1. J. S. Hwang, H. C. Lin, K. I. Lin and Y. T. Lu Department of Physics, National Cheng Kung University, Tainan, Taiwan 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

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

3. What is Terahertz Radiation (THz or T-ray) ? What is Terahertz Radiation (THz or T-ray) ? Terahertz region : 0.1 ~ 30 THz 1 THz = 10 12 Hz ~ 300 µm ~ 4.1 meV ~ 47.6 K THz Gap

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

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

7. Powder distribution in an envelope

11. 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. 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.

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. Voltage source Semiconductor crystal THz pulse optical beam reflected optical beam & THz pulse E1E1    E t E2E2 Laser pulse  THz pulse E THz (t,  ) (1) laser pulse + semiconductor (2) create transient photocurrent (3) far field THz radiation

14. ZnTe Wollaston polarizer [1,-1,0] [1,1,0] /4 plate pellicle probe beam THz beam s p polarizer detector II

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. t=t 1 t=t 2 t signal t Porbe beam pulse THz pulse t=t 0 t t=t 1 t=t 2 t=t 0

17. Time-domain THz spectroscopy FFT of THz spectroscopy

18. 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

19. 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 n ph (t) : the number of photo-excited carriers μ : carrier mobility E loc : 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.

20. Sample Structures Sample Structures In 0.52 Al 0.48 As SIN + InP (100) Semi-insulated In 0.52 Al 0.48 As (100) 1μm Si-doped 1*10 18 cm -3 In 0.52 Al 0.48 As (100) Thickness d d = 200, 120, 50, 20 nm GaAs SIN + GaAs (100) Semi-insulated 1μm n-doped 1*10 18 cm -3 GaAs (100) Thickness d d = 100 nm

21. Time domain THz radiation spectrum: Frequency domain THz radiation (FFT) spectrum:

22. 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 !

23. 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; I o : 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. I 0 : maintained at 200mW over an area with radius of 500μm.

24. 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. We have :

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

26. The critical electric field : depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset ) in the semiconductor. Why is E THz independent of E loc ? 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

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

28. 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. In 0.52 Al 0.48 As SIN + d (nm) Field (kV/cm) 200 120 50 20 47.25 53.33 122.90 255.30 GaAs SIN + d (nm) Field (kV/cm) 100 61.15 These results have been published in APL 87,121107 (2005).

29. GaAs SIN + GaAs (100) Semi-insulated 1μm n-doped 1*10 18 cm -3 GaAs (100) Thickness d d = 800 nm THz Amplitude v.s. Thickness

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

31. THz Amplitude and v.s. Thickness

32. THz Amplitude and v.s. Thickness

33. 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.

34. 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).

35. The End. Thanks for your attention !

36. ZnTe Crystal Z(001) X(100) Y(010) K p, K THz (110) E THz EpEp Probe beam intensity Refraction index of ZnTe Electro-optical coefficient of ZnTe Thickness of ZnTe

38. ZnTe e = 11; n g = 3.2 v g (800 nm) = v p (150 μ m) E g = 2.2 eV v phonon = 5.3 THz E  = 89 V/cm f > 40 THz; t < 30 fs r 41 = 4 pm/V Visible pulse experiences different THz induced refractive-index Change for different polarizations

39. Phase matching condition  k=0, optical group velocity = THz phase velocity

40. Spectra absorptionα(ω) (abs.vs.frequency) Refractive index n(ω) (time delay vs. frequency