STEM Cathodoluminescence of Individual GaN/AlN Quantum Disks within - - PowerPoint PPT Presentation

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STEM Cathodoluminescence of Individual GaN/AlN Quantum Disks within a single Nanowire Luiz Fernando Zagonel 1 , Mathieu Kociak 1* , Stefano Mazzucco 1 , Odile Stphan 1 , Marcel Tenc 1 , Katia March 1 , Romain Bernard 1 , Benoit Laslier 1 ,

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STEM Cathodoluminescence of Individual GaN/AlN Quantum Disks within a single Nanowire

Luiz Fernando Zagonel1, Mathieu Kociak1*, Stefano Mazzucco1, Odile Stéphan1, Marcel Tencé1, Katia March1, Romain Bernard1, Benoit Laslier1, Maria Tchernycheva2, Lorenzo Rigutti2, Francois Julien2, R. Songmuang3

1- Laboratoire de Physique des Solides, CNRS/UMR8502, Université Paris-Sud, Orsay, 91405, France 2- Institut d’Electronique Fondamentale, CNRS/UMR 8622, Université Paris-Sud, Orsay, 91405, France 3- CEA-CNRS group ”Nanophysique et Semiconducteurs”, Institut Nel, Grenoble, France

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24 Jun 2011 2

Introduction

Conduction Band Valence Band

Quantum Confinement

d

Egap Ea-b

Correlation of quantum object dimension and its transition energy.

  • L. F. Zagonel et al.
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24 Jun 2011 3

2 nm

8 nm

CdSe nanoparticles

Introduction

CdSe ZnS

  • L. F. Zagonel et al.
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24 Jun 2011 4

Electron – Matter interactions

Direct Beam Inelasticity scattered electrons Elastically Scattered electrons Back Scattered electrons Auger Electrons Bremsstrahlung X-rays Caracteristic X-rays UV-Vis-IR Light

Cathodoluminescence

  • L. F. Zagonel et al.
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24 Jun 2011 5

Cathodoluminescence

  • D. Spirkoska et al., PHYSICAL REVIEW B 80, 245325 2009

zinc-blende/wurtzite GaAs nanowire heterostructures

  • L. F. Zagonel et al.
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24 Jun 2011 6

Micrometric screws for 3D Position adjustment

150 mm Parabolic Mirror

Positioning the mirror is crucial to its efficiency.

Sample Mirror Objective Lens Aperture

Cathodoluminescence System

  • L. F. Zagonel et al.
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24 Jun 2011 7

Cathodoluminescence System

An Optimized CL detector:

 High collecting angle (35% of 4p sr).  High energy resolution (~5 meV).  High throughput.

  • L. F. Zagonel et al.
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24 Jun 2011 8

Cathodoluminescence System

Microscopic Probe – STEM:

  • STEM HB 501 by VG operating at 60kV
  • Spatial resolution of <1 nm.
  • Probe current of about ~200pA.
  • Liquid Nitrogen Sample stage (150K).
  • 3D Spectrum-Image acquisition.

0 1 2 1 0 1 2 4 2 1 2 1 0 1 X position (nm) Energy (eV)

I = F(x,y,E)

  • L. F. Zagonel et al.
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24 Jun 2011 9

AlN shell Unstrained bulk GaN Bulk GaN in a AlN shell GaN QDs

Sample: GaN quantum discs

GaN nanowires with GaN quantum discs with AlN barrier and shell. AlN barrier GaN QDs

  • L. F. Zagonel et al.
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24 Jun 2011 10

Sample: GaN quantum discs

Catalyst-free growth on Si (111) (RF-PA-MBE). GaN nanowires with GaN quantum discs with AlN barrier and shell. Nanowire length = ~ 2.1 µm Nanowire diam. = 20 to 50 nm Number of QD’s = 20. Disc Thickness: ~3.5 nm Barrier Thickness: ~4.1 nm

  • L. F. Zagonel et al.
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24 Jun 2011 11

Emissions below and above the GaN Band gap

Sample: GaN quantum discs

  • L. Rigutti et al. Nano Lett. 2010, 10, 2939–2943
  • L. F. Zagonel et al.
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24 Jun 2011 12

Sample: GaN quantum discs

Where do they come from?

