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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 Motivation System for generation and detection of THz radiation Experimental Results and Discussions Summary What is Terahertz Radiation (THz or T-ray) ? THz Gap Terahertz region : 0.1 ~ 30 THz 1 THz = 1012 Hz ~ 300 µm ~ 4.1 meV ~ 47.6 K 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 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 THz imaging Science, vol. 297, 763 (2002) Powder distribution in an envelope 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. System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ Pulse width : 80 fs; Semiconductor crystal Laser pulse THz pulse ETHz(t, W) Dt E THz ( t ) Voltage source reflected optical beam & THz pulse THz pulse Et E1 q1 qo optical beam E2 q2 J ( t ) t (1) laser pulse + semiconductor E g (2) create transient photocurrent J( t ) n ( t )eE b (3) far field THz radiation E THz ( t ) J ( t ) t DI L D n3r41 ETHz I detector s [1,-1,0] ZnTe DI p Wollaston polarizer /4 plate probe beam pellicle E E polarizer System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ Pulse width : 80 fs; Porbe beam pulse signal THz pulse t=t0 t t t=t1 t=t2 Intensity (a.u) t=t2 t=t0 t=t1 0 2 4 Time delay (ps) 6 t Time-domain THz spectroscopy FFT of THz spectroscopy GaAs Amplitude Intensity (a.u) GaAs 0 2 4 6 Time delay (ps) 8 10 12 0 1 Frequency (THz) 2 3 System for generation and detection of THz radiation Ti:Sapphire pulse laser (Tsunami, Spectro-Physics) Power : 700 mw (max); Wavelength : 790 nm ; Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ Pulse width : 80 fs; 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. ETHz (t ) J n ph (t ) eEloc t t 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 . Sample Structures In0.52Al0.48As SIN+ GaAs SIN+ 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 d = 200, 120, 50, 20 nm GaAs (100) Semi-insulated d = 100 nm Time domain THz radiation spectrum: (a) Amplitude (arb.units) 1.5 GaAs wafer + GaAs SIN d=100nm + InAlAs SIN d=104nm + InAlAs SIN d=200nm 1.0 0.5 0.0 -0.5 -1.0 0 2 4 6 8 10 time delay (ps) Frequency domain THz radiation (FFT) spectrum: Phase (rad) 50 Amplitude (arb. unit) 3.0 2.5 40 30 20 10 2.0 0 0.0 1.5 0.5 1.0 1.5 2.0 2.5 3.0 Frequency (THz) 1.0 GaAs wafer + GaAs SIN d=100nm + InAlAs SIN d=104nm + InAlAs SIN d=200nm 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Frequency (THz) 3.0 3.5 4.0 Intensities of THz radiation from InAlAs SIN+ structures with various intrinsic layer thicknesses d : 2.5 GaAs SIN + etching from 100 nm 0.2 40 60 80 100 1.5 0.0 1.0 Top layer thickness (nm) InAlAs SIN + 0.5 Built-in electric field (kV/cm) 0.4 2.0 THz amplitude (nA) 0.6 Surface field of different SIN Structure GaAs (Etched from 100 nm) InAlAs ( As Grown ) InAlAs ( Etched from 200 nm ) 350 THz amplitude (nA) 0.8 20 + 1.0 300 250 200 150 100 etching from 200 nm as grown 50 0.0 -200 -100 0 100 Top layer thickness (nm) 200 -200 -100 0 100 Intrinsic Layer thickness d (nm) 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 ! 