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Spin diffusion and transport in (110) GaAs microcavity structures

Spin diffusion and transport in (110) GaAs microcavity structures. K. Biermann, R. Hey, and P. Santos. Paul-Drude-Ins titu t für Festkörperelektronik Berlin. outline. „Spin diffusion and transport in (110) GaAs microcavity structures“. motivation, basic concepts MBE growth

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Spin diffusion and transport in (110) GaAs microcavity structures

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  1. Spin diffusion and transport in (110) GaAs microcavity structures K. Biermann, R. Hey, and P. Santos Paul-Drude-Institut für FestkörperelektronikBerlin DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  2. outline „Spin diffusion and transport in (110) GaAs microcavity structures“ • motivation, basic concepts • MBE growth • surface acoustic waves (SAWs) • in microcavity structures • spin diffusion and spin transport measurements • - external applied magnetic field • - Hanle effect measurement • - intense SAW fields • summary / outlook DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  3. motivation light in light out rf S IDT M V undoped QW gate N SAW electron photons Aim: transport and manipulation of spins up to LN2 temperature (2007-2009) / RT (2009-2011) Basic concept: Requirements: a) long spin-lifetimes b) effective conversion of circularly polarized light into spin-polarized carriers & vice versa DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  4. requirement a: long spin-lifetimes hwL • Transport by SAW: • -> enhanced carrier lifetime • -> enhanced spin lifetime • (reduced exchange interaction of electrons and holes -> • suppression of theBir-Aronov-Pikus spin dephasing mechanism) • Transport of spin-polarized carriers in (110) QWs: • dominant spin-lifetime limitting process (Dyakonov-Perel mechanism) • in semiconductors without inversion symmetry: •  spin induced splitting of the conduction band •  effective magnetic field Bint (ke)of spin-orbit-interaction • (001) QW: (110) QW: DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  5. requirement b:effective light-carriers conversion -> insertion of the QW into a micro-resonator(cavity) structure ________________ ___upper DBR____ cavity with QW _ ___lower DBR____ ____substrate____ lC -> precise control of layer thicknesses mandatory! DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  6. MBE growth: challenges MBE challenges: a) Growth of high structural perfection on (110) oriented GaAs substrates <-> spin-dephasing <-> transport of carriers (eg. recombination at local potential minima) b) Growth of exactly tuned cavity structures at working temperature: QW-emission wavelength lQW<=> cavity resonance wavelength lC DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  7. MBE growth of high-quality (110) cavity structures • Compared to (001) GaAs based • structures: • stronger tendency to facetting • reduced critical thickness • Growth parameter for (110) GaAs • based structures: • low Tg (~ 490 °C) • high V/III BEP ratio (~ 45) • MEE grown smoothing buffer layer • growth interruptions • in-situ annealing steps • whole cavity structure is composed • of short-period-super-lattices (SPSLs) 10K-PL of a GaAs-QW in a cavity (out of resonance) dQW~20 nm: DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  8. MBE growth of exactly tuned cavities in-situ reflection measurement Real-time growth rate corrections Deviation of the cavity resonancewavelength from the nominalvalue (lc) issmaller than the MBE inherent lateralthickness variation (due to flux inhomogeneities) DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  9. SAWs in modified cavity structures 1-l ZnO • `Al-reduced´cavity structure: • reduction of Al-content in every layer • ¾-l Ga-rich layers • 2-l (instead of 1-l) Ga-rich cavity • -> propagation of SAWs is supported QW Additional deposition of a 1-l thick ZnO cap layer (sputtering) to increase the piezoelectric potential of the SAWs. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  10. spin diffusion and spin transport measurements B B measurements with in-plane magnetic field B T = 80 K spin diffusion spin transport Il Ir v = 3 µm/ns y || [001] y || [001] generation point G generation point G SAWs IDT switched off IDT switched on DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  11. spin transport B > 0 -> spin precession -> in-plane component sy ty << 8 ns Lamor frequency ge = -0.36, W = 1.27/ns Spin polarisation d1/e = 25.2 µm T = 80K PSAW = + 8.6 dBm Pl=790nm= 150 µW (100µm pinhole, 20x objective) B = 0 spins || z no in-plane spin component tz ~ 8 ns DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  12. Hanle effect measurement x [1-10] y [001] T = 80 K, Pl=790nm= 25 µW Measurement of spin diffusion along [001] in dependence of B[1-10] Bext s Precession leads to an in-plane spin component. -> t = f(B) if ty <> tz -> allows for an estimation of ty T2* = 1.3 ns (g = -0.36) in-plane: ts,y 0.7 ns out-of-plane: ts,z 8.4 ns (@ 8.6 dBm) T = 80K DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  13. influence of strain fields transport by intense SAW fields: T = 80K PSAW =15 , 23 dBm Pl=790nm = 58 µW (100µm pinhole, 20x objective) SAW strain field -> internal magnetic field along (1-10) -> strong influence on spin polarization DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

  14. summary /outlook summary • growth and processing of high quality (110) GaAs • cavity structures, that support SAW propagation • spin diffusion and tranport measurements at T=80K • - effect of an external magnetic field and of SAWs on • spin polarisation • - estimation of the in-plane (1 ns) and out-of-plane • spin-lifetimes (8 ns). • processing of narrow lateral channels (along [001]) • to increase in-plane spin lifetime • (deep etching / metal stripes on top of samples) • implementation of electric / magnetic gates • replace GaAs QW by InGaAs QW • (higher confinement) outlook DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008

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