gaas band gap engineering by colloidal pbs quantum dots · 2017-02-01 · gaas band gap engineering...
TRANSCRIPT
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
20 nm
Particle size:
2.0±0.4 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
0
50
100
150
200
250
0.9 1.0 1.1 1.2 1.3 1.4
PL
in
ten
sity
(a
rb. u
nit
s)
Energy (eV)
120 W/cm2
90 W/cm2
72 W/cm2
60 W/cm2
48 W/cm2
30 W/cm2
18 W/cm2
12 W/cm2
6 W/cm2
5 K
Experimental setup
Photo-doping
1.155
1.160
1.165
1.170
1.175
1.180
0 20 40 60 80 100 120 140
EP
L (
eV)
Iex
(W/cm2)
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
0
1
2
3
4
1.0 1.1 1.2 1.3 1.4 1.5 1.6
Tra
nsm
itta
nce
(n
orm
ali
zed
)
Energy (eV)
300 K
200 K
100 K
10 K
GaAsPbS/GaAs
Absorption edge manipulation
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.0 1.1 1.2 1.3 1.4 1.5 1.6
TR
(n
orm
ali
zed
)
Energy (eV)
10 KRT
Eg
PbS/GaAs
GaAs
Slope of the edge
-1.10
-1.00
-0.90
-0.80
-0.70
-0.60
-0.50
-0.40
-0.30
0 50 100 150 200 250 300 350
Norm
ali
zed
slo
pe
(1/e
V)
Temperature (K)
GaAs
PbS/GaAs
Band gap shift
-80
-70
-60
-50
-40
-30
-20
-10
0
1.35 1.36 1.37 1.38 1.39 1.40 1.41 1.42
dT
R/d
E (
1/e
v)
Energy (eV)
2.0 nm QDs
(CO2)
3.0 nm QDs
(Spin-coating)
SI-GaAs
PbS/GaAs
RT
?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!