IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 49, NO. 7, JULY 2013
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Graphene-Si Schottky IR Detector Mina Amirmazlaghani, Farshid Raissi, Omid Habibpour, Josip Vukusic, and Jan Stake, Senior Member, IEEE Abstract— This paper reports on photodetection properties of the graphene-Si schottky junction by measuring current–voltage characteristics under 1.55-µm excitation laser. The measurements have been done on a junction fabricated by depositing mechanically exfoliated natural graphite on top of the prepatterned silicon substrate. The electrical Schottky barrier height is estimated to be (0.44–0.47) eV with a minimum responsivity of 2.8 mA/W corresponding to an internal quantum efficiency of 10%, which is almost an order of magnitude larger than regular Schottky junctions. A possible explanation for the large quantum efficiency related to the 2-D nature of graphene is discussed. Large quantum efficiency, room temperature IR detection, ease of fabrication along with compatibility with Si devices can open a doorway for novel graphene-based photodetectors. Index Terms— Graphene, Si, Schottky diode, Detector.
I. I NTRODUCTION
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RAPHENE is a two-dimensional material which has attracted much attention due to its remarkable optical, thermal and electronic properties since its discovery in 2004 [1]–[3]. This single atom layer can absorb the incident light in a broadband range of frequencies through interband and intraband transitions [4], [5]. Based on these two transitions, in addition to the photo-thermoelectric effect in graphene [6]–[9], different kinds of detectors have been fabricated working at different wavelength ranges [5], [10]–[19]. Among these different frequency ranges of detectors, photo detection properties in the wavelength range from C band (1528–1561 nm) to L band (1561–1620 nm) are of great interest and importance from optical communication point of view [20]. Graphene-FET detectors have been reported to operate at 1.55μm laser illumination with a photo responsivity of 0.5mA/W at a gate bias of 80 V and internal quantum efficiency of (6 to 16)% [11]. This responsivity has been improved to (1.5 to 6.1) mA/W at 15 V gate bias in [12], by creating a wider photo-detection region and providing higher E-field using finger-shaped gates and an asymmetric metallization scheme. However, the responsivities of these photo detectors are restricted by the limited optical absorption Manuscript received December 5, 2012; revised February 26, 2013 and April 10, 2013; accepted April 18, 2013. Date of publication May 3, 2013; date of current version May 31, 2013. M. Amirmazlaghani and F. Raissi are with the Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran 16314, Iran (email:
[email protected];
[email protected]). O. Habibpour, J. Vukusic, and J. Stake are with the Department of Microtechnology and Nanoscience, Terahertz and Millimetre Wave Laboratory, Chalmers University of Technology, Göteborg 412 96, Sweden (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2013.2261472
in graphene, short carrier lifetime and small effective photodetection area [11]–[13]. Different methods have been applied to compensate the limited optical absorption in graphene and recently complete optical absorption in graphene is reported in [21]. Nevertheless, the effects of short carrier life time and small effective photodetection area in graphene detectors have not been solved yet. In order to detect radiation intended for optical communication in Si-based chips, other IR detectors like Germanuimbased photo detectors, all-Si photodetectors and Schottky detectors are usually used [20], [22]–[31]. Germanuim-based photo detector in these wavelength range; which is one of the best choices, is not compatible with Si chips due to the thermal mismatch and their aggressive substrate cleaning processes [23], [24]. All-Si photodetectors, in the best case, have a responsivity peak of 0.08mA/W at 1.55μm [20] while Schottky IR detectors have a limited efficiency ( 30% is also predicted for graphene devices illuminated by a 532nm laser while a strong electric field is applied at the metal-graphene interface [10], [15]. One important feature which can determine QEint is the collection mechanism of photo-generated carriers in the device. Most of the reported photo responsivities of graphene are based on graphene-metal contacts, so the collecting mechanism of photo-generated carriers for the most part is limited by short carrier lifetime and a small photo detection area [10]–[15]. On the other hand, the enhancement of internal QE in graphene devices will occur when applying a new method to separate the absorbed photo-carriers other than the electric field of the graphene-metal contacts in [10]–[15].
