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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

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 (1017cm-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

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)!

slide15

A polarisation sensitive THz detector

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

simultaneous measurement of orthogonal field components

92° (90±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)

summary
Summary

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

[email protected]

www-THz.physics.ox.ac.uk

multi energy ion implantation
Multi energy ion implantation

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

ion implanted inp o inp fe
Ion-implanted InP:O, InP:Fe
  • 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)

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

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.

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:

(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

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

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

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).
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