200 On the other hand, the number of photo-excited free charged carriers can be estimated as function of the intrinsic layer thickness d by (1 R ) ds n ph (d) I 0 exp( x )dx cos q 0 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 S 2 0 r / eN where r 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. We have : 2.5 GaAs SIN + etching from 100 nm 0.6 0.4 0.2 20 40 60 80 100 2.0 1.5 0.0 1.0 Top layer thickness (nm) InAlAs SIN + 0.5 etching from 200 nm as grown 0.0 -200 -100 0 100 200 Top layer thickness (nm) 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. THz amplitude (nA) 0.8 THz amplitude (nA) 1.0 Let’s come back to the equation: ETHz (t ) J n ph (t ) eEloc t t In the instantaneous photo-excited case: t c (~1ps) Carrier life time >> laser pulse duration (~80fs) n ph (t ) n phe t /t c 1 ETHz (t ) ( )n ph (t )eEloc tc ETHz (0) n ph The THz amplitude: 2.5 GaAs SIN + etching from 100 nm 0.6 0.4 0.2 20 40 60 80 100 2.0 1.5 0.0 1.0 Top layer thickness (nm) InAlAs SIN + 0.5 etching from 200 nm as grown 0.0 -200 -100 0 100 Top layer thickness (nm) 200 THz amplitude (nA) 0.8 THz amplitude (nA) 1.0 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. 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) The surface fields in our samples exceed their corresponding critical electric fields In0.52Al0.48As SIN+ d (nm) Field (kV/cm) + Surface field of different SIN Structure GaAs (Etched from 100 nm) InAlAs ( As Grown ) InAlAs ( Etched from 200 nm ) Built-in electric field (kV/cm) 350 300 250 200 47.25 120 53.33 50 122.90 20 255.30 200 150 GaAs SIN+ 100 d (nm) 50 -200 -100 0 100 Intrinsic Layer thickness d (nm) Field (kV/cm) 200 100 61.15 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). GaAs SIN+ THz Amplitude v.s. Thickness 16 GaAs (100) Thickness d GaAs (100) Semi-insulated 12 THz Amplitude (nA) GaAs (100) 1μm n-doped 1*1018cm-3 14 10 8 6 4 2 0 -4000 -2000 0 2000 4000 Thickness(Å) d = 800 nm 6000 8000 THz Carrier 12 8 10 6 8 6 4 THz Amplitude and Carriers v.s. Thickness 4 2 2 0 0 -4000 -2000 0 2000 4000 6000 8000 Thickness (Å) THz Field 16 300 THz Amplitude and Field v.s. Thickness 12 250 10 200 8 150 6 100 4 50 2 0 0 -4000 -2000 0 2000 Thickness(Å) 4000 6000 8000 Field (kV/cm) 14 THz Amplitude (nA) THz Amplitude (nA) 14 10 8 16 12 Carrier Number (10 ) 18 14 n E THz Amplitude and v.s. Thickness Carrier Field THz 14 THz Amplitude (nA) 20 10 15 8 6 10 4 5 2 0 0 -4000 -2000 0 2000 Thickness(Å) 4000 6000 8000 9 12 CarrierField (10 kV/cm) 25 n Eeffective THz Amplitude and v.s. Thickness Carrier Number Effective Field 25 9 14 kV/cm) THz 16 12 15 8 6 10 4 5 2 0 0 -4000 -2000 0 2000 4000 Thickness(Å) 6000 8000 Carrier Number THz Amplitude (nA) 10 Effective Field (10 20 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. 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). The End. Thanks for your attention ! ZnTe Crystal ETHz Z(001) Ep Y(010) X(100) Kp , KTHz (110) DI ( , ) I p n3 ETHz r41L 2c Ip n r41 L (cos sin 2 2 sin cos 2 ) Probe beam intensity Refraction index of ZnTe Electro-optical coefficient of ZnTe Thickness of ZnTe DI ( , ) I p n3 ETHz r41L 2c (cos sin 2 2 sin cos 2 ) 900 8 10 Eprobe // ETHz 6 Eprobe ETHz 8 6 Intensity (arb. units) Intensity (arb. units) 4 2 0 -2 -4 4 2 0 -2 -4 -6 -6 -8 -8 -10 0 90 180 Azimuthal angle (degrees) 270 360 0 90 180 Azimuthal angle (degrees) 270 360 vphonon= 5.3 THz E= 89 V/cm f > 40 THz; t < 30 fs Eg= 2.2 eV ZnTe e = 11; ng = 3.2 r41 = 4 pm/V vg(800 nm) = vp(150 μm) Visible pulse experiences different THz induced refractive-index Change for different polarizations Phase matching condition Dk=0, optical group velocity = THz phase velocity Dk k op THz k op k THz k op THz k op k THz THz THz THz k k THz THz THz op dn THz nopt opt opt |opt nTHz d Lc c Dk THz nopt c dnopt opt |opt nTHz d k () n () / c Spectra absorptionα(ω) (abs.vs.frequency) Refractive index n(ω) (time delay vs. frequency