High-speed and broadband terahertz wave ... - OSA Publishing

2 downloads 0 Views 427KB Size Report
cautiously selecting the gate dielectric materials in a large-area graphene-based field-effect transistor (GFET). An ultrathin Al2O3 film (∼60 nm) is deposited by ...
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS

5649

High-speed and broadband terahertz wave modulators based on large-area graphene field-effect transistors Qi Mao,1 Qi-Ye Wen,1,* Wei Tian,1 Tian-Long Wen,1 Zhi Chen,2 Qing-Hui Yang,1 and Huai-Wu Zhang1 1

2

State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China

National Key Laboratory of Science and Technology of Communications, University of Electronic Science and Technology of China, Chengdu 610054, China *Corresponding author: [email protected] Received July 17, 2014; revised August 25, 2014; accepted August 30, 2014; posted September 2, 2014 (Doc. ID 216913); published September 24, 2014 We present a broadband terahertz wave modulator with improved modulation depth and switch speed by cautiously selecting the gate dielectric materials in a large-area graphene-based field-effect transistor (GFET). An ultrathin Al2 O3 film (∼60 nm) is deposited by an atomic-layer-deposition technique as a high-k gate dielectric layer, which reduces the Coulomb impurity scattering and cavity effect, and thus greatly improves the modulation performance. Our modulator has achieved a modulation depth of 22% and modulation speed of 170 kHz in a frequency range from 0.4 to 1.5 THz, which is a large improvement in comparison to its predecessor of SiO2 -based GFET. © 2014 Optical Society of America OCIS codes: (230.4110) Modulators; (300.6495) Spectroscopy, terahertz; (160.4236) Nanomaterials; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/OL.39.005649

Technologies generating, detecting, and manipulating an electromagnetic wave at the frequency of terahertz (THz) have been broadly investigated to fulfill their great potential in fundamentals as well as in many practical applications such as communication, remote sensing, imaging, and astronomy [1–4]. On the other hand, nanomaterials—especially carbon nanotubes and graphene— have attracted extensive interest for applications in THz technologies to make devices with superior performance over traditional materials, including THz source, detectors, and modulators [5–8]. In an advanced THz system, one of the key components mostly desired is high-speed modulators that actively control the spatial transmission (reflection) of an incident THz wave [9–12]. A graphenebased field effect transistor (GFET) can be used to tune the carrier concentration in graphene by applying a voltage at the gate, making it possible to modulate the absorption/transmission of THz waves through the devices. In most GFET devices, ∼300 nm SiO2 was used as the gate dielectric on top of a p-type silicon (p-Si) substrate. However, when used as a THz broadband modulator, GFET with graphene∕ ∼ 300 nm SiO2 ∕p − Si structures has drawbacks such as high switching voltage, small modulation depth, and slow modulation speed [13,14]. Alternatively, Al2 O3 with a dielectric constant of 7.5 is a high-K material with numerous outstanding dielectric properties in comparison with SiO2 [15]. It was reported that the transconductance (gm ) of GFET using ∼72 nm Al2 O3 as a gate dielectric is higher than that of ∼300 nm SiO2 [15], and the Coulomb impurity scattering can also be greatly reduced at the graphene∕Al2 O3 interface [16]. All these features infer a better THz modulation performance if we replace the graphene∕SiO2 interface with a graphene∕Al2 O3 interface in a GFET THz modulator. In this work, we propose a high efficiency broadband THz wave modulator with quicker modulation speed and larger modulation depth by using an Al2 O3 -based largearea graphene GFET device. The modulator consists of graphene monolayer∕ ∼ 60 nm Al2 O3 ∕p-Si structures. 0146-9592/14/195649-04$15.00/0

In our devices, an intensity modulation depth of 22% (1% per 1.36 V) and a modulation speed of 170 kHz have been successfully achieved, which are notable improvements from previously reported 15% and 20 kHz, respectively, in the broadband modulators with graphene∕ ∼95 nm SiO2 ∕Si structures [13]. Large modulation depth could also be achieved by metamaterial-based devices [17]. However, they are inherently narrowband in terms of amplitude/phase modulation. Our work suggests that a GFET modulator with graphene∕Al2 O3 ∕p-Si structures provides a feasible method for quick modulation of a THz wave. Here Al2 O3 fabricated by an atomic layer deposition (ALD) technique was chosen because it is a high-κ material that can be made ultrathin, smooth, and pinholefree over a large area. The native oxide layer on the (100) p-Si (ρ ∼ 1– 10 Ω cm) substrate was dissolved by a buffered oxide etching solution to make a naked hydrophobic silicon surface. An Al2 O3 film was deposited on top of the silicon substrate by an ALD technique at 120°C using trimethylaluminum (TMA) and O2 as the source [18]. A total of 480 cycles was performed to fabricate an Al2 O3 film with a thickness of 60 nm. Figure 1(a) shows the x-ray diffraction (XRD) of the Al2 O3 thin film on an Si substrate scanned by the Cu Kα radiation. The strong peak at 69° is attributed to the silicon substrate, and the sharp peak at ∼33° coincides with the reflection of the α-Al2 O3 phase, which indicates the Al2 O3 film is single crystalline. Figure 1(b) shows the cross section of the sample characterized by a scanning electron microscope (SEM). As shown in Fig. 1(b), the deposited Al2 O3 layer is dense and smooth with a thickness of ∼60 nm. The thickness of the Al2 O3 layer was precisely controlled at a deposition rate of 0.125 nm per cycle. Both an XRD spectrum and an SEM image confirm Al2 O3 films were well prepared by the ALD system. Large-area monolayers of graphene films were synthesized by chemical vapor deposition (CVD) on a copper foil, which were then transferred onto the ∼60 nm © 2014 Optical Society of America

5650

OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014

voltage at the gate in a GFET. The intraband and interband transitions play a crucial role to determine the electromagnetic wave absorption by graphene. The optical conductivity σω of graphene is given by σω  σ intra ω  σ inter ω;

Fig. 1. (a) θ − 2θ scan of Al2 O3 based GFET by XRD with CuKα radiation and (b) SEM image of the cross section of a singlecrystalline ∼60 nm Al2 O3 film on the Si substrate. The inset shows the surface of graphene∕Al2 O3 obtained by an optical microscope. The dashed lines are guides for the eye.

Al2 O3 ∕p-Si substrate and 300 nm SiO2 ∕p-Si substrate, respectively [19,20]. The SiO2 ∕p-Si substrate was used to fabricate the reference sample. The transferred graphene monolayers on the Al2 O3 ∕p-Si and SiO2 ∕p-Si substrate were characterized by a Raman spectroscopy with a 514 nm laser in Fig. 2(a). The two obvious peaks in the Raman spectra of graphene on the Al2 O3 ∕p-Si substrate are the G peak at ∼1591 cm−1 and 2D peak at ∼2687 cm−1 . The peak intensity ratio I G ∕I 2D is ∼0.43, and the full width at half-maximum (FWHM) of the 2D peak is about 38 cm−1 . In addition, the intensity of the D peak at 1341 cm−1 is low with an I D ∕I G ratio of ∼0.18, indicating that the transferred graphene is a high-quality monolayer which is similar to the sample grown on the SiO2 ∕p-Si substrate [21,22]. The optical microscope shows the top view of the graphene∕Al2 O3 in the inset of Fig. 1(b), confirming that the device surface is smooth, uniform, and pinhole-free over large areas. Figure 2(b) shows the conical band structures of graphene, whose Fermi level can be shifted by applying a

where ω is angular frequency of the photon [13]. In the THz range, intraband transitions dominate. For a broadband modulator, the Drude model can be used to describe the relation between the optical conductivity and the DC electrical conductivity of graphene, namely, σω  σ DC E F ∕1  ω2 τ2 , where σ DC E F  is the DC electrical conductivity at the Fermi level E F and τ is the carrier scattering time [13]. As a result, the transmittance of the THz wave could be modulated by varying the Fermi level of graphene, which is realized by applying different bias voltages at the gate. Ambient atmosphere and processing residual on the surface of graphene often deviate the Dirac point of graphene from the zero voltage point [23]. To tune the transmittance of THz wave through the graphene, a bias at gate was applied to deviate E F further from the charge neutrality point (CNP) where carrier density of state and thus σ DC E F  and σω are minimized because of the conical band structure. To evaluate the CNP of monolayer graphene in the Al2 O3 and SiO2 -based GFET, silver paste was used as the source and drain electrodes, respectively. In addition, p-Si was used as the back gate to modulate the carrier concentration and Fermi level of graphene, whose geometry is depicted in Fig. 2(c). Figure 2(d) shows the total resistance between source and drain (Rtotal ) as a function of back gate voltage (V bg ). In the Al2 O3 -based GFET, the maximum Rtotal occurs at 18 V, where Fermi level is located at the CNP for this devices. At the CNP, carrier concentration is minimized while the THz transmittance is maximized respectively. The fabrication of SiO2 -based GFET is similar to that of the Al2 O3 -based devices. The CNP of SiO2 based GFET is ∼15 V, which is very close to Al2 O3 based devices. The carrier mobility (μ) in GFET can be calculated by μ  gm L∕WCg V ds ;

Fig. 2. (a) Raman spectra of the monolayer graphene on the 300 nm SiO2 ∕p − Si and 60 nm Al2 O3 ∕p − Si substrate (b) conical band structure of graphene and the sweeping of the Femi level. (c) The scheme of GFET and THz transmission (W  5 mm, L  5 mm). (d) Total resistance Rtotal as a function of back gate voltage V bg in the GFET.

(1)

(2)

where gm  dIds ∕dVbg jVdsconstant is obtained from the Rtotal − V bg curve shown in Fig. 2(d); both the length (L) and width (W ) of graphene channel are 5 mm; the constant V ds is 1 V; the back-gate capacitance per unit area, C g , is 11.9 nF∕cm2 for SiO2 GFET and 110.625 nF∕cm2 for Al2 O3 GFET [24]. With these parameters the carrier mobility of graphene on Al2 O3 and SiO2 were calculated to be 682.4 cm2 V−1 s−1 and 546.2 cm2 V−1 s−1 , respectively. It can be seen that the carrier mobility of GFET on Al2 O3 is larger than that on SiO2 . More importantly, the gm of GFET on Al2 O3 (∼75.5 μS) is calculated to be 11 times higher than that on SiO2 (∼6.5 μS) [15]. According to Eq. (2), the huge difference in gm indicates a large discrepancy in the backgate capacitance (C g ) and consequent the carrier density variation for Al2 O3 -and SiO2 -based GFET. Because of the larger gm and higher carrier mobility, a larger modulation

October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS

depth and higher modulation speed can be anticipated in an Al2 O3 -based GFET modulator, respectively. To compare the THz modulation performance of SiO2 and Al2 O3 based GFET modulator, voltages ranging from −20 to 10 V at an increment of 5 V were applied at the back gate for both modulators. Breakdown voltage is the main concern in selecting the applied voltage. The effect of the substrate was subtracted from the whole devices by measuring a blank p-Si sample as a reference, so that modulation purely from graphene can be obtained. In the following analysis, signals from p-Si substrate are removed, and the transmission of THz wave through the graphene was normalized to the blank p-Si substrate. We measured the spectral transmission of the modulators by standard THz-time domain spectroscopy (TPS Spectra 3000, TERAVIEW). Figure 3(a) shows the normalized intensity of a THz wave at the frequency from 0.4 to 1.5 THz after transmitting through a GFET with graphene∕ ∼ 300 nm SiO2 ∕p-Si sandwich structures. A small change in THz wave transmission at different V bg was observed, indicating a small swing of Fermi level of graphene under different V bg between 20 and −10 V. To study the maximum transmittance, here modulation depth is defined as jT10 V − T−20 V∕T10 Vj at 1 THz, which has a minimum transmission of 88% at −20 V and a maximum of 90% at 10 V respectively for SiO2 -based GFET. The modulation depth was calculated to be ∼2% (1% per 15 V). The relatively thick SiO2 dielectric layer would increase the cavity effect along the transmission direction, leading to the weaker modulation [13]. Figure 3(b) shows the modulation curves measured for Al2 O3 -based GFET. A distinctive variation in THz wave transmission was observed at different gate voltage from −20 to 10 V. At 1 THz, the transmission exhibited a minimum of 71% at −20 V and reached a maximum of 91.3% at 10 V. The total modulation depth was calculated to be

Fig. 3. Normalized intensity of transmitted THz wave through the (a) SiO2 and (b) Al2 O3 -based GFET at different back gate voltage. The modulation depth of Al2 O3 -based GFET as a function of applied gate voltage is shown in (c), and (d) the comparison of the amplitudes of the THz wave transmission through the GFET modulators with ∼300 nm SiO2 and ∼60 nm Al2 O3 dielectric at 1 THz.

5651

22%. The amplitude of THz transmission is approximately flat from 0.4 to 1.5 THz at each gate voltage, indicating that the Al2 O3 based GFET is an efficient broadband THz modulator. Figure 3(c) shows the extracted modulation depth of Al2 O3 -based GFET from 0.4 to 1.5 THz at different applied gate voltages. It is clear that the maximum modulation depth of 22.5% occurs at 0.85 THz with V bg  −20 V. The transmission of the THz wave is primarily determined by carrier density, which in turn can be precisely tuned by V bg . Figure 3(d) compares the transmitted amplitudes of THz wave at the frequency of 1 THz through the GFET with an SiO2 and Al2 O3 gate dielectric at different V bg . As shown in Fig. 3(d), the modulation of THz wave transmission can be greatly improved by replacing the SiO2 with Al2 O3 as dielectric materials in GFET. Dynamic modulation characteristics of Al2 O3 -based GFET were further investigated at 340-GHz carrier. The measurement setup consists of a VDI (Virginia Diodes) continuous-wave (CW) THz source with a central output in the 340 GHz, and a 240–400 GHz zero-bias Schottky diode intensity detector. The modulation speed of the modulator was monitored by the responding waveform to a square modulation voltage. The modulation voltage is −10 and 10 V at the minimum and maximum, respectively, with different frequencies. Figure 4(a) shows the typical responding waveform of Al2 O3 -based GFET THz modulator under a frequency of 5 kHz. The normalized modulation magnitude at different modulation frequency was summarized in Fig. 4(b), which indicates a 3 dB bandwidth (f c ) is ∼170 kHz. In addition, the RC time constant of the device is also an important parameter that affects the modulation speed. As the total resistance of the Si substrate is negligibly small compared with that of graphene, the resistance of the device was estimated to be 261Ω by extracting the average resistance of that measured at the different back voltage from −10 to 10 V. The capacitance (C) can be calculated by C  Aεε0 ∕d, where ε 7.5 is the relative dielectric constant of the ALD deposited Al2 O3 film, ε0 is the permittivity of free space, A is the effective area of active graphene device of 5 mm × 5 mm, d is thickness of the Al2 O3 film of 60 nm. The capacitance C is calculated, according to Ref. 15, to be ∼27.7 nF. As a result, the calculated RC time constant is ∼138.7 kHz, which is consistent with the directly measured 3 dB bandwidth. These results

Fig. 4. (a) Modulated responding waveform to a square pulse (f  5 kHz) applied at back gate and measured at the Schottky diode detector for a carrier frequency of 340 GHz. (b) Normalized modulation magnitude at different modulation frequency, showing a 3 dB operation bandwidth of ∼170 kHz.

5652

OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014

confirm that an Al2 O3 -based GFET possesses a higher modulation speed than the previously reported graphene THz wave modulators [13]. Our results show that Al2 O3 is superior to SiO2 as a gate dielectric for GFET THz modulation. As previous reports [15], impurity scattering would significantly reduce the gm of charge carriers in GFET. The high-k dielectrics reduce the Coulomb impurity scattering to achieve a high gm in GFET and thus to obtain a high THz modulation depth [25]. On the other hand, a thinner dielectric layer is necessary to eliminate the strong cavity effect by reducing the cavity dimension along the transmission direction, giving rise to an improvement of gm , too. In this circumstance, high-k materials with small thicknesses and a large-area uniformity, such as HfO2 grown by ALD technology [26], are also suitable for GFET based THz modulators. In conclusion, we demonstrated that the replacement of a 300 nm SiO2 with a 60 nm Al2 O3 as gate dielectric can significantly improve the performance of GFET THz wave modulators. Compared to the 300 nm SiO2 , an enhancement in the transconduction and consequently the modulation depth was observed by using 60 nm Al2 O3 as gate dielectric. Furthermore, the modulation speed has increased from 20 to 170 kHz as well. Our work provides an effective method to fabricate a high-quality GFET THz wave modulator, which is vital for fundamental researches and many THz technology applications such as THz communication, imaging, and beam shaping. Improvement could be made with different high-K materials and patterned GFET arrays [12]. Such works are in progress. This work is financially supported by the National Nature Science Foundation of China (No. 61131005), the Keygrant Project of Chinese Ministry of Education (No. 313013), National High-tech Research and Development Projects (No. 2011AA010204), the New Century Excellent Talent Foundation (No. NCET-11-0068), the Sichuan Youth S & T foundation (No. 2011JQ0001), and the start-up fund from UESTC. References 1. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, Semicond. Sci. Tech. 20, S266 (2005). 2. R. Kohler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, Nature 417, 156 (2002). 3. H. Li and J. C. Cao, Semicond. Sci. Technol. 26, 95029 (2011).

4. M. Tonouchi, Nat. Photonics 1, 97 (2007). 5. B. Heshmat, H. Pahlevaninezhad, and T. E. Darcie, IEEE Photon. J. 4, 970 (2012). 6. B. Heshmat, H. Pahlevaninezhad, M. C. Beard, C. Papadopoulos, and T. E. Darcie, Opt. Express 19, 15077 (2011). 7. L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H. A. Bechtel, X. Liang, A. Zettl, Y. R. Shen, and F. Wang, Nat. Nanotechnol. 6, 630 (2011). 8. L. Vicarelli, M. S. Vitiello, D. Coquillat, A. Lombardo, A. C. Ferrari, W. Knap, M. Polini, V. Pellegrini, and A. Tredicucci, Nat. Mater. 11, 865 (2012). 9. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, Nature 444, 597 (2006). 10. Q. Y. Wen, H. W. Zhang, Y. X. Xie, Q. H. Yang, and Y. L. Liu, Appl. Phys. Lett. 95, 241111 (2009). 11. Q. Y. Wen, H. W. Zhang, Q. H. Yang, Y. X. Xie, K. Chen, and Y. L. Liu, Appl. Phys. Lett. 97, 21111 (2010). 12. C. M. Watts, D. Shrekenhamer, J. Montoya, G. Lipworth, J. Hunt, T. Sleasman, S. Krishna, D. R. Smith, and W. J. Padilla, Nat. Photonics 8, 605 (2014). 13. B. Sensale-Rodriguez, R. Yan, M. M. Kelly, T. Fang, K. Tahy, W. S. Hwang, D. Jena, L. Liu, and H. G. Xing, Nat. Commun. 3, 780 (2012). 14. I. Maeng, S. Lim, S. J. Chae, Y. H. Lee, H. Choi, and J. Son, Nano Lett. 12, 551 (2012). 15. L. Liao, J. Bai, Y. Qu, Y. Huang, and X. Duan, Nanotechnology 21, 15705 (2010). 16. C. Jang, S. Adam, J. H. Chen, E. D. Williams, S. D. Sarma, and M. S. Fuhrer, Phys. Rev. Lett. 101, 146805 (2008). 17. R. Yan, B. S. Rodriguez, L. Liu, D. Jena, and H. G. Xing, Opt. Express 20, 28664 (2012). 18. M. D. Gronera, J. W. Elama, F. H. Fabreguette, and S. M. George, Thin Solid Film 413, 186 (2002). 19. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, Science 324, 1312 (2009). 20. J. Liu, P. Li, Y. Chen, Z. Wang, J. He, H. Tian, F. Qi, B. Zheng, J. Zhou, W. Lin, and W. Zhang, J. Alloys Compd. 615, 415 (2014). 21. L. M. Malard, M. A. Pimenta, G. Dresselhaus, and M. S. Dresselhaus, Phys. Rep. 473, 51 (2009). 22. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, Phys. Rev. Lett. 97, 187401 (2006). 23. X. Liang, B. A. Sperling, I. Calizo, G. Cheng, C. A. Hacker, Q. Zhang, Y. Obeng, K. Yan, H. Peng, Q. Li, X. Zhu, H. Yuan, A. R. H. Walker, Z. Liu, L. Peng, and C. A. Richter, ACS Nano 5, 9144 (2011). 24. F. Schwierz, Nat. Nanotechnol. 5, 487 (2010). 25. A. Konar, T. Fang, and D. Jena, Phys. Rev. B 82, 115452 (2010). 26. J. H. Lee, I. H. Yu, S. Y. Lee, and C. S. Hwang, J. Vac. Sci. Technol. B 32, 03D109 (2014).