power Shows Rise and Saturation of the Signal for STO:  Time Dependent SHG as a Function of Power Shows Rise and Saturation of the Signal for STO 351 mW 292 mW 272 mW 212mW Interpreting the STO Measurements:  Interpreting the STO Measurements 2 1  y = 1+2(1-exp(-x/)) 1: the non-time dependent interface contribution χ(2) 2: represents the time-dependent part of the SHG signal (χ(3)) τ : time constant for trapping the electrons ∆1 and ∆2 increase approximately with the square of the incident laser beam power for the STO sample:  ∆1 and ∆2 increase approximately with the square of the incident laser beam power for the STO sample I 2 (t)  |(2) + (3) EDC (t) |2 (I  )2 The Band Diagram of STO:  The Band Diagram of STO Band offsets are important to device design.:  Band offsets are important to device design. Silicon on hafnium oxide Band Offset = 1.1 eV + 1.5 eV = 2.6 eV Example: Slide13:  Band offsets are crucial parameters in spin-carrier dynamics and spin-polarized carrier transport across semiconductor heterostructures. For many semiconductor interfaces under investigation for spintronics application these band offsets are experimentally not well characterized. Band offsets studies help us understand the photon-energy dependent measurements of optically excited spin carrier tunneling (Spin Leakage Currents) through thin insulators through the band bending due to charge redistribution. Motivation Slide14:  At Vanderbilt, we have successfully measured band offsets at semiconductor interfaces using both IPE and SHG. We are now in the process of applying these techniques to spin interesting materials. Band Offset Measurements Approaches Slide15:  OPA or FEL Sample A Low intensity Metal contacts Tunable output from Samples provided by Univ. Würzburg Band Offset Measurements Using IPE Slide16:  Internal Photoemission from InAs/CdMnSe Structure Electrons tunneling through the ZnTe interlayer These figures tell us that the photocurrent is linear with power. Slide17:  First Band Offset Measurements of InAs/CdMnSe Using Internal Photoemission This value is consistent with literature value of the InAs gap plus the difference in the conduction band energies. Spintronics:  Spintronics A spin device uses not only the electronic charge of its carriers, like conventional devices, but also their spin degree of freedom. Quantum Computing Basics:  In conventional computing, the fundamental carrier of information is the bit. It has possible states of either “0” or “1”. However, the fundamental carrier of quantum information, the qubit, has possible qubit states of any superposition described by the wavefunction:  = a |0 + b |1 Quantum Computing Basics Shor’s Algorithm:  Shor’s Algorithm A quantum computer able to perform Peter Shor's algorithm could break present cryptography methods in just a few seconds. IBM demonstrated the algorithm by factoring the number 15 using 7 qubits. Slide21:  Spintronics: The Characterizaton of Spin Relaxation in GaAs Work at IBM Almaden Slide22:  Optical Detection of Spin Polarization in Quantum Wells With this light, we are able to determine if the electrons that traveled to the well were last spin up or spin down, and thus can get a measure of spin relaxation. GaAs GaAs InGaAs h n electrons holes An efficient place to trap electrons and holes Wavelength is shifted, thus light is able to travel through substrate Good separation between light and heavy holes HH LH Slide23:  Spectra Physics Ti-Sapphire Model 3900S attenuator Jobin Yvon Spectrometer camera   8 7mW Temperature Control Magnet Power Supply Remote Source Meter Pump: Spectra Physics Argon Ion Laser Model 2020 CCD vert. pol. Variable Retarder /4 Variable Retarder a /4 b 3/4 lens lens lin. pol. sample Oxford Spectromag Experimental Setup for Measuring Spin Relaxation in GaAs Slide24:  Intensity vs. Wavelength plots show change in polarization (5 quantum wells with 8 nm width, separated by 15 nm intrinsic GaAs) The letters “a” and “b” stand for right and left circularly polarized light detection. Each will detect either - or +, corresponding to spin of ±½. S is the polarization, measured by the difference of the intensity peaks over the sum. S = --+ -++ Slide25:  Injection into bulk GaAs Light hole resonance Temperature and wavelength dependence plots show relaxation in polarization in bulk GaAs. Heavy hole resonance Injection into quantum well subbands Two mechanisms: Hole relaxation at > 5K [occurs in both GaAs (800 nm)and QW(900 nm)] Electron relaxation at < 5K (bulk GaAs) The error for each point is ± .01. Slide26:  What Did We Learned at IBM? Our optical method is very useful in determining spin polarization. We see