Measuring ultrafast current of atomically thin photodetector

2 mins read

A team of engineers from Cornell University has come up with an inventive way of measuring the near-instantaneous electrical current generated using a light detector made from an atomically thin material.

The team, headed by Farhan Rana, associate professor in the School of Electrical and Computer Engineering, measured the ultrafast response of their two-dimensional photodetector using a strobe-like process called two-pulse photovoltage correlation.

Prof Rana said: "He [Haining Wang] came up with this idea of essentially hitting the device with an optical pulse [to initiate an electrical charge] and after a small delay, hitting it with the pulse again. By varying the time between the first and second pulse, and looking at the response of the device as a result, you can sort of see what the intrinsic speed of the device is."

Prof Rana's team used a three-atoms-thick sheet of molybdenum disulphide (MoS2), a material Prof Rana and others have tested previously in photodetection studies. Photodetection is used in various high-speed optoelectronic applications, including optical fibre networks.

According to Wang's experimentation, the MoS2 photodetector had intrinsic response times as short as 3ps. Wang said the speed at which the MoS2 detector responds is superior to current technology, and is partly due to the short distance the charges generated by light must travel before making it out of the device and into the external electrical circuit.

Wang said: "State-of-the-art optical communication links work at around 10GHz per channel, so if you make 10 channels in parallel, you have a 100GHz optical communication link. We find that this single device can work up to 300GHz, which is an amazing speed."

Wang also said that, despite being just three-atoms thick, MoS2 is "extremely easy to make" and relatively inexpensive.

As with all photodetectors, however, the downside is the low quantum efficiency, which is a measure of the number of charges generated by the detector in the external circuit per incident photon.

In the team's work, only a small percentage of the light-generated charges - 1 to 2% - was able to escape the photodetector and make it into the external circuit; most recombined inside the device, producing heat. Market-available photodetector materials such as silicon and gallium arsenide, while generally much slower, have efficiencies of anywhere from 50 to 90%.

Further research by the group will include coating the sample with a chemical that they say will virtually eliminate the recombination after a group from the University of California, Berkeley recently reported an efficiency of 95% using their chemically coated MoS2 photodetector.

Prof Rana said the photodetection technology will play a major role in emerging fields, such as LiFi - using light as a source of wireless communication. He said windows and walls could be coated with atomically thin layers of material that would interact with light and carry Internet signals.