Fabrication of Sub-THz Oscillators Using Resonant Tunneling Diodes Integrated with Slot Antennas

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1 Fabrication of Sub-THz Oscillators UsingResonant Tunneling Diodes Integrated with Slot Antennas N. Orihashi, T. Abe, T. Ozono, and M. Asada Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo , Japan Abstract We fabricated sub-thz oscillators using resonant tunneling diodes integrated with slot antennas. Oscillations at 90GHz and 136GHz were obtained for the slot antennas with the lengths of 1mm and 800µm. Theoretical calculation shows that the maximum oscillation frequency up to ~1.2THz is expected in this structure with short antenna. 1. Introduction The terahertz (THz) frequency range is remaining almost undeveloped, although several applications are expected. Main reason is that compact and coherent solid-state light sources are lacking. light sources with continuous operation at room temperature is desired. Quantum cascade lasers (1) and p-ge lasers (2) oscillating in the THz range at low temperatures have been reported. Development of electron devices toward THz range is being done from the low frequency side. (3)-(6) We have also been studying a possibility of three-terminal amplifier devices in the THz range (7), THz laser gain due to the photon-assisted tunneling (8), and oscillation of resonant tunneling diodes (s). The has been considered as one candidate for the THz frequency. (3)-(4) In this paper, we report oscillation of s integrated with slot antennas at millimeter waves and estimation of maximum oscillation frequency showing a possibility of extending to the THz range. 2. Device structure and Oscillation characteristics We used made from GaInAs/AlInAs material system (GaInAs quantum well, AlInAs barrier) on InP substrate. The structure of the fabricated device is shown in Fig. 1. GaInAs/AlInAs double barrier s are integrated with a planar slot antenna. The negative differential resistance (NDR) in the compensates a radiation resistance, resulting in the oscillation, as shown in the equivalent circuits in Fig. 1(b). The area of is 1.8µm1.8µm. Three s are

2 Vdc Idc 1mm 1mm SI-InP substrate 0.5eV -Rd Slot antenna L C r slot radiation antenna resistance (a) Fig. 1 (a) Schematic structure of the oscillator with GaInAs/InAlAs double barrier and slot antenna. (b) Equivalent circuit of the oscillator. (b) (a) (b) (e) upper electrode (c) (d) bottom electrode Fig. 2 Fabrication process and SEM image of the fabricated device.

3 i n t e n s i t y f = 90 GHz 3.20mm l e n g t h /mm f = 136GHz Fig. 3 Current-voltage characteristics of the fabricated device. parallelly connected at the center of the slot antenna. The fabrication process is as follows. A 700-nm-high mesa was fabricated by a series of electron-beam (EB) lithography, deposition of Ti/Au/Pt/Ti, CH 4 and H 2 reactive ion etching (RIE), and wet etching (H 2 O: H 2 SO 4 :H 2 O 2 =40:1:1), as shown in Fig. 2(a). the bottom electrode of the antenna was also fabricated by EB lithography and deposition of Ti/Au/Pt/Ti, as shown in Fig. 2(b). The substrate was coated by benzo-cyclo-butane (BCB) as an insulator, and etched BCB to expose top of the mesa by CF 4 and O 2 RIE, as shown in Fig. 2(c). Then the upper electrode was fabricated by EB lithography and Au/Cr deposition, and the bottom was exposed i n t e n s i t y 2.19mm l e n g t h /mm Fig.4 Oscillation characteristics of 90GHz and 136GHz measured by a simple Fabry-Perot interferometer. admittance Re(Y rtd (f)) [S] S=1.8µm 1.8µm electrode by CF 4 and O 2 RIE, as shown in Fig. 2(d). Figure 2(e) is a SEM image of the fabricated device THz frequency [THz] Fig. 5 Frequency dependence of the real part of the admittance of, Re(Y rtd (f)), which is negative at f 1.35THz.

