Plasma wave resonant detection of femtosecond pulsed terahertz ...

6 downloads 292 Views 80KB Size Report
The work at RPI was supported by the STTR grant by. ARO (subcontract from SET, Inc.). The research at Montpel- lier 2 University was supported by the French ...
APPLIED PHYSICS LETTERS 87, 022102 共2005兲

Plasma wave resonant detection of femtosecond pulsed terahertz radiation by a nanometer field-effect transistor F. Teppea兲 Center for Broadband Data Transport and Electrical, Computer and System Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 and GES CNRS-Université Montpellier2 UMR 5650 34900 Montpellier, France

D. Veksler Center for THz Research and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180

V. Yu. Kachorovski and A. P. Dmitriev Center for Broadband Data Transport and Electrical, Computer and System Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180

X. Xie and X.-C. Zhang Center for THz Research and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180

S. Rumyantsev Center for Broadband Data Transport and Electrical, Computer and System Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180

W. Knap Center for Broadband Data Transport and Electrical, Computer and System Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 and GES CNRS-Université Montpellier2 UMR 5650 34900 Montpellier, France

M. S. Shur Center for Broadband Data Transport and Electrical, Computer and System Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 and Center for THz Research and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180

共Received 5 April 2005; accepted 16 May 2005; published online 7 July 2005兲 We report on the room-temperature, resonant detection of femtosecond pulsed terahertz radiation obtained by optical rectification in a ZnTe crystal. The detection was realized using a 250 nm gate length GaAs/ AlGaAs heterostructure field-effect transistor. We show that physical mechanism of the detection is related to the plasma waves excited in the transistor channel. The detection is strongly enhanced by increasing the drain current and driving the transistor into the plasma wave instability region. Our results clearly show that plasma wave nanometer transistors can be efficient and fast detectors for terahertz spectroscopic imaging based on the femtosecond pulsed THz sources. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1952578兴 The pulsed terahertz 共THz兲 sources based on the physical mechanism of optical rectification1,2 共OR兲 are well known to be one of most important sources in modern terahertz spectroscopy. A large variety of terahertz detectors include bolometers,3 pyroelectric detectors, Schottky diodes,4 or photoconducting dipole antennas.5 Most of these detectors are cumbersome and/or require cryogenic temperature operation. The development of optical rectification imaging spectroscopy suffers from the lack of compact and fast detectors that would allow constructing detector matrixes/cameras for THz space and time-resolved imaging. Nonlinearities related to plasma wave excitations in twodimensional electron gas in a nanometer-size high electron mobility transistor 共HEMT兲 were proposed by Dyakonov and Shur as a way to realize selective resonant and voltage tunable terahertz detectors.6 Recently, the resonant detection of THz radiation by two-dimensional plasma waves was demonstrated using a commercial GaAs/ AlGaAs field-effect a兲

Electronic mail: [email protected]; [email protected]

transistor 共FET兲 at cryogenic7 and room temperature,8 and in a double quantum well field-effect transistor.9 In addition, plasma wave THz emission was reported from InGaP / InGaAs/ GaAs pseudomorphic HEMT.10 In this letter, we present the experimental evidence of the plasma wave-based detection of pulsed terahertz radiation by GaAs FETs working at room temperature. The regime of operation of the FET in which we apply a constant drain bias allowed us to increase the device responsivity by more than two orders of magnitude. We demonstrate that this way one can obtain the room-temperature resonant detection of femtosecond pulsed THz radiation by nanometer-size transistors. These detectors are small and fast and therefore can be efficiently used for construction of matrixes/cameras allowing THz space- and time-resolved imaging. The pulsed THz radiation used as a source in the experiments was based on physical mechanism of optical rectification.11–13 The laser used in this experiment is a Ti/ sapphire amplifier 共Coherent Hurricane兲 with 800 nm central wavelength, 800 ␮J per pulse energy, 130 fs pulse duration,

0003-6951/2005/87共2兲/022102/3/$22.50 87, 022102-1 © 2005 American Institute of Physics Downloaded 19 Oct 2005 to 162.38.137.110. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

022102-2

Teppe et al.

