Hongseok Oh1 2 3 JunBeom Park1 2 3 Woojin Choi4 Heehun Kim1 2 3 Youngbin Tchoe1 2 3 Arpana Agrawal1 2 3 Shadi Dayeh4 Gyu-Chul Yi1 2 3

1, Seoul National University, Seoul, , Korea (the Republic of)
2, Seoul National University, Seoul, , Korea (the Republic of)
3, Seoul National University, Seoul, , Korea (the Republic of)
4, University of California, San Diego, La Jolla, California, United States

Flexible electronics have recently attracted much attention for use in wearable devices and biomedical applications. For the bendable, stretchable and wearable devices, organic materials have been widely employed as channels due to due to their excellent scalability and flexibility. However, unsatisfactory electrical properties such as low electron mobility, easy degradation and poor integration density have hindered their widespread use. On the other hand, single crystalline inorganic materials have been widely used for high-performance electronic devices. For example, nanowire based field-effect transistor exhibited excellent electronic properties such as large Imax/Imin ratio, high carrier mobility and a small subthreshold swing. Nevertheless, requirements for single crystalline substrates such as SiC or sapphire have hindered to implement them on flexible electronics. To resolve this problem, a hybrid material system composed of one dimensional (1-d) inorganic nanostructures on two dimensional (2-d) nanomaterials such as graphene films have recently been proposed. The well-controlled inorganic nanostructures could serve as efficient channels for electronics with better electron mobility and stability. Great scalability and flexibility can be offered from the graphene substrates. Furthermore, precise position control of each individual nanostructures provides an advantage for realizing the high-density integration of devices, as well as integration with other electronics for flexible and/or wearable device applications. However, to utilize them as electronic devices, three-dimensional architectures are required since channels are grown vertically from the graphene substrates in contrast to conventional devices with lateral channels.
Here, we report the fabrication of vertical field-effect transistor(VFET) arrays using position- and morphology-controlled ZnO nanotube arrays grown on graphene films. For the fabrication of the VFET, single crystalline ZnO nanotubes were heteroepitaxially grown on graphene films with controlled position and dimension. The fabricated devices exhibited good electrical characteristics such as the small subthreshold swing of 110 mV/dec, high Imax/Imin ratio of 106 and a transconductance of 170 nS/um, thanks to its novel surrounding gate structures and single crystalline ZnO nanotube channels. Furthermore, fabricated VFET arrays could be transferred onto flexible substrates via simple mechanical lift-off process. The performance of the devices remained the same on flexible substrates, even at highly bent conditions. This research offers a general route to construct high-performance electronics for flexible and wearable device applications.