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Controlling Carrier Dynamics in THz Photonic Devices. E. Castro-Camus 1 , J. Lloyd-Hughes 1 , L. Fu 2 , S.K.E. Merchant 1 , Y. J. Wang 1 , H. H. Tan 2 , C. Jagadish 2 , and Michael B Johnston 1 . 1 University of Oxford, Department of Physics. 2 EME Australian National University.

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Controlling carrier dynamics in thz photonic devices

Controlling Carrier Dynamics in THz Photonic Devices

E. Castro-Camus1, J. Lloyd-Hughes1, L. Fu2, S.K.E. Merchant1, Y. J. Wang1, H. H. Tan2, C. Jagadish2, and Michael B Johnston1.

1University of Oxford, Department of Physics.

2EME Australian National University.

Introduction to THz technology

Time resolved conductivity (OPTPS)

Tayloring materials for THz devices(passivation, ion-implantation)

A polarisation sensitive THz detector

Non-contact conductivity of nanowires

www-THz.physics.ox.ac.uk


Why use light of thz frequencies
Why use light of THz frequencies

1THz  33cm-1  4.1meV  47.6 K  300m

  • THz band (0.04 - 40meV) is the Energy Range of:

  • Plasmons, Phonons, Cooper pairs and Excitons in solid state systems

  • Rotational modes in molecules and collective vibrational modes in macromolecules and biomolecules


Thz spectroscopy and imaging is now commercial
THz spectroscopy and imaging is now commercial

TeraViewwww.teraview.com

Picometrixwww.advancedphotonix.com



Three forms of terahertz spectroscopy

Terahertz emission spectroscopy

  • Probes surface electric fields directly.

  • Indirect probe of ultrafast carrier dynamics.

Terahertz time-domain spectroscopy

  • Measures complex refractive index/conductivity of a sample over a broad frequency range (50GHz-10THz).

-1

Optical-pump terahertz-probe spectroscopy (OPTPS)

  • Dynamic conductivity response of material, from ~100fs to ~1ns.

10

328 ps

s(t,w) /W-1cm-1

5.9 ps

-2

THz

10

1.2 ps

IR

0.75 ps

0

10

20

30

40

Time t /ps

Three forms of terahertz spectroscopy

THzEMITTER

0.2mm <110> ZnTeon 6mm <100> ZnTe


Two important thz photonic devices

-V

IR

h

e

THz

+V

Two important THz photonic devices

  • Photoconductive THz Detector

  • Photoconductive THz Emitter

Devices are typically fabricated from bulk III-V semiconductors


Thz emitters how to increase power and bandwidth

-V

IR

h

e

  • Generation rate

  • Acceleration under electric field

THz

+V

  • Momentum scattering

  • Recombination rate

  • Capture rate

Electric

field

Time

THz emitters: How to increase power and bandwidth:

[More realistic carrier dynamics & THz emission modelling: Phys. Rev. B 65, 165301 & Phys. Rev. B 71, 195301]

Performance: Power & BandwidthHigh mobility & short carrier lifetime


Photoconductive thz detectors
Photoconductive THz detectors

Integrating mode(SI-GaAs device)

Direct mode(LT-GaAs device)

Performance: Responsivity & SNRHigh mobility & short carrier lifetime


Ideal materials for thz devices
Ideal Materials for THz Devices

  • High mobility

    • For improved emitter power

    • For improved detector sensitivity

  • Short carrier lifetime

    • Improved bandwidth of emitters

    • Improve damage threshold

    • Improved SNR of receivers

      So we want a material that is extremely conductive for a short period after photo-excitation and highly resistive at other times

  • III-Vs have been materials of choice

    • Compatible with Ti:Sapphire lasers

    • High mobility at room temperature

    • Short carrier lifetime


  • Tailoring carrier dynamics

    ECB

    EF

    EVB

    Tailoring Carrier Dynamics

    • Surface modifications

      • Passivation

      • Patterning

    • Ion implantation

      • Implanted Si on Sapphire

      • GaAs:As+, GaAs:H, InP:Fe+

    • Low Temperature Growth

      • LT-GaAs, LT-InGaAs


    Time resolved conductivity of passivated gaas

    Doubling of mobility of photoexcited carriers due to reduced carrier-defect scattering near surface.

    Bulk lifetime ~15ns.

    Time-resolved conductivity of Passivated GaAs

    (100) GaAs and InSb etched with 5:1:1 H2SO4:H2O2:H2O to remove oxides, then passivated by dipping in (NH4)2S for 10 min.

    Initial lifetime 390ps, c.f. 190ps for reference.

    Long-lived bulk carriers give non-zero conductivity before pulse. (Lowers resistivity of devices).

