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

2. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 2 outline 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 „Spin diffusion and transport in (110) GaAs microcavity structures“

3. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 3 motivation Aim: transport and manipulation of spins up to LN 2 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 IDT S rf N undoped QW SAW electron photons V gate M light inlight out

4. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 4 requirement a: long spin-lifetimes 1)Transport by SAW: -> enhanced carrier lifetime -> enhanced spin lifetime (reduced exchange interaction of electrons and holes -> suppression of the Bir-Aronov-Pikus spin dephasing mechanism ) hLhL 2)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 B int (k e ) of spin-orbit-interaction (001) QW: (110) QW:

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

6. 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 QW cavity resonance wavelength C

7. 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) d QW ~20 nm:

8. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 8 in-situ reflection measurement Deviation of the cavity resonance wavelength from the nominal value ( c ) is smaller than the MBE inherent lateral thickness variation (due to flux inhomogeneities) Real-time growth rate corrections MBE growth of exactly tuned cavities

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

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

11. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 11 T = 80 K P SAW = + 8.6 dBm P=790nm = 150 µW (100µm pinhole, 20x objective) spin transport B = 0 spins || z no in-plane spin component  z ~ 8 ns Lamor frequency g e = -0.36,  = 1.27/ns Spin polarisation d 1/e = 25.2 µm B > 0 -> spin precession -> in-plane component s y  y << 8 ns

12. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 12 Hanle effect measurement Measurement of spin diffusion along [001] in dependence of B [1-10] B ext y [001] x [1-10] s Precession leads to an in-plane spin component. ->  = f(B) if  y <>  z -> allows for an estimation of  y T = 80 K, P=790nm = 25 µW in-plane: s,y 0.7 ns out-of-plane:  s,z 8.4 ns (@ 8.6 dBm) T 2 * = 1.3 ns (g = -0.36) T = 80K

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

14. DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 14 summary /outlook 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 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). summary