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

STEM Cathodoluminescence of Individual GaN/AlN Quantum Disks within a single Nanowire Luiz Fernando Zagonel 1 , Mathieu Kociak 1* , Stefano Mazzucco 1 , Odile Stéphan 1 , Marcel Tencé 1 , Katia March 1 , Romain Bernard 1 , Benoit Laslier 1 , Maria Tchernycheva 2 , Lorenzo Rigutti 2 , Francois Julien 2 , R. Songmuang 3 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

Introduction Quantum Confinement Conduction Band E a-b E gap Valence Band d Correlation of quantum object dimension and its transition energy. 24 Jun 2011 L. F. Zagonel et al. 2

Introduction CdSe nanoparticles 8 nm 2 nm CdSe ZnS 24 Jun 2011 L. F. Zagonel et al. 3

Cathodoluminescence Electron – Matter interactions Back Scattered electrons Bremsstrahlung X-rays Caracteristic X-rays Auger Electrons UV-Vis-IR Light Elastically Inelasticity Scattered scattered electrons Direct electrons Beam 24 Jun 2011 L. F. Zagonel et al. 4

Cathodoluminescence zinc-blende/wurtzite GaAs nanowire heterostructures D. Spirkoska et al., PHYSICAL REVIEW B 80, 245325 2009 24 Jun 2011 L. F. Zagonel et al. 5

Cathodoluminescence System Positioning the mirror is crucial to its efficiency. Parabolic Mirror 150 mm Sample Mirror Aperture Micrometric screws for Objective 3D Position adjustment Lens 24 Jun 2011 L. F. Zagonel et al. 6

Cathodoluminescence System An Optimized CL detector:  High collecting angle (35% of 4 p sr).  High energy resolution (~5 meV).  High throughput. 24 Jun 2011 L. F. Zagonel et al. 7

Cathodoluminescence System Microscopic Probe – STEM: I = F(x,y,E) • STEM HB 501 by VG operating at 60kV • Spatial resolution of <1 nm. Energy (eV) • Probe current of about ~200pA. 0 1 2 1 0 1 2 4 2 1 • Liquid Nitrogen Sample stage (150K). 0 1 2 1 0 • 3D Spectrum-Image acquisition. X position (nm) 24 Jun 2011 L. F. Zagonel et al. 8

Sample: GaN quantum discs GaN nanowires with GaN quantum discs with AlN barrier and shell. GaN QDs AlN shell Bulk GaN in a Unstrained AlN shell bulk GaN GaN QDs AlN barrier 24 Jun 2011 L. F. Zagonel et al. 9

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

Sample: GaN quantum discs Emissions below and above the GaN Band gap L. Rigutti et al. Nano Lett. 2010, 10, 2939 – 2943 24 Jun 2011 L. F. Zagonel et al. 11

Sample: GaN quantum discs Where do they come from? L. Rigutti et al. Nano Lett. 2010, 10, 2939 – 2943 24 Jun 2011 L. F. Zagonel et al. 12

GaN Quantum discs 5 0 0 n m 5 0 0 n m Bright field image CL Spectrum Image Dark field image 24 Jun 2011 L. F. Zagonel et al. 13

GaN quantum discs 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 24 Jun 2011 L. F. Zagonel et al. 14

GaN quantum discs HAADF Image and Single pixel spectra. the cathodoluminescence datacube. 24 Jun 2011 L. F. Zagonel et al. 15

GaN quantum discs 20 nm 1 2 3 4 5 6 7 ~15 nm Spectrum Image is projected perpendicular to the wire for better visualization. 24 Jun 2011 L. F. Zagonel et al. 16

GaN quantum discs Only the QDs and the bulk 20 nm GaN are emitting. 1 2 3 4 5 6 7 Energy higher than Gap Band Gap: Dominated by Quantum Confinement Energy lower than Gap Band Gap: Dominated by Stark Effect ~15 nm Spectrum Image is projected perpendicular to the wire for better visualization. 24 Jun 2011 L. F. Zagonel et al. 17

Nanowire Morphology Quantum confinement effect is more important in smaller discs. Unstrained Bulk GaN in a GaN QDs bulk GaN AlN shell AlN Barrier AlN shell Disc Diameter: Increases from 1 nm (4 ML) to 3.4 nm (13 ML) 24 Jun 2011 L. F. Zagonel et al. 18

Quantum confinement Stark Effect Gap GaN = 3.47 eV Conduction Band Gap AlN = 6.20 eV The band bending due to internal electric field GaN is the origin of the Band Gap observed QCSE and therefore the redshift emission for large QDs. Valence Band 24 Jun 2011 L. F. Zagonel et al. 19

Identifying single QD emissions The partial overlapping of all QD emission can be distinguished by fitting. L. F. Zagonel et al., Nano Lett. 2011, 11, 568 – 573 24 Jun 2011 L. F. Zagonel et al. 20

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 24 Jun 2011 21

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 24 Jun 2011 22

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 24 Jun 2011 L. F. Zagonel et al. 23

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

Relation QD thickness vs. emission energy The dispersion in the curve is possibly caused by strain. L. F. Zagonel et al., Nano Lett. 2011, 11, 568 – 573 24 Jun 2011 L. F. Zagonel et al. 25

Effect of the AlN Shell At equal thickness, QD’s emission red-shift from the begin to the end of the QD stack. L. F. Zagonel et al., Nano Lett. 2011, 11, 568 – 573 24 Jun 2011 L. F. Zagonel et al. 26

Emission and absorption with electrons Monochromatic Electron beam Metalic Nano Particle Si 3 N 4 substrate D E Electron Induced Radiation Emission (Cathodoluminescence) Electron Energy Loss Spectrum Absorption by EELS Emission by CL 27

Plasmon modes on Au Triangles STEM VG Absorption (EELS) Absorption (EELS) Emission (Cathodo) DF Image mode at ~2.4 eV mode at ~2.7 eV 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. 28

Summary Quantum confinement and Emission and absorption on Stark effect is clearly the same metallic nanopartiple evidenced in individual GaN has been performed. quantum discs. Analysis, simulations and new High localization of CL signal experiments are in progress. is shown. 24 Jun 2011 L. F. Zagonel et al. 29

Thank you for your attention! Merci de votre attention! Acknowledgements Financial Support 24 Jun 2011 L. F. Zagonel et al. 30