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Experimental Condensed Matter Physics
• Henry Fenichel Holographic Imaging
• Howard Jackson Semiconductor Nano
• Young Kim Hi-TC/Strongly Cor. e-
• David Mast Near-field Microwave
• Richard Newrock Josephson/1D Transport
• Phillippe DeBray
• Leigh Smith Semiconductor Spins/Nano
• Hans-Peter Wagner 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
Based on Bimberg (1999)
BulkQuantum
WellQuantum
WireQuantum
Dot
Energy
D(E
)
Energy
D(E
)
1 32 4
Energy
D(E
)
1,1 1,2 1,3
Energy
D(E
)
Confining the electron motion in at least one spatial dimension affects the energy levels and the density of states…
Nano-Photonics: Controlling the Electromagnetic Field
Circular grating on GaN, pitch is 1 m. 500 pA, 5 min.Circular grating on GaN, pitch is 1 m. 500 pA, 5 min.
“Pocket Guide” to our Group
• Imaging
“Developing new techniques for directly imaging small things”
• Spectroscopy
“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
Selectively oxidized layers
Light Output
InjectionCurrent
p-DBR
n-DBR
GaAs
AlGaAs
8 nm QWsin 1 cavity
• Square mesas etched past active layers via RIE
• Lateral oxidation of high Al content layer forms the aperture
• 10 µm square aperture leads to transverse multimode structure
Experimental Setup
He-NeBeam
Dither Piezo
Contact pad
Optical fiber to spectrometer
Emission from VCSEL
Spectrometer & CCD
Y
X
Z
Scanning Stage
• Subwavelength tip aperture (80~100 nm) for spatially resolved information
• Near field collection (<20 nm from surface) for a spatial picture of modes at surface
• Spectral resolution for transverse mode differentiation
848 849 850 851
0
2000
4000
6000
8000
10000
Co
un
ts
Wavelength (nm)
Y (microns)
X (
mic
rons
)
Y (microns)
X (
mic
ron
s)
Y (microns)
X (
mic
ron
s)
Y (microns)
X (
mic
rons
)
Y (microns)
X (
mic
rons
)
1-0, 849.6nm 0-1, 849.72nm
0-0, 850nm 0-2, 849.40nm
2-0, 849.23 nm
Transverse modes at 5mATransverse modes at 5mATransverse modes at 5mATransverse modes at 5mA
• Expect states with strong binding (confinement) to CdSe dots.
• Strain, alloying, and dot-layer morphology very important.
Strain Driven Quantum Dot Growth
~ 50 nm ZnSe cap
~ 1 m ZnSe
GaAs Substrate
z - direction
CdSe
ZnSe
EC
V
ECdSe
EZnSe
ZnSe
Atomic-Force Microscopy
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 !
Characterization of CdSe SAQDs
Observations
• 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
Scanning Tunneling Electron Microscopy
Phys. Rev. Lett. 85, 1124 (2000).
1.5 ML
2.6 ML
Photoluminescence Spectroscopy
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.
CB
VB
E
k
ħ=Eg
CB
VB
ħ= Eg-Eex
Eex
Exciton band
Looking for single dots
GaAs Substrate
ZnSe buffer layer (~1 m)
ZnSe capping layer (~50 nm)Apertures
Al Pad
SAQDs
Laser Beam
Fig. 1
From thousands to tens…
2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26
0
10000
20000
30000
PL
INT
EN
SIT
Y (
arb
. un
its)
PHOTON ENERGY (eV)
2.18 2.20 2.22 2.24 2.26 2.28 2.300
3000
6000
90002.24437 eV
2.27967 eV
PL
INT
EN
SIT
Y (
arb
.un
its)
PHOTON ENERGY (eV)
A new high-resolution imaging tool with 200 nm resolution
•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 data-cube 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
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
x-position (microns)
y-p
osi
tion
(m
icro
ns)
CdTe Nonresonant Excitation
2.094 2.095 2.096 2.097 2.098 2.099 2.100 2.101 2.102
0.0
0.5
1.0
Nor
mal
ized
Inte
nsity
(a.
u.)
Energy (eV)
1 2 3a3b3c
-1 0 128000
29000
30000
31000
32000
33000
34000
Inte
nsity
(a.
u.)