In the case of graphene-FETs, photo-generated carriers are captured by the electric field of graphene-metal contacts, so only a small fraction of carriers which have been generated around the contacts can be collected by the external circuit. The rest of the carriers will recombine without any contribution to the external photo current. As a result, the effective photo detection area in graphene FETs are restricted to narrow regions adjacent to the graphene-metal interface [12]. But in graphene-Si Schottky diode, photo-generated carriers transport vertically which is completely different from transport mechanism in graphene-FETs detectors. In a graphene-Si Schottky junction, all of the photo carriers in graphene, independent of their excitation location; have a similar chance to be absorbed by Si. So, the effective photo detection area in graphene-Si schottky diodes is only restricted by the contact area between graphene and Si. It is worth noting that the relatively short lifetime of the photo-generated carriers of graphene are not a detriment to graphene-Si schottky diode performance because the thin graphene layer is shorter than the mean recombination length of the carriers. Thus, the photo-generated carriers in graphene in all parts of the junction can be absorbed before recombining. In other words, in graphene-Si schottky diode, the transport of photo-generated carriers is vertical which means if the generated carriers pass through the thin layer of graphene, they will be absorbed by Si. But, in graphene-FETs, the photo-generated carriers transport horizontally and lots of them recombine before reaching to the metal contact. This slow collecting behavior of graphene-FETs, not only decrease the speed of the device but also decrease the responsivity. As graphene-Si schottky diode doesn’t suffer from such problems, it collects the carriers faster and has higher responsivity in comparison with graphene-FET detectors. IV. D ISCUSSION Regular Schottky diode detectors have a limited internal quantum efficiency (less than 1% in most of the cases) compared to graphene-Si Schottky diodes which can be defined by modified Fowler theory [26]–[31] . Modified Fowler theory [26]–[31] provides an equation for the internal quantum efficiency of Schottky detectors given by 1 (hυ − φ B )2 (4) 8φ B hυ where hν is the photon energy, and φB is the Schottky barrier [26]–[31]. This equation is obtained as the ratio of the holes in the metal, which can escape over the barrier into the semiconductor substrate, to the number of total holes, which are created due to the absorption of photons. Shown graphically, this ratio corresponds to the volume of the cap in Fig 5.a divided by the total volume of the sphere in momentum space [26]–[28]. As shown in Fig. 5.a, only a fraction of photo carriers, holes in our case, whose kinetic energy normal to the surface is sufficient to overcome the potential barrier are injected into the substrate and results in internal QE less than 1% in most cases [25], [26]. The graphene-Si schottky diodes’ QE exceeds the theoretical limit provided by (4). In graphene, there is a linear relation between energy and momentum and ηi =
AMIRMAZLAGHANI et al.: GRAPHENE-Si SCHOTTKY IR DETECTOR
it is not possible to directly apply the relation (4) for graphene samples. A possible explanation for the high internal quantum efficiency of graphene-Si Schottky detectors is based on the two dimensionality of graphene and the presence of π orbitals in graphene which are normal to Si surface. These orbitals provide only two directions for graphene carriers’ momentum which are in the same or opposite directions of Si. So, almost half of graphene’s carriers have a chance to enter the Si and the quantum efficiency will not follow the previous limitation. In a two dimensional system such as graphene, the spheres of Fig. 5.a can be reduced to two circles one corresponding to a momentum perpendicular to the semiconductor pointing toward it and another to a momentum opposite that as shown in Fig. 5.b. In such a case the ratio, which is calculated corresponding to those carriers, which are pointing toward the semiconductor and have energies, which exceed the potential barrier. These carriers, which also have a linear energy-momentum relation; occupy a ring in one of the circles and equation (4) is reduced to: 1 (hυ)2 − φ 2B (5) ηi = 2 (hυ)2 This equation allows for much larger efficiencies compared to three-dimensional materials. On the other hand, it is possible that the other graphene carriers which momentums are in the opposite direction of Si; change their momentum direction due to the scattering by graphene sidewalls, such as the case of thin film metals [25], [26]. We reiterate that the external quantum efficiency is related to the number of photons that graphene can absorb, which is small. To take practical advantage of the increased internal efficiency the photon collection efficiency must be increased as well. The incident photon energy in our experiments is about 0.8 eV and with a potential barrier of 0.449 eV we obtain the efficiency about 34%. Considering side wall scattering [25], [26], internal QE larger than 34% is possible for graphene-Si schottky diodes under 1.55μm excitation laser which practically can be achieved by applying reported methods [21] to get the complete optical absorption in graphene. The presented model also corresponds to the modified Fowler models when the thickness of silicide approaches zero in Si-based schottky diodes. In this case the internal efficiency is given by [31] hυ − φ B (6) ηi ≈ hυ It is important to note that this equation is considered to be applied to systems with parabolic energy bands, but in the case of graphene; which has a linear energy-momentum relation, equation (6) changes to (5) as explained above. The increase of quantum efficiency to values more than the theoretical limit has been observed in other unconventional Schottky junctions, in which the escape cap of Fig. 5.a can be increased to several caps and can in the limit cover the whole volume between the two spheres. Such a behavior occurs in PtSi/porous Si Schottky junctions. Because of the porosity of Si, the photo-excited holes effectively see several surfaces in different directions in their close proximity.
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Thus, the single allowed cap for transfer of holes in Fig. 5 effectively increases to several caps in different directions normal to every surface [26]. It is important to noting that application of an electric field to the graphene layer via an isolated gate, making a gated graphene-Si schottky contact, can change the responsivity at different wavelengths by modifying the Fermi level. V. C ONCLUSION Schottky contact made of graphene and Si is sensitive to 1.55μm IR laser with an efficiency which is larger than the other graphene-based photo detectors as well as regular Schottky junctions. Larger responsivities compared to other graphene-based detectors are obtained due to larger effective photo detection area and fast collecting behavior of schottky contact. Based on its 2-D nature and linear energy-momentum relation of graphene, an internal quantum efficiency of more than 34% is predicted for graphene-Si Schottky diodes under 1.55μm laser. Experiments are underway to extend the study of graphene-Si detection properties to Terahertz frequencies. R EFERENCES [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science, vol. 306, no. 5696, pp. 666–669, 2004. [2] A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene,” Rev. Mod. Phys., vol. 81, no. 1, pp. 109–162, 2009. [3] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science, vol. 320, no. 5881, p. 1308, 2008. [4] J. Dawlaty, S. Shivaraman, J. Strait, P. George, M. Chandrashekhar, F. Rana, M. G. Spencer, D. Veksler, and Y. Chen, “Measurement of the optical absorption spectra of epitaxial graphene from terahertz to visible,” Appl. Phys. Lett., vol. 93, no. 13, pp. 131905-1–131905-13, 2008. [5] B. Sensale-Rodriguez, R. Yan, M. M. Kelly, T. Fang, K. Tahy, W. S. Hwang, D. Jena, L. Liu, and H. G. Xing, “Efficient terahertz electro-absorption modulation employing graphene plasmonic structures,” Appl. Phys. Lett., vol. 101, no. 26, p. 26115-126115–3, 2012. [6] P. A. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene,” Nano Lett., vol. 8, no. 12, p. 4248, 2008. [7] D. Sun, Z.-K. Wu, C. Divin, X. Li, C. Berger, W. A. de Heer, P. N. First, and T. B. Norris, “Hot dirac fermions in epitaxial graphene,” Phys. Rev. Lett., vol. 101, no. 15, p. 157402, 2008. [8] X. Xu, N. M. Gabor, J. Alden, A. van der Zande, and P. L. McEuen, “Photo-thermoelectric effect at a graphene interface junction,” Nano Lett., vol. 10, pp. 562–566, Jan. 2010. [9] N. M. Gabor, J. C. W. Song, Q. Ma, N. L. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L. S. Levitov, and P. Jarillo-Herrero, “Hot carrier–assisted intrinsic photoresponse in graphene,” Science, vol. 334 no. 6056 pp. 648–652, 2011. [10] F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photon., vol. 4, pp. 611–622, Aug. 2010. [11] F. N. Xia, T. Mueller, Y.-M. Lin, A. Valdes-Garcia, and P. Avouris, “Ultrafast graphene photodetector,” Nat. Nanotechnol., vol. 4, no. 12, pp. 839–843, 2009. [12] T. Müller, F. Xia, and P. Avouris, “Graphene photodetectors for highspeed optical communications,” Nature Photon., vol. 4, p. 297, Feb. 2010. [13] F. Xia, T. Mueller, R. Golizadeh-Mojarad, M. Freitag, Y. Lin, J. Tsang, V. Perebeinos, and P. Avouris, “Photocurrent imaging and efficient photon detection in a graphene transistor,” Nano Lett., vol. 9, no. 3, pp. 1039–1044, 2009.
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Farshid Raissi received the B.S. degree in electrical engineering from Louisiana State University, Baton Rouge, LO, USA, in 1988, and the M.S. and Ph.D. degrees from the University of Wisconsin, Madison, WI, USA, in 1992 and 1995, respectively. He has been a Faculty Member with the K. N. Toosi University of Technology, Tehran, Iran, since 1996. His current research interests include superconducting amplifying and detector devices, MEMS, semiconductor field effect devices, and IR detectors.
Omid Habibpour received the B.S. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, in 2002, the M.S. degree in electrical engineering from the Amirkabir University of Technology, Tehran, in 2004, and the Ph.D. degree in graphene electronics from the Chalmers University of Technology, Gothenburg, Sweden. He is currently with the Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology.
Josip Vukusic received the Diploma and Ph.D. degrees in photonics from the Chalmers University of Technology, Göteborg, Sweden, in 1997 and 2003, respectively. He has been with the Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, since 2004, where he is involved with terahertz technology. He is currently involved in modeling, fabrication, and characterization of frequency multipliers for terahertz generation.
Jan Stake (S’95–M’00–SM’06) was born in Uddevalla, Sweden, in 1971. He received the M.Sc. degree in electrical engineering and the Ph.D. degree in microwave electronics from the Chalmers University of Technology, Göteborg, Sweden, in 1994 and 1999, respectively. He was a Research Assistant with the University of Virginia, Charlottesville, VA, USA, in 1997. From 1999 to 2001, he was a Research Fellow with the Millimetre Wave Group, Rutherford Appleton Laboratory, Didcot, U.K. He then joined Saab Combitech Systems AB, where he was a Senior RF/Microwave Engineer in 2003. From 2000 to 2006, he held different academic positions with Chalmers University of Technology, and from 2003 to 2006, was the Head of the Nanofabrication Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology. In 2007, he was a Visiting Professor with the Submillimeter Wave Advanced Technology Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. He is currently a Professor and the Head of the Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience (MC2). He is the co-founder of Wasa Millimeter Wave AB, Göteborg. His current research interests include sources and detectors for terahertz frequencies, high-frequency semiconductor devices, graphene electronics, and terahertz measurement techniques and applications. Prof. Stake is a Topical Editor for the IEEE T RANSACTIONS ON T ERA HERTZ S CIENCE AND T ECHNOLOGY .
