Nano Electronic Materials Research Group
XSTM Study of InGaAs Quantum Dots
1. Motivation
The shape and size of the QDs, and their spatial correlations, composition profile - critical parameters in determining their optoelectronic properties.
Previously people used TEM, XRD and RHEED, but images in TEM are contrast distorted by strain field; RHEED and XRD only provide indirect information averaged over substantially larger areas or volumes.
XSTM directly image the properties corresponding to individual atoms on semiconductor interfaces or nanostructures buried under a capping layer.
2. Chemically resolved atomic structure
Filled state image (anion lattice) Empty state image (cation lattice)
3. XSTM of the MEE grown QDs
Spatially Anti-correlated QDs
Plan View Side View
Image Contrast Mechanism
Plan View Side View
- Contrast due to Outward Relaxation
XSTM reveals non-uniform distribution of In atoms in the QDs, Inverted Triangle Shape of In Core
STM Image of a 10ML MEE grown QD Contour plot of the QD (Range- 5.5 ? Step-0.5 ?
Modeling
Assuming: (Inverted-Triangle In Composition Profile)
1) a fixed growth shape; 2) no bulk diffusion; 3) strain depends only on the scaled position; 4) 2D model
4. Strain distribution of the 10 ML MEE-QD
5. Quantitative Composition Analysis
Chemically resolved In atoms
Center Edge
6. Heterogeneous Droplet Epitaxy(HDE) QDs
In distribution is more uniform in HDE QDs
HDE QD
MEE QD
a) Empty state STM image b) curvature image
c) combined image of (a) and (b); d) marked In sites
By Composition Profile Analysis on HDE QDs, smaller QDs have higher In Concentration and larger QDs have lower In concentration (Self-compensation).
7. Summary & Conclusion
XSTM has demonstrated to be a powerful tool for the investigation of semiconductor heterostructurs at the atomic scale.
We have found:
1) size, shape, spatial correlation for MEE grown InGaAs QDs;
2) nonuniform composition profile: Inverted-triangle shape In-rich core
3) mechanism of QD formation: size and shape evolution, depleted WL near the base of the QDs, and In segregation effect;
4) self-compensation effect for HDE grown InGaAs QDs => sharp PL peaks.
Nano Electronic Material Research Group
