Photoresponse on Cu-Cu₂O-Cu Flexible Photodetectors Fabricated Using Laser-Induced Digital Oxidation

Optoelectronic devices can improve the quality of life by providing optical communication and the transport of information. In particular, photodetectors (PDs) refer to optoelectronic devices that detect a light signal by converting photons into a precise electrical signal. Recently, several studies have been conducted on flexible PDs to apply the new applications, such as oximeters, flexible cameras, and e-eyes. For mechanical stability, various materials, such as ZnO quantum dots and GaS nanosheets have been applied as flexible PDs. However, those materials have a slower response time and require a complex fabrication process. To overcome these problems, we proposed a controllable method to fabricate ultra-thin copper (Cu)-based metal-semiconductor-metal (MSM) PDs using laser-induced oxidation. Particularly, Cu is low price, excellent electrical conductivity, and its flexibility on flexible substrates, so it is a material suitable for the metal part of the MSM structure. Moreover, Cu can simply be converted to Cu oxide compounds (Cu_xO) using a laser oxidation process, which is possible to fabricate a site-selective oxidation. The Cu_xO is a promising p-type semiconductor with high stability and good photovoltaic characteristics. The structure of Cu-Cu_xO-Cu can improve its rectifying characteristics by providing a wider space-charge region.<br/>However, despite research on various Cu-based PDs, the amount of photocurrent changes according to channel scaling and charge-transport mechanism is still a lack of understanding. In recent years, scanning photocurrent microscopy (SPCM) has been verified as measurement technique to investigate the charge-transport mechanism in semiconductors. Therefore, we utilized to verify how the amount of photocurrent changes according to channel scaling. We confirmed that the photocurrent declined farther from the interface and large number of photo-excited carriers were created near the interface. Also, the photocurrent increases as the length of the Cu₂O decreases. Because the long channel-length devices exhibit more gradual band slopes. In addition, the amount of photocurrent increases as the width of the Cu₂O decreases was confirmed. Because the quasi-fermi level of photo-excited electrons shifts toward the conduction band. Furthermore, bending tests were performed to prove the flexibility of our devices. The bending radius is ~ 4 mm and it was tested at room temperature. Despite various numbers of bending cycles (up to 10,000 times), the resistance of our devices does not change. These results mean that Cu-based MSM PDs have substantial flexibility. Our findings could contribute an important guideline for understanding the mechanisms of other flexible PDs.<br/><b>Acknowledgments</b><br/>This work was supported by the National Research Foundation of Korea (NRF) grants funded by the MSIT (2019R1F1A1061883 and 2019M3C1B8090840), by the DGIST R&D Program of the Ministry of Science and ICT (19-CoE-BT-03), and by the Samsung Electronics University R&D program. J.S. and K.K acknowledge the support of the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (2020R1C1C1011219) as well as the research fund (1.190128.01 and 1.210035.01) of UNIST (Ulsan National Institute of Science and Technology). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.