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Department of Materials Science & Engineering University of California at Berkeley. TEM and XSTM Characterization of Embedded InAs Quantum Dots. Matt Lowry Presented for NSEC203/EEC235 April 16, 2008. Department of Materials Science & Engineering University of California at Berkeley. - PowerPoint PPT Presentation
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TEM and XSTM Characterization ofEmbedded InAs Quantum Dots
Department of Materials Science & Engineering University of California at Berkeley
Matt Lowry
Presented for NSEC203/EEC235
April 16, 2008
Motivation
Department of Materials Science & Engineering University of California at Berkeley
Optical & electrical properties depend on shape, size, composition, and symmetry
AFM vs. XSTM vs. TEM
Changes due to capping
In segregation
TEM averaging effects
XSTM cleavage planes
Optimize growth strategies for potential applications
Good microscopy is difficult
Cross-Sectional Scanning Tunneling Microscopy (XSTM)
Department of Materials Science & Engineering University of California at Berkeley
http://www.insp.upmc.fr/axe1/Dispositifs%20quantiques/AxeI2_more/PRINCIPLE/STSprin.HTM
Raster tip
I ~ exp (-kz)
Image or topography
Shape and Size Determination
Department of Materials Science & Engineering University of California at Berkeley
Full and truncated pyramidal quantum dots
Cleaved on (110) planes
Only model 2 was observed
D.M. Bruls, et al., Appl. Phys. Lett. 81, 1708 (2002).
Composition Determination
Department of Materials Science & Engineering University of California at Berkeley
Stress relaxes on free surface due to 7% lattice mismatch
InxGa1-xAs alloy with linear In gradient
60%-100% In gradient from bottom to top
Lattice constant
Increase of 35pm across dot due to In content increase
D.M. Bruls, et al., Appl. Phys. Lett. 81, 1708 (2002).
Multi-Directional Analysis of an Isolated Quantum Dot
Department of Materials Science & Engineering University of California at Berkeley
Multi-directional analysis to detect asymmetries
Splitting of exciton states
InAs dot with GaAs cap
Focused Ion Beam used to isolate a quantum dot in a micropillar
80nm X 80nm pillar with 5nm amorphous region
T. Kita, et al., Appl. Phys. Lett. 90, 041911 (2007).
TEM of an Isolated Quantum Dot
Department of Materials Science & Engineering University of California at Berkeley
T. Kita, et al., Appl. Phys. Lett. 90, 041911 (2007).
Along [110] : 21nm Along [-110] : 23nm
Anisotropic growth due to faceting
Preferential strain in [-110]
Asymmetry of diffraction patterns
[110] Zone Axis [-110] Zone Axis
Conclusions
Department of Materials Science & Engineering University of California at Berkeley
Cross-sectional STM has allowed for the determination of shape, size, and composition of InAs quantum dots embedded in GaAs.
A combined FIB/TEM approach has allowed for the determination of asymmetric crystal strain in an isolated InAs quantum dot embedded in GaAs.
Both procedures are applicable to other systems
Better characterization will hopefully aid in fulfilling the promise of applications
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