Spin diffusion and transport
in (110) GaAs
microcavity structures
K. Biermann, R. Hey, and P. Santos
Paul-Drude-Institut
für Festkörperelektronik
Berlin
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
1
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
2
motivation
Aim:
transport and manipulation of spins up to
LN2 temperature (2007-2009) / RT (2009-2011)
Basic concept:
rf
light in
IDT
light out
V
M
undoped
QW
SAW
gate
electron
photons
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
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requirement a: long spin-lifetimes
1) Transport by SAW:
-> enhanced carrier lifetime
hwL
-> enhanced spin lifetime
(reduced exchange interaction of electrons and holes ->
suppression of the Bir-Aronov-Pikus spin dephasing mechanism)
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:
laser
Bint
s
(110) QW:
laser
s
s
z
Bint
z
x
x
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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requirement b: effective light-carriers conversion
insertion of the QW into a micro-resonator (cavity) structure
M4-2142 I z = 18.9 nm I no of DBRs: 18/ 4
dTmatch
dlC
K
nm
1
840
(e
lC
-hh
1
)
860
~ 10
Tmatch
QW
________________
___upper DBR____
cavity
with QW
_
___lower DBR____
____substrate____
l(nm)
->
s.
e
r
y
it
820
lC
cav
100
200
T (K)
300
-> precise control of layer thicknesses mandatory!
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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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
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MBE growth of high-quality (110) cavity structures
Compared to (001) GaAs based
structures:
• stronger tendency to facetting
• reduced critical thickness
(GaAs)/(Al,Ga)As (110)
TG= 480C
PL-Int. (arb. u.)
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:
FWHM  0.7 meV
1911
1.520
1.525
1.530
Energy (eV)
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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MBE growth of exactly tuned cavities
M4_2147 measured
in-situ reflection measurement
calculated
400
350
Filmetrics F30
lC
light out
1l- cavity
light source
300
growth time (min)
‘Y’- fiber
Spectrometer
5 mirror
pairs
reflected light
heatable
viewport
(Createc
HW-63-40)
PC
rotating
sample
250
18.5 mirror
pairs
200
( Al0.20Ga0.80As /
Al0.95Ga0.05As )
150
100
MBE
growth
chamber
(V80)
50
0
#M4-2147 (at RT)
750 800 850 900 950 750 800 850 900 950 GaAs (110)
substrate
l (nm)
l (nm)
lc
reflectivity
1.0
Real-time growth rate corrections
0.5
Deviation of the cavity resonance wavelength
from the nominal value (lc) is smaller than the
MBE inherent lateral thickness variation
(due to flux inhomogeneities)
787.8 801.4
center
edge
(of a 2" wafer)
0.0
700
800
l (nm)
900
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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SAWs in modified cavity structures
1-l ZnO
l/4 Al0.60Ga0.40As
QW
x4
Ga0.93Al0.07As
l-cavity
Ga0.93Al0.07As
l/4 Al0.60Ga0.40As
3l/4 Ga0.93Al0.07As
l/4 Al0.60Ga0.40As
l/4 Ga0.93Al0.07As
l/4 Al0.60Ga0.40As
l/4 Ga0.93Al0.07As
l/4 Al0.60Ga0.40As
x5
• 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
Additional deposition of a 1-l thick ZnO cap layer
(sputtering) to increase the piezoelectric potential of
the SAWs.
M4_2170 + 424nm ZnO
normalized intensity ( w/2 )
l/4 Ga0.93Al0.07As
`Al-reduced´cavity structure:
1
10
(110)-GaAs (220)
0
10
-1
10
-2
ZnO (0002)
10
17.2
SPSLs
-3
10
ZnO (0004)
36.3
-4
1x10
-5
1x10
-6
10
15
20
25
30
35
w (°)
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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spin diffusion and spin transport measurements
T = 80 K
spin diffusion
IDT
switched off
Il  I r

Il  I r
v=
3 µm/ns
B
generation
point G
y || [001]
generation
point G
Il
Ir
y || [001]
B
spin transport
SAWs
IDT
switched on
measurements with in-plane magnetic field B
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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spin transport
B=0
B = 40 mT
0.15
T = 80 K
PSAW = + 8.6 dBm
l1/e = 25.2 µm (8.4 ns)
0.10
Pl=790nm = 150 µW
(100µm pinhole, 20x objective)
B=0
spins || z
no in-plane spin component
tz ~ 8 ns

0.05
0.00
-0.05
0
5
10
15
20
25 30
y[001] (µm)
35
40
B>0
-> spin precession ->
in-plane component sy
ty << 8 ns
F:\biermann\data\Probendaten\R0619\M4_2170\M4_2170B_A12\Jan2408-002_003-f(y B) Graph1 16.04.2008 17:27:27
Lamor frequency
Spin polarisation

ge  B B

ge = -0.36,  = 1.27/ns
  x 

s z  exp( x / d1/ e )  cos
 vSAW 
d1/e = 25.2 µm
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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Hanle effect measurement
T = 80 K, Pl=790nm = 25 µW
Measurement of spin diffusion
along [001] in dependence of B[1-10]
s
x [1-10]
0.03

Bext
0.04
0.02
0.01
y [001]
-40
Precession leads to an in-plane
spin component.
-> t = f(B) if ty <> tz
-> allows for an estimation of ty
g B B


t S ,Z  t S ,Y 
1 1  1
1
 

*
T2 2  t S ,Y t S , Z




-20
0
20
B (mT)
40
 (0) 2)2)-1
s(B)
( B)= s,0 * (1s + (T*
ns
T2* = 1.3 T*
ns2 =(g1.3
= -0.36)
 = g  B/(h/2) * 2
B

1  T

1/T*2 = 1/tr+1/ts 2
tr electron scattering lifetime
ts spin lifetime
in-plane:
ts,y
out-of-plane: ts,z
F:\2008_02_29 group meeting\M4_2170b_d11\Feb1408-002
 0.7 ns
 8.4 ns (@ 8.6 dBm)
T = 80K
1 *
T2  t S ,Y  0.7 ns
2
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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influence of strain fields
transport by intense SAW fields:

0.10
PSAW = 0 (B = 0)
PSAW = 15 dBm (B = 0)
0.05
T = 80 K
PSAW = 15 , 23 dBm
Pl=790nm = 58 µW
(100µm pinhole, 20x objective)
0.00
0
10
20
30
40
y[001] (m)
0.10
PSAW = 0 (B = 0)
PSAW = 23 dBm (B = 0)
SAW strain field
-> internal magnetic field
along (1-10)
S 

0.05
0.00
0
10
20
30
40
1
C3  k x  s XZ

-> strong influence on
spin polarization
y[001] (m)
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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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).
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)
DFG SPP1285 Treffen der Projektgruppen, Hannover 6./7. Juni 2008
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