1. Introduction

In optical data communications, graphene has attracted much attention as a versatile optoelectronic material because this one-atom-thick carbon sheet packed in a honeycomb crystal lattice can guide, control, modulate, and detect light [1–6]. By using its transparent thin metal-like characteristics, so-called plasmonic waveguides and polarization controllers have been developed [2, 3]. A strong electroabsorption effect and generation of electron-hole pairs in graphene allow us to develop graphene-based optical modulators and photodetectors [4, 5]. With the aid of the linear relation between the energy and momentum in graphene, graphene-based photonic devices can be operated in ultra-broadband wavelength light in the infrared to visible range.

Based on graphene’s versatile properties, the concepts of all-graphene photonic integrated circuits (PICs) have recently been proposed for developing on-chip optoelectronic systems [7]. To configure the photonic system completely, several elementary graphene photonic devices, such as plasmonic waveguides and modulators, have been demonstrated [8, 9]. Most investigations on the graphene PD have been based on the normal incidence of light [5, 10–12]. However, a grapene photodetector (PD) for detecting a horizontally incident beam is called for. Recently chip-integrated graphene PDs have demonstrated their ability to detect a horizontally incident beam [13–15]. However, these devices are like a combination of a graphene PD and a dielectric waveguide so they require hybrid integration.

In this paper, we developed graphene-based plasmonic photodectors (PDs) based on a graphene plasmonic waveguide structure. The optoelectronic characteristics were investigated at a wavelength of 1.55 μm. To detect light that propagates along the waveguide in the horizontal direction, the graphene PD structure was configured with an all-in-one graphene plasmonic waveguide. The photocurrent profile between the source and drain electrodes was measured with respect to the position of the graphene plasmonic waveguide on the PD. We also investigated the temporal behavior of the photocurrent of the fabricated graphene PD while light was switched on and off.

2. Architectural concept and fabrication

Figure 1(a) exhibits a schematic view of the proposed graphene-based plasmonic PD. It consists of a long, narrow graphene stripe for the plasmonic waveguide and a rectangular graphene ribbon for the photodetector. The two graphene structures cross each other and are configured by only one graphene film. The width of the graphene stripe ranges from 1 to 10 µm and the length varies according to the device length. The width and length of the graphene ribbon are W and L, respectively. D is the distance between the source metal contact and the point where the graphene stripe and the graphene ribbon touch. The proposed planar-type PD is a novel device concept that is different from conventional plasmonic PDs and chip-integrated graphene PDs [14–17]. In the plasmonic PDs, light is guided through metallic features but photon-detection is performed by semiconductor materials such as GaAs, InAs, or doped Si [14]. Chip-integrated graphene PDs require hybrid integration of a lightwave guiding medium and a graphene PD [15–17]. Compared to the hybrid graphene PD, the proposed graphene-based plasmonic PD provides easy and low cost mass production by configuring the whole device structure with one material.

 

Fig. 1 (a) Schematic view of the proposed graphene-based plasmonic potodetector for detecting a horizontally incident beam. (b) Fabricated device. Small metal pads between the two large metal pads are the source and drain contact pads. The inset shows the close-up of the fabricated graphene plasmonic PD structure, where the graphene stripe for the waveguide and graphene ribbon for the PD cross each other between the source and drain metals.

Download Full Size | PDF

A long, narrow graphene stripe extending along the device structure serves as a plasmonic waveguide. In a way similar to metal stripe-based plasmonic waveguides [18], light propagates along the graphene stripe in the form of surface plasmon polaritions [8]. A rectangular graphene ribbon with drain and source metal contact pads is employed as a graphene PD. When the propagating light along the graphene stripe reaches the graphene PD structure, it is absorbed by the graphene PD and generates electron-hole pairs. Consequently, a photocurrent I_ph is generated when an electric potential is applied between the source and drain. An alternative photocurrent can be obtained if the intensity of the guided light is periodically changed.

To fabricate the proposed plasmonic PD, we first formed a 20 µm-thick polymer dielectric on the Si substrate. Then, Graphene film that was synthesized on a 300 nm-thick Ni catalyst by chemical vapor deposition (CVD) was transferred on the cladding. Consequently, Ti/Au (5/100 nm) for the source and drain contact pads were formed on the graphene by e-beam evaporation through a shadow mask. Crossroad-type graphene structures were defined by an O₂ plasma ash process followed by a standard lithographic technique. Finally, 20 µm-thick cladding material was spin-coated again and the metal electrodes were opened for external electric contact. The refractive index of the polymer dielectric was 1.45. The propagation loss and the birefringence (n_TE – n_TM) of the optical polymer material at a wavelength of 1.55 µm were less than 0.35 dB/cm and 0.001, respectively.

Figure 1(b) shows the fabricated graphene-based plasmonic PD. The small metal pads between two large rectangular pads (alignment keys) serve as the source or drain metal contact pads. The graphene stripe for the waveguide is formed along the widthwise direction of the sample (passing through the metal pads) and the graphene ribbon for the PD is configured along the lengthwide direction (connecting the two metal pads). This is clearly shown in the inset of Fig. 1(b), where the 5 µm-wide graphene stripe crosses the 10 µm-wide 140 µm-long graphene PD between the source and drain contact pads, where D is 10 µm. Graphene PDs whose intersection point (D) increases with a 10 µm step were fabricated on the same substrate.

Our previous studies confirmed that the graphene film used in this study consists of various graphene domains, with sizes up to 20 µm, and the number of layers ranged from 1 to 8 [8, 9, 19]. The propagation loss and mode field diameter (MFD) varied according to the graphene strip width [8]. The narrowest graphene plasmonic waveguide provided the lowest propagation loss but its MFD was as large as 50 µm. Therefore, the measurement of the photocurrent profile was not easy. For a wide graphene stripe, the MFD was small but the propagation loss was high. Considering the trade-off between the loss and MFD, we set the graphene width as 5 µm with an MFD and loss of about 20 µm and 1.6 dB/mm [7].

3. Optoelectronic characteristics

Figure 2 presents the measured optoelectronic characteristics of the fabricated planar-type graphene PD. While the 1.55 µm wavelength light that is amplified by an erbium-doped fiber amplifier (EDFA) is launched at the input facet of the graphene plasmonic waveguide of the graphene PD using a single-mode polarization maintaining fiber (PMF), the photocurrents between the drain and source are measured.

 

Fig. 2 The measured optoelectronic characteristics of the fabricated planar-type graphene PD. (a) Current-bias voltage characteristics. The inset shows the dependence of I_ph on the input light intensity. (b) Photocurrent line scan profile. The inset exhibits what D is. (c) Dependence of the photocurrent on the length L and width W of the graphene PD.

Download Full Size | PDF

Figure 2(a) exhibits current-voltage (I_ds-V_ds) characteristics of the fabricated graphene PD. The inset shows the photocurrent according to the intensity of the input light. When the light excitation is turned off, the dark current (I_dark) of the graphene PD as a function of the bias voltage V_ds is yielded as I_ds = I_dark = V_ds / R_g, where V_ds is the applied bias voltage and R_g is the graphene’s resistance. On the other hand, when light coupled to the end-face of the graphene stripe via butt-coupled fiber is turned on, light propagates along the stripe and electron-hole pairs are generated in the graphene PD. Due to a Schottky-like barrier effect at the interface between the graphene and metal contact pads, the electron-hole pairs are separated by an external electric field, which leads to a photocurrent I_ph. Consequently, I_ds increases as I_ds = I_dark + I_ph. For a 38 mW (16.3 dB) optical input power, the averaged 3.16 µA photocurrent is added to the dark current. Shown as the inset of Fig. 2(a), the photocurrent increases with an increase in the input light intensity because high intensity light generates more electron-hole pairs. The measured external response is about 64 µA/W.

