QUANTUM DOT COM GaAs Ray Murray Why Quantum Dots? Novel atom-like electronic structure Immunity to environment Epitaxial growth Well established device fabrication Scalable Single Photon Sources Potential as qubits Density of states bulk QWi
QW QD Growth of Quantum Dots Scanning TEM image Molecular Beam Epitaxy Ga Ascapped t> 1.7 ML GaAs substrate t< 1.7 ML In
Escape 350 Wcm 0 0 2 4 -2 6 2000 4000 6000 0 time (ns) Time (ps)
2000 Time Single photon sources Santori et al. Phys Rev Lett 86, 1502 (2001) Single Photon Emitting Diode single photon emission Conventional p-i-n diode containing layer of quantum dots quantum dot layer n-contact electron injector
p-contact 1. Electrically driven (easy to use) 2. Fab. similar to LED (cheap) insulator hole injector substrate/buffer aperture mesa image of quantum dot layer in an Atomic Force Microscope quantum dots n-contact
p-contact Science 295, 102 (2002) 15 x 5 nm 1 m Toshiba Research Controlling dot density InAs/GaAs QD growth under typical conditions yields QD densities of ~2-5 x 1010 cm-2 For single photon devices need QD density of ~108 cm-2 Reduction in InAs Alloing et al. Appl Phys Lett 86, 101908 (2005) deposition rate leads to
reduction in QD density PL from etched mesas X 300 K Reflectivity from planar cavity 4.2 K PL from a 2-m diameter etched pillar incorporating a low density QD layer emission from single QDs can be resolved Electron spin S as qubit B(z) S1 Why Spin? QD y S 2
E(x) aB V(x,y) -a a x Burkard, Loss and DiVincenzo Phys.Rev.B 1999 QM property interaction only with QM forces No interaction with electrostatic forces Easy to create, manipulate and detect spins in semiconductors Spin states in III-V semiconductors
Energy -1/2 p + p-antibonding +1/2 - s-antibonding Eg p-bonding s -3/2
so VB Spin conservation in QDs- Pauli blocking HN: no spin conservation HN X1 GS HS (q11 ) - Spin is irrelevant to the dynamics - Spin need not be conserved during relaxation HS: spin is always conserved - Spin lifetimes are long compared to radiative lifetimes
GS X1 GS (q 20 ) - Spin is conserved during relaxation 0.30 T=10 K 0.25 (q 21 ) Ratio I(X1) / I(GS) X1 0.20
0.15 0.95 1.00 1.05 1.10 1.15 Energy (eV) 0.10 0.05 Le Ru et al. Phys.Stat.Sol. (2003) T=10 K
0.00 Integrated PL Intensity (a. u.) Probing spin states with light rel<100ps rad~500ps Ts~900ps Gotoh et al. J.J.Appl.Phys. 42 (2003) Spin lifetime reduced by acoustic phonon scattering Spin-LED structure Itskos et al. Appl.Phys.Lett. 88 (2006) Emission Fe n-AlGaAs InAs/GaAs
QDs p-AlGaAs Fe InAs QDs Inject electrons through Schottky diode into n-i-p LED (injected polarisation from Fe ~ 45%) Ballistic transport: AlGaAs barriers Rotating the spins B=0 Magnetisation axis Injected spin Faraday Geometry B>1.4T
Oblique Hanle Geometry B<<1T Faraday geometry rotates spins in the metal Oblique Hanle geometry rotates spins in the semiconductor The oblique Hanle effect Sz S0x S 45 B field Initially, no overall component of the spin in the direction of the emission Apply oblique magnetic field: spin precesses about the field Introduces a component of the spin in z-direction Leading to circularly polarised emission
Experiment 1/4 lin pol monochromator Spin Spin injection lifetime of the Fe ground state exciton from into semiconductor From Hanle half-width B1/2 obtain TS
g * B B1/ 2 using g* =-1.7, obtain spin lifetime of ~300 ps S 0 x (7.5 0.7)%. TS Spin polarisation in the dots ~ 7.5% Spin injection from the Fe to AlGaAs of 20 3% Spin relaxation mechanisms 1. Dyakonov-Perel k3 term splits the conduction band 2. Elliott-Yafet band mixing through k.p interaction 3. Exchange interaction connecting electrons/holes of opposite spin 4. Hyperfine interaction with nucleii Investigating spin decoherence A similar device emits at lower currents Oscillations with
magnetic field Cascade process Further work needed: PL data Dyakonov and Perel, in Optical Orientation Further work Single Photon Sources Lower dot density Investigate regular arrays of QDs Target 10% efficient fibre compatible sources Spin LED Faraday geometry measurements Current dependence
Temperature dependence Optical injection: oblique Hanle effect P-doped quantum dots Acknowledgements Grigorios Itskos, Edmund Clarke, Patrick Howe, Edmund Harbord, Peter Spencer, Richard Hubbard and Matthew Lumb Paul Stavrinou Steve Clowes and Lesley Cohen Martin Ward and Andrew Shields Toshiba Research Europe Wim Van Roy and Peter Van Dorpe IMEC
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