Inkjet-printed non-volatile memory arrays using ferroelectric and resistive elements

Abstract

Non-volatile memory devices are crucial to printed electronic applications such as RFIDs and large-area sensors. At Palo Alto Research Center, we have developed both ferroelectric and resistive memories to facilitate integration of memory with printed systems. Ferroelectric transistor memory was demonstrated as short-term data storage elements, while resistive memory was used as control devices in a threshold detector that did not require active circuit components or battery. Inkjet printing was the patterning method, in which the electrode contacts were printed with silver nanoparticles. The organic feFETs were fabricated with poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the gate dielectric material. Application of an electric field across the ferroelectric insulator induced polarization of the dielectric layer. The remnant polarization alignment provided a record of the input voltage and thus served as memory states. The feFETs were combined with addressing TFTs into active-matrix memory arrays in order to avoid pixel cross talking. The memory feFETs were shown to retain 50% of output current over seven days. The transistor characteristics were monitored to understand the limiting factors to data retention time. We studied the degradation in the dielectric layer and in the semiconductor layer separately to understand the origins of possible device instabilities. We have examined the changes in semiconductor mobility m, threshold voltage VT, and dielectric capacitance Cf, with time. The measurement results indicated that shifts in VT was the main limit, as the current decay of feFET hysteresis was dominated by charge trapping. The use of resistive memory can simplify circuit designs and reduce power requirements for printed electronics. Here metal/oxide/metal junctions were the memristive devices (named from memristors, short for memory resistors), and they were characterized to infer ionic and electronic transport parameters such as mobilities and ion distribution. The flux dependence was measured for an individual junction as well as for junctions connected in a series. It was found that dopant redistribution was continuous and led to a peak in conductance. The mutable conductance of memristive junctions was utilized to demonstrate a threshold detector, in which the printed memristive junctions were connected with a piezo voltage-pulse input and an electrophoretic display output. The memristive circuit would switch the color of display pixels depending on the number of input pulses sensed by the piezo. This demonstration used only passive elements and no battery and illustrated the potentials of using memristive elements in printed electronics.