Terahertz and infrared photodetectors based on multiple graphene layer and nanoribbon structures INVITED PAPER V. RYZHII *1,2 , N. RYABOVA 1,2 , M. RYZHII 1 , N.V. BARYSHNIKOV 2 , V.E. KARASIK 2 , V. MITIN 3 , and T. OTSUJI 4 1 Computational Nanoelectronics Laboratory, University of Aizu, Ikki-machi, 965-8580 Aizu-Wakamatsu, Japan 2 Centre for Photonics and Infrared Engineering, Bauman Moscow State Technical University, 5 Vtoraya Baumanskaya Str., 105-005 Moscow, Russia 3 Department of Electrical Engineering, University at Buffalo, Buffalo, NY 1460-1920, U.S.A. 4 Research Institute for Electrical Communication, Tohoku University, Komada, 980-8577 Sendai, Japan We consider new concepts of terahertz and infrared photodetectors based on multiple graphene layer and multiple graphene nanoribbon structures and we evaluate their responsivity and detectivity. The performance of the detectors under consider- ation is compared with that of photodetectors made of the traditional structures. We show that due to high values of the quan- tum efficiency and relatively low rates of thermogeneration, the graphene-based detectors can exhibit high responsivity and detectivity at elevated temperatures in a wide radiation spectrum and can substantially surpass other detectors. The detector being discussed can be used in different wide-band and multi-colour terahertz and infrared systems. Keywords: terahertz, infrared, photodiode, graphene, nanoribbon. 1. Introduction Among 3-, 2-, 1-, and 0-dimentional (3D, 2D, 1D, and 0D) carbon structures, whose variety is shown schematically in Fig. 1, graphene layers (GLs), i.e., monolayers of carbon atoms forming a dense honeycomb two-dimensional crystal structure, as well as non-Bernal stacked multiple graphene layers (MGLs) have attracted a considerable attention due to their unique features. In particular, GLs and MGLs exhibit very specific optical properties associated with the gapless energy spectrum and linear dispersion law for electrons and holes (see, for instance, an extensive review by Castro Neto et al. [1]). Owing to a rather high quantum efficiency of interband transitions in a single GL (SGL), graphene na- noribbons (GNRs), and graphene bilayers (GBLs) [1,2], they are very promising for detectors of terahertz (THz) and infrared radiation (IR) [3–8]. Indeed, the probability of absorption b, of a photon incident on a GL (at the zero tem- perature) is expressed via the fundamental constants: b = p pa e c 2 0 023 h = @ . , where e is the electron charge, h is the reduced Planck constant, c is the speed of light in vacu- um, and a = @ e c 2 1 137 h . The quantity b exceeds the pro- bability of the intersubband (intraband) photon absorption in quantum wells by more than the order of magnitude. Moreover, as demonstrated recently (see the paper by Sprinkle et al. [9] as well as the review paper by Orlita and Potemski [10] and the references therein), the stacks of dis- oriented the non-Bernal stacked GLs epitaxially grown on a C-terminated surface of 4H-SiC, exhibit virtually the same optical spectral characteristics as individual GLs, but much stronger interband absorption. This is because, in contrast to the MGL structures with the Bernal stacking (which nor- mally constitute graphite), the rotational stacking results in the electron decoupling of GLs in the non-Bernal stacked MGLs. As a result, each GL in such a structure exhibits the energy spectrum of electrons and holes similar to that in individual GLs, i.e., the gapless spectrum [10]. Due to the contributions of each GL to the optical absorption, MGLs can exhibit fairly high absorption coefficient. Such a system of MGLs can be considered as a new form of carbon, which constitutes a novel material (which, for brevity, could be referred to as grapheneplex, Ref. 8) for optoelectronics. One of the remarkable feature of this material is high quality of GLs which characterized by fairly long electron and hole momentum relaxation time. The latter provides relatively low intraband (Drude) absorption of radiation. The use of the MGL structures in question opens up the prospect of farther enhancement of capabilities of graphene- -based optoelectronic devices, in particular, THz and IR photodiodes. The GNR structures also ex-hibit the proper- ties which are very useful for THz and IR detectors. The res- Opto-Electron. Rev., 20, no. 1, 2012 V. Ryzhii 15 OPTO-ELECTRONICS REVIEW 20(1), 15–25 DOI: 10.2478/s11772-012-0009-y * e-mail: v-ryzhii@u-aizu.ac.jp