4 admittance [S] slot length = 1mm Re(Y(f)) Im(Y(f)) f = 112GHz frequency [GHz] admittance [S] LW=20 4µm Re(Y(f)) Im(Y(f)) frequency [THz] 1.16THz Fig. 6 Frequency dependence of the real and imaginary parts of the total admittances, Re(Y(f)) and Im(Y(f)). Figure 3 shows the static current-voltage characteristics at room temperature of fabricated device. The peak current density is 154kA/cm 2 and the peak-to-valley ratio is To suppress the low-frequency parasitic oscillation with the external circuits, a resistance was bridged across the electrodes at the outside of the antenna using a bithmas film. The device oscillated at 90GHz and 136GHz. The used detector was the Gollay cell. Figure. 4 shows oscillation characteristics of 90GHz and 136GHz measured by a simple Fabry-Perot interferometer. The equivalent refractive index of the slot antenna is approximated as ne 1+ ε r = =2.56 where ε r is the relative 2 dielectric constant of the substrate. The equivalent wavelengths λ e =λ/n e are estimated Fig. 7 Frequency dependence the real and imaginary parts of the total admittances, Re(Y(f)) and Im(Y(f)), for a slot antenna with L W=20µm4µm. from this value as 1.25mm at 90GHz and 855µm at 136GHz. From this estimation, the oscillation is attributed to the resonance of the antenna length l=(3/4) λ e 3.Estimation of maximum oscillation frequency The oscillation condition is given by Re(Y(f)) 0 and Im(Y(f))=0, where Y(f) is the sum of the admittances of the (Y rtd (f)) and antenna (Y ant (f)). From Fig. 5, the frequency dependence of Re(Y rtd (f)) has negative values at f<1.35thz, showing the inherent ability of oscillating up to this frequency. Calculated results for Y(f) of the integrated with 1-mm-long slot antenna is shown in Fig. 6, which corresponds to the fabricated device oscillating at 90GHz. The oscillating point expected from the

5 calculation is about 110GHz, as shown in Fig. 6, in reasonable agreement with the measurement. To explore the THz-frequency oscillation, we calculated the case of a short slot antenna combined with a. For the slot length L of 20µm and the width W of 4µm, the estimation shows the oscillation at 1.16THz, as shown in Fig. 7. Higher oscillation frequency is expected for s with high current density. (9) 4. Conclusions We fabricated sub-thz oscillators using resonant tunneling diodes integrated with slot antennas. Oscillations at 90GHz and 136GHz are obtained for the slot antennas with the lengths of 1mm and 800µm. Theoretical calculation shows that the maximum oscillation frequency up to ~1.2THz is expected in this structure with short antenna. and T.C.McGill: Appl. Phys. Lett., 58, 2291(1991). (4)M.Reddy, S.C.Martin, A.C.Molnar, R. E.Muller, R.P.Smith, P.H.Siegel, M. J. Mondry, M. J. W. Rodwell, H. Kroemer, and S.J.Allen,Jr: IEEE Electron Device Lett., 18, 218 (1997). (5) Q. Lee, S. C. Martin, D. Mensa, R. P. Smith, J. Guthric, S. Jaganathan, Y. Bester, T.Mathew, S. Krishanan, L. Samoska, and M. Rodwell: 57th Device Research Conference, Post-deadline paper V.B.-6, June 1999, Santa Barbara/CA. (6) T. W. Crowe, T. C. Grein, R. Zimmermann, and P. Zimmermann: IEEE Microwave and Guided Wave Lett., 6, 207(1996). (7)M.Asada: Japan. J. Appl. Phys., 35, L685 (1996). (8)M.Asada, Y.Oguma, and N.Sashinaka: Appl. Phys. Lett., 77, 618 (2000). (9) T. P. E. Broekaert and C. G. Fonstad, J. Appl. Phys., 68, 4310(1990). Acknowledgements The authors thank Professor K. Masu and Mr. Y. Yokoyama of the Tokyo Institute of Technology for useful discussions and assistance with the electromagnetic analysis of the slot antenna. References (1)S. Komiyama: Phys. Rev. Lett., 48, 271(1982). (2)R. Kohler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, and R. C. lotti: nature, 417, 156(2002). (3)E.R.Brown, J.R.Soderstrom, C.D. Parker, L.J.Mahoney, K.M.Molvar,

Direct intensity modulation of resonant-tunneling-diode terahertz oscillator up to ~30 GHz

This article has been accepted and published on J-STAGE in advance of copyediting. Content is final as presented. IEICE Electronics Express, Vol.* No.*,*-* Direct intensity modulation of resonant-tunneling-diode