The Nanolab
http://people.roma2.infn.it/~nanolab/index.html
Prof. A. Balzarotti
MBE
HREELS
UPS-XPS
LEED
Prof. F. Patella
Prof. M. Fanfoni
MBE Lab
Prof. A. Sgarlata
Dr. F. Arciprete
Dr. E. Placidi (CNR)
Dr. L. Persichetti (PhD student)
STM Lab
Primo Workshop del Dipartimento di Fisica dell’Università di Roma Tor Vergata,
7 – 8 Giugno 2011
InAs/GaAs(001) Quantum Dots
grown by Molecular Beam Epitaxy
F. Arciprete, E. Placidi, F. Patella, M. Fanfoni, and
A. Balzarotti
n  Investigation of the basic microscopic
mechanisms driving the self-assembling
of InAs/GaAs(001) QDs at each stage of
their evolution (PRIN 1997, 2000, 2002 –
MADESS II (CNR))
n  Site-controlled nucleation: selective
growth of a single Quantum Dot on
400nm
selected areas of the SiO2/GaAs(001)
(PRIN 2005, 2007 – FIRB 2002)
n  Growth and morphological characterization
of Diluted Magnetic Semiconductors:
Ga(In)1-xMnxAs/GaAs(001) thin films and
nanostructures. (FP7 – FET No. 225926)
Semiconductor Heteroepitaxy “Road-map”
Richness and variety of III-V’s ⇒ high-performance "band-gap
engineered" heterostructures and devices with optical and electronic
properties difficult to achieve in other materials.
Growth Processes
q  Epitaxy : the film grows following the same lattice of the
substrate ⇒ single crystalline layers grow on a single crystal
surface:
q  LPE, VPE: near-equilibrium technique; fast,
inexpensive, poor thickness/interface control.
q  MBE (Molecular Beam Epitaxy): slower, accurate
structure and composition control, better than the
single atomic monolayer (~0.3 nm):
Epitaxy
Amorphous, Polycrystalline
Lattice mismatch and deformation
"
The lattice mismatch ε is the
cause of the deformation
"
The elastic energy per unit area
accumulated in an isotropic film of
thickness d is:
The favored growth morphology of
the film is that of minimum Gibbs
free energy:
F2D-coh = kε2d + γfilm
FSK-coh = Rkε2d + γfilm + Δγ3D
ΔEfilm~kε2d
"
"
When no dislocations appear we
have a “coherently strained” or
“pseudomorphic” growth.
F2D-MD = (2EMD/do) + γfilm
low lattice mismatch
partial relaxation
high lattice mismatch
ε=
d InAs − d GaAs
=7%
d GaAs
for d > dc coherent islands can nucleate
Stranski-Krastanow (SK) growth mode
Molecular Beam Epitaxy
Base Pressure : 5×10-11mbar
7 effusion cells
Ga, In, As4 (2), Al, Si, Mn
InAs/GaAs(001) prototypical of high-mismatched growth which
proceeds in Stranski-Krastanow coherent mode
AFM/STM characterization of QDs during their complete evolution cycle
§ 
Wetting Layer composition
§ 
Evolution of the 2D-3D transition
§  Size distribution and the scaling
properties of QDs
1.57 ML - InAs/GaAs(001)
§ 
Shape of islands
§ 
Lateral ordering and Selective Nucleation of QDs
1.54 ML
QDs density evolution
1.0 × 0.5 µm2
Small QDs
1.57 ML
Large QDs
Appl. Phys. Lett. 89, 041904 (2006)
1.61 ML
Step erosion
QDs nucleated onto the step edge determine erosion of the steps for a volume > 0.3 ML
2.40 ML
Cover page for
issue 24 vol. 86
Appl. Phy. Lett. 86, 241913 (2005)
The compressive strain around 3D islands significantly reduces the In attachment/detachment
rate ratio in that region: step erosion takes place
process evolution
Site-controlled nucleation
2 x 2 µm2
1x1 µm2
200 nm
400nm
200nm
J. of Nanophotonics 3, 031995 (2009)
InAs QDs
µPL on patterned holes
1.159 eV
µPL - 5 µm spatial resolution
4×4 µm2
500 nm diameter hole
20 µm spaced
FWHM=4 meV
8.0µm
1.177 eV
40×40 µm2
1 single hole is probed (~110 dots)
0.2 % of the PL spot area
T=10 K
FWHM<1 meV
4×4 µm2
100 nm diameter holes
100 nm spaced
~490 holes are probed
20 % of the PL spot area
Appl. Phys. Lett. 93, 231904 (2008)
The emission observed is
compatible with emission from
single dots
Nanoscale selective growth of InAs QDs
InAs/SIO2/GaAs
100×100 nm2 square holes
3-4 InAs dots nucleated on the
GaAs at the bottom
100 nm holes:
<b> = 25 nm < 30-40 nm
<h> = 2 nm < 3-4 nm
h
b
QDs selfassembled on
extended GaAs
surfaces
The sizes of QDs inside nanoscale holes
are smaller than those nucleated on the
extended GaAs surface:
-  confined area diffusion?
-  different role for nanoscale finite-area
WL?
Appl. Phys. Lett. 93, 231904 (2008)
A different kinetics for spatially confined growth at the nanoscale
Holes spacing L = 150 nm
120 nm
600 × 600 nm2
Collaborations
Università degli Studi di Modena e Reggio Emilia eand Centro S3 (CNR) –
Istituto di Nanoscienze:
Ab initio studies and MonteCarlo simulation
Prof. R. Magri, Dr. M. Rosini
Institute for Photonic and Nanotechnologies of Rome (CNR):
Dr. A. Gerardino, Dr. E. Giovine
Electron Beam Litography
Max Planck Institute for Solid State Research - Stuttgart
Dr. J. Honolka, Prof. K. Kern
DMS - Magnetic Characterization
Università di Firenze e European Laboratory for Non-linear spectroscopy:
Prof. A. Vinattieri, Prof. M. Gurioli, Dr. L. Cavigli, Dr. M. Abbarchi
macro-PL & µPL
Istituto di Struttura della Materia (CNR)
Dr. S. Colonna, Dr. F. Ronci, Dr. A. Cricenti
Diluted Magnetic Semiconductors