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Tuning eigenstate-energies of InGaAs Quantum-Dots using lateral electric fields

Tuning eigenstate-energies of InGaAs Quantum-Dots using lateral electric fields. W. Prestel , H. Krenner, J. J. Finley St. Petersburg – JASS 2004. Outline. Introduction Growth of self-assembled Quantum Dots (SAQDs) electric fields on QDs

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Tuning eigenstate-energies of InGaAs Quantum-Dots using lateral electric fields

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  1. Tuning eigenstate-energies of InGaAs Quantum-Dots using lateral electric fields W. Prestel, H. Krenner, J. J. Finley St. Petersburg – JASS 2004

  2. Outline • Introduction • Growth of self-assembled Quantum Dots (SAQDs) • electric fields on QDs • Work in progress:single QDs in lateral electric fields • Benefit of lateral electric fields • structural information about QDs • Implementation of CNOT Gate

  3. Volmer-Weber Frank-van der Merwe Stranski-Krastanov Self-Assembly of Quantum Dots   • similar to rain drops on window • used for „usual“ heterostructures: • unstrained material systems i.e. GaAs/AlAs • strained material systems i.e. In(Ga)As/GaAs • particular growth conditions i.e. temperature, In content, growth rate formation ofpseudomorpic layer: Wetting Layer (WL) growth of islands: strain relaxes in islands

  4. In(Ga)As Quantum Dots Lattice constant: • GaAs: 0.57nm • InAs: 0.61nm Lattice mismatch ε = 7% typical surface densities: 0 - 1.000 µm-2

  5. In Ga Growth on unrotated substrate constantIn:Garatio gradually changingIn:Garatio

  6. Overgrowth for optical application Intermixing of materials further processing • no surface states • low band-gap material surrounded by high band-gap matrix material • 0-dimensional confinement • occurs naturally • can also be driven by thermal annealing • change of confinement potential

  7. "real atom" single QD: "artificial atom" Quantum Dots – artificial atoms Band Gap (300K) • Eg,GaAs= 1.411eV • Eg,InAs= 0.356eV » ΔEg up to ~ 1eV

  8. z x,y X E x,y SAQDs – confinement for excitons • optically active exciton (X)states are bound • shell structure • parabolic potential • few particle interaction n = 2 n = 1 n = 1 n = 2

  9. 10nm growth direction Electric fields on QDs pintrinsic dipolecpolarizability QCSE: Quantum Confined Stark Effect vertical ( ) fields: • well investigated • intrinsic dipole p 0 • weak polarizability c lateral ( ) fields: • not investigated in detail • intrinsic dipole p = 0 expected • high polarizability c further investigation

  10. 10nm growth direction ΔE(F) F Fvertical ??? Flateral Electric fields on QDs pintrinsic dipolecpolarizability QCSE: Quantum Confined Stark Effect

  11. Work in progress • Sample Design • model calculations • strength of electrical field • Setup + crash course in PL & PC • Characterization of sample

  12. 2µm Sample Design • Substrate: • In0.5Ga0.5As – QDs in GaAs • surface density: ~ 1.000 QDs/µm2 • undoped substrate • Contact-Design • split-gates • standard optical lithography • contacts-on-top design (2µm gap)

  13. d First Approach • put QDs in Capacitor • 1. order approximation:homogeneous lateral field • realisation of metal-semiconductor junction(pinning) • expected field: U

  14. Stability Problems structure died during measurement

  15. Vacuum GaAs Vacuum Vacuum GaAs GaAs Model calculations on different Designs

  16. Model calculations on different Designs Elateral Evertical

  17. Model calculations – contacts on top • decreasing d increases field • considering homogeneity • trade-off: d = 2µm • extraction of geometry factor • fmidgap ≈ 0.75

  18. temperature dependent IV-Curves Dark current measurement max. fields: 80-130 kV/cm

  19. µPL/µPC - Setup • Spatial Resolution (1µm Spot) • Bias dependent optical spectroscopy(PL and PC) • Temperature: down to 4.2K U

  20. negative external voltage (V) 1.308 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 WL 1.306 Ensemble of QDs 1.304 single QD 1.302 energy (eV) 1.300 1.298 1.296 PL PC 20 30 40 50 60 70 80 90 100 110 electric field (kV/cm) Crash course PL & PC

  21. Bias dependent PL-Spectra • HeNe-excitation (632.8nm) • PL disappears @ 13 kV/cm (3.5V)

  22. Bias dependent PL & PC HeNe excitation (632.8nm)

  23. PC resonant excitation

  24. Sample Design – future plans • 4-terminal-µCapacitor • different crystal directions • top and back contactsforeseen top view

  25. Application • Investigation of shape and alloy profile of buried Dots • Goal in further future:Implementation of CNOT gate

  26. Shape and alloy profile of QDs • no non-invasive characterization of overgrown QDs possible • structural properties determine electro-optical properties

  27. 10nm Definition of Qubits QM implementation of CNOT • 1-Qbit-System:X0 in QD  |10 empty QD  |0 • 2-Qbit-System: Quantum Dot Molecule (QDM):empty dots |00; X0 in lower dot |10; … |01; … |11 • coupling of X0 in QDM via dipole-dipole interaction:  Applying lateral fieldmeans control of ΔE 10nm E|11 = E|01 + E|10 + ΔE

  28. 1 0 0 1 0 CNOT Gate initialization applying gate operation readout 1 on off control bit switches NOT-operation on target bit control bit unaffected by CNOT 00 --> 00 01 --> 01 10 --> 11 0 on off 11 --> 10 target bit target bit changed if control bit is 1 consideration purely classical and logic so far: quantum mechanicalimplementation

  29. w wa+wb wab wba wa wb Implementation initialization applying gate operation readout control of dot occupation • Rabi-oscillation • differentX0-GS-energies PC-Meas b a The above term scheme can be taylored for our needs by applying vertical & lateral fields!!!

  30. Rabi Oscillation

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