Transcript Experimental Condensed Matter Physics in the Nanoscale

Experimental Condensed Matter
Physics
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Henry Fenichel
Howard Jackson
Young Kim
David Mast
Richard Newrock
Phillippe DeBray
Leigh Smith
Hans-Peter Wagner
Holographic Imaging
Semiconductor Nano
Hi-TC/Strongly Cor. eNear-field Microwave
Josephson/1D Transport
Semiconductor Spins/Nano
Semiconductor Nonlinear
Experimental Condensed Matter
Physics in the Nanoscale
Leigh M. Smith
Howard Jackson
Jan Yarrison-Rice
Sebastian Mackowski
Aditi Sharma
Kapila Hewaparakrama Nguyen Tuan
Tak Gurung
Amensisa Abdi
Firoze Haque Anthony Wilson
The year(s) of the nano
Reduced Dimensionality
Confining the electron motion in at least one spatial
dimension affects the energy levels and the density of states…
Quantum
Well
e1 e2
Energy
Based on Bimberg (1999)
Quantum
Wire
e3
e4
Energy
Quantum
Dot
r0D(E)
r1D(E)
r3D(E)
r2D(E)
Bulk
e1,1
e1,2
e1,3
Energy
Energy
Nano-Photonics: Controlling the
Electromagnetic Field
Circular grating on GaN, pitch is 1 m. 500 pA, 5
min.
“Pocket Guide” to our Group
• Imaging
• Spectroscopy
“Developing new
techniques for directly
imaging small things”
“Using optical spectroscopy
to look at the interactions
and dynamics of the
electronic and vibronic
states in nanostructures”
Nano-Imaging: How to see things much
smaller than the wavelength of light
• NSOM: Scanned nano-apertures
• Fixed Apertures
• Solid Immersion Lenses
VCSEL Structure
Light Output
• Square mesas etched
p-DBR
GaAs
AlGaAs
Injection
Current
8 nm QWs
in 1 l cavity
n-DBR
past active layers via
RIE
• Lateral oxidation of
Selectively
oxidized
layers
high Al content layer
forms the aperture
• 10 µm square
aperture leads to
transverse multimode
structure
Experimental Setup
Optical fiber to spectrometer
Spectrometer &
CCD
Dither
Piezo
Z
X
He-Ne
Beam
Contact pad
Emission from
VCSEL
Scanning
Stage
• Subwavelength tip
aperture (80~100 nm)
for spatially resolved
information
Y
• Near field collection
(<20 nm from surface)
for a spatial picture of
modes at surface
• Spectral resolution for
transverse mode
differentiation
Transverse modes at 5mA
1-0, 849.6nm
X (microns)
X (microns)
0-1, 849.72nm
10000
Counts
8000
Y (microns)
6000
Y (microns)
4000
2000
0
849
850
851
X (microns)
X (microns)
Wavelength (nm)
X (microns)
848
Y (microns)
0-0, 850nm
Y (microns)
0-2, 849.40nm
Y (microns)
2-0, 849.23 nm
Strain Driven Quantum Dot Growth
EZnSe
ZnSe
ECdSe
~ 50 nm ZnSe cap
CdSe
~ 1 m ZnSe
DEC
GaAs Substrate
DEV
z - direction
• Expect states with strong binding (confinement) to CdSe dots.
• Strain, alloying, and dot-layer morphology very important.
ZnSe
Characterization of CdSe SAQDs
Atomic-Force Microscopy
Scanning Tunneling Electron Microscopy
1.5 ML
2.6 ML
Phys. Rev. Lett. 85, 1124 (2000).
Observations
• “Pancake” in shape
• Somewhate uniform in size
height ~ 2-4 nm, diameter ~ 10-20 nm
• Distinguishable from surface variations
• Number density is about 1000 m-2 !
Observations
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Even at 1.5 ML, CdSe layer not uniform
Variation in size both laterally and vertically
Co-existence of 2-D platelets and 3-D islands
Dots extend above and below the interface
Photoluminescence Spectroscopy
E
CB
CB
k
ħ=Eg
VB
Exciton
band
Eex
ħ= Eg-Eex
VB

A laser excites electrons from the valence band into the conduction
band, creating electron-hole pairs.

