Tailoring broadband photoresponse in phase change vanadium dioxide thin films.
Tailoring broadband photoresponse in phase change vanadium dioxide thin films.

Most light sensors or photodetectors are made from silicon because it is cheap, high speed, very responsive and can be integrated easily with other electronic devices or systems. But silicon’s indirect bandgap limits its optical absorption range. Other III-V compound semiconductors can stretch absorption into the near-infrared and infrared ranges but are expensive and complex to fabricate. Now researchers may have come up with an alternative solution in the form of vanadium dioxide (VO2) [Kabir et al., Applied Materials Today 21 (2020) 100833, https://doi.org/10.1016/j.apmt.2020.100833 ].

This inorganic compound presents an intriguing proposition for optoelectronics because of its unique band structure and reversible phase transition from an insulator at room temperature to a metal above 68°C. The change can be triggered by light, heat, or electrically, while the phase transition temperature can vary according to film thickness, crystal grain size, substrate, or annealing conditions. As a small bandgap material, VO2 absorbs light from the ultraviolet to the IR range. However, most of the photogenerated charge carriers recombine, so Madhu Bhaskaran and her colleagues wanted to find ways to boost the performance of simple planar VO2 thin film devices.

“Enhancement of photoresponse is improved by two unique steps: firstly, with the miniaturization of device size and, secondly, with elevated temperature,” explains Sumaiya Kabir, first author of the study.

Because of the unique phase transition behavior of VO2, the researchers looked to improve photoresponse by raising the temperature. In both the 50-60°C and 65°C temperature regimes, the VO2 devices showed significantly increased photocurrent at all wavelengths of irradiation. The material can, therefore, be used in broadband detectors.

“Our fabricated VO2 devices can sense light in a broad wavelength range from UV to NIR at both room temperature and elevated temperature. In addition, the performance parameters are comparable to conventional and other oxide-, graphene-, and chalcogenide-based photodetectors,” she says.

The researchers also identified a size dependency effect in the insulator-to-metal transition temperature, which they believe is related to grain size and grain boundary density, and the presence of intermediate states that underpin the broadband photoresponse of VO2.

“An added advantage [of our approach] is the use of standard fabrication techniques,” she points out. “These results suggest that VO2 can be a potential candidate in different light sensing applications.”

The researchers now plan to work towards practical demonstrations of the fabrication and characterization of imaging arrays. The new insights into the sensing mechanism of VO2 photodetectors provide useful guidance on improving performance further and lowering power consumption.