Graphene photo transistor covers light, UV and IR

Zhaohui ZhongA photo-sensor that spans infra-red to ultra-violet without cryo-cooling is the result of novel graphene research. Wide band cameras could result.

Created at the University of Michigan, the device is a form of transistor whose room-temperature mid-infrared (3.2nm wavelength) responsivity is claimed to be “comparable with state-of-the-art infrared photodetectors”, said the team in a Nature Nanotechnology paper.

Graphene is known to be photo-sensitive, but with poor sensitivity because the one atom thick layer can only absorbs 2.3% of ambient light, and electron-hole pairs are apt to re-combine.

“The challenge for the current generation of graphene-based detectors is that their sensitivity is typically very poor. It’s a hundred to a thousand times lower than what a commercial device would require,” said Michigan professor Zhaohui Zhong.

Rather than detecting excitons directly, the researchers built a sandwich structure consisting of two graphene layers separated by a few nm of intrinsic silicon as an insulator.

The insulator is thin to allow hot electrons to tunnel through, which they do more easily than holes can tunnel.

Photons hitting the top graphene layer produce electron-hole pairs, from which electrons preferentially tunnel into the lower layer. Importantly, this process leaves a positive charge on the top graphene layer.

By building source and gate electrodes into the bottom layer, the bottom layer can be used as the channel of a mosfet, with the top layer as its photo-sensitive floating gate.

“The graphene channel has high carrier mobility and is very sensitive to external electrostatic perturbation, while the thin oxide film in this device design not only favours hot carrier tunnelling, but also induces high interlayer dielectric capacitance,” the team told Nature, adding: “The approach allowed the sensitivity of a room-temperature graphene device to compete with that of cooled mid-infrared detectors for the first time.”

Tunnelled electrons make no real difference to channel flow, with a few pA of tunnelling current joining tens of µA in the channel.

Michigan gives the sensitivity of graphene alone “tens of mA/W”, rising to 1A/W (mid-IR) in its transistor.

This is a second-generation device. Silicon was chosen to allow the less-energetic electrons produced by infra-red photons to tunnel across the barrier.

Its initial device used 5nm of tantalum oxide instead of silicon, which made it three orders of magnitude less sensitive to IR, but more sensitive to visible light.

The prototype detectors are under 20µm across, and are thought of as single pixels in a future image sensors.

“We are working on a room temperature infra-red camera,” Zhong told Electronics Weekly.

He also said modelling indicates sensitivity of these or similar structures should go out to beyond 10µm infra-red, and thinks THz operation is on the cards.

The device is described in a paper titled “Graphene photodetectors with ultra-broadband and high responsivity at room temperature,” which appears online in Nature Nanotechnology on March 16.

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