These electrons and holes recombine and emit a photon.

We measure the number of emitted photons (intensity) as a
function of energy.
Looking for single dots
Al Pad
Apertures
ZnSe buffer layer (~1 m)
GaAs Substrate
Fig. 1
Laser
Beam
ZnSe capping layer (~50 nm)
SAQDs
9000
30000
PL INTENSITY (arb.units)
PL INTENSITY (arb. units)
From thousands to tens…
20000
10000
2.24437 eV
2.27967 eV
6000
3000
0
2.19
2.20
2.21
2.22
2.23
2.24
PHOTON ENERGY (eV)
2.25
2.26
0
2.18
2.20
2.22
2.24
2.26
PHOTON ENERGY (eV)
2.28
2.30
A new high-resolution imaging
tool with 200 nm resolution
l
•Using a truncated solid immersion
lens we can directly image up to a
5x5 micron region of a sample with
200 nm resolution. The excitation
laser is de-focused to a 20 micron
radius spot.
•The entrance slit is imaged onto the
CCD camera so that each CCD image
contains both x-position and
wavelength information.
•Then the sample is scanned across
the entrance slit in the y-direction, an
image taken at each point.
•This results in a 100x100x2000 datacube with x, y and energy along each
axis.
•Such a high-spectral and spatial
resolution image can be taken in less
than 20 minutes with an appropriate
sample.
x
y
CdTe Nonresonant Excitation
•Shown here are grey-scale and contourplot images of a 3x3-micron region of the
CdTe QDs selected over a limited
(0.1 nm) spectral range
3.5
3c
1
3b
2 3a
3
2.5
1.0
Normalized Intensity (a.u.)
y-position (microns)
3.0
1
2.0
1.5
1.0
2
0.5
•The dots marked 1 and 2 exhibit single
emission lines at 2.0987 and 2.0989 eV,
while dot 3 exhibits a cluster of at least 3
dots within 500 nm (partially resolved):
two with single emission lines (3a and 3b)
and a doublet (3c) presumably from an
assymetric dot.
0.5
0.0
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
2.094
x-position (microns)
2.095
2.096
2.097
2.098
2.099
2.100
2.101
2.102
Energy (eV)
3.5
34000
•Spatial scans of dot 1 show 200
nm resolution along y and 350
nm resolution along x.
3.0
2.9E4
2.5
2.8E4
2.0
3.0E4
3.0E4
2.9E4
1.5
2.9E4
3.0E4
2.9E4
31000
y
2.8E4
x
l
29000
2.9E4
2.9E4
2.9E4
3.0E4
0.0
0.0
32000
30000
2.9E4
2.8E4
1.0
0.5
2.9E4
Intensity (a.u.)
y-position (microns)
33000
0.5
1.0
1.5
2.0
x-position (microns)
2.5
28000
-1
3.0
0
DX (microns)
1
x
y
Position sensitivity of dots
QD 1 (A and B)
0.4
0.6
0.8
1.0
1.2
2.0
2.0
B
C
A
D
1.8
1.6
1.5
1.4
1.2
1.0
1.0
0.5
1.0
x-position (microns)
QD 1 (A and C)
0.4
0.6
0.8
1.0
2.0
y-position (microns)
1.8
1.6
1.5
1.4
1.2
1.0
0.5
1.0
x-position (microns)
0.8
0.6
0.4
0.2
0.0
2.07
2.08
2.09
2.10
Energy (eV)
1.2
2.0
1.0
•By spectrally selecting
particular emission peaks
one can look at the emission
profile of each peak.
•Note that peaks A and B
collected near QD1 show
clearly that they are
separated by 200 nm along
the y-direction and and 200
nm along the x-direction
Normalized Intensity (a.u.)
y-position (microns)
1.0
•On the other hand, peaks A
and C are aligned (both in
size and position within less
than 20 nm. Are A and C
from the same dot
(biexcitons perhaps), or are
they two dots separated by
less than 20 nm?
l
x
y
2.11
Position sensitivity (continued)
QD 2 (C and D)
1.6
1.8
2.0
2.0
2.2
2.0
1.6
1.5
D
1.0
Normalized Intensity (a.u.)
1.8
y-position (m)
C
D
2.4
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
1.5
2.0
x-position (m)
2.08
2.09
2.10
Energy (eV)
•In another example there are two emission lines
(C and D above) emitted near QD 2. These two
peaks are separated spatially by 75 nm along y
68 nm along x.
1.0
2.5
l
x
y
Nano-Spectroscopy: Using spectroscopy to
look inside small things
• Polarized Photoluminescence
• Magneto-Photoluminescence
• Excitation Spectroscopy
9000
30000
PL INTENSITY (arb.units)
PL INTENSITY (arb. units)
From thousands to tens…
20000
10000
2.24437 eV
2.27967 eV
6000
3000
0
2.19
2.20
2.21
2.22
2.23
2.24
PHOTON ENERGY (eV)
2.25
2.26
0
2.18
2.20
2.22
2.24
2.26
PHOTON ENERGY (eV)
2.28
2.30
Some dots are different than
others….
1800
PL INTENSITY (arb. units)
PL INTENSITY (arb.units)
B=0 T
2.24437 eV
B=0 T
6000
3000
2.27967 eV
1700
1600
2.2437
2.2446
2.2455
PHOTON ENERGY (eV)
2.2790
2.2795
2.2800
PHOTON ENERGY (eV)
Symmetric Quantum Dot
Asymmetric Quantum Dot
2.2805
2.2810
Magneto-PL
100
1.6
PL Intensity (a.u.)



Energy Difference(eV)

1.4
1.2
1.0
3 Tesla
0.8
0.6
0.4
0.2
0 Tesla

80
60
40
diamagnetic shift
20
2.24156 eV
0
-20

0.0
2.2410
2.2415
0.0
2.2420
0.5
1.0
1.5
2.0
2.5
Magnetic Field (Tesla)
Energy (eV)
1
DE   B   B g * B
2
2


3.0
Photoluminescence Spectroscopy
Laser energy
Continuum states
Electronic
bound state
CB
E
k
hexcitation
hemission
VB
PL Intensity
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

A laser excites electrons from the valence band into the conduction band,
creating electron-hole pairs
These electrons and holes recombine (annihilate) and emit a photon.
We measure the number of emitted photons (intensity) as a function of
energy.
1st
LO
Laser
2nd LO
3rd
LO
PL Intensity (a.u.)
PL Intensity (a.u.)
PLE spectra for single CdTe QDs
CdTe QDs
T=6K
on 0.8 m aperture
PL
ELO
0.00
2.02
2.04
2.06
2.08
0.02
2.10
 Intense and narrow lines in the PLE spectrum
originate from direct excitation into an excited
state
 Excitation spectra vary from dot to dot in
ensemble
PL intensity (a.u.)
 Broad resonances in both PL and PLE spectra are
related to LO phonon-assisted absoprtion
0.04
0.06
Eex-Edet (eV)
Energy (eV)
 The sharp peaks of about 200 eV linewidth in the
PL spectrum reflect quasi-zero
dimensional
densities of state of the quantum dots
PLE
PLE
PL
0.00
0.08
0.10
Eex-Edet (eV)
0.12
QD and Electron-Phonon
Coupling
PL intensity (a.u.)
Low temperature
non-resonant PL
1.96
2.00
2.04
2.08
2.12
Energy (eV)
2.16
2.20
Recent Publications (2003-2004)
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“Exciton spin relaxation in CdTe/ZnTe self-assembled quantum dots,” S. Mackowski, T.A. Nguyen, T. Gurung, K. Hewaparkarama,
H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, and G. Karczewski, submitted to Physical Review B.
“Optically-induced magnetization of CdMnTe self-assembled quantum dots,” S. Mackowski*, T. Gurung, T.A. Nguyen, H.E. Jackson,
L.M. Smith, G. Karczewski and J. Kossut, submitted to Applied Physics Letters (2004).
“Optically controlled magnetization of zero-dimensional magnetic polarons in CdMnTe self-assembled quantum dots,” S. Mackowski,
T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, J. Kossut and G. Karczewski, to be published in physica status solidi (b) (March,
2004)
“Optical Studies of Spin Relaxation in CdTe Self-Assembled Quantum Dots,” S. Mackowski, T. Gurung, T.A. Nguyen, K.P.
Hewaparakrama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, and G. Karczewski, to be published in physica status solidi
(b) (March, 2004).
“Exciton-LO-phonon interaction in II-VI self-assembled quantum dots,” T.A. Nguyen, S. Mackowski, H.E. Jackson, L. M. Smith, G.
Karczewski, and J. Kossut, M. Dobrowolska and J. Furdyna, to be published in physica status solidi (b) (March, 2004).