Appl. Phys. Lett. 87, 022102 共2005兲

FIG. 3. Detector photoresponse under broadband THz radiation as a function of gate voltage for different values of dc drain current 共13.5– 22 mA兲. FIG. 1. THz generation by optical rectification in ZnTe crystal. The upper figure 共a兲 shows temporal profile of a THz pulse measured by EO sampling and the lower figure 共b兲 shows its frequency profile obtained by fast Fourier transform method. Superimposed curve 共dotted line兲 is a fitting of the spectrum with Lorentzian function and shows maximum amplitude at 0.64 THz and a width of 0.9 THz.

and 1 kHz repetition rate. We used a 2 mm thick 具110典oriented ZnTe crystal as a THz emitter. The laser power on the emitter was 300 mW. Figure 1共a兲 shows temporal profile of the THz pulse measured by electro-optical 共EO兲 sampling method1,2 and Fig. 1共b兲 shows its frequency profile obtained by the fast Fourier transform method. The superimposed curve 共dotted line兲 is a fitting of the spectrum with the Lorentzian function w / 关共f − f m兲2 + w2兴, where f m is the maximum frequency and w is the full width at half-maximum兲. We obtained f m = 0.64 THz and w = 0.9 THz. The THz radiation was collimated and focused on the 250 nm gate length GaAs commercial HEMT14 using parabolic mirrors. The current voltage characteristics are shown in the inset in Fig. 2. The threshold voltage, extracted from the I-V curves is Uth ⬇ −0.4 V. The sample was mounted on a quartz plate, to avoid any parasitic interferences and reflections, and was placed on an adjustable sample holder.

The HEMT was covered by polyethylene filters to avoid the effect of any parasitic visible and infrared background illumination. No special antennas were used and the radiation was coupled to the device through the contact pads. The radiation intensity was modulated with a mechanical chopper at 80 Hz. The device was tuned by a gate-to-source voltage and a drain-to-source current. The source terminal was always grounded and the induced dc drain voltage Uds, which appeared in response to the THz radiation 共detector signal兲, was measured by a standard lock-in technique. The response to the THz radiation is shown in Fig. 2 as a function of applied current for different values of the gate voltage 共from −0.3 to − 0.1 V兲. One can see that the detection is strongly increased by increasing the drain current and driving the transistor into the current saturation region. One can see that a threshold-like behavior is observed—after some threshold current the signal increases rapidly with current. This behavior is similar to that observed in the case of the resonance emission reported in Ref. 10. It indicates that the important increase of the signal is caused by driving the transistor closer to the plasma wave instability region. Figure 3 shows the detector response as a function of Ugs for different values of drain current 共13– 22 mA兲. As seen, the maximum signal increases with the applied drain current, reaches the maximum value of ids = 19 mA corresponding to Ugs = −10 mV, and then decreases for higher values ids. The physical mechanism of detection was described in the work of Dyakonov and Shur. They have shown that even in the absence of the drain current, a FET subject to electromagnetic radiation can develop a constant drain-source voltage, which has a resonant dependence on the radiation frequency f = ␻ / 2␲ with maxima at the plasma oscillation frequencies f N = ␻N / 2␲.15 The fundamental frequency of plasma wave oscillations and odd harmonics can be easily tuned by changing the gate voltage Ugs. In the gradual channel approximation, the carrier density in the channel is related to the gate voltage as n = C共Ugs − Uth兲 / e, where Uth is the threshold voltage. For a gate length Lg, the fundamental plasma frequency, f 0 = ␻0 / 共2␲兲 is given by f 0 = 共1 / 4Lg兲冑共e共Ugs − Uth兲 / m*兲. Here m* is the electron effective mass and e is the electronic charge. The width of the resonance curve is determined by the inverse time of the electron momentum relaxation, 1 / ␶. In the regime such that

FIG. 2. Threshold behavior of the drain response as a function of applied current for different values of gate voltage 共from −0.3 to − 0.1 V兲. Inset shows I-V characteristics of the device at different values of the gate voltage 共Vg = 0 V, down to −0.3 V兲. Downloaded 19 Oct 2005 to 162.38.137.110. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

022102-3

Appl. Phys. Lett. 87, 022102 共2005兲

Teppe et al.