    • 1D diffusion equation model [following Beard et al., Phys. Rev. 62 15764] yields:

      S0 = 2.0x105 cm s-1 (passivated)

      S0 = 1.2x106 cm s-1 (reference)

    • i.e. defect/trap density reduced to 17% by passivation.

    • Laser: 10fs Ti:sapphire, 790nm, 9nJ per pulse, 13ns period (75MHz repetition rate).

    • J. Lloyd-Hughes et al., Appl. Phys. Lett. 89 232102 (2006).


    Improved thz emission from passivated thz emitter

    0 carrier-defect scattering near surface.

    10

    -1

    -1

    1

    10

    10

    2

    2

    -2

    -2

    10

    10

    passivated

    -3

    -3

    10

    10

    -1

    /arb. units

    -4

    -4

    10

    10

    0

    -V

    10

    0

    E(t) /kVm

    -5

    -5

    2

    10

    10

    1

    )|

    n

    1

    1

    -6

    -6

    |E(

    |E()|2 / arb. units

    -0.5

    10

    10

    IR

    h

    0.5

    -7

    -7

    10

    10

    (b)

    (a)

    -1

    -1

    -8

    -8

    /arb. units

    10

    10

    ref

    e

    E(t) /kVm

    -9

    10

    2

    -2

    0

    2

    )|

    4

    6

    0

    2

    4

    6

    8

    10

    n

    n

    Time t /ps

    Frequency

    /THz

    0

    0

    |E(

    0

    1

    2

    THz

    (b)

    (a)

    Frequency  /THz

    +V

    -9

    10

    0

    2

    4

    6

    8

    10

    n

    Time t /ps

    Frequency

    /THz

    Improved THz emission from passivated THz emitter

    400m gap.

    150V at 21kHz.

    • Laser: 10fs Ti:sapphire, 790nm, 9nJ per pulse, 13ns period (75MHz repetition rate).

    Enhanced ETHz J/t ~ /t

    • J. Lloyd-Hughes et al., Appl. Phys. Lett. 89 232102 (2006).


    Ion implantation inp fe

    Vacancy concentration (10 carrier-defect scattering near surface.17cm-3)

    Ion Implantation (InP:Fe+)

    Optical-pump, terahertz probe Carrier lifetime extracted from decay in conductivity - the perfect characterisation tool!

    Ion implantation

    InP:Fe, 1x1013cm-2 at 2MeV and 2.5x1012cm-2 at 0.8MeV.

    Annealed at 500°C for 30min.

    We have also performed similar measurements to optimise annelling conditions (Activation Energies extracted from Arrhenius plots)


    Ion implanted thz detectors
    Ion implanted THz detectors carrier-defect scattering near surface.

    THz Spectrum of SI-GaAs THz emitter taken with InP:Fe detectors

    Differentiatedphotocurrent

    Deconvolved(“true” spectrum)

    So OPTPS data not only allows device optimisation, but in addition allows spectral response correction (via deconvolution)!


    Controlling carrier dynamics in thz photonic devices

    A polarisation sensitive THz detector carrier-defect scattering near surface.

    Appl. Phys. Lett. 86:254102 (2005)


    Simultaneous measurement of orthogonal field components

    92° (90 carrier-defect scattering near surface.±5°)

    49° (45±5°)

    -3° (0±5°)

    Simultaneous measurement of orthogonal field components

    EV (arb. units)

    E. Castro-Camus et al.Appl. Phys Lett86, 254102 (2005)


    Controlling carrier dynamics in thz photonic devices

    1THz (zero order) quarter waveplate carrier-defect scattering near surface.


    Time resolved conductivity of nanostructures
    Time resolved conductivity of nanostructures carrier-defect scattering near surface.


    Summary
    Summary carrier-defect scattering near surface.

    Terahertz spectroscopy enables conductivity of sample to be measured

    without applying contacts to a sample

    with sub-picosecond time resolution

    Complex conductivity is measured information about capacitance and inductance

    Frequency depended AC conductivity is measured  information about carrier dynamics

    • Surface passivation improves THz emitter performance

    • Ion implantation may be used to optimise photo-excited carrier lifetimes in THz detectors

    • Time resolved conductivity measurements (photoexcited with similar laser pulses to those used with the operating device) used

      • to optimise detector materials

      • in deconvolution of detector signal

    • Polarisation resolved THz spectroscopy now available

    • THz conductivity of GaAs nanowires studied

    M.Johnston@physics.ox.ac.uk

    www-THz.physics.ox.ac.uk


    Controlling carrier dynamics in thz photonic devices

    Analysing the polarisation state(s) carrier-defect scattering near surface.

    R

    L


    Multi energy ion implantation
    Multi energy ion implantation carrier-defect scattering near surface.

    Vacancy concentration in InP dual energy implanted with Fe+(SRIM)


    Colloquium outline
    Colloquium Outline carrier-defect scattering near surface.


