InGaAs Schottky barrier diode array detector for a real-time compact terahertz line scanner Sang-Pil Han,1,3 Hyunsung Ko,1 Jeong-Woo Park,1,3 Namje Kim,1 Young-Jong Yoon,1,3 Jun-Hwan Shin,1,3 Dae Yong Kim,1 Dong Hun Lee,2 and Kyung Hyun Park1,3,* 2
1 THz Photonics Creative Research Center, ETRI, Daejeon 305-700, South Korea Convergence Components & Materials Research Laboratory, ETRI, Daejeon 305-700, South Korea 3 School of Advanced Device Technology, UST, Daejeon 305-350, South Korea * [email protected]
Abstract: We present a terahertz (THz) broadband antenna-integrated 1 × 20 InGaAs Schottky barrier diode (SBD) array detector with an average responsivity of 98.5 V/W at a frequency of 250 GHz, which is measured without attaching external amplifiers and Si lenses, and an average noise equivalent power (NEP) of 106.6 pW/√Hz. The 3-dB bandwidth of the SBD detector is also investigated at approximately 180 GHz. For implementing an array-type SBD detector by a simple fabrication process to achieve a high yield, a structure comprising an SiNx layer instead of an air bridge between the anode and the cathode is designed. THz line beam imaging using a Gunn diode emitter with a center frequency of 250 GHz and a 1 × 20 SBD array detector is successfully demonstrated. ©2013 Optical Society of America OCIS codes: (110.6795) Terahertz imaging; (110.0110) Imaging systems; (110.2970) Image detection systems; (040.0040) Detectors; (040.1240) Arrays.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). A. Rogalski and F. Sizov, “Terahertz detectors and focal plane arrays,” Opto-Electron. Rev. 19(3), 346–404 (2011). N. Oda, A. W. M. Lee, T. Ishi, I. Hosako, and Q. Hu, “Proposal for real-time terahertz imaging system, with palm-size terahertz camera and compact quantum cascade laser,” Proc. SPIE 8363, 83630A (2012). N. Karpowicz, H. Zhong, C. Zhang, K.-I. Lin, J.-S. Hwang, J. Xu, and X.-C. Zhang, “Compact continuous-wave subterahertz system for inspection applications,” Appl. Phys. Lett. 86(5), 054105 (2005). C. Sydlo, O. Cojocari, D. Schçnherr, T. Goebel, P. Meissner, and H. L. Hartnagel, “Fast THz detectors based on InGaAs Schottky diodes,” Frequenz 62(5-6), 107–110 (2008). A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320 × 240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18(13), 1415– 1417 (2006). R. Han, Y. Zhang, Y. Kim, D. Y. Kim, H. Shichijo, E. Afshari, and K. K. O, “Active terahertz imaging using Schottky diodes in CMOS array and 860-GHz pixel,” IEEE J. Solid-St. Circulation 99, 1–14 (2013). F. Schuster, D. Coquillat, H. Videlier, M. Sakowicz, F. Teppe, L. Dussopt, B. Giffard, T. Skotnicki, and W. Knap, “Broadband terahertz imaging with highly sensitive silicon CMOS detectors,” Opt. Express 19(8), 7827– 7832 (2011). V. V. Popov, D. M. Ermolaev, K. V. Maremyanin, N. A. Maleev, V. E. Zemlyakov, V. I. Gavrilenko, and S. Y. Shapoval, “High-responsivity terahertz detection by on-chip InGaAs/GaAs field-effect-transistor array,” Appl. Phys. Lett. 98(15), 153504 (2011). U. V. Bhapkar, Y. Li, and R. J. Mattauch, “InGaAs-InP heteroepitaxial Schottky barrier diodes for terahertz applications,” Proc. ISSTT, 661–677 (1991). I. Oprea, A. Walber, O. Cojocari, H. Gibson, R. Zimmermann, and H. L. Hartnagel, “183 GHz mixer on InGaAs Schottky diodes,” Proc. ISSTT, 159–160 (2010). A. Semenov, O. Cojocari, H.-W. Hübers, F. Song, A. Klushin, and A.-S. Müller, “Application of zero-bias quasi-optical Schottky-diode detectors for monitoring short-pulse and weak terahertz radiation,” IEEE Electron Device Lett. 31(7), 674–676 (2010). J. L. Hesler and T. W. Crowe, “Responsivity and noise measurements of zero-bias Schottky diode detectors,” Proc. ISSTT, 89–92 (2007).
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25874
14. J. L. Hesler, L. Liu, H. Xu, Y. Duan, and R. M. Weikle, “The development of quasi-optical THz detectors,” IRMMW-THz 15–19(Sep), 1–2 (2008). 15. N. Kim, Y. A. Leem, M. Y. Jeon, C. W. Lee, S.-P. Han, D. Lee, and K. H. Park, “Widely tunable 1.55 µm detuned dual mode laser diode for compact continuous-wave THz emitter,” ETRI J. 33(5), 810–813 (2011). 16. N. Kim, H.-C. Ryu, D. Lee, S.-P. Han, H. Ko, K. Moon, J.-W. Park, M. Y. Jeon, and K. H. Park, “Monolithically integrated optical beat sources toward a single-chip broadband terahertz emitter,” Laser Phys. Lett. 10(8), 085805 (2013). 17. S.-P. Han, H. Ko, N. Kim, H.-C. Ryu, C. W. Lee, Y. A. Leem, D. Lee, M. Y. Jeon, S. K. Noh, H. S. Chun, and K. H. Park, “Optical fiber-coupled InGaAs-based terahertz time-domain spectroscopy system,” Opt. Lett. 36(16), 3094–3096 (2011). 18. S.-P. Han, N. Kim, H. Ko, H.-C. Ryu, J. W. Park, Y.-J. Yoon, J.-H. Shin, D. H. Lee, S.-H. Park, S.-H. Moon, S.W. Choi, H. S. Chun, and K. H. Park, “Compact fiber-pigtailed InGaAs photoconductive antenna module for terahertz-wave generation and detection,” Opt. Express 20(16), 18432–18439 (2012).
1. Introduction Real-time, compact, and cost-effective terahertz (THz) imaging systems are required for use in outdoor or portable environments for security, nondestructive testing, postal inspection, measurement of paint thickness, and skin diagnosis [1–4]. Schottky barrier diodes (SBDs), which directly rectify the input THz signal and vary the current-voltage (I-V) characteristics, are vintage candidates for such THz detectors [5]. For use in THz-band applications, however, SBD detectors must exhibit a high-frequency response under noise-level operation. In addition, array-type SBD detectors can be used for high-speed operation. The imaging speed of one-dimensional (1-D) scanning using array-type detectors is understandably higher than that of two-dimensional (2-D) scanning using single-channel detectors. Various types of THz array detectors such as uncooled microbolometers, CMOS-based SBD arrays, silicon CMOS detectors, and InGaAs/GaAs field-effect transistor arrays have also been developed [6–9]. InGaAs-based SBDs are typically used for zero-bias operation, while GaAs-based SBDs have a large turn-on voltage. Zero-bias operation results in a superior noise figure and lower power consumption. InGaAs SBDs also reduce the conversion losses at higher frequencies because of their higher mobility, which results in a lower series resistance [10–13]. CMOSbased SBDs have the ability to integrate various additional devices in the same chip, such as amplifiers, multiplexers, and patch antennas [7]; however, InGaAs SBDs are more favorable for device development for mass customization because of the initial fabrication facilities, considering the development costs of CMOS SBDs. In this paper, we present the fabrication and experimental results of broadband antennaintegrated 1 × 20 InGaAs SBD array detectors as well as single-channel InGaAs SBD detectors. The THz pulse imaging using single-channel SBD detectors is described. Moreover, 1-D line beam THz CW imaging using 1 × 20 InGaAs SBD array detectors is demonstrated for real-time, compact, portable THz scanners. 2. Fabrication and characteristics of 1 × 20 InGaAs SBD detectors We have designed a structure comprising a silicon nitride (SiNx) layer instead of an airbridge-type connection structure between the anode and the cathode in order to implement an SBD array detector achieving good uniformity and high yield as a THz detector. The designed fabrication process is described as follows. For the fabrication of an SBD device, epitaxial (epi) layers are grown by metal organic chemical vapor deposition (MOCVD). On the top of a semi-insulating (SI) InP substrate, a 0.1-μm-thick InP buffer layer, 0.8-μm-thick n + In0.53Ga0.47As layer, and 0.1-μm-thick n-In0.53Ga0.47As layer are grown at 650°C. The doping level of the n + In0.53Ga0.47As layer is 2 × 1018 cm−3, and that of the n-In0.53Ga0.47As layer is 5 × 1016 cm−3. We have determined the doping level and growth conditions of each layer and confirmed the ohmic and Schottky contact performance. Figure 1(a) shows the fabrication process of the SBD device. First, an anode mesa and isolation mesa structure is formed by reactive ion etching (RIE). After the mesa isolation process, an SiNx film is
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25875
deposited on the entire epi using plasma-enhanced chemical vapor deposition (PECVD). For forming contacts between the cathode and the anode, the SiNx layer above the cathode and anode contact areas is etched using a magnetically enhanced reactive ion etch (MERIE). After the MERIE process, Cr and Au are deposited using an e-beam evaporator to form a cathode contact. Further, rapid thermal annealing (RTA) is performed to form an ohmic contact. Pt and Au are deposited using an e-beam evaporator to form an anode contact. After the anode contact process, the entire epi is covered with SiNx using PECVD again, and the SiNx layer above the electrode pads is etched by an MERIE. The ideality factor of the fabricated SBD array samples is 1.26, and the series resistance typically is 110 Ω.
Fig. 1. (a) Fabrication process of a 1 × 20 InGaAs Schottky barrier diode (SBD) detector, (b) SEM image of a fabricated SBD array sample, and (c) its I-V characteristics.
Figure 1(b) shows an SEM image of a fabricated 1 × 20 InGaAs SBD array chip sample with an anode diameter of 3 μm. A square spiral antenna-integrated 1 × 20 InGaAs SBD array with a cell pitch of 0.5 mm is adopted to build a small-size array chip. As a result, the I-V characteristics are uniform throughout the 1 × 20 SBD array sample, as shown in Fig. 1(c). We have investigated the device performance such as the responsivity, noise equivalent power (NEP), and 3-dB bandwidth of the fabricated SBD detectors. The responsivity is measured by using a Gunn diode emitter (VDI) with a center frequency of 250 GHz and a
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25876
calorimeter (VDI, PM4). The NEP is measured by using a preamplifier (SRS, SR560) and an oscilloscope (Agilent, DSO1004A). As a result, the average responsivity is 98.5 V/W, and the average NEP is 106.6 pW/√Hz for a 1 × 20 SBD array detector sample. These values are lower when compared with those of other commercial SBD devices [13,14]; however, these detector devices are promising in terms of simple fabrication process and high yield. We have also measured the frequency response of the single-channel SBD using a fibercoupled CW THz spectroscopy system [15,16]. The CW THz system is shown in Fig. 2(a). The dual-wavelength optical signal emitted from the optical beating source is amplified at the EDFA, and it is then launched at the THz emitter to generate a THz wave. While tuning the optical wavelength, the single-channel SBD detector detects the amount of THz signal. According to the measurement results, the 3-dB band frequency of the fabricated single channel SBD sample is approximately 180 GHz, as shown in Fig. 2(b). A higher bandwidth can be achieved if the SiNx layer between the anode and the cathode is changed a material having a lower dielectric constant and higher thickness.
Fig. 2. (a) Fiber-coupled CW THz spectroscopy system setup and (b) detected THz signal measured by using the CW THz system.
3. Terahertz pulse imaging We have performed THz pulse imaging using a fiber-coupled THz pulse imaging system consisting of a THz-TDS emitter (which includes a femtosecond laser with a pulse width of 70 fs, an InGaAs photoconductive antenna (PCA)-integrated emitter, and a dispersioncompensation fiber), an InGaAs SBD detector, a computer-controlled 2-D stage, a sine-wave function generator, and a lock-in amplifier [17,18], as shown in Fig. 3(a). As shown in Fig. 3(b), the THz radiation bandwidth and maximum center frequency of the THz-TDS emitter are 3 THz and 180 GHz, respectively. For the InGaAs SBD detector, there is less scope for selection between the four different anode sizes for slope efficiency with regard to the SBD IV curves; however, an anode diameter of 1 μm achieves the highest turn-on voltage. Highefficiency THz pulse detection, with a difference in the detected SBD currents with and #196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25877
without THz pulse radiation generated from the THz-TDS emitter at a center frequency of approximately 180 GHz, is performed for anode diameters of 2 μm and 3 μm. We have measured the spot size of the THz wave beam along the THz propagation direction by a knife-edge method to determine the optimal position for THz imaging. According to the results, we have selected an image target position at a distance of 15 mm from the THz-TDS emitter. We have obtained the THz images using a THz pulse imaging system with a THz-TDS emitter and a single-channel SBD detector, as shown in Fig. 3(a), for three different anode size configurations to investigate the imaging performance. Figure 3(c) shows the THz imaging results of a medical knife behind a high-density polyethylene (HDPE) sheet with a thickness of 2 mm, where a cell size of 0.5 × 0.5 mm2, a pixel resolution of 110 × 30, and a bias voltage of 0 V are used.
Fig. 3. (a) THz pulse imaging system setup using a THz-TDS emitter and single-channel InGaAs SBD detector, (b) measured THz pulse spectrum of the THz-TDS emitter, with the inset showing the results on a logarithmic scale, and (c) photograph of a medical knife and its THz images measured by using the THz pulse imaging system.
Zero bias is applied to the SBD detector because an excellent image can be obtained owing to the relatively decreased noise. For an anode diameter of 3 μm, the THz imaging of the medical knife is finely performed. We find that the SBD detector with an anode diameter of 3 μm is more suitable for the THz-TDS emitter with a center frequency of 180 GHz. 4. Terahertz continuous-wave imaging We have designed an SBD chip board (CB) for easy connection to the other boards. Figures 4(a) and 4(b) show the SBD array chip bonded onto the SBD-CB along with a cylindrical Si lens placed on the SBD array chip and the transmitted IR image of the SBD array chip bonded onto the SBD-CB obtained by using an IR camera, respectively. Using a flip-chip bonding process, the SBD array chip and SBD-CB are connected electrically and mechanically. The solder pads of the SBD array chip and the solder balls of the SBD-CB are bonded in a self-passive alignment at a defined temperature and time [18].
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25878
Fig. 4. (a) SBD array chip bonded onto an SBD-CB and a cylindrical Si lens placed on the SBD array chip and (b) transmitted IR image of the SBD array chip bonded onto the SBD-CB obtained by using an IR camera.
We have also designed an SBD evaluation board (EB) to evaluate the 1 × 20 SBD array detector bonded onto an SBD-CB. In the SBD-EB, a 32 × 1 multiplexer (MUX) to switch the cells in the SBD array, a microcontroller unit (MCU) to control the address of the MUX, and an RS232 device to communicate with a personal computer (PC) are set up. Further, we have provided the setup to measure the THz signal of the Gunn diode emitter by focusing the signal on the SBD array chip using an HDPE biconvex lens. We have examined the THz signals measured by channel 10 of the SBD array as a function of the number of transparent tapes (Scotch) attached to the surface of the SBD array chip for antireflection (AR). By varying the number of transparent tapes, the highest THz signal is detected for four pieces of overlapped transparent tape, as shown in Fig. 5. The thickness of the four pieces of overlapped transparent tape agrees with the general AR coating condition, λ/4/neff, where λ and neff denote the wavelength of the Gunn diode emitter and the effective index of all the materials including the substrate, AR coating layer, and air, respectively. Furthermore, the THz signal in the case of a cylindrical Si lens with four pieces of overlapped transparent tape is 2.1 times greater than that in the case of four pieces of overlapped transparent tape only. However, it is noted that alignment between the SBD array chip and the cylindrical Si lens is required when this tape used. Thus, four pieces of overlapped transparent tape are attached to the surface of the SBD array chip during the experiment.
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25879
Fig. 5. THz signal detected by a single-channel SBD as a function of the number of transparent tapes with the parameters of the transparent tape only and a cylindrical Si lens with four pieces of overlapped transparent tape.
We have measured the focused radiation beam patterns of the Gunn diode emitter using 1D scanning by all the channels of the SBD array detector and 2-D scanning by channel 10 of the SBD array detector to investigate its feasibility. Figures 6(a) and 6(b) show the results of 20-channel 1-D scanning and single-channel 2-D scanning, respectively, where the vertical axis denotes the array chip direction, and the horizontal axis denotes the mechanical scanning direction. As a result, in the case of the single-channel 2-D scanning, the horizontal full width at half maximum (FWHM) and vertical FWHM are 5.1 mm and 5.1 mm, respectively. In the case of the 20-channel 1-D scanning, the horizontal FWHM is 5.1 mm, which is the same as in the 2-D scanning, while the vertical FWHM is smaller at 4.1 mm because of the different characteristics of the channels in the SBD array detector, such as detection capacity and crosstalk. In addition, the different scanning schemes along the horizontal and vertical axes may also contribute to these differences in vertical FWHM.
Fig. 6. THz radiation beam patterns of the Gunn diode emitter measured by using (a) all the channels and (b) only channel 10 of a 1 × 20 SBD array detector.
We have set up a THz CW imaging system using a Gunn diode emitter and a 1 × 20 SBD array detector, as depicted in Fig. 7(a), where a pixel size of 0.5 × 0.5 mm2, a pixel resolution of 120 × 40, and a bias voltage of 0 V are used. To obtain a line beam, an HDPE convex lens and cylindrical plastic lens are used, and two HDPE convex lenses are used in the system for focusing the image on the 1 × 20 SBD array detector. Figure 7(b) shows a photograph of a metal ring and clip used as samples. Figures 7(c) and 7(d) show the THz images of the samples measured by using all the channels and channel 10 only, respectively, of the 1 × 20 SBD array detector. In the THz imaging scheme using all the channels of the 1 × 20 SBD array detector, the calibrated 20 channels of the SBD are simultaneously scanned along the
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25880
horizontal direction. In the case of channel 10 only, the single channel of the 1 × 20 SBD is scanned along the vertical direction after scanning along the horizontal direction. Therefore, 1-D scanning by the 1 × 20 SBD array is faster than 2-D scanning by the single-channel SBD. However, blurring is observed from the imaging results of 1-D scanning by the 1 × 20 SBD array, as shown in Fig. 7(c). This is because all the cells of the 1 × 20 SBD array exhibit different responsivities and crosstalk characteristics, while all the cells exhibit the same characteristics during scanning for the single-channel 2-D scan method. However, almost same results are observed after image processing using a fast Fourier transform (FFT), as shown in Figs. 7(c) and 7(d). Furthermore, the SNR performance of THz imaging is enhanced because of the ability to perform frequency modulation up to 30 kHz when direct modulation of the Gunn diode emitter is used instead of a mechanical chopper in the system, as shown in Fig. 7(a).
Fig. 7. (a) THz CW imaging measurement setup, (b) photograph of a metal ring and clip, and their THz images measured by using the THz CW imaging system with (c) all the channels and (d) only channel 10 of the 1 × 20 SBD array detector.
5. Summary We have fabricated and characterized InGaAs SBD detectors. An average responsivity of 98.5 V/W, an average NEP of 106.6 pW/√Hz, and a 3-dB bandwidth at approximately 180 GHz for a 1 × 20 SBD array detector sample were measured. The structure comprising an SiNx layer instead of an air-bridge-type connection structure between the anode and the cathode achieved good uniformity and high yield as an array chip. We determined that the THz (f = 250 GHz) detection efficiencies of the SBD detector covered with four pieces of overlapped transparent tape and a cylindrical Si lens including four pieces of overlapped
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25881
transparent tape were enhanced by 86% and 298%, respectively, when compared with a bare SBD chip. THz pulse detection using a THz-TDS emitter with a center frequency of 180 GHz was effectively performed for anode diameters of 2 μm and 3 μm. The SBD detector with an anode diameter of 3 μm was more suitable for the THz-TDS emitter. The THz imaging of a medical knife behind an HDPE sheet was finely performed by using a THz pulse imaging system. We also successfully demonstrated THz CW imaging using 1-D scanning by the 1 × 20 SBD array detector and Gunn diode emitter. Blurring was observed from the imaging results; however, almost the same results were observed for 2-D scanning by a single-channel SBD after image processing using FFT. We found that the 1 × 20 SBD array detector is suitable for a real-time compact line scanner. Acknowledgments This work was partly supported by the IT R&D program of MOTIE/KEIT [10045238, Development of the portable scanner for THz imaging and spectroscopy], Joint Research Projects of ISTK, the Public Welfare & Safety Research Program through the National Research Foundation of Korea (NRF) Technology (NRF-2010-0020822), and Nano·Material Technology Development Program through the NRF of Korea (NRF-2012M3A7B4035095).
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25882
1 THz Photonics Creative Research Center, ETRI, Daejeon 305-700, South Korea Convergence Components & Materials Research Laboratory, ETRI, Daejeon 305-700, South Korea 3 School of Advanced Device Technology, UST, Daejeon 305-350, South Korea * [email protected]
Abstract: We present a terahertz (THz) broadband antenna-integrated 1 × 20 InGaAs Schottky barrier diode (SBD) array detector with an average responsivity of 98.5 V/W at a frequency of 250 GHz, which is measured without attaching external amplifiers and Si lenses, and an average noise equivalent power (NEP) of 106.6 pW/√Hz. The 3-dB bandwidth of the SBD detector is also investigated at approximately 180 GHz. For implementing an array-type SBD detector by a simple fabrication process to achieve a high yield, a structure comprising an SiNx layer instead of an air bridge between the anode and the cathode is designed. THz line beam imaging using a Gunn diode emitter with a center frequency of 250 GHz and a 1 × 20 SBD array detector is successfully demonstrated. ©2013 Optical Society of America OCIS codes: (110.6795) Terahertz imaging; (110.0110) Imaging systems; (110.2970) Image detection systems; (040.0040) Detectors; (040.1240) Arrays.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). A. Rogalski and F. Sizov, “Terahertz detectors and focal plane arrays,” Opto-Electron. Rev. 19(3), 346–404 (2011). N. Oda, A. W. M. Lee, T. Ishi, I. Hosako, and Q. Hu, “Proposal for real-time terahertz imaging system, with palm-size terahertz camera and compact quantum cascade laser,” Proc. SPIE 8363, 83630A (2012). N. Karpowicz, H. Zhong, C. Zhang, K.-I. Lin, J.-S. Hwang, J. Xu, and X.-C. Zhang, “Compact continuous-wave subterahertz system for inspection applications,” Appl. Phys. Lett. 86(5), 054105 (2005). C. Sydlo, O. Cojocari, D. Schçnherr, T. Goebel, P. Meissner, and H. L. Hartnagel, “Fast THz detectors based on InGaAs Schottky diodes,” Frequenz 62(5-6), 107–110 (2008). A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320 × 240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18(13), 1415– 1417 (2006). R. Han, Y. Zhang, Y. Kim, D. Y. Kim, H. Shichijo, E. Afshari, and K. K. O, “Active terahertz imaging using Schottky diodes in CMOS array and 860-GHz pixel,” IEEE J. Solid-St. Circulation 99, 1–14 (2013). F. Schuster, D. Coquillat, H. Videlier, M. Sakowicz, F. Teppe, L. Dussopt, B. Giffard, T. Skotnicki, and W. Knap, “Broadband terahertz imaging with highly sensitive silicon CMOS detectors,” Opt. Express 19(8), 7827– 7832 (2011). V. V. Popov, D. M. Ermolaev, K. V. Maremyanin, N. A. Maleev, V. E. Zemlyakov, V. I. Gavrilenko, and S. Y. Shapoval, “High-responsivity terahertz detection by on-chip InGaAs/GaAs field-effect-transistor array,” Appl. Phys. Lett. 98(15), 153504 (2011). U. V. Bhapkar, Y. Li, and R. J. Mattauch, “InGaAs-InP heteroepitaxial Schottky barrier diodes for terahertz applications,” Proc. ISSTT, 661–677 (1991). I. Oprea, A. Walber, O. Cojocari, H. Gibson, R. Zimmermann, and H. L. Hartnagel, “183 GHz mixer on InGaAs Schottky diodes,” Proc. ISSTT, 159–160 (2010). A. Semenov, O. Cojocari, H.-W. Hübers, F. Song, A. Klushin, and A.-S. Müller, “Application of zero-bias quasi-optical Schottky-diode detectors for monitoring short-pulse and weak terahertz radiation,” IEEE Electron Device Lett. 31(7), 674–676 (2010). J. L. Hesler and T. W. Crowe, “Responsivity and noise measurements of zero-bias Schottky diode detectors,” Proc. ISSTT, 89–92 (2007).
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25874
14. J. L. Hesler, L. Liu, H. Xu, Y. Duan, and R. M. Weikle, “The development of quasi-optical THz detectors,” IRMMW-THz 15–19(Sep), 1–2 (2008). 15. N. Kim, Y. A. Leem, M. Y. Jeon, C. W. Lee, S.-P. Han, D. Lee, and K. H. Park, “Widely tunable 1.55 µm detuned dual mode laser diode for compact continuous-wave THz emitter,” ETRI J. 33(5), 810–813 (2011). 16. N. Kim, H.-C. Ryu, D. Lee, S.-P. Han, H. Ko, K. Moon, J.-W. Park, M. Y. Jeon, and K. H. Park, “Monolithically integrated optical beat sources toward a single-chip broadband terahertz emitter,” Laser Phys. Lett. 10(8), 085805 (2013). 17. S.-P. Han, H. Ko, N. Kim, H.-C. Ryu, C. W. Lee, Y. A. Leem, D. Lee, M. Y. Jeon, S. K. Noh, H. S. Chun, and K. H. Park, “Optical fiber-coupled InGaAs-based terahertz time-domain spectroscopy system,” Opt. Lett. 36(16), 3094–3096 (2011). 18. S.-P. Han, N. Kim, H. Ko, H.-C. Ryu, J. W. Park, Y.-J. Yoon, J.-H. Shin, D. H. Lee, S.-H. Park, S.-H. Moon, S.W. Choi, H. S. Chun, and K. H. Park, “Compact fiber-pigtailed InGaAs photoconductive antenna module for terahertz-wave generation and detection,” Opt. Express 20(16), 18432–18439 (2012).
1. Introduction Real-time, compact, and cost-effective terahertz (THz) imaging systems are required for use in outdoor or portable environments for security, nondestructive testing, postal inspection, measurement of paint thickness, and skin diagnosis [1–4]. Schottky barrier diodes (SBDs), which directly rectify the input THz signal and vary the current-voltage (I-V) characteristics, are vintage candidates for such THz detectors [5]. For use in THz-band applications, however, SBD detectors must exhibit a high-frequency response under noise-level operation. In addition, array-type SBD detectors can be used for high-speed operation. The imaging speed of one-dimensional (1-D) scanning using array-type detectors is understandably higher than that of two-dimensional (2-D) scanning using single-channel detectors. Various types of THz array detectors such as uncooled microbolometers, CMOS-based SBD arrays, silicon CMOS detectors, and InGaAs/GaAs field-effect transistor arrays have also been developed [6–9]. InGaAs-based SBDs are typically used for zero-bias operation, while GaAs-based SBDs have a large turn-on voltage. Zero-bias operation results in a superior noise figure and lower power consumption. InGaAs SBDs also reduce the conversion losses at higher frequencies because of their higher mobility, which results in a lower series resistance [10–13]. CMOSbased SBDs have the ability to integrate various additional devices in the same chip, such as amplifiers, multiplexers, and patch antennas [7]; however, InGaAs SBDs are more favorable for device development for mass customization because of the initial fabrication facilities, considering the development costs of CMOS SBDs. In this paper, we present the fabrication and experimental results of broadband antennaintegrated 1 × 20 InGaAs SBD array detectors as well as single-channel InGaAs SBD detectors. The THz pulse imaging using single-channel SBD detectors is described. Moreover, 1-D line beam THz CW imaging using 1 × 20 InGaAs SBD array detectors is demonstrated for real-time, compact, portable THz scanners. 2. Fabrication and characteristics of 1 × 20 InGaAs SBD detectors We have designed a structure comprising a silicon nitride (SiNx) layer instead of an airbridge-type connection structure between the anode and the cathode in order to implement an SBD array detector achieving good uniformity and high yield as a THz detector. The designed fabrication process is described as follows. For the fabrication of an SBD device, epitaxial (epi) layers are grown by metal organic chemical vapor deposition (MOCVD). On the top of a semi-insulating (SI) InP substrate, a 0.1-μm-thick InP buffer layer, 0.8-μm-thick n + In0.53Ga0.47As layer, and 0.1-μm-thick n-In0.53Ga0.47As layer are grown at 650°C. The doping level of the n + In0.53Ga0.47As layer is 2 × 1018 cm−3, and that of the n-In0.53Ga0.47As layer is 5 × 1016 cm−3. We have determined the doping level and growth conditions of each layer and confirmed the ohmic and Schottky contact performance. Figure 1(a) shows the fabrication process of the SBD device. First, an anode mesa and isolation mesa structure is formed by reactive ion etching (RIE). After the mesa isolation process, an SiNx film is
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25875
deposited on the entire epi using plasma-enhanced chemical vapor deposition (PECVD). For forming contacts between the cathode and the anode, the SiNx layer above the cathode and anode contact areas is etched using a magnetically enhanced reactive ion etch (MERIE). After the MERIE process, Cr and Au are deposited using an e-beam evaporator to form a cathode contact. Further, rapid thermal annealing (RTA) is performed to form an ohmic contact. Pt and Au are deposited using an e-beam evaporator to form an anode contact. After the anode contact process, the entire epi is covered with SiNx using PECVD again, and the SiNx layer above the electrode pads is etched by an MERIE. The ideality factor of the fabricated SBD array samples is 1.26, and the series resistance typically is 110 Ω.
Fig. 1. (a) Fabrication process of a 1 × 20 InGaAs Schottky barrier diode (SBD) detector, (b) SEM image of a fabricated SBD array sample, and (c) its I-V characteristics.
Figure 1(b) shows an SEM image of a fabricated 1 × 20 InGaAs SBD array chip sample with an anode diameter of 3 μm. A square spiral antenna-integrated 1 × 20 InGaAs SBD array with a cell pitch of 0.5 mm is adopted to build a small-size array chip. As a result, the I-V characteristics are uniform throughout the 1 × 20 SBD array sample, as shown in Fig. 1(c). We have investigated the device performance such as the responsivity, noise equivalent power (NEP), and 3-dB bandwidth of the fabricated SBD detectors. The responsivity is measured by using a Gunn diode emitter (VDI) with a center frequency of 250 GHz and a
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25876
calorimeter (VDI, PM4). The NEP is measured by using a preamplifier (SRS, SR560) and an oscilloscope (Agilent, DSO1004A). As a result, the average responsivity is 98.5 V/W, and the average NEP is 106.6 pW/√Hz for a 1 × 20 SBD array detector sample. These values are lower when compared with those of other commercial SBD devices [13,14]; however, these detector devices are promising in terms of simple fabrication process and high yield. We have also measured the frequency response of the single-channel SBD using a fibercoupled CW THz spectroscopy system [15,16]. The CW THz system is shown in Fig. 2(a). The dual-wavelength optical signal emitted from the optical beating source is amplified at the EDFA, and it is then launched at the THz emitter to generate a THz wave. While tuning the optical wavelength, the single-channel SBD detector detects the amount of THz signal. According to the measurement results, the 3-dB band frequency of the fabricated single channel SBD sample is approximately 180 GHz, as shown in Fig. 2(b). A higher bandwidth can be achieved if the SiNx layer between the anode and the cathode is changed a material having a lower dielectric constant and higher thickness.
Fig. 2. (a) Fiber-coupled CW THz spectroscopy system setup and (b) detected THz signal measured by using the CW THz system.
3. Terahertz pulse imaging We have performed THz pulse imaging using a fiber-coupled THz pulse imaging system consisting of a THz-TDS emitter (which includes a femtosecond laser with a pulse width of 70 fs, an InGaAs photoconductive antenna (PCA)-integrated emitter, and a dispersioncompensation fiber), an InGaAs SBD detector, a computer-controlled 2-D stage, a sine-wave function generator, and a lock-in amplifier [17,18], as shown in Fig. 3(a). As shown in Fig. 3(b), the THz radiation bandwidth and maximum center frequency of the THz-TDS emitter are 3 THz and 180 GHz, respectively. For the InGaAs SBD detector, there is less scope for selection between the four different anode sizes for slope efficiency with regard to the SBD IV curves; however, an anode diameter of 1 μm achieves the highest turn-on voltage. Highefficiency THz pulse detection, with a difference in the detected SBD currents with and #196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25877
without THz pulse radiation generated from the THz-TDS emitter at a center frequency of approximately 180 GHz, is performed for anode diameters of 2 μm and 3 μm. We have measured the spot size of the THz wave beam along the THz propagation direction by a knife-edge method to determine the optimal position for THz imaging. According to the results, we have selected an image target position at a distance of 15 mm from the THz-TDS emitter. We have obtained the THz images using a THz pulse imaging system with a THz-TDS emitter and a single-channel SBD detector, as shown in Fig. 3(a), for three different anode size configurations to investigate the imaging performance. Figure 3(c) shows the THz imaging results of a medical knife behind a high-density polyethylene (HDPE) sheet with a thickness of 2 mm, where a cell size of 0.5 × 0.5 mm2, a pixel resolution of 110 × 30, and a bias voltage of 0 V are used.
Fig. 3. (a) THz pulse imaging system setup using a THz-TDS emitter and single-channel InGaAs SBD detector, (b) measured THz pulse spectrum of the THz-TDS emitter, with the inset showing the results on a logarithmic scale, and (c) photograph of a medical knife and its THz images measured by using the THz pulse imaging system.
Zero bias is applied to the SBD detector because an excellent image can be obtained owing to the relatively decreased noise. For an anode diameter of 3 μm, the THz imaging of the medical knife is finely performed. We find that the SBD detector with an anode diameter of 3 μm is more suitable for the THz-TDS emitter with a center frequency of 180 GHz. 4. Terahertz continuous-wave imaging We have designed an SBD chip board (CB) for easy connection to the other boards. Figures 4(a) and 4(b) show the SBD array chip bonded onto the SBD-CB along with a cylindrical Si lens placed on the SBD array chip and the transmitted IR image of the SBD array chip bonded onto the SBD-CB obtained by using an IR camera, respectively. Using a flip-chip bonding process, the SBD array chip and SBD-CB are connected electrically and mechanically. The solder pads of the SBD array chip and the solder balls of the SBD-CB are bonded in a self-passive alignment at a defined temperature and time [18].
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25878
Fig. 4. (a) SBD array chip bonded onto an SBD-CB and a cylindrical Si lens placed on the SBD array chip and (b) transmitted IR image of the SBD array chip bonded onto the SBD-CB obtained by using an IR camera.
We have also designed an SBD evaluation board (EB) to evaluate the 1 × 20 SBD array detector bonded onto an SBD-CB. In the SBD-EB, a 32 × 1 multiplexer (MUX) to switch the cells in the SBD array, a microcontroller unit (MCU) to control the address of the MUX, and an RS232 device to communicate with a personal computer (PC) are set up. Further, we have provided the setup to measure the THz signal of the Gunn diode emitter by focusing the signal on the SBD array chip using an HDPE biconvex lens. We have examined the THz signals measured by channel 10 of the SBD array as a function of the number of transparent tapes (Scotch) attached to the surface of the SBD array chip for antireflection (AR). By varying the number of transparent tapes, the highest THz signal is detected for four pieces of overlapped transparent tape, as shown in Fig. 5. The thickness of the four pieces of overlapped transparent tape agrees with the general AR coating condition, λ/4/neff, where λ and neff denote the wavelength of the Gunn diode emitter and the effective index of all the materials including the substrate, AR coating layer, and air, respectively. Furthermore, the THz signal in the case of a cylindrical Si lens with four pieces of overlapped transparent tape is 2.1 times greater than that in the case of four pieces of overlapped transparent tape only. However, it is noted that alignment between the SBD array chip and the cylindrical Si lens is required when this tape used. Thus, four pieces of overlapped transparent tape are attached to the surface of the SBD array chip during the experiment.
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25879
Fig. 5. THz signal detected by a single-channel SBD as a function of the number of transparent tapes with the parameters of the transparent tape only and a cylindrical Si lens with four pieces of overlapped transparent tape.
We have measured the focused radiation beam patterns of the Gunn diode emitter using 1D scanning by all the channels of the SBD array detector and 2-D scanning by channel 10 of the SBD array detector to investigate its feasibility. Figures 6(a) and 6(b) show the results of 20-channel 1-D scanning and single-channel 2-D scanning, respectively, where the vertical axis denotes the array chip direction, and the horizontal axis denotes the mechanical scanning direction. As a result, in the case of the single-channel 2-D scanning, the horizontal full width at half maximum (FWHM) and vertical FWHM are 5.1 mm and 5.1 mm, respectively. In the case of the 20-channel 1-D scanning, the horizontal FWHM is 5.1 mm, which is the same as in the 2-D scanning, while the vertical FWHM is smaller at 4.1 mm because of the different characteristics of the channels in the SBD array detector, such as detection capacity and crosstalk. In addition, the different scanning schemes along the horizontal and vertical axes may also contribute to these differences in vertical FWHM.
Fig. 6. THz radiation beam patterns of the Gunn diode emitter measured by using (a) all the channels and (b) only channel 10 of a 1 × 20 SBD array detector.
We have set up a THz CW imaging system using a Gunn diode emitter and a 1 × 20 SBD array detector, as depicted in Fig. 7(a), where a pixel size of 0.5 × 0.5 mm2, a pixel resolution of 120 × 40, and a bias voltage of 0 V are used. To obtain a line beam, an HDPE convex lens and cylindrical plastic lens are used, and two HDPE convex lenses are used in the system for focusing the image on the 1 × 20 SBD array detector. Figure 7(b) shows a photograph of a metal ring and clip used as samples. Figures 7(c) and 7(d) show the THz images of the samples measured by using all the channels and channel 10 only, respectively, of the 1 × 20 SBD array detector. In the THz imaging scheme using all the channels of the 1 × 20 SBD array detector, the calibrated 20 channels of the SBD are simultaneously scanned along the
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25880
horizontal direction. In the case of channel 10 only, the single channel of the 1 × 20 SBD is scanned along the vertical direction after scanning along the horizontal direction. Therefore, 1-D scanning by the 1 × 20 SBD array is faster than 2-D scanning by the single-channel SBD. However, blurring is observed from the imaging results of 1-D scanning by the 1 × 20 SBD array, as shown in Fig. 7(c). This is because all the cells of the 1 × 20 SBD array exhibit different responsivities and crosstalk characteristics, while all the cells exhibit the same characteristics during scanning for the single-channel 2-D scan method. However, almost same results are observed after image processing using a fast Fourier transform (FFT), as shown in Figs. 7(c) and 7(d). Furthermore, the SNR performance of THz imaging is enhanced because of the ability to perform frequency modulation up to 30 kHz when direct modulation of the Gunn diode emitter is used instead of a mechanical chopper in the system, as shown in Fig. 7(a).
Fig. 7. (a) THz CW imaging measurement setup, (b) photograph of a metal ring and clip, and their THz images measured by using the THz CW imaging system with (c) all the channels and (d) only channel 10 of the 1 × 20 SBD array detector.
5. Summary We have fabricated and characterized InGaAs SBD detectors. An average responsivity of 98.5 V/W, an average NEP of 106.6 pW/√Hz, and a 3-dB bandwidth at approximately 180 GHz for a 1 × 20 SBD array detector sample were measured. The structure comprising an SiNx layer instead of an air-bridge-type connection structure between the anode and the cathode achieved good uniformity and high yield as an array chip. We determined that the THz (f = 250 GHz) detection efficiencies of the SBD detector covered with four pieces of overlapped transparent tape and a cylindrical Si lens including four pieces of overlapped
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25881
transparent tape were enhanced by 86% and 298%, respectively, when compared with a bare SBD chip. THz pulse detection using a THz-TDS emitter with a center frequency of 180 GHz was effectively performed for anode diameters of 2 μm and 3 μm. The SBD detector with an anode diameter of 3 μm was more suitable for the THz-TDS emitter. The THz imaging of a medical knife behind an HDPE sheet was finely performed by using a THz pulse imaging system. We also successfully demonstrated THz CW imaging using 1-D scanning by the 1 × 20 SBD array detector and Gunn diode emitter. Blurring was observed from the imaging results; however, almost the same results were observed for 2-D scanning by a single-channel SBD after image processing using FFT. We found that the 1 × 20 SBD array detector is suitable for a real-time compact line scanner. Acknowledgments This work was partly supported by the IT R&D program of MOTIE/KEIT [10045238, Development of the portable scanner for THz imaging and spectroscopy], Joint Research Projects of ISTK, the Public Welfare & Safety Research Program through the National Research Foundation of Korea (NRF) Technology (NRF-2010-0020822), and Nano·Material Technology Development Program through the NRF of Korea (NRF-2012M3A7B4035095).
#196790 - $15.00 USD Received 30 Aug 2013; revised 28 Sep 2013; accepted 2 Oct 2013; published 22 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.025874 | OPTICS EXPRESS 25882
