dfg spp1285 treffen der projektgruppen, hannover 6./7. juni 2008 1 spin diffusion and transport in...

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 FestkörperelektronikBerlin


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“


motivation

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

IDT Srf

N

undoped QW

SAWelectron

photons

V

gate

M

light in light out


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)

hL

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 Bint (ke) of spin-orbit-interaction

(001) QW: (110) QW:B int sB int slaser

zs

x x

laser

z

s

B int

x

laser

z

s

B int


-> 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!


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


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:


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


SAWs in modified cavity structures

/4 G a Al As0 .93 0 .07

x 5

/4 A l G a As0 .60 0.40

G a Al As0 .93 0 .07

-cavity

/4 A l G a As0 .60 0.40

/4 G a Al As0 .93 0 .07x 4

/4 A l G a As0 .60 0.40

/4 G a Al As0 .93 0 .07

/4 A l G a As0 .60 0.40

/4 A l G a As0 .60 0.40

3 /4 G a Al As0 .93 0 .07

G a Al As0 .93 0 .07

`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


spin diffusion and spin transport measurements

generationpoint G

spin diffusion

IDTswitched off

spin transport

generationpoint G

IDTswitched on

SAWs

B

measurements with in-plane magnetic field B

B y |

| [0

01]

y |

| [0

01]

v = 3 µm/ns

IlIr

rl

rl

II

II

T = 80 K


T = 80 KPSAW = + 8.6 dBm

P=790nm = 150 µW

(100µm pinhole, 20x objective)

spin transport

B = 0spins || zno in-plane spin componentz ~ 8 ns

Bg BeLamor frequency ge = -0.36, = 1.27/ns

Spin polarisation d1/e = 25.2 µm

SAWez v

xdxs cos)/exp( /1

B > 0-> spin precession ->in-plane component sy

y << 8 ns


Hanle effect measurement

Measurement of spin diffusionalong [001] in dependence of B[1-10]

Bext

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

2*21

)0()(

TB s

Bg B

in-plane:s,y 0.7 ns

out-of-plane: s,z 8.4 ns (@ 8.6 dBm)

ZSYST ,,*2

11

2

11

YSZS ,, nsT YS 7.02

1,

*2

T2* = 1.3 ns (g = -0.36)

T = 80K


influence of strain fields

T = 80 KPSAW = 15 , 23 dBmP=790nm = 58 µW

(100µm pinhole, 20x objective)

XZxS skC 3

1

SAW strain field-> internal magnetic field along (1-10)

-> strong influence on spin polarization

transport by intense SAW fields:


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

dfg spp1285 treffen der projektgruppen, hannover 6./7. juni 2008 1 spin diffusion and transport in...

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