Highly Efficient and Flexible Photosensors with GaN Nanowires Horizontally Embedded in a Graphene Sandwich Channel

Abstract

In this study, we report highly efficient and flexible photosensors with GaN nanowires (NWs) horizontally embedded in a graphene sandwich structure fabricated on polyethylene terephthalate. GaN NWs and the graphene sandwich structure are used as light-absorbing media and the channel for carrier movement, respectively. To form uniform high-quality crystalline GaN NWs on Si(111) substrates, the initial nucleation behavior of the NWs was manipulated by applying the new growth technique of Ga predeposition. High-resolution transmission electron microscopic images obtained along the vertical direction of GaN NWs showed that stacking faults, typically observed in Si-based (In,Ga)As NWs, were rare. Consequently, narrow and strong optical emission was observed from the GaN NWs at wavelengths of 365.12 nm at 300 K. The photocurrent and photoresponsivity of the flexible photosensor with 802 nm long GaN NWs horizontally embedded in the graphene sandwich channel were measured as 9.17 mA and 91.70 A/W, respectively, at the light intensity of 100 mW/cm2, which are much higher than those previously reported. The high optical-to-electrical conversion characteristics of our flexible photosensors are attributed to the increase in the effective interface between the light-absorbing media and the carrier channel by the horizontal distribution of the GaN NWs within the graphene sandwich structure. After 200 cyclic-bending test of the GaN NW photosensor at the strain of 3%, the photoresponsivity under strain was measured as 89.04 A/W at 100 mW/cm2, corresponding to 97.1% of the photoresponsivity obtained before bending. The photosensor proposed in this study is relatively simple in device design and fabrication, and it requires no sophisticated nanostructural design to minimize the resistance to metal contacts.

Fast Response Characteristics of Flexible Ultraviolet Photosensors with GaN Nanowires and Graphene

“…The spatial densities of GaN NWs for the photosensors are almost the same. In addition, the effect of the length and spatial density of GaN NWs on the device performances of the photosensors was reported in our previous work …”

“…The average diameter and height of the undoped GaN NWs were measured as 63.8 ± 1.3 nm and 1.1 ± 0.1 μm, respectively. High-resolution TEM images and diffraction patterns obtained along the vertical direction of GaN NWs showed that stacking faults were rare . This indicates the formation of high-crystalline GaN NWs.…”

“…Figure b shows an optical microscopic image of the fabricated photosensors (top) and a magnified image of a single device (bottom). As mentioned in our previous work, the photosensor proposed in the present study is a relatively simple device in terms of design and fabrication, and it requires no sophisticated nano-structural design to minimize the resistance of its metal contacts Figure c shows a cross-sectional FE-SEM image of the GaN NWs grown using plasma-assisted molecular-beam epitaxy.…”

“…This indicates the formation of high-crystalline GaN NWs. Additional details about the growth conditions of the GaN NWs are available elsewhere Figure d shows the θ–2θ X-ray diffraction (XRD) rocking curve of the GaN NWs.…”

“…To achieve flexible III-nitride NW-based photosensors, various device structures have been proposed. , For instance, Shi et al demonstrated flexible photosensors using vertically aligned GaN NWs fabricated on a polyethylene terephthalate (PET) substrate . The photosensors were flexible; however, some of their electrical properties, such as their photocurrent, EQE, and photoresponsivity, were strongly influenced by agglomeration between adjacent GaN NWs and chemical contamination by metal electrodes or catalysts. , In our previous work, we demonstrated a new approach to improve the optical-to-electrical conversion efficiency of flexible UV photosensors by fabricating devices using highly crystalline GaN NWs horizontally embedded in a graphene sandwich structure …”