1. Controlled Synthesis of InAs Wires, Dot and Twin-Dot Array configurations by Cleaved Edge Overgrowth
E. Uccelli, M. Bichler, S. Nürnberger, G. Abstreiter, A. Fontcuberta i Morral
Nanotechnology 19, 2008

2. What is it all about?
Control of size and thereby emission properties of QDs is possible but control of position is more challenging
In situ cleaved edge overgrowth to control the nucleation of InAs QDs
Uccelli, Bichler, Nürnberger, Abstreiter and Fontcuberta i Morral

3. Cleaved Edge Overgrowth - Fabrication I
Quantum well structures in (001) direction – turn into stripes on a (110) facet which is natural cleavage plane
Quantum structures originate at intersection of two or three quantum wells which can be defined with atomic precision – thus this precision is transferred to the lower dimensional structures.
Left: L. Pfeiffer et al., Appl. Phys. Lett. 56, 1697 (1990) Right: G. Schedelbeck et al., Science 278, 1792 (1997)

4. Growth of InAs Structures on Cleaved GaAs/AlAs - Fabrication II
AlAs stripes of thicknesses ranging from 3-42 nm with 90 nm GaAs in between
Different structures achieved – nanowires, QD chains
Stranski-Krastanov growth mode

5. Phenomenological Growth Models
γs = γsf + γf cosφ
Layer-by-layer
Frank-van der Merve
Layer-plus-island
Stranski-Krastanov
Island
Vollmer-Weber
γs ≥ γsf + γf + CkBT ln(p0/p)
γs < γsf + γf + CkBT ln(p0/p)

6. Mechanism for Growth 1 ML = 2.83 Å
Selective mass redistribution of InAs towards AlAs stripes – consequence of reduced mobility of and lifetime of adatoms on AlAs (110) surface compared to the GaAs (100) surface (around 1 micrometer)
In/As adatoms slow down over AlAs regions which increases probability of precipitation and covalent binding on the surface
Thickness of InAs material deposited on AlAs region is larger than nominal thickness and will depend on the stripe width (so will onset of formation of InAs QDs) – onset of QD formation occurs at lower thickness than planar case (for Stranski-Krastanov growth model)
Slope changes right when QD formation occurs – heigth of QDs is higher than nominal thickness indicating strong migration of InAs from the neighbouring areas
The value of the slope is a convolution of the diffusion length on (110) oriented AlAs stripes and on the growing InAs layers
Difference in slopes before and after QD formation is geometrical effect due to rearrangement from wires to dots
1 ML = 2.83 Å

7. Twin-Dot Arrays

8. BUT! Also growth on GaAs regions

9. Conclusion
Atomic precison of MBE used for positioning of QDs and other structures
Technique seems limited to simple geometries because of the use of cleavage planes
Small window for preferential growth on AlAs but it is there!
Questions?