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Terahertz Radiation from InAlAs and GaAs Surface Intrinsic-N + Structures and the Critical Electric Fields of Semiconductors J. S. Hwang, H. C. Lin, K. I. Lin and Y. T. Lu Department of Physics, National Cheng Kung University, Tainan, Taiwan. Outline. Introduction to Terahertz (THz) Radiation

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  1. Terahertz Radiation from InAlAs and GaAs Surface Intrinsic-N+ Structures and the Critical Electric Fields of SemiconductorsJ. S. Hwang, H. C. Lin, K. I. Lin and Y. T. LuDepartment of Physics,National Cheng Kung University, Tainan, Taiwan

  2. Outline • Introduction to Terahertz (THz) Radiation • Motivation • System for generation and detection of THz radiation • Experimental Results and Discussions • Summary

  3. THz Gap • What is Terahertz Radiation (THz or T-ray) ? Terahertz region : 0.1 ~ 30 THz 1 THz = 1012Hz ~ 300 µm ~ 4.1 meV ~ 47.6 K

  4. Application of Terahertz Radiation • Material characterization ex: carriers dynamics (concentration, mobility..), refraction index, superconductor characterizations… • THz Imaging ex: security screening, distinguish cancerous tissue … • Biomedicine application ex: molecule (or protein) vibration modes in THz range, cancer detection, genetic analysis… • THz Laser

  5. TeraView.Ltd (2001 UK) => http://www.teraview.com • medical imaging and diagnosis : • cancer (oncology) , cosmetics , oral healthcare • pharmaceutical applications : • drug discovery & formulation , proteomics • security • non-destructive testing

  6. THz imaging Science, vol. 297, 763 (2002)

  7. Powder distribution in an envelope

  8. Motivation During the past ten years, the research activities in our lab are mainly concentrated in the field of modulation spectroscopy of photoreflectance. Three years ago, we started to set up the system for the generation and detection of THz radiation. We did not have any fund to buy the equipments for THz image or THz spectroscopy. In addition, we are unable to grow any semiconductor microstructures or devices. Therefore, we put all the semiconductor samples we have studied in the modulation spectroscopy to the THz system as the emitter. We tried to find the most effective THz emitter or to find any new physical mechanism involved in the THz radiation. Thank to Prof. Hao-Hsiong Lin, Dept. of Electric Engineering, National Taiwan University. Prof. Jen-Yin Chyi, Dept. of Electric Engineering, National Central University.

  9. System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ

  10. E t E1 E2 q1 qo q2 Photoconduction Semiconductor crystal Laser pulse THz pulseETHz(t, W) Dt Voltage source (1) laser pulse + semiconductor reflected optical beam & THz pulse THz pulse (2) create transient photocurrent (3) far field THz radiation optical beam

  11. Free-Space Electro-Optic Sampling detector DI s p [1,-1,0] Wollaston polarizer [1,1,0] /4 plate ZnTe THz beam probe beam pellicle polarizer

  12. System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ

  13. t=t2 Porbe beam pulse signal t=t0 THz pulse t=t1 t t=t0 t t t=t2 t=t1

  14. Time-domain THz spectroscopy FFT of THz spectroscopy

  15. System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ

  16. Generation : • Photoconductive: • 1. Ultra-fast laser pulse with photo energy greater than semiconductor band gap. • Electron-hole pairs created. • 2. Static electric field at surface or interface. • 3. Carriers driven by field form a transient photocurrent. • 4. The accelerated charged carrier or fast time-varying current radiates electromagnetic waves. • where • J : transient current • e : the electron charge • nph(t) : the number of photo-excited carriers • μ : carrier mobility • Eloc :the built-in electric field or external bias over the sample surface illuminated by the pump beam • Detection : • Electro-Optical Sampling • 1. Stop THz pulse => rotate λ/4 wave-plate => balance s- , p-polarized intensity . • 2. While THz pulse and Probe pulse arrived ZnTe at the same time • => optical axis of ZnTe will be rotated => balance detector measures a difference signal ΔI . • 3. ΔI is proportional to THz Field .

  17. In0.52Al0.48As (100) Thickness d GaAs (100) Thickness d In0.52Al0.48As (100) 1μm Si-doped 1*1018cm-3 GaAs (100) 1μm n-doped 1*1018cm-3 InP (100) Semi-insulated GaAs (100) Semi-insulated Sample Structures In0.52Al0.48As SIN+ GaAs SIN+ d = 200, 120, 50, 20 nm d = 100 nm

  18. Time domain THz radiation spectrum: • Frequency domainTHz radiation (FFT) spectrum:

  19. Intensities of THz radiation from InAlAs SIN+ structures with various intrinsic layer thicknesses d : It is widely believed that the amplitude of THz is proportional to the surface electric field. However, compared with the electric fields measured from PR spectroscopy, the amplitude is not proportional to the surface field !

  20. On the other hand, the number of photo-excited free charged carriers can be estimated as function of the intrinsic layer thickness d by where R : the reflectivity of the emitter; α : the absorption coefficient; η : the quantum efficiency; d : the thickness of the intrinsic layer in the SIN+ structure used as an emitter, : the photon energy of the pump beam; Θ : the incident angle of the pump beam; γ : the repetition rate of the pump beam; Io : the pump beam power; S : the width of the charge depletion layer defined by where is the dielectric constant of the semiconductor and is the potential barrier height across the interface or the charge depletion layer on surface. I0 : maintained at 200mW over an area with radius of 500μm.

