GaAs band gap engineering by
colloidal PbS quantum dots
Bruno Ullrich
Instituto de Ciencias Físicas,
Universidad Nacional Autónoma de
México, Cuernavaca, Morelos C.P.
62210, Mexico
Acknowledgements
•
•
•
•
Joanna Wang (WPAFB)
Akhilesh Singh (UNAM)
Puspendu Barik (UNAM)
DGAPA-UNAM PAPIIT project
TB100213-RR170213 (PI Bruno
Ullrich)
Motivation
• Work in 2009 showed that PbS
quantum dots (QDs) notably alter the
emission of GaAs
• Tailored photonic applications?
• Ullrich et al., J. Appl. Phys. 108,
013525 (2010)
Presentation’s outline
Essentially, two points will be covered:
a) Optical properties of colloidal PbS
QDs on GaAs
b) Absorption edge engineering of
GaAs with PbS QDs
Sample preparation
Oleic acid capped PbS QDs are
dispersed on GaAs either by a
supercritical CO2 method*) or by
spin coating.
*)Wang et al., Mat. Chem. Phys.
141, 195 (2013).
Why PbS, why GaAs?
• PbS possesses a large Bohr radius
(20 nm). Emission covers the
attractive range for optical fibers
• GaAs is “fast” and meanwhile a main
player in optoelectronics
SEM image of a typical sample
Particle size:
2.00.4 nm
20 nm
We are dealing with a size-hybrid
• Sample can be considered – to a
certain extend – as free standing
(regularly arranged) energy
confinement potentials with
similarities to superlattices.
• Indeed, electronic states of the QDs
are coupled via tunneling.
Photoluminescence
250
PL intensity (arb. units)
2
120 W/cm
200
5K
2
90 W/cm
2
72 W/cm
150
2
60 W/cm
2
48 W/cm
100
2
30 W/cm
2
18 W/cm
2
50
12 W/cm
2
6 W/cm
0
0.9
1.0
1.1
1.2
Energy (eV)
1.3
1.4
Experimental setup
Photo-doping
1.180
E
PL
(eV)
1.175
1.170
1.165
1.160
1.155
0
20
40
60
80
2
I (W/cm )
ex
100
120
140
Burstein-Moss effect
• Doping by excited charge carriers
increases the QD band gap
• Generation of at least one electron-hole
pair per QD
• Reversible band gap alteration
proportional to Iex2/3
• Ullrich et al., J. Appl. Phys. 115, 233503
(2014)
Transmittance (normalized)
Transmittance
300 K
4
3
200 K
100 K
2
10 K
1
PbS/GaAs
0
1.0
1.1
1.2
1.3
1.4
Energy (eV)
GaAs
1.5
1.6
Absorption edge manipulation
1.2
E
TR (normalized)
1.0
g
GaAs
0.8
0.6
PbS/GaAs
0.4
RT
0.2
0.0
1.0
1.1
1.2
1.3
1.4
Energy (eV)
10 K
1.5
1.6
Slope of the edge
Normalized slope (1/eV)
-0.30
-0.40
GaAs
-0.50
-0.60
-0.70
PbS/GaAs
-0.80
-0.90
-1.00
-1.10
0
50
100
150
200
250
Temperature (K)
300
350
Band gap shift
0
dTR/dE (1/ev)
-10
3.0 nm QDs
(Spin-coating)
-20
RT
-30
-40
-50
2.0 nm QDs
(CO )
2
-60
-70
-80
1.35
SI-GaAs
PbS/GaAs
1.36
1.37
1.38
1.39
1.40
Energy (eV)
1.41
1.42
?Reasons?
•
•
•
•
•
Charge transfer (Urbach tail alteration)
Superposition of absorption spectra
Interfacial impurities
Vibronic mode manipulation
Influence of preparation method and
doping of the substrate (currently
ongoing studies)
• Change of reflectance
Conclusion and future
• QDs alter the optical properties of the host
• Concentration on the emission properties for
technological applications (emission from the
interface?)
• Possible influence of the QD size on the
optical properties of the host
• Formation of opto-electronically active
junctions
Thank you!
bruno@fis.unam.mx