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