  21. We have : • Surprisingly the dependence of the number of the photo-excited • carriers is the same as the dependence of the THz amplitude on • the intrinsic layer thickness.

  22. In the instantaneous photo-excited case: Carrier life time (~1ps) >> laser pulse duration (~80fs) The THz amplitude: Let’s come back to the equation:

  23. Why is ETHz independent of Eloc ? • The critical electric field introduced by Leitenstorfer et al. in • Appl. Phys. Lett. 74 (1999) 1516. • Phys. Rev. Lett. 82 (1999) 5140. • In low field limit : the maximum drift velocity is proportional to the electric field • In high-field limit (as the field rises above the critical electric field) : • the maximum drift velocity declines slightly as the field increases. • The drift velocity of free carrier reaches its maximum at the critical electric field The critical electric field : depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset ) in the semiconductor.

  24. The critical electric field: Appl. Phys. Lett. 74 (1999) 1516 : GaAs : ΔE = 330meV, Ec = 40 kV/cm Phys. Rev. Lett. 82 (1999) 5140: InP : ΔE = 600meV, Ec = 60 kV/cm Solid State Electron. 43 (1999) 403 : InAlAs : ΔE = 430meV, Ec ~ 47 kV/cm (estimated)

  25. In0.52Al0.48As SIN+ d (nm) Field (kV/cm) 47.25 200 120 53.33 50 122.90 255.30 20 GaAs SIN+ d (nm) Field (kV/cm) 61.15 100 • The surface fields in our samples exceed • their corresponding critical electric fields All the surface fields are larger than their corresponding critical fields, therefore; the amplitudes of THz are independent of the surface field. • These results have been published in APL 87,121107 (2005).

  26. GaAs (100) Thickness d GaAs (100) 1μm n-doped 1*1018cm-3 GaAs (100) Semi-insulated GaAs SIN+ THz Amplitude v.s. Thickness d = 800 nm

  27. THz Amplitude and Carriers v.s. Thickness THz Amplitude and Field v.s. Thickness

  28. THz Amplitude and v.s. Thickness

  29. THz Amplitude and v.s. Thickness

  30. Summary • THz radiation from series of GaAs and InAlAs SIN+ structures without external bias was studied. • The amplitude of THz waves radiated is independent of the built-in electric field when the built-in electric field exceeds the critical electric field. • The THz amplitude is proportional to the number of photo-excited free charged carriers. (while bias field exceeds the critical electric field). • If the critical electric field determined from the THz amplitude as a function of the electric field => It would be to determine the Γ to L valley splitting in semiconductors. • The most efficient SIN+ structure THz emitter would be the built-in electric field equal to the critical field while the thickness of the intrinsic layer equal to the penetration depth of pump laser.

  31. References 1. X. C. Zhang and D. H. Auston: J. Appl. Phys. 71 (1992) 326. 2. K. Liu, A. Krotkus, K. Bertulis, J. Z. Xu and X. C. Zhang: J. Appl. Phys. 94 (2003) 3651. 3. P. Gu, M. Tani, S. Kono and K. Sakai: J. Appl. Phys. 91 (2002) 5533. 4. M. B. Johnston, D. M. Whittaker, A. Corchia, A. G. Davies and E. H. Linfield: Phys. Rev. B 65 (2002) 165301. 5. J. S. Hwang, S. L. Tyan, W. Y. Chou, M. L. Lee, D. Weyburne and Z. Hang: Appl. Phys. Lett. 64 (1994) 3314. 6. J. S. Hwang, W. C. Hwang, Z. P. Yang and G. S. Chang: Appl. Phys. Lett. 75 (1999) 2467. 7. J. S. Hwang, W. Y. Chou and M. C. Hung, J. S. Wang and H. H. Lin: J. Appl. Phys. 82 (1997) 3888. 8. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 68 (1996) 1604. 9. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 70 (1997) 1784. 10. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 71 (1997) 1285. 11. J. N. Heyman, N. Coates and A. Reinhardt: Appl. Phys. Lett. 83 (2003) 5476. 12. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox: Appl. Phys. Lett. 74 (1999) 1516. 13. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox: Phys. Rev. Lett. 82 (1999) 5140. 14. R. Dittrich and W. Schroeder: Solid State Electron. 43 (1999) 403. 15. S. M. Sze: Semiconductor Device Physics and Technology (Wiley, New York, 1985).

  32. The End. Thanks for your attention !

  33. ZnTe Crystal Z(001) ETHz Ep Y(010) X(100) Kp , KTHz (110) Probe beam intensity Refraction index of ZnTe Electro-optical coefficient of ZnTe Thickness of ZnTe

  34. ZnTe Property of ZnTe Eg= 2.2 eV vphonon= 5.3 THz e = 11; ng = 3.2 r41 = 4 pm/V Ep= 89 V/cm vg(800 nm) = vp(150 μm) f > 40 THz; t < 30 fs Visible pulse experiences different THz induced refractive-index Change for different polarizations

  35. Coherent Length Lc Phase matching conditionDk=0, optical group velocity = THz phase velocity

  36. Time-Domain THz Transmission Spectroscopy Spectra absorptionα(ω) (abs.vs.frequency) Refractive index n(ω) (time delay vs. frequency

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