GaAs band gap engineering by colloidal PbS quantum dots · PDF file 2017-02-01 ·...

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Transcript of GaAs band gap engineering by colloidal PbS quantum dots · PDF file 2017-02-01 ·...

  • 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

    P L

    i n

    te n

    si ty

    ( a

    rb . u

    n it

    s)

    Energy (eV)

    120 W/cm 2

    90 W/cm 2

    72 W/cm 2

    60 W/cm 2

    48 W/cm 2

    30 W/cm 2

    18 W/cm 2

    12 W/cm 2

    6 W/cm 2

    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

    E P

    L (

    eV )

    I ex

    (W/cm 2 )

  • 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 Iex 2/3

    • 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

    T ra

    n sm

    it ta

    n ce

    ( n

    o rm

    a li

    ze d

    )

    Energy (eV)

    300 K

    200 K

    100 K

    10 K

    GaAs PbS/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

    T R

    ( n

    o rm

    a li

    z ed

    )

    Energy (eV)

    10 KRT

    E g

    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

    N o rm

    a li

    ze d

    s lo

    p e

    (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

    d T

    R /d

    E (

    1 /e

    v )

    Energy (eV)

    2.0 nm QDs

    (CO 2 )

    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!

    [email protected]