␻0␶ Ⰷ 1, the FET operates as a resonant detector. When ␻0␶ ⬍ 1, the plasma oscillations are overdamped and the FET response is a smooth function of ␻ as well as of the gate voltage 共nonresonant broadband detection兲. In any case, the dimensionless parameter determining the detector behavior is ␻0␶. The results shown in Figs. 2 and 3 are similar to those obtained for the same device using a coherent 600 GHz source,8 except that the linewidths in Fig. 3 are somewhat larger due to a broad spectrum excitation. The physical origin of the shift of the resonance with the drain current was discussed in Ref. 8. The shift of the maxima with the drain current can be qualitatively explained by the effect of the voltage drop across the source access resistance: f0 =

1 冑共e共Ugs − Uth − IRs兲/m*兲, 4Lg

where I is the drain current and Rs the source access resistance. An increase of the current leads to increase of the voltage drop on the access resistance that—for experiments with the constant excitation frequency—has to be compensated by the increase of the measured resonant gate voltage Vg. This formula shows that the resonant frequency can be shifted/ tuned either by the gate voltage or by the drain current. The maximum frequency f max can be estimated as f max = 共1 / 4Lg兲冑共eUth / m*兲 and is around 1 THz for our transistor. This frequency can be increased by increasing the gate bias or using a transistor with a shorter gate. As mentioned earlier, the width of the resonance line is determined by the scattering rate 1 / ␶. As shown in Ref. 16, in the presence of the current, ␶ should be replaced by ␶eff, where 1/␶eff = 1/␶ − 2v0/L

共1兲

and v0 is the effective electron drift velocity in the channel. 共This expression coincides with the expression for the decrement of the Dyakonov-Shur instability derived in Ref. 17.兲 Therefore, the line shrinks with increasing the drain current 共and, hence, the electron drift velocity兲. On the qualitative level, this mechanism might explain why one can observe the resonant detection for relatively low mobility samples by pushing the transistor into the saturation region.8 However, fitting of the observed results requires the values of the effective electron velocity vo, which are much higher than the electron saturation velocity. The reason for this discrepancy is that Eq. 共1兲 was derived assuming the constant velocity in the device channel and constant momentum relaxation time. These assumptions are not valid in the transistor saturation region. In the saturation region hot electron physics might play an essential role. In particular, transit time effects,18 interaction with optical phonons,19 and/or stratification of electron flow20 might enhance the resonance by diminishing the plasma wave decay. A detailed analysis of the transit time effects will be published elsewhere. In conclusion, we present the experimental evidence of the plasma wave detection of pulsed terahertz radiation by

GaAs FETs working at room temperature. The regime of operation of the FET with a constant dc drain bias has allowed us to increase the device responsivity by more than two orders of magnitude. We demonstrate that one can obtain the room-temperature detection that is resonant, voltage tunable, and can be efficiently used for the THz spectroscopy with femtosecond pulsed THz sources. These detectors are small and fast and therefore can be used for construction of matrixes/cameras allowing THz space- and time-resolved imaging. The work at RPI was supported by the STTR grant by ARO 共subcontract from SET, Inc.兲. The research at Montpellier 2 University was supported by the French ministry of scientific research and by the CNRS research group 共GdR兲 “Solid State Detectors and Emitters of Terahertz Radiation.” The work at Ioffe Physico-Technical Institute was supported by RFBR, a Grant of the RAS, and a Grant of Russian Scientific School 2192.2003.2. 1