“Tuning the optical and magnetic properties of II-VI quantum dots by post-growth rapid thermal annealing,” T. Gurung, S.
Mackowski*, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut and G. Karczewski, to be published in physica status solidi (b) (March,
2004).
S. Mackowski, L.M. Smith, H.E. Jackson, W. Heiss, J. Kossut, and G. Karczewski, “Optical properties of annealed CdTe self-assembled
quantum dots”, Applied Physics Letters, 83, 254 (2003).
T.A. Nguyen, S. Mackowski, H.E. Jackson, L.M. Smith, M. Dobrowolska, J. Furdyna, K. Fronc, J. Wrobel, J. Kossut, G. Karczewski,
“Resonant Spectroscopy of II-VI Self-Assembled Quantum Dots: Excited States and Exciton-LO Phonon Coupling”, submitted to Phys.
Rev. B. (2003).
“Tuning the Properties of Magnetic CdMnTe Quantum Dots,” S. Mackowski, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut, and G.
Karczewski, Applied Physics Letters, 83, 3575 (2003).
"Nano-photoluminescence of CdSe self-assembled quantum dots: experiments and models," R.A. Jones, Jan M. Yarrison-Rice, L.M.
Smith, Howard E. Jackson, M. Dobrowolska, and J.K. Furdyna, Phys. Rev. B 68, 125333 (2003).
“Magneto-photoluminescence measurements of symmetric and asymmetric CdSe/ZnSe self-assembled quantum dots,” K.P.
Hewaparakrama, N. Mukolobwiez, L.M. Smith, H.E. Jackson, S. Lee, M. Dobrowolska, J. K. Furdyna, in “Proceedings of the 26th
International Conference on the Physics of Semiconductors, Edinburgh, 2002,” (World Scientific, 2003).
“Resonant and non-resonant PL and PLE spectra of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots,” T.A. Nguyen, S.
Mackowski, L.M. Robinson, H. Rho, H.E. Jackson, L. M. Smith, M. Dobrowolska, J.K. Furdyna, and G. Karczewski, in
“Proceedings of the 26th International Conference on the Physics of Semiconductors, Edinburgh, 2002,” (World Scientific, 2003).
“Exciton Spin Relaxation in Quantum Dots Probed by Continuous-Wave Spectroscopy,” S. Mackowski, T. A. Nguyen, H. E. Jackson,
L. M. Smith, J. Kossut, and G. Karczewski, Applied Physics Letters, 83, 5524 (2003).
“Optical Properties of Semimagnetic Quantum Dots,” S. Mackowski, T. A. Nguyen, H. E. Jackson, L. M. Smith, J. Kossut, and G.
Karczewski, and W. Heiss, Quantum Confined Semiconductor Nanostructures. Symposium (Mater. Res. Soc. Symposium Proceedings
Vol.737) 65-70 (2003).
“Resonant photoluminescence and excitation spectroscopy of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots,” T. A. Nguyen,
S. Mackowski, H. Rho, H. E. Jackson, L. M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, M. Dobrowolska and J. Furdyna,
Quantum Confined Semiconductor Nanostructures. Symposium (Mater. Res. Soc. Symposium Proceedings Vol.737) 71-6 (2003).