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!

bruno@fis.unam.mx