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).