  • L. Rigutti et al. Nano Lett. 2010, 10, 2939–2943
  • L. F. Zagonel et al.
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24 Jun 2011 13

GaN Quantum discs

5 n m 5 n m

CL Spectrum Image Bright field image Dark field image

  • L. F. Zagonel et al.
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24 Jun 2011 14

HADF acquired simultaneously with CL spectrum-image

20 nm

Monochromatic image extracted from the Datacube at 300 nm. Datacube from 271 to 390 nm.

Spatial sampling: 0.6 nm per pixel Spectral sampling: 2 nm per pixel Dwell time: 20 ms per pixel Spectrum image size: 256 x 64 x 256 pixels

GaN quantum discs

  • L. F. Zagonel et al.
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24 Jun 2011 15

GaN quantum discs

HAADF Image and the cathodoluminescence datacube. Single pixel spectra.

  • L. F. Zagonel et al.
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24 Jun 2011 16

GaN quantum discs

1 2 3 4 5 6 7 Spectrum Image is projected perpendicular to the wire for better visualization.

20 nm

~15 nm

  • L. F. Zagonel et al.
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24 Jun 2011 17 Spectrum Image is projected perpendicular to the wire for better visualization.

20 nm

Energy higher than Gap Band Gap: Dominated by Quantum Confinement Energy lower than Gap Band Gap: Dominated by Stark Effect

1 2 3 4 5 6 7

~15 nm

Only the QDs and the bulk GaN are emitting.

GaN quantum discs

  • L. F. Zagonel et al.
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24 Jun 2011 18

Nanowire Morphology

AlN shell Unstrained bulk GaN Bulk GaN in a AlN shell GaN QDs AlN Barrier Disc Diameter: Increases from 1 nm (4 ML) to 3.4 nm (13 ML)

Quantum confinement effect is more important in smaller discs.

  • L. F. Zagonel et al.
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24 Jun 2011 19

Quantum confinement Stark Effect

Gap GaN = 3.47 eV Gap AlN = 6.20 eV The band bending due to internal electric field is the origin of the

  • bserved QCSE and

therefore the redshift emission for large QDs.

Conduction Band Valence Band GaN Band Gap

  • L. F. Zagonel et al.
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24 Jun 2011 20

Identifying single QD emissions

The partial

  • verlapping of all

QD emission can be distinguished by fitting.

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

  • L. F. Zagonel et al.
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24 Jun 2011 21

Identifying single QD emissions

Each individual QD appears as a peak in the combined spatial-spectral plot. High spatial and spectral resolutions and sampling are need to find and distinguish each of these peaks.

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

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24 Jun 2011 22

Identifying single QD emissions

Previous micro-PL data compare well with spatially integrated signal on CL.

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

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24 Jun 2011 23

Measurement of QD thickness

HAADF HR-STEM images were acquired to determine the thickness of each QD.

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

  • L. F. Zagonel et al.
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24 Jun 2011 24

Relation QD thickness vs. emission energy

Each data point represents a single QD of known size and emission energy! The Quantum Confinement is clearly evidenced by the relation of QD thickness and emission energy.

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

  • L. F. Zagonel et al.
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24 Jun 2011 25

The dispersion in the curve is possibly caused by strain.

Relation QD thickness vs. emission energy

  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

  • L. F. Zagonel et al.
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24 Jun 2011 26

Effect of the AlN Shell

At equal thickness, QD’s emission red-shift from the begin to the end

  • f the QD stack.
  • L. F. Zagonel et al.,

Nano Lett. 2011, 11, 568–573

  • L. F. Zagonel et al.
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Emission and absorption with electrons

Absorption by EELS Emission by CL DE

Monochromatic Electron beam Electron Energy Loss Spectrum Electron Induced Radiation Emission (Cathodoluminescence)

Metalic Nano Particle Si3N4 substrate

27

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Plasmon modes on Au Triangles

STEM VG DF Image Absorption (EELS) mode at ~2.4 eV Absorption (EELS) mode at ~2.7 eV Emission (Cathodo) mode at ~2.3 eV Collaboration with Luis M. Liz-Marzán

  • J. B. Rodriguez

Vigo University Spain

24 Jun 2011

  • L. F. Zagonel et al.
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24 Jun 2011 29

Summary

Quantum confinement and Stark effect is clearly evidenced in individual GaN quantum discs. High localization of CL signal is shown. Emission and absorption on the same metallic nanopartiple has been performed. Analysis, simulations and new experiments are in progress.

  • L. F. Zagonel et al.
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24 Jun 2011 30

Thank you for your attention! Merci de votre attention! Acknowledgements Financial Support

  • L. F. Zagonel et al.