X (microns)
2.8E4
2.9E4 2.9E4
2.8E4
2.9E4
2.8E4
3.0E4
2.9E42.9E4
2.9E4
3.0E4
2.9E4
3.0E42.9E4 3.0E4
2.9E4
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
y-p
osi
tion
(m
icro
ns)
x-position (microns)
1
2
3
xy
•Shown here are grey-scale and contour-plot images of a 3x3-micron region of the CdTe QDs selected over a limited (0.1 nm) spectral range
•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.
•Spatial scans of dot 1 show 200 nm resolution along y and 350 nm resolution along x.
x
y
Position sensitivity of dots
0.4 0.6 0.8 1.0 1.2
1.0
1.2
1.4
1.6
1.8
2.0
0.5 1.01.0
1.5
2.0
y-po
sitio
n (m
icro
ns)
x-position (microns)
QD 1 (A and C) 2.07 2.08 2.09 2.10 2.11
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity
(a.
u.)
Energy (eV)
DABC0.4 0.6 0.8 1.0 1.2
1.0
1.2
1.4
1.6
1.8
2.0
0.5 1.01.0
1.5
2.0
y-po
sitio
n (m
icro
ns)
x-position (microns)
QD 1 (A and B)
•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
•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?
x
y
Position sensitivity (continued)
1.6 1.8 2.0 2.2 2.4
1.0
1.2
1.4
1.6
1.8
2.0
1.5 2.0 2.51.0
1.5
2.0
y-po
sitio
n (
m)
x-position (m)
QD 2 (C and D)D C
2.08 2.09 2.10
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity
(a.
u.)
Energy (eV)
D
•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 y68 nm along x.
x
y
Nano-Spectroscopy: Using spectroscopy to look inside small things
• Polarized Photoluminescence
• Magneto-Photoluminescence
• Excitation Spectroscopy
From thousands to tens…
2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26
0
10000
20000
30000
PL
INT
EN
SIT
Y (
arb
. un
its)
PHOTON ENERGY (eV)
2.18 2.20 2.22 2.24 2.26 2.28 2.300
3000
6000
90002.24437 eV
2.27967 eV
PL
INT
EN
SIT
Y (
arb
.un
its)
PHOTON ENERGY (eV)
Some dots are different than others….
2.2437 2.2446 2.2455
3000
6000
2.24437 eVB=0 T
PL
INT
EN
SIT
Y (
arb
.un
its)
PHOTON ENERGY (eV)2.2790 2.2795 2.2800 2.2805 2.2810
1600
1700
1800
2.27967 eVB=0 T
PL
INT
EN
SIT
Y (
arb
. un
its)
PHOTON ENERGY (eV)
Symmetric Quantum Dot
Asymmetric Quantum Dot
Magneto-PL
0.0 0.5 1.0 1.5 2.0 2.5 3.0
-20
0
20
40
60
80
100
diamagnetic shift
2.24156 eV
En
erg
y D
iffe
ren
ce(
eV)
Magnetic Field (Tesla)
BgBE B*2
2
1
Photoluminescence Spectroscopy
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.
CB
VB
Electronic bound state
hexcitationhemission
E
k
PL Intensity
Laser energy
Continuum states
2.02 2.04 2.06 2.08 2.10
PL
Inte
nsi
ty (
a.u
.)
Energy (eV)
PLE spectra for single CdTe QDs
0.00 0.02 0.04 0.06
CdTe QDsT=6Kon 0.8 m aperture
PLE
PL
PL
In
ten
sit
y (
a.u
.)
Eex
-Edet
(eV)
The sharp peaks of about 200 eV linewidth in the PL spectrum reflect quasi-zero dimensional densities of state of the quantum dots
Broad resonances in both PL and PLE spectra are related to LO phonon-assisted absoprtion
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
ELO
1st LO2nd LO
Laser
3rd LO
QD and Electron-Phonon Coupling
1.96 2.00 2.04 2.08 2.12 2.16 2.20
non-resonant PL
Low temperature
PL
inte
nsi
ty (
a.u
.)
Energy (eV)
Recent Publications (2003-2004)
“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).
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