Functionalized organic memory devices toward sensor integrated applications

Abstract

Organic electronic technology offers promising form factors and some of the functionalities key to future electronics. In particular, organic devices with memory attributes can carry multiple roles at once with their innate functionality. In this work, such possibilities are explored mainly in two cases of organic memory devices based on i) a floating gate flash memory device and ii) a resistive memory device. As a first example, organic photomemory devices are proposed in a flash-memory-like geometry where an isolated charge storage node or floating gate is replaced with an isolated photo-absorption zone embedded between upper and lower insulator layers without directly interfacing neither with an organic channel semiconductor nor a gate electrode. In order to preserve the integrity of $C_{70}$ as a photo-absorption zone, insulator layers enclosing $C_{70}$ are prepared with polymer dielectric layers deposited by an initiated chemical vapor deposition (iCVD) method. This isolated photo-absorption zone then allows the device to operate in electrically ‘on’ state, in which the high electric-field region can have a maximal spatial overlap with the illuminated area for efficient and facile light-programming. In an effort to understand the mechanism of the proposed organic photomemory device, programming time, quantum efficiencies, and the exposure responses are characterized as photosensors and memory devices. With the proposed approach, a significant threshold voltage shift is achieved even with the exposure time as short as 5 ms, which is readily compatible with modern photography. High quality dielectric layers prepared by iCVD ensure erasing to occur only with electrical signal in a controlled manner. Retention time up to 700s is demonstrated. The endurance up to 50 times of light-programming/ erasing cycles is demonstrated. As a second case, the functionality integration based on a resistive memory is performed from the perspectives of memory-and-sensor integration. A multi-level resistive memory device is demonstrated for the proposed scheme based on iCVD-grown polymer dielectrics called ‘pV3D3’. A 10 nm-thick $MoO_x$ interlayer between a copper top electrode and pV3D3 is employed to expand the working environmental conditions of the proposed resistive memory devices to those deficient of oxygen sources. In advance to an integration with sensors, a multi-level set operation of the proposed resistive memory device is demonstrated by limiting compliance current or applying current pulses. The proposed device exhibit six states of resistances by conditioned ‘set’ processes, thus proving the feasibility to exhibit multi-bit data in a single device. A simple integration to achieve strain sensing in a digitized manner is then performed with a strain sensor based on compression bandages. A logarithmic change of resistance is confirmed in this sensor device thanks to grainy silver-based conducting paths on the rough surface of bandage fabrics. By combining one resistive sensor device and one memory device, the proposed schematic showed strain-sensitive memory characteristics within 4.5 % of strain and about 105 of on/off ratio. Lastly, a photo-sensing and memorizing devices based on a resistive memory device is proposed by a series connection of a photodiode with the tested resistive memory device. It is confirmed that a photodiode is applicable not only as a light-sensitive part but also as a selector device by exploiting its diode-like nature and by incorporating bipolar resistive memory operation. By fully utilizing operation quadrants of a photodiode, both light-sensitive ‘set’ and electrical ‘reset’ in the dark are demonstrated. As a result, the operation of the proposed schematic as a functional light-sensitive memory pixel that can express six levels of the irradiance is confirmed with a Si photodiode by 4 V and 1 ms of ‘set’ pulses, and the feasibility of this concept is also proved with an organic photodiode which results in 2-bit operation with 6 V of low voltage ‘set’ operation.