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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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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-Institut

für FestkörperelektronikBerlin

DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008


Outline

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


Motivation

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


Requirement a long spin lifetimes

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


Requirement b effective light carriers conversion

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


Mbe growth challenges

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


Mbe growth of high quality 110 cavity structures

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


Mbe growth of exactly tuned cavities

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


Saws in modified cavity structures

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


Spin diffusion and spin transport measurements

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


Spin transport

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


Hanle effect measurement

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


Influence of strain fields

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


Summary outlook

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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