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:2.00.4 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

inte

nsi

ty (

arb

. uni

ts)

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

(nor

mal

ized

)

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

aliz

ed)

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

Nor

mal

ized

slo

pe (

1/eV

)

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

dTR

/dE

(1/

ev)

Energy (eV)

2.0 nm QDs(CO

2)

3.0 nm QDs(Spin-coating)

SI-GaAsPbS/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