Investigation of Single Semiconductor
Nanowire Heterostructures Using Polarized
Imaging Spectroscopy
(CdS, GaAs/AlGaAs)
Thang Ba Hoang
Department of Physics, University of Cincinnati, Cincinnati, OH
Nanowires for opto-electronic nano-devices
Nanowire highperformance FET
Hybrid single-nanowire
photonic crystal
Nanowire biosensor
Nanowire avalanche
photodiode
Single Photon
Emitter
Nanowire related publications
(Source: ISI Web of Knowledge - http://isiknowledge.com/)
Content
 Introduction
 Nanowires and nanowire growth
 Micro-photoluminescence techniques for studying single nanowires
 Optical properties of single CdS nanowires
 Defect and surface related states
 Temperature dependent photoluminescence
 Photoluminescence imaging of defect states
 Optical properties of single core-shell GaAs/AlGaAs nanowires
 General photoluminescence properties
 Polarization of photoluminescence
 Resonant excitation photoluminescence
 Polarized resonant excitation
 Model for spin relaxation in single nanowires
Nanowires
 Nanowires are nanostructures which have diameters ranging
from 30 nm to 150 nm, length from 5-20 mm
 No significant quantum confinement because wire’s diameter > Bohr exciton
diameter
• For example: CdS Bohr exciton radius ~2.9 nm
GaAs Bohr exciton radius ~10 nm
 What make a nanowire different from bulk material?
 Huge surface-to-volume ratio: (~108 m-1 for nanowires
compared to ~102 m-1 for bulk materials)
 strong sensitivity of the excitons to surface states and
defects and structural inhomogeneities
 Single nanowire measurements are required
Nanowire growth
Vapor-Liquid-Solid (VLS) growth
GaAs Nanowires
Pre-growth
Au
AsH3
Ga
AsH3
450oC, 30min
reactants
GaAs
600oC, 10 min
desorb surface
contaminants
and form eutectic alloy.
wire diameter: controllable by Au catalyst
Core-Shell GaAs/AlGaAs
core GaAs
shell AlGaAs (650oC, 15 min)
Shell AlGaAs: to increase the quantum
efficiency by reducing non-radiative
surface recombination
Field-Emission Scanning
Electron Microscope
(FESEM) image: nanowires
have tapered shape.
Single nanowire studies
GaAs
AlGaAs
~80 nm
~40 nm
Core-shell GaAs/AlGaAs
AFM
Nanowires were removed
from the growth substrate
into solution and deposited
onto a silicon substrate
Photoluminescence (PL)
Conduction band (CB)
E
k
ħ~Eg
Photoluminescence
Valence band (VB)
A light source (laser) excites electrons from the valence band
to the conduction band, leaving holes in valence band.
Electrons and holes recombine to emit photons.
Excitons
 Electron-hole correlation: Exciton
eh+
 Hydrogen-like bound state of an
electron-hole pair
 Smaller binding energy (1/1000)
E
 Larger Bohr radius (100 times)
 Energy spectrum of an exciton
Ebin
En  E g  E K 
Eg
K
vacuum
Eex
n2
Experimental setup
Spectrometer
1.5 mm spatial
resolution
L - Lens
BS - Beam Splitter
DP - Dove Prism
Defocusing
lens
laser
2D CCD image
CCD
DP
BS
spatial
L
Y
Tunable
E
emission energy
T=10 K
X-Y-Z
translation stage
Slit-confocal microscopy
Integrated PL (a.u)
sample
Energy
Dove prism to rotate PL image
Dove Prism (DP) can be used to rotate
image of a nanowire (rotate an image
twice the angle that it rotates through )
PL image
from sample
Dove prism

PL image
rotated
y
slit
Image on
CCD camera
(Edmund Optics Co. http://www.edmundoptics.com/ )
Single CdS nanowires
CdS nanowires
SEM
 Majority of nanowires are
straight and uniform
 Few have significant
irregularities
 Individual nanowire:
~ 50 – 200 nm in diameter
~ 10 – 15 μm long
 Nanowires were removed from the growth substrate into
solution and deposited onto a silicon substrate (for single
wire study)
Nanowire morphology and optical properties
Room temperature
Low temperature
Irregular-shaped wires
Uniform wires
FX
2.45
Room temperature emission
is similar regardless of wire
morphology: Near Band
Edge emission
FX
T=10K
PL Intensity (a. u.)
PL Intensity (a. u.)
T=10K
2.50
Energy (eV)
2.55
2.45
2.50
2.55
Energy (eV)
Low temperature PL differs significantly
Single nanowire studies
We show two representative nanowires:
AFM images of the two nanowires:
Uniform Wire
Irregular Wire
~45 – 50 nm
~100 – 200 nm
A
B
B’
A’
40
AA'
150
30
45 nm
20
10
0
0
2
4
(mm)
6
8
straight and uniform
Height (nm)
Height (nm)
50
BB'
100
150 nm
50
0
0
2
4
6
(mm)
8
with morphological
irregularities
Room-Temp. PL vs Low-Temp. PL
Room temperature
Eg
T=295K
NBE
irregular wire
T=295K
2.35
2.40
2.45
2.50
T=10K
uniform wire
PL Intensity (arb. units)
PL Intensity (arb. units)
uniform wire
Low temperature
2.55
Energy (eV)
• Room Temperature: the PL
spectra of the 2 wires are alike
• Single NBE (Near Band Edge)
line emission
NBE
irregular wire
2.43
2.46
2.49
T=10K
2.52
2.55
2.58
Energy (eV)
• Low Temperature: the PL
properties of the 2 wires differ
significantly
• Sharp lines are attributed to
defect or surface state related
emission
Low-Temp. PL imaging
Only NBE emission with
occasional small energy variation
Compositional/strain fluctuation
Narrow lines occur at specific
localized positions along the wires
Excitons localized to particular
positions along the wire
Time-resolved photoluminescence
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
2.455
Time (ps)
Decay time (ps)
Wavelength (nm)
2.477
2.505
2.530
2.556
505
500
495
490
485
2.455
2.477
2.505
2.530
2.556
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
Energy (eV)
NBE
Energy (eV)
Defect-related
emission
Time-resolved photoluminescence
Normalized PL intensity (a.u.)
Temperature dependence
irregular wire
23
4
1
NBE
T=5K
b)
30K
90K
295K
2.36 2.40 2.44 2.48 2.52 2.56 2.60
Energy (eV)
- Narrow lines start decreasing in
intensity at 30 K and disappear by
90 K
- NBE emission becomes the only
peak as the temperature increases
- Energies of the NBE emission and
the localized states follow the band
edge as temperature increases
- Indicates that localized states are not
deep levels but are excitonic
This is consistent with time-resolved PL measuring of lifetime!
Single Core/shell nanowires
core GaAs
(40nm)
shell AlGaAs
(~20nm)
Core-shell GaAs/AlGaAs
Intensity (counts)
80000
GaAs nw Pls
T=10K
60000
core-shell
40000
20000
uncoated
0
1.44
1.47
1.50
1.53
1.56
Energy (eV)
Bare (uncoated) GaAs nanowires:
low quantum efficiency due to
nonradiative surface recombination
Undoped MBE-grown GaAs epilayer (PL at
5K)
(Heiblum et al. J. Vac. Sci. Tech. B2 233 (1984))
Single nanowire imaging
2D image and extracted PLs:
•
•
•
•
Broad spectrum (FWHM ~25 meV)
Emission energy and line-shape
are uniform
Lower energy shoulder: defectrelated
No evidence of quantum
confinement
Temperature dependent PL
•
•
PL quenches at T > 140K (activation energy ~17 meV)
Presence of non-radiative centers
Dielectric mismatch
Dielectric mismatch:
Excitation:
“Perfect, infinitely long cylinder”
E||  E||0
E||0
2 0
E 
E 0
  0
(Ruda et al.
PRB 72 115308)
Emission:
I||
I
For GaAs:
  0
  12

  0 

2
E||
E 0
E
0

 2 02
I ||
6 02
P
I||  I 
I||  I 
~ 93%
Laser into
page

0
I
PL out of
page
GaAs/AlGaAs NW: PL Polarization
Excitation polarization
photoluminescence
P = 96%
30000
20000
excite parallel
excite perpendicular
40000
10000
0
1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56
Energy (eV)
Intensity (arb)
Intensity (arb)
40000
30000
20000
Emission polarization
P = 82%
detect parallel
detect perpendicular
10000
laser
0
1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56
Energy (eV)
Non equilibrium spin dynamics
Our motivation: dielectric “confinement” of exciton dipole field (D<<):
┴
NW
Exciton
Exciton densities
N║ = N┴
║
Photoluminescence intensities
I║ >> I┴
Spin relaxation time
t║ << t┴
We are interested in exciton spin dynamics
in single nanowires
ts ?
Resonant excitation
core GaAs
Tune excitation energy, E Laser ,
record PL intensity (PLE)
Elaser
shell AlGaAs
AlGaAs
AlGaAs
GaAs
E
2-LO
1-LO
resonances
GaAs
PL
r
real space
hexcitation
hemission
k-space
Resonant excitation
25000
single core-shell
GaAs/AlGaAs nw
36meV
Intensities (a.u)
20000
T=10K
15000
37meV
10000
5000
0
60meV
PL
PLE
1.44 1.48 1.52 1.56 1.60 1.64 1.68 1.72 1.76
Energy (eV)
Clear resonances at 36, 73 and ~133 meV
above free exciton energy.
Resonant excitation
wire 2
PL Intensity (a. u.)
PLE
1-LO and 2-LO GaAs
phonons
PL
Resonance at ~133 meV:

LO
PL
-60 -30 0
Defect-AlGaAs related.
wire 1
LO
or
PLE
30 60 90 120 150 180 210 240

Bottom of AlGaAs band -Al
concentration ~10%,
instead of nominal growth
concentration 26%
Eexcitation - EX (meV)
How does the polarization depend on
excitation energy?
Excitation energy dependent polarization
Intensity (a.u)
wire 2
PL
PLE
1.45 1.50 1.55 1.60 1.65 1.70 1.75
Excitation energy (eV)
Intensity (a.U)
Excited: 1.553 eV
Excited: 1.653 eV
Excited: 1.589 eV
P~92%
P~76%
P~82%
x2
x2
1.44 1.46 1.48 1.50 1.52 1.54
1.44 1.46 1.48 1.50 1.52 1.54
Emission energy (eV)
Emission energy (eV)
x2
1.46
1.48
1.50
1.52
Emission energy (eV)
Polarization changes with excitation energy!
1.54
Resonant excitation creates nonequilibrium exciton spin distributions
Emission
Polarization (%)
110
100
90
wire 1
excite
80
70
ideal wire
wire 2
60
50
 As excitation comes closer to
free exciton energy:
N||=N
excite
40
PL Intensity (arb. units)
wire 2
PLE
PL
LO
wire 1
LO
PLE
PL
-60 -30
0
30 60 90 120 150 180 210 240
Eexcitation - EX (meV)
•
Excite parallel: polarization
increases
•
Excite perpendicular:
polarization decreases
 Wire 2: thermal equilibrium
N║ = N┴
Modeling exciton dynamics
ts
y
ty
x
t nr
tx
ts
z
t nr
tz
Gy
0
Fermi’s golden rule
t x, z
1  s 
t y

 2 
t y  t vac
t x, z
I
 t y  t nr ,t s and
 1
I||
2
At thermal equilibrium (highest energies) assume:
3 0 c
 3 2
exc Dexc
3
0
nx, z  ny
I t y

I|| t x, z
Spin relaxation time
dnx, y, z / dt  0
t s I  1  P 

)  1 for | |
t nr I || 1  P 
t s I  1  P 

 1 for 
t nr I || 1  P 
tstnr
Steady state:
1
0.1
0.01
Spin relaxation time depends
on excitation energy
tnr ~ 50 ps
wire 1
wire 2
wire 2
50
ts ~ 1 - 50 ps
100
150
200
250
Eexcitation- EX (meV)
“Non-Equilibrium Exciton Spin Dynamics in Resonantly Pumped Single
Core-Shell GaAs-AlGaAs Nanowires”
Thang. B. Hoang, L.V. Titova, J. M. Yarrison-Rice , H. E. Jackson, , A. O. Govorov, Y.
Kim, H. J. Joyce, H. H. Tan, C. Jagadish, L. M. Smith
Nano Letters 7 588 (2007)
Summary
We study optical properties of single CdS and GaAs/AlGaAs Nws:
 CdS nanowires:

Room temperature nanowire PL not sensitive to morphological irregularities or defects.

Low temperature (< 20 K) extremely sensitive to such defects.

Spatially resolved PL imaging of defect states

Time-resolved PL shows quantum efficiency can be increased by removing such defects
or surface states

Low temperature PL provides quick and non-destructive method for rapidly
characterizing NW growth
 Exciton spin dynamics from resonantly excited single GaAs-AlGaAs
nanowires:

Resonances observed at 1-LO and 2-LO and ~133meV (AlGaAs related) above the PL
emission line

Dependence of PL polarization on excitation energies

Spin relaxation time varies from ~1 ps at high energies to ~50 ps near resonance
Rate equations
ts
y
ty
ts
x
t ns
z
t ns
tx
tz
Gy
0
dnx
nx nx
nx ny nz
 Gx  
2   ,
dt
t x t nr
ts ts ts
dny
dt
 Gy 
ny
ty

ny
t nr
2
ny
ts

nx
ts

nz
ts
ny ny
ny nx nz
dnz
 
2   ,
dt
t z t nr
ts ts ts
,
Optical “bright” states
- “Bright” states: Δm = ±1
E
Conduction Band
mJ  
1
J
2
1
2
1 3
 ,
2 2

Eg
k
hh
Valance Band
3
J
2
ESO
lh
lh (SO)
mJ  
mJ  
1
2
1
2
mJ  
3
2
1 1
,
2 2
1 3
 ,
2 2
1 1
 ,
2 2
- Ignore the split-off bands
Acknowledgements