A. AbdiA. Abdi, T. B. Hoang, S. Mackowski, L. M. Smith and H. E. , T. B. Hoang, S. Mackowski, L. M. Smith and H. E. JacksonJackson

Department of Physics, University of Cincinnati, Ohio 45221-0011

J. M. Yarrison-RiceJ. M. Yarrison-RiceDepartment of Physics, Miami University, Oxford, OH

J. Kossut and G. KarczewskiJ. Kossut and G. KarczewskiInstitute of Physics Polish Academy of Sciences, Warsaw, Poland

Supported by NSF(United States) and CELDIS (Poland).

ProbingProbing Excited States in Excited States in Self-aSelf-assembledssembled Quantum Dots with Quantum Dots with

Resonant Raman ScatteringResonant Raman Scattering

QD samples – as grown and annealedQD samples – as grown and annealed

CdTe = 4 ML

ZnTe ~50 nm

GaAs

ZnTe ~1 m

CdTe ~4 m

Z

Effects of annealingEffects of annealing

Eg(CdTe)Eg (ZnTe)

ZnTe ZnTeCdTe

Eg(Cd1-xZnxTe)

Eg (Zn1-xCd xTe)

Zn1-xCd xTeCd1-xZnxTe

As-grown QDs Annealed QDs

inter-diffusion of Zn and Cd increases average dot size Shallower confining potential Shifts energy to higher value

Zn1-xCd xTe

Resonant Raman Intensity Resonant Raman Intensity

sβiαα,βissi ωΕωΕ

Τεεω)I(ω Σ

124

• Our group has studied excited states of CdTe QDs by resonant PL and a

single QD PLE techniques at 6K. T. A. Nguyen, S. Mackowski, H. E. Jackson and L.M. Smith Phys. Rev. B 70 (2004)

• Can we study the excited state of QDs by Resonant Raman Scattering at room temperature?

hexcitation

hWLO

hscattered

Typical Resonant Raman Typical Resonant Raman spectrum at room temperaturespectrum at room temperature

0 100 200 300 400 500 6001000

2000

3000

4000

5000

6000

7000

8000

9000

10000

2LO of ZnTe

590 nm

Inte

nsi

ty (

a.u

.)

Raman shift (cm -1)

Sample: Annealed CdTe

1LO of ZnTe

• 1LO and 2LO ZnTe phonon appear at 202 cm-1 and 404 cm-1

• We analyze 1LO resonance to probe excited state in CdTe QDs

ZnTe 1LO Raman intensity vs. excitation energy for as-grown sample

160 180 200 220 240 260 280 3000

100

200

300

400

500

600

700

800

Inte

nsit

y (a

.u.)

Raman Shift (cm-1)

575 nm 580 nm 585 nm 590 nm 595 nm 600 nm 605 nm 612 nm 613 nm 615 nm

as-grown CdTe at T~300 KZnTe 1LO

ZnTe 1LO Raman intensity vs. excitation energy for annealed sample

180 200 220 240 260-100

0

100

200

300

400

500

600

700

800

900

1000

Inte

ns

ity

(a.u

.)

Raman shift (cm-1)

575 nm 580 nm 585 nm 590 nm 595 nm 600 nm 605 nm

annnealed CdTe at T~300 K

Photoluminescence temperature dependence

0 100 200 3001.95

2.00

2.05

2.10

PL

En

erg

y (e

V)

Temperature (K)

1.962 eV

as-grown sample

• Room temperature ground state at 1.962 eV

Raman and non-resonance PL Raman and non-resonance PL intensities vs. excitation energy:intensities vs. excitation energy:

as-grown CdTe QDsas-grown CdTe QDs

1.8 1.9 2.0 2.1 2.20

200

400

600

800

Energy [eV]

1st

LO

ph

on

on

inte

nsi

ty (

a.u

.) 110 meV Shifted PL RR G fit to RR

As-grown CdTe QDs at T~300K

Raman and non-resonance PL Raman and non-resonance PL intensities vs. excitation energy: intensities vs. excitation energy:

annealed CdTe QDsannealed CdTe QDs

1.8 1.9 2.0 2.1 2.20

100

200

300

400

500

600

700

800

900

Energy [eV]

Annealed CdTe QDs at T~300 K

1st

LO

ph

on

on

in

ten

sity

(a.

u.)

80 meV Shifted PL RR G fit to RR

Comparison of as grown and Comparison of as grown and annealed excited statesannealed excited states

1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20

0

200

400

600

800

1000

Inte

nsi

ty (

a.u

.)

Energy (eV)

110 meV80 meV

Effects of annealingEffects of annealing

Eg(CdTe)Eg (ZnTe)

ZnTe ZnTeCdTe

Eg(Cd1-xZnxTe)

Eg (Zn1-xCd xTe)

Zn1-xCd xTeCd1-xZnxTe

As-grown QDs Annealed QDs

inter-diffusion of Zn and Cd increases average dot size Shallower confining potential Shifts energy to higher value

Zn1-xCd xTe

SummarySummary

We have utilized Resonant Raman scattering to observe the excited state energy distribution from as-grown and annealed CdTe QDs.

The ground state - excited state energy difference is larger for as-grown sample, as expected.

The excited state energy distribution for

annealed QDs is narrower vs. as-grown QDs.

Evidence for excited stateEvidence for excited state

2.0 2.1 2.2 2.3 2.4

shifted PL spectrum ES-GS excitation

ZnTebarrier

~100meV

CdTe QDs

Nor

mal

ized

Inte

nsity

Emission Energy [eV]

Excited state is 100 mev above ground state Profile of excited state is similar with that of ground state

T. A. Nguyen, S. Mackowski, H. E. Jackson and L.M. Smith Phys. Rev. B 70 (2004)

Photoluminescence excitation SpectrumPhotoluminescence excitation Spectrum

T. A. Nguyen, S. Mackowski, H. E. Jackson and L.M. Smith Phys. Rev. B 70 (2004)