GaAs QUANTUM DOT COM Ray Murray

Why Quantum Dots? Novel “atom-like” electronic structure Immunity to environment Epitaxial growth Well established device fabrication Scalable Single Photon SourcesPotential as qubits

Density of states bulk QW QWi QD

Molecular Beam Epitaxy substrate As In Ga Growth of Quantum Dots t< 1.7 ML GaAs ‘capped’ t> 1.7 ML Scanning TEM image

Optical Properties Relaxation Escape E0E0 E1E1 E2E2 Wavelength (Å) Intensity (arb. units) E0E0 E1E1 E2E PL intensity time (ns)

Single photon sources Santori et al. Phys Rev Lett 86, 1502 (2001)

image of quantum dot layer in an Atomic Force Microscope n-contact p-contact electron injector quantum dot layer substrate/buffer hole injector insulator single photon emission mesa aperture n-contact Conventional p-i-n diode containing layer of quantum dots Science 295, 102 (2002) 1  m quantum dots 15 x 5 nm Single Photon Emitting Diode 1. Electrically driven (easy to use) 2. Fab. similar to LED (cheap) Toshiba Research p-contact

Controlling dot density InAs/GaAs QD growth under typical conditions yields QD densities of ~2-5 x cm -2 For single photon devices need QD density of ~10 8 cm -2 Reduction in InAs deposition rate leads to reduction in QD density Alloing et al. Appl Phys Lett 86, (2005)

PL from etched mesas 4.2 K PL from a 2-µm diameter etched pillar incorporating a low density QD layer  emission from single QDs can be resolved X 300 K Reflectivity from planar cavity

x V(x,y) -a a S1S1 S2S2 B(z) E(x) y aBaB QD Electron spin S as “qubit” Why Spin? QM property – interaction only with QM forces No interaction with electrostatic forces Easy to create, manipulate and detect spins in semiconductors Burkard, Loss and DiVincenzo Phys.Rev.B 1999

Spin states in III-V semiconductors p s p-antibonding s-antibonding s-bonding p-bonding CB VB EgEg Energy k hh lh so Energy k J=3/2 J=1/2 so -3/2 +3/2 hh -1/2 +1/2 σ+ σ- -1/2 +1/2 lh lh

H N : no spin conservation - Spin is irrelevant to the dynamics - Spin need not be conserved during relaxation H S : spin is always conserved - Spin lifetimes are long compared to radiative lifetimes - Spin is conserved during relaxation X1 GS X1 GS X1 GS HNHN HSHS Spin conservation in QDs- Pauli blocking Le Ru et al. Phys.Stat.Sol. (2003)

Probing spin states with light  rad ~500ps  rel <100ps Ts~900ps Spin lifetime reduced by acoustic phonon scattering Gotoh et al. J.J.Appl.Phys. 42 (2003)

Spin-LED structure InAs QDs  Fe Emission Fe n-AlGaAs InAs/GaAs QDs p-AlGaAs Inject electrons through Schottky diode into n-i-p LED (injected polarisation from Fe ~ 45%) Ballistic transport: AlGaAs barriers Itskos et al. Appl.Phys.Lett. 88 (2006)

Rotating the spins Faraday Geometry B=0 Oblique Hanle Geometry B>1.4T B<<1T Faraday geometry rotates spins in the metal Oblique Hanle geometry rotates spins in the semiconductor Magnetisation axis Injected spin

The oblique Hanle effect 45° B field S SzSz 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 S 0x

Experiment 1/4 monochromator lin pol Spin injection from Fe into semiconductor Spin lifetime of the ground state exciton

Spin polarisation in the dots ~ 7.5% From Hanle half-width B 1/2 obtain using g* =-1.7, obtain spin lifetime of ~300 ps Spin injection from the Fe to AlGaAs of 20 ± 3%

Spin relaxation mechanisms 1.D’yakonov-Perel – k 3 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 D’yakonov and Perel, in Optical Orientation

Further work Faraday geometry measurements Current dependence Temperature dependence Optical injection: oblique Hanle effect P-doped quantum dots Single Photon Sources Lower dot density Investigate regular arrays of QDs Target 10% efficient fibre compatible sources Spin LED

Acknowledgements Steve Clowes and Lesley Cohen Grigorios Itskos, Edmund Clarke, Patrick Howe, Edmund Harbord, Peter Spencer, Richard Hubbard and Matthew Lumb Paul Stavrinou Wim Van Roy and Peter Van Dorpe IMEC Martin Ward and Andrew Shields Toshiba Research Europe