M. Bass, P. A. Franken, J. F. Ward, and G. Weinreich, Phys. Rev. Lett. 9, 446 共1962兲. 2 B. Ferguson and X.-C. Zhang, Nat. Mater. 1, 26 共2002兲. 3 B. S. Karasik, W. R. McGrath, M. E. Gershenson, and A. V. Sergeev, J. Appl. Phys. 87, 7586 共2000兲. 4 S. M. Marazita, W. L. Bishop, J. L. Hesler, K. Hui, W. B. Bowen, and T. W. Crowe, IEEE Trans. Electron Devices 47, 1152 共2000兲. 5 J. Q. Wang, P. L. Richards, J. W. Beeman, and E. E. Haller, Appl. Opt. 26, 4767 共1987兲. 6 M. Dyakonov and M. S. Shur, IEEE Trans. Electron Devices 43, 380 共1996兲. 7 W. Knap, Y. Deng, S. Rumyantsev, and M. S. Shur, Appl. Phys. Lett. 81, 4637 共2002兲; W. Knap, Y. Deng, S. Rumyantsev, J.-Q. Lü, M. S. Shur, C. A. Saylor, and L. C. Brunel, Appl. Phys. Lett. 80, 3433 共2002兲. 8 F. Teppe, D. Veksler, V. Yu. Kachorovski, A. P. Dmitriev, S. Rumyantsev, W. Knap, and M. S. Shur 共unpublished兲. 9 X. G. Peralta, S. J. Allen, and M. C. Wankee, Appl. Phys. Lett. 81, 1627 共2002兲; X. Peralta, S. J. Allen, M. C. Wanke, N. E. Harff, M. P. Lilly, J. A. Simmons, J. L. Reno, P. J. Burke, J. P. Eisenstein, Wojtek Knap, Y. Deng, S. Rumyantsev, J.-Q. Lü, and M. S. Shur, in Frontiers in Electronics: Future Chips, Proceedings of the 2002 Workshop on Frontiers in Electronics (WOFE-02), St. Croix, Virgin Islands, 15 January 2003 共World Scientific, Singapore, 2003兲, Vol. 26 关Int. J. High Speed Electron. Syst. 12, 925 共2002兲兴. 10 W. Knap, J. Lusakowski, T. Parenty, S. Bollaert, A. Cappy, V. V. Popov, and M. S. Shur, Appl. Phys. Lett. 84, 2331 共2004兲. 11 T. Yajima and K. Inoue, IEEE J. Quantum Electron. QE-5, 140 共1969兲. 12 X.-C. Zhang, Y. Jin, and X. F. Ma, Appl. Phys. Lett. 61, 2764 共1992兲. 13 A. Rice, Y. Jin, X. F. Ma, X. C. Zhang, D. Bliss, J. Perkin, and M. Alexander, Appl. Phys. Lett. 64, 1324 共1994兲. 14 Fujitsu Microwave Semiconductor Databook 共1999兲, Fujitsu Compound Semiconductor Inc., 2355 Zanker Rd., San Jose, CA 95131-1138. 15 M. Dyakonov and M. S. Shur, in the Proceedings of 22d International Symposium on GaAs and Related Compounds, Cheju, Korea, 28 Aug.–1 Sep. 1995 关Inst. Phys. Conf. Ser. 145, 785 共1996兲兴. 16 D. Veksler, F. Teppe, A. P. Dmitriev, V. Yu. Kachorovskii, W. Knap, and M. S. Shur 共unpublished兲. 17 M. Dyakonov and M. S. Shur, Phys. Rev. Lett. 71, 2465 共1993兲. 18 A. Satou, I. Khmyrova, V. Ryzhii, and M. S. Shur, Semicond. Sci. Technol. 18, 460 共2003兲. 19 V. L. Kustov, V. I. Ryzhii, and Yu. S. Sigov, Sov. Phys. JETP 52, 1207 共1980兲. 20 V. Yu. Kachorovskii, I. S. Lyubinskiy, and L. D. Tsendin, Phys. Rev. B 68, 033308 共2003兲.

Downloaded 19 Oct 2005 to 162.38.137.110. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp