LARGE-AREA IMAGING: a-Si/MoS2 photodetector has faster photoresponse

Two engineers at the University of California, Berkeley (UC Berkeley) have created an amorphous silicon (a-Si) metal-semiconductor-metal heterojunction photodetector with added molybdenum disulfide (MoS₂) that they say could speed up medical imaging at low cost.¹ Molybdenum disulfide is well known as a dry lubricant.

Many photodetectors in large-area imaging devices use a-Si because it absorbs light well and is relatively inexpensive to process. But a-Si has defects that prevent the fast, ordered movement of electrons, leading to slower operating speeds and more exposure to radiation. Getting better performance requires more expensive, high-temperature processing, adding to the cost of the imaging device.

Sayeef Salahuddin and Mohammad Esmaeili-Rad solved this problem by pairing a thin film of MoS₂ with a sheet of a-Si. By forming a diode with the a-Si, the MoS₂ allows the photogenerated electrons it collects to travel ten times faster through the a-Si. The detector has a photoresponsivity of 210 mA/W for green light—two to four times higher than usual for a-Si devices.

The researchers say that because these materials are easy and inexpensive to handle, the cost of speeding up photodetectors would be minimal. Unlike conventional semiconductors like Si, MoS₂ consists of individual nanosheets that can be torn off like pages in a book. These sheets can be used to make thin, novel electronic devices or to improve existing ones.

Mechanically peeled flake

The structure consists of a 60-nm-thick mechanically exfoliated (peeled) MoS₂ flake on a silicon dioxide (SiO₂) substrate, with the flake covered by a 100-nm-thick a-Si film using plasma-enhanced chemical vapor deposition (see figure). Gold (Au) and titanium contacts were also added via electron-beam evaporation. A control device without the MoS₂ flake was also fabricated.

The photoresponse of the devices was measured using standard blue, green, and red LEDs as light sources at an incident power of about 0.4 mW/cm². The photoresponse to light from the three LEDs was much higher for the MoS₂-containing device. The response of the detectors to transients was also measured, showing a 3 to 5 ms residual conductivity for the a-Si devices but a lack of this residual response in the MoS₂-containing detector. As a result, say the researchers, the imaging speed of the new detector is increased by a factor of ten over the conventional a-Si detector.

This new photodetector structure could be used in large-area biomedical imaging, the researchers note. In one example, the fast response of the a-Si/MoS₂ photodetector could allow flat-panel x-ray imagers to reach frame rates of up to several kilohertz—much faster than the 10 to 1000 Hz frame rates of conventional a-Si-based x-ray imagers. But for practical use, large-area fabrication approaches such as chemical vapor deposition should be developed, replacing mechanical exfoliation. Ultimately, integration with MoS₂ transistors could be achieved, leading to large monolithic optoelectronic imaging devices.

REFERENCE
1. M. R. Esmaeili-Rad and S. Salahuddin, Sci. Rep., 3, 2345 (Aug. 2, 2013); doi:10.1038/srep02345.