Figure 2(b) shows the photocurrent line scan profile depending on D, which is the distance between the drain metal contact and the point where the graphene stripe and the graphene ribbon touch. The gray regions indicate the metal contact pads. Similar to previous investigations on graphene PD [11–13], the fabricated planar-type graphene-based plasmonic PD shows an opposite polarity in the photocurrent profile. The profile is the same as that of a p-type doped graphene PD without gate bias voltage [12]. Most graphene film on a substrate loses its electron and behaves like a p-doped material and the metallic electrode behaves like an n-type material because of its sufficient free electrons. Therefore, p-n junctions are formed at the metal-graphene interface. While an electric field is applied between the drain and source electrode and while light is irradiated at the drain part, electrons drift to the nearby metal electrode and holes toward the bulk of graphene [12]. If light is launched at the source part, holes are attracted by the metal electrode and electrons go to graphene. Thus, the major carrier changes from an electron to a hole. As a result, an opposite polarity in the photocurrent profile is measured as shown in Fig. 2(b). For the fabricated device, the polymer cladding embedding the graphene PD is UV-curable polymer resin, which is solidified by a photo-initiator. When the resin is under UV light, free radicals with unpaired electrons are generated and they accept free electrons from double bonds in the resin [20]. During that procedure, the free radicals take electrons from the graphene and the graphene PD behaves as a p-type doped material. As a result, a similar photocurrent profile with an opposite polarity is obtained for the fabricated graphene PD. When light is illuminated at the center of the graphene PD, there is nearly no photocurrent. This is attributed to the absence of a strong electric field that separates the photoexcited electron-hole pairs. The photoexcitons are recombined efficiently in the region. The photocurrent profile depicts the in-plane electric field in the channel [12]. Because of the large MFD of the guide mode in the graphene plasmonic waveguide, an exact field distribution cannot be obtained. However, the energy band diagram at the metal-graphene interface can be roughly validated with this profile [12].

Figure 2(c) shows the dependence of the photocurrent on the width W and length L of the graphene PD. The photocurrent increases as W increases and as L decreases. These characteristics are attributed to the longer light-graphene interaction and the efficient separation of the electron-hole pairs. With an increase in the graphene PD width W, the interaction length between the graphene and photons increases. The number of the electron-hole pairs that are converted to current increases, and consequently, the photocurrent increases. The reduction of L increases the local electric field strength in the graphene PD so that more electron-hole pairs are separated into charged carriers and consequently, the photocurrent increases. In particular, the photocurrent decreases rapidly when L is 30 µm. This is attributed to the mode field diameter (MFD: the electric and magnetic field strengths are reduced to 1/e of their maximum values) of the graphene plasmonic waveguide. The MFD of the 5 µm-wide graphene plasmonic waveguide is about 20 µm. Thus, the field can reach the drain-graphene interface. Different from the source-graphene interface where a positive current flow is generated, the drain-graphene interface generates a negative current because the charge carrier is different. The amount of the negative courrent is less than that of the positive current. As a result, the photocurrent decreases extremely for the 30 µm-long graphene PD.

We now investigate the temporal behavior of the photocurrent to evaluate the fabricated graphene PD as an optical data receiver while the intensity of the light source was switched on and off. Figure 3(a) shows the normalized current response to on/off light irradiation. The graphene PD exhibits reproducible light detection without signal degradation under repeated light source on and off. The dynamic response is described by I(t) = I_dark + D{1–exp[–(t–t₀)/τ]} and I(t) = I_dark + Dexp[–(t–t₀)/τ] for rising and falling, respectively, where I_dark is the dark current, D is the scaling constant, t₀ is the light on/off time, and τ is the time constant. By fitting the experimental data to the above equation, we obtained the time constants of 39.7 ms and 31.5 ms for rising and falling, respectively. These time constants imply that modulation of frequencies in excess of 30 Hz could be detected. This time response to the pulsed optical input is much slower compared to that reported for single layer graphene [16]. This is attributable to the low carrier transport that arises from inhomogeneous layer distribution of the graphene film. The graphene film used in this study consists of various graphene, domains with sizes up to 20 µm and the number of layers ranges from 1 to 8 [8, 9, 19]. In addition, discontinuities with ripples, wrinkles, and contaminants may be generated when the graphene film is transferred to the polymer surface. This physical inhomogeneity acts as an obstacle of charge carrier transport in graphene. Application of single-layer graphene without those defects may result in realization of satisfactory optical devices [15].

 

Fig. 3 Temporal behavior of the fabricated graphene plasmonic PD. (a) The normalized current response to on/off light irradiation. Time response of photocurrent for (b) rising and (c) falling.

Download Full Size | PDF

4. Conclusion

To develop all-graphene photonic integrated circuits (PICs), a graphene-based plasmonic photodetector (PD) was developed. With a guiding light through the all-in-one graphene plasmonic waveguide, the fabricated graphene PD can detect a horizontally incident light signal. The photocurrent profile of the graphene PD has an opposite polarity because the major carrier is the electron and the hole for the drain and source part, respectively. The amount of the photocurrent is maximum at the graphene-metal interface and increases with an increase in the metal-graphene interface length. The time constants for the time response are below 39.7 ms. Considering our experimental results, we concluded that the proposed planar-type graphene PD can be exploited further for application in on-chip PICs.