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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
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“
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 3
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
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)
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
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!
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
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
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
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 9
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
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 10
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
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 11
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
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 12
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
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008 13
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:
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
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