NTT and Japan's National Institute for Materials Science (NIMS) achieved the world's fastest zero-bias operation1(220 GHz) of a graphene photodetector (PD).
The research clarifies the optical-to-electrical (O-E) conversion process in graphene for the first time. Graphene has high sensitivity and high-speed electrical response to a wide range of electromagnetic waves, from terahertz (THz) to ultraviolet (UV). Thus, it is a promising photodetection material for enabling high-speed O-E conversion at wavelength ranges where existing semiconductor devices cannot operate. However, until now, the demonstrated zero-bias operating speed has been limited to 70 GHz due to conventional device structure and measurement equipment. For this reason, the challenge for graphene PDs is to demonstrate 200-GHz operation speeds and clarify graphene's inherent properties, such the process of optical-to-electrical conversion.
In this study, NTT and NIMS demonstrated high-speed operation with a 3dB bandwidth of 220 GHz by removing the current delay caused by the device structure by using zinc oxide (ZnO) thin film as the gate material and by using on-chip THz spectroscopy technology to read out the current at high speed. The research also found a trade-off between operating speed and sensitivity by comparing the characteristics of PDs fabricated with graphene of different qualities. The findings will enable graphene PDs to be optimized according to their intended use, such as in optical sensors prioritizing sensitivity or O-E signal converters prioritizing speed.
The research was published online in the British scientific journal Nature Photonics on August 25th, 2022.