    Ion implanted inp o inp fe
    Ion-implanted InP:O, InP:Fe carrier-defect scattering near surface.

    • Typical damage profile for multi energy implants :

    Vacancy concentration (1017cm-3)

    James Lloyd-Hughes, Oxford Terahertz Photonics Group 17th October 2005


    Time resolved thz spectroscopy of inp fe

    600C (114ps) carrier-defect scattering near surface.

    500C (24.7ps)

    E (arb. units)

    400C (1.35ps)

    Time resolved THz spectroscopy of InP:Fe

    Dose dependence

    Anneal temperature dependence

    Unimplanted (328ps)

    (5.94ps)

    E (arb. units)

    (1.24ps)

    (0.75ps)

    James Lloyd-Hughes, Oxford Terahertz Photonics Group 17th October 2005


    Arrhenius plot for low dose inp fe

    carrier-defect scattering near surface.Activation energy for thermal

    annealing Ea = 1.20§0.06 eV

    (c.f. Ea = 1.27§0.05 eV from TRPL)

    [Carmody et. al., JAP 94 1074]

    Arrhenius plot for low-dose InP:Fe

     = 0eEa/kT

    T=336°C should have =0.1ps (for this dose)

    James Lloyd-Hughes, Oxford Terahertz Photonics Group 17th October 2005


    Surface defects

    STM image of 110 surface of GaAs. carrier-defect scattering near surface.

    http://www.mse.berkeley.edu/groups/weber/

    No Fermi-level

    pinning

    Fermi-level

    pinning

    Bulk

    Surface

    ECB

    ECB

    EF

    EF

    Defect states

    EVB

    EVB

    ~100nm

    ~1nm

    Surface defects

    • Surface states trap and scatter carriers.

    • Critical in surface and nano-scale semiconductor physics, e.g. in polymer transistors, nanowires.

    2m

    GaAs nanowires with AlGaAs shells.

    Titova et al., Appl. Phys. Lett. 89 173126 (2006).


    Surface passivation

    Samples: carrier-defect scattering near surface.

    (100) GaAs and InSb etched with 5:1:1 H2SO4:H2O2:H2O to remove oxides, then passivated by dipping in (NH4)2S for 10 min.

    Reference samples prepared without passivation step, and left to oxidise in air.

    Similar results using Na2S.9H2O.

    LEDs

    Ga

    Kamiyama et al., Appl. Phys. Lett. 58 2595 (1991).

    As

    Etch & passivate

    Ga

    S

    Ga

    Solar cells

    As

    Mauk et al., Appl. Phys. Lett. 54 213 (1989).

    Ga

    Ga

    As

    S

    THzemitters?

    Surface passivation

    V.N. Bessolov and M.V. Lebedev, Semiconductors 32 1141 (1998).

    Ga

    Ga

    -

    As

    oxides

    Ga

    Ga

    -

    Ga

    -

    As

    Ga

    Ga

    oxides

    Ga

    Ga

    As


    Surface terahertz emission

    Surface carrier-defect scattering near surface.

    emitter

    Terahertz emission spectroscopy

    • Probes surface electric fields directly.

    • Indirect probe of ultrafast carrier dynamics.

    THz

    pump

    surface field

    THz

    IR

    photo-Dember

    -1

    10

    328 ps

    s(t,w) /W-1cm-1

    5.9 ps

    -2

    THz

    10

    1.2 ps

    IR

    0.75 ps

    0

    10

    20

    30

    40

    Time t /ps

    Surface terahertz emission

    Surface THz emitters

    0.2mm <110> ZnTeon 6mm <100> ZnTe


    Surface terahertz emission1
    Surface terahertz emission carrier-defect scattering near surface.

    passivated

    InSb

    ref.

    passivated

    GaAs

    ref.

    ref.

    passivated

    • Laser: 10fs Ti:sapphire, 790nm, 9nJ per pulse.

    Further details on simulation:

    • M.B. Johnston et al., Phys. Rev. B 65 165301 (2002),

    • J. Lloyd-Hughes et al., Phys. Rev. B 70 235330 (2004).

    • J. Lloyd-Hughes et al., Appl. Phys. Lett. 89 232102 (2006).


    Surface terahertz emission2
    Surface terahertz emission carrier-defect scattering near surface.

    passivated

    InSb

    ref.

    passivated

    GaAs

    ref.

    ref.

    passivated

    • Laser: 10fs Ti:sapphire, 790nm, 9nJ per pulse.

    Further details on simulation:

    • M.B. Johnston et al., Phys. Rev. B 65 165301 (2002),

    • J. Lloyd-Hughes et al., Phys. Rev. B 70 235330 (2004).

    • J. Lloyd-Hughes et al., Appl. Phys. Lett. 89 232102 (2006).