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Controlling Carrier Dynamics in THz Photonic Devices

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

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  1. 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

  2. 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

  3. THz spectroscopy and imaging is now commercial TeraViewwww.teraview.com Picometrixwww.advancedphotonix.com

  4. Single-Cycle Time-Domain Spectroscopy

  5. 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

  6. -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

  7. -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

  8. Photoconductive THz detectors Integrating mode(SI-GaAs device) Direct mode(LT-GaAs device) Performance: Responsivity & SNRHigh mobility & short carrier lifetime

  9. 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

  10. 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

  11. 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).

  12. 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).

  13. 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)

  14. 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)!

  15. A polarisation sensitive THz detector Appl. Phys. Lett. 86:254102 (2005)

  16. 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)

  17. 1THz (zero order) quarter waveplate

  18. Time resolved conductivity of nanostructures

  19. 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 M.Johnston@physics.ox.ac.uk www-THz.physics.ox.ac.uk

  20. Analysing the polarisation state(s) R L

  21. Multi energy ion implantation Vacancy concentration in InP dual energy implanted with Fe+(SRIM)

  22. Colloquium Outline

  23. 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

  24. 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

  25. 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

  26. 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).

  27. 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

  28. 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

  29. 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).

  30. 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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