Photodetectors play a pivotal role in almost all photonic applications. For example, in optical-communications links, photodetectors convert information carrying optical bit streams to electrical signals. In digital cameras, images are also recorded using photodetector arrays. Therefore, improving photodetector performance is essential for the advancement of photonics.
Photodetectors are usually made from semiconductor materials with a certain band gap. Only photons with higher energies than the band gap of the photodetecting material can be absorbed and converted to electrical signals. Graphene, a 2D carbon material assembled in a honeycomb lattice, can potentially be used to detect photons with any energy, because its band gap is zero. In fact, single- and few-layer graphenes have recently attracted significant attention from electrical engineers because of their unique band structures and extraordinary carrier mobility, which can be as high as 100,000cm2/Vs (volt second) at room temperature.
Fortunately, the optical properties of graphene are equally impressive. A single layer of graphene (nominally 0.34nm thick) can, in principle, absorb 2.3% of incident photons regardless of their energy. In comparison, 20nm of indium gallium arsenide (In0.53Ga0.47As) is needed to absorb 2.3% of the incident light at a wavelength of 1.55μm. Moreover, the carrier saturation velocity in graphene can be as high as 5×107cm/s, even under a very moderate electric field. Thus, graphene photodetectors are also ideal for high-speed applications. As a result, they may be useful in various applications, such as high-speed optical communications, security, surveillance, and long-wavelength imaging.
We performed photocurrent-imaging experiments1 in graphene field-effect transistors (FETs) to demonstrate the graphene photodetection function. In this experiment, we recorded the photocurrent generation in a graphene FET while a laser spot (from a helium neon laser, wavelength 0.633μm) was scanned across the device to obtain a 2D image. Around the metal-graphene contact, the photo-generated electron-hole pairs are separated by the built-in electric field, leading to a photocurrent in the external circuit: see Figure 1 (right). We performed similar experiments at 0.532, 1.55, and 3μm, and observed strong photocurrent responses at all wavelengths, demonstrating the ultrawide operational wavelength range of graphene photodetectors.
Figure 1. (left) Scanning-electron micrograph (SEM) of a graphene field-effect transistor (FET). (middle) Reflection image of the same FET. (right) Resulting photocurrent image.
Carriers in graphene can travel at very high speeds (up to 5×107cm/s) in even a moderate electric field because of high carrier mobility, and so the photocurrent is expected to respond to very fast light-intensity modulations. We measured the high-frequency photoresponse in graphene FETs up to a light-intensity modulation frequency of 40GHz and observed no photoresponse degradation.2This clearly reveals the ultrahigh-bandwidth nature of the photocurrent response in graphene.
In these simple graphene FET-based photodetectors, the effective photodetection area is limited to the vicinity of the metal-graphene contact area, resulting in a rather low external photoresponsivity. To enhance this, we introduced an interdigitated finger structure (see Figure 2) to expand the effective photodetection area. At an incident-light wavelength of 1.55μm, we obtained a maximum external responsivity of 6.1mA/W, a very impressive value given that only a single layer of atoms is involved in photon detection. We further applied such a graphene photodetector as a photon-detection element in a 10Gbit/s optical-communications link.3 We converted the 10Gbit/s optical bit streams to electrical pulses using the graphene photodetector: see Figure 2 (left). We generated the eye diagram in Figure 2 (right) by repeatedly plotting the recovered electrical pulses on the oscilloscope. The completely open eye indicates error-free detection.
Figure 2. (left) Scanning-electron micrograph of a metal-graphene-metal photodetector with an interdigitated finger structure. (right) Eye diagram obtained using a metal-graphene-metal photodetector as the light-detection element.
In summary, we have shown that graphene, a novel 2D carbon material, can find numerous photodetection applications because of its unique band structure and extraordinary optical properties. The major issue remaining now is its relatively low responsivity due to the limited light absorption in single- and few-layer graphene (2–10%). We are actively pursuing integration of graphene and optical components such as waveguides and optical cavities. Photodetectors with ultrahigh bandwidth, wide operational wavelength range, and good external responsivity (or efficiency) are expected to mature in the near future. Moreover, we recently showed that a direct band gap up to 150meV can be created in a biased bilayer graphene (see Figure 3).4This opens up an avenue for the realization of other advanced graphene photonic devices such as LEDs and terahertz emitters.
Figure 3. Biased bilayer graphene. E: Electric field. A1 atoms in the upper layer are located directly on top of B2 atoms in the bottom layer.
IBM Thomas J. Watson Research Center
Yorktown Heights, NY
Fengnian Xia is a research staff member. His research interests include integrated nanophotonics and nanoelectronics based on novel material systems. Currently, he focuses on group IV materials such as carbon (nanotubes and graphene), silicon, and germanium. He is also interested in carrier-transport processes in low-dimensional systems.
1. F. Xia, T. Mueller, R. Golizadeh-Mojarad, M. Freitag, J. Tsang, V. Perebeinos, P. Avouris, Photocurrent imaging and efficient photon detection in a graphene transistor, Nano Lett. 9, pp. 1039-1044, 2009. doi:10.1021/nl8033812