evidence of this from the low polarization from the quantum well and the dependence on magnetic fields. Spin relaxation does occur with an external magnetic field. Relaxation is strongest around 800 nm, which relates to inside the bulk GaAs. Not as much relaxation occurs in the quantum wells. This is important, because we want to understand the electrical properties of the GaAs, not the quantum wells. At temperatures greater than about 5 K, something causes the polarization to be the opposite sign of thermal equilibrium for electrons. This is probably due to the effect of thermally induced hole spin relaxation. We note that our optical method injects both electrons and holes into the sample, whereas the electrical setup would only inject electrons. Slide27:  Measurements of Spin Dynamics and Band Offsets at Interfaces at Vanderbilt University Motivations :  Motivations Multilayer III-IV semiconductors doped with Mn and digital Mn-based semiconductor multilayers reveal optically induced ferromagnetism (dilute magnetic semiconductors) Due to the interface sensitivity of SHG, pump-probe SHG is a unique tool for monitoring the ultrafast spin and carrier dynamics with interlayer spatial resolution. The pump-probe SHG directly monitors interface magnetism and will uniquely be able to probe the effects of the interfaces on the spin dephasing and g-factor values. Pump-probe Method allows Separation of Injection from SHG:  Pump-probe Method allows Separation of Injection from SHG GaAs(100nm)/GaSb(400nm) 2. GaAs(100nm)/GaSb(500nm)/InAs(20nm) All samples grown by Molecular Beam Epitaxy Separation of injection from second-harmonic generation Can follow directly carrier movement while injection beam is blocked Slide30:  Second Harmonic Generation Dependence on Interfacial Electric-fields and Magnetic-fields Nonlinear polarization at the frequency 2 in the presence of quasi-dc interfacial field Total Intensity Extract electric- and magnetic-field-induced contributions Induced SHG intensity Linear and Nonlinear Optical Probing of Spin-Polarized Carrier Dynamics in Multilayer Semiconductors:  Linear and Nonlinear Optical Probing of Spin-Polarized Carrier Dynamics in Multilayer Semiconductors Semiconductor Heterostructures Photoinduced magnetization Magneto-optical Kerr Effect Magnetization-induced Second Harmonic Generation 2w w (a) Ti:Sapphire (150 fs) (b) OPAs (0.06-6.0 eV) (c) FEL (0.1-1.2 eV) Slide32:  Induced magnetization Not Observed in GaAs/GaSb heterostructures GaAs(100 nm)/GaSb(400 nm); T=4.3 K Pump: +-polarized 1.55-eV light (120 mW) Probe: p-polarized 1.55-eV light (120 mW) Slide33:  Induced magnetization Observed in GaAs/GaSb/InAs heterostructures GaAs(100 nm)/GaSb(500 nm)/InAs(20 nm) Pump: +-polarized 1.55 eV light (120 mW) Probe: p-polarized 1.55 eV light (120 mW) Slide34:  InAs GaSb GaAs 0.41 eV 0.81 eV 1.52 eV E C E V Band-gap alignment for GaAs/GaSb/InAs heterostructure and optically induced charge separation at the interfaces Bending of initial energy profiles lead to coherent spin transport across GaAs/GaSb/InAs heterostructure:  Bending of initial energy profiles lead to coherent spin transport across GaAs/GaSb/InAs heterostructure GaAs GaSb InAs MFISH Conclusions:  MFISH Conclusions First application of pump-probe SHG measurements for monitoring the optically-induced magnetization in non-magnetic multilayer semiconductor Pump-Probe SHG measures Local magnetic fields while MOKE and Faraday Rotation measures Bulk magnetic fields Coherent spin transport across GaAs/GaSb/InAs heterostructure Magnetization dynamics involves spin density evolution at the interfaces and spin dephasing Interfacial electric fields from charge separation which arise due to the thermalization/cooling of electrons and holes at the interfaces within ~ 2 ps Transport and Injection times for spin polarized electrons ~15 ps Spin relaxation times in the InAs layer ~100 ps Summary:  Summary Electron and hole carrier dynamics - Electric field-induced second harmonic (EFISH) generation - Internal photoemission (IPE) Spin carrier dynamics - Photoluminescence - Magnetic field-induced second harmonic (MFISH) generation Thank you!:  Thank you! SiO2 has a low dielectric constant! :  SiO2 has a low dielectric constant! In the MOS device, the gate electrode, gate oxide, and silicon substrate form a capacitor. High capacitance is required to produce an efficient transistor. Cgate = K0A / d K = dielectric constant, 0 = permittivity of vacuum A = area of capacitor, d = dielectric thickness Experimental Setup for IPE:  Experimental Setup for IPE Current Amplifier Boxcar Integrator Computer Ge Attenuator Doubling Crystal Water Cell Sample Signal Reference Polarizer Lens Lens Detector IBM 7 Qubit Quantum Computer:  IBM 7 Qubit Quantum Computer IBM chemists designed and made a new molecule that has seven nuclear spins -- the nuclei of five fluorine and two carbon atoms -- which can interact with each other as qubits, be programmed by radio frequency pulses and be detected by nuclear magnetic resonance (NMR) instruments similar to those commonly used in hospitals and chemistry labs. Nuclear magnetic resonance involves the interaction of atomic nuclei placed in an external magnetic field with an applied electromagnetic field oscillating at a particular frequency. Magnetic conditions within the material are measured by monitoring the radiation absorbed and emitted by the atomic nuclei. Entanglement:  Entanglement Entangled qubits do not have individual quantum states. Entangled objects act as though they are connected to each other, no matter how far apart. This cannot be used to send signals faster than the speed of light. Error Correcting:  Error Correcting Quantum states are fragile, easily destroyed by noise or stray interactions. Repetition Strategy which works for classical computing (seen right) does not work for quantum computing due to the fact that qubits in unknown states cannot be perfectly cloned. Slide44:  h e h n InGaAs GaAs: n i i p Optical Detection of Spin Polarization Produced by Tunnel Injection CoFe CoFe GaAs/InGaAs/GaAs hot e Alumina tunnel barrier external magnetic field This is a very simple spin-selective device. Electrons of one angular momentum are favored as they travel past the Schottky barrier due to the external magnetic field and spin filtering in the CoFe. They then fall into the quantum well and recombine with holes. Emission from the quantum well gives a good probe of spin. Slide45:  Selection Rules for Optical Transitions Optical selection rules provide a means of monitoring spin polarization of electrically and optically-injected carriers via polarized luminescence. They also allow polarized spins to be created optically via laser excitation. GaAs E-k Diagram:  GaAs E-k Diagram The GaAs bandgap at 0 K is 1.51 eV and at 300 K it is 1.43 eV. Two Spin Relaxation Mechanisms:  Two Spin Relaxation Mechanisms D’Yakonov-Perel: (is dominant in InGaAs at room temperature) - It is based on the breaking of spin degeneracy as a consequence of spatial inversion The spin-orbit interaction then results in a small splitting of the conduction band Equivalent to a magnetic field, which causes a precession of spins Scattering of the electrons change momentum which leads to rotation and allows spins to flip Elliot-Yafet: Considers the effect of spin-orbit coupling on normal electron-phonon and electron-impurity scattering Consequence of mixing of the valence band wave functions with conduction band wave functions at finite k values This mixing allows electrons to spin flip due to momentum scattering from optical and acoustic phonons and/or impurities Properties of GaAs :  Properties of GaAs  Atoms/cm3  4.42E22  Atomic Weight  144.63  Breakdown field (V/cm)  ~4E5  Crystal structure  Zincblende  Density (g/cm3) 5.32 Dielectic Constant 13.1 Nc (cm-3) 4.7E17 Nv (cm-3) 7.0E18 Effective Mass, m*/m0   Electrons 0.067   Holes                m*lh 0.08                m*hh 0.45 Electron affinity, c (V) 4.07 Energy gap (eV) at 300K 1.424 Intrinisic carrier conc. (cm-3) 1.79E6 Intrinsic Debye length (um) 2250 Intrinsic resistivity (W-cm) 1E8 Lattice constant (A) 5.6533 Linear coefficient of thermal expansion,  DL/(L DT) (C-1) 6.86E-6 Melting point (C) 1238 Minority carrier lifetime (s) ~1E-8 Mobility (drift)  (cm2/V-s) 8500 400 Optical-phonon energy (eV) 0.035 Phonon mean free path (A) 58 Specific heat (J/g C) 0.35 Thermal conductivity (W/cmC) 0.46 Thermal diffusivity (cm2/s) 0.44 Faraday Effect Kerr Effect:  Faraday Effect Kerr Effect Results when light is: - transmitted through a sample reflected off a sample Larmor frequency Rotation in plane of polarization Ellipticity change (used when sample not transparent) longitudinal polar transverse Accomplishments By David Awschalom:  Accomplishments By David Awschalom Photoluminescence used to measure Magnetization Electron spin packets are mobile D. Awschalom et al., Phys. Today 52, 33 (1999). The CMASS MFISH experiments will be expanded, by applying an external magnetic field to the system. The SHG signal is expected to exhibit coherence oscillations in near-interface magnetization similar to that observed in the bulk by David Awschalom. This will be the first time these oscillations will have